Radiant Barriers, simple explanation

When I talk to a lot of folks about radiant barriers, I get the sense that their level of understanding is about where mine was five years ago. They sort of get it, but they sort of don't. Energy Efficiency Man knows that blocking this particular form of heat transfer (radiant energy) is incredibly important in hot climates, and is highly underutilized out there in the USA, costing us lots of unnecessary energy use and reducing our summertime comfort.

So what is a radiant barrier? Put in the simplest terms, it's 100% shade for your house. That radiant barrier is keeping sunlight from heating up your house the same way a huge oak tree that shades the entire roof would. In fact, it's probably doing a better job; depending on the angles involved, that pesky sun is probably sneaking around at least part of that theoretical tree and heating your house at some point during the day. The "shade" from the radiant barrier, the area under it hidden from the sun, moves less during the day since it's right down at the level of your attic, rather than a certain number of feet higher up where a shade tree canopy would be. If you picture the sun moving across the sky and an two objects casting shadows, one tall and one short, you will realize that the higher up (or taller) an object is, the further its shadow is going to move along the ground as the sun crosses the sky. So if you want the shadow to cover your living space for more of the day (and you do!), you want the shading object as low over the living space as possible.
To put it in slightly more detailed and accurate terms, a radiant barrier is like 97% shade (the rating for a good foil barrier), except that unlike the shade tree which absorbs the energy of the light to produce nutrients for the tree, the barrier reflects the energy back the way it came.
Another typical question: "OK, I understand the shade thing, but the radiant barrier is inside your attic. It's already in the shade. Plus, I've read that it can be facing down, and still work. I can see how it would work if it were like a mirror facing up, but if you install it on the underside of the rafters, it's facing the wrong way. How can that even be possible?" My answer? You've got me. I just know it does work, really, REALLY well, and I've got the numbers to prove it! If I were to guess, the simplification most of us make to think of photons as particles that bounce off of the barrier and get reflected probably just isn't accurate enough. Light is also like a wave; perhaps we're talking about a dramatic change in the index of refraction of the medium which is transmitting the photons causing the energy to be reflected (even though the reflective side is facing the wrong way). Comments from folks with real physics knowledge are welcome. One clue as to the physics might be that the radiant barrier needs to have an air gap next to it to operate; in effect, the reflective side must be facing an empty air space of at least an inch, or the barrier won't work.
For me, it's good enough to stick with the shade analogy. Who wouldn't want to put their house in 97% shade, without having to wait 20 years for a good shade tree to grow, to say nothing of trimming the limbs, raking leaves, and worrying about things falling on your house?
If you've read this far, you don't have a radiant barrier, you have less than 97% natural shade on your house, and you live in a hot climate, this post is for you. You know what to do.

Heat Transfer, revisited

I have thought a lot about how I have managed to reduce my electricity usage by over 50%, with a good bit of that reduction occurring before I added any additional insulation, the one thing people usually think of when talking about home energy efficiency. I think I can boil it down to three major factors, followed by some explanations:

1. My attic lies between my entire living space and the sun
   
2. My attic used to work against me, trapping the sun's energy
3. My attic now works for me to reject the sun's energy
   

Let's examine my attic space's performance with regard to the 3 mechanisms of heat transfer (explanation here) before any improvements were made:

1. Conduction: I had roughly R-20 insulation in the attic, enough to slow conduction somewhat, though far below code. This was probably my "least bad" heat transfer problem.
2. Radiation: I had a composite shingle roof which gathered the radiant energy of sunlight all day, heating up and then radiating its own energy down into the attic all day and all night, where it heated up the air and the insulation. The energy that hit the insulation turned into heat in the insulation that eventually conducted into the living space, costing me energy to remove via air conditioning. This was probably tied with #3 as my worst heat transfer problem.
3. Convection: I had very little air intake into the attic since my soffit vents were mostly blocked, and too few outflow vents on the roof, and those that I had were badly placed (not at the peak). Thus, all the air heated up by the radiant energy in #2 tended to stay in the attic for very long periods of time. In short, my attic was working as a rather effective solar oven to heat up a bunch of air, then hold it right next to my living space for a very long time. Again, this was probably tied with #2 as my worst heat transfer problem.

As you can see, in my first 9 years or so in this house, from 1996-2005, I was expending way more energy than I should have needed to, because my attic was working against me in every mode of heat transfer.

Contrast that to now, when through some rather simple improvements, I've seen the following:

1. Conduction: In this most recent year, I've improved my insulation to about an R-49 level, current to today's building codes, from an R-20 level. This should be helping me reduce conduction gains in the summer (and losses in the winter) by some 50%. Again, my feel is that this is the least significant improvement for summertime (winter is another matter), which is one reason I did it last, but it should be helpful. Another reason to do this one last is that all the insulation gets in your way when you're trying to work in the attic!
   
2. Radiation: In the last two years, I completed installing the radiant barrier. Instead of acting to gather the sun's heat and put it into my attic air and insulation, my attic now acts to reject over 90% of the sun's heat right back out through the roof, before it can warm anything other than the shingles.
   
3. Convection: By opening more soffit vent intakes and installing a continuous vent along the ridgeline of the roof, I'm allowing convection to work for me to actively cool the attic by replacing air that heats up and rises through the ridge vent with cooler air that is at the outside air temperature. Since my attic routinely got over 150 degrees, and even on hot days, the outside air is around 100 degrees at the hottest, this has been a huge help.

As you can see, rather than having 2 of the 3 heat transfer mechanisms actively working against me (radiation and convection), I have largely stopped radiation, and convection is now working for me rather than against me. These two things alone dropped my yearly electric usage by 50%. The attic insulation that I've added to address conduction should help as well by reducing the magnitude of the conduction heat gain.

All this talk of heat and 100 degree days seems odd right now in December. The temperature outside is in the 40's, and I do wonder what effect the improvements will have on my house's wintertime performance. Too much attic ventilation, after all, will keep that attic colder and will increase my conductive heat flow from the living space to the attic. The radiant barrier, while reflecting any radiated heat from the top of the insulation back down, is also rejecting the sun's rays that would be nice to have during this cold time of year. Both of these factors should be mitigated by the additional insulation I've added. After all, these improvements are a balancing act, hopefully well-tuned to the requirements of my local climate. Rest assured, efficiency enthusiasts, that this too will be analyzed in a few months as data comes in!

Usage Comparison: 2005 - 2009

Well, the data is all but in for 2009. Although I don't have my electricity usage for December measured (or even completed yet), I can estimate it pretty well because it is consistent from year to year, plus the value is so low since there is no cooling demand that I can be off by a large percentage and it won't particularly change my results. So I'll go with 11 months of real data and 1 month (December) copied from the 2008 data. Here are my results (drum roll please):

As you can see, the usage for 2009 is well below the 2008 usage, even though 2009 was the hottest year here in central Texas, EVER. The improvements that helped lower the usage this year included completing the radiant barrier (which was only about 40% done in 2008) and installing enough attic insulation to bring the R-value up close to the current code of R-49. In all previous years, the insulation was an estimated R-20.
The numbers:
2005 usage = 13866 kWh
2009 usage = 5731 kWh
Reduction for all improvements (read the Executive Summary to see them all): 58%

For just the 2008-2009 comparison:
2008 usage = 6982 kWh
2009 usage = 5731 kWn
Reduction for 2009 improvements (completed barrier, added insulation): 18%

One thing a lot of people focus on when you talk about home energy efficiency is insulation, but as you can see above, from 2005-2008 I reduced my electricity usage by half without adding an ounce of insulation. There is far more to reducing heat flow than adding insulation. Remember, there are 3 ways that heat flows into (or out of!) your house: convection, conduction, and radiation. Traditional insulation addresses conduction quite well, and possibly convection, but it fails miserably at reducing radiative heat gain. Of course, to handle radiation, you need a radiant barrier. More on all of this shortly; I think that a basic understanding of heat flow is critical to the efficiency enthusiast.

Holidays and SIPs

Apologies to my "multitudinous" followers about the lack of posts lately. The busy holiday season is upon us, along with end-of-the-year business tasks, and time for posting has been reduced. For those who are interested, I'd recommend the following to fill your web-browsing needs:

Diverging a bit from my usual focus on retrofits, I read about a simple construction idea gaining more popularity: using the same materials we already build with, but rearranging them a bit, can produce incredible energy savings. Building with Structural Insulated Panels (SIPs) can reduce energy use of a home by 75%. The trick: instead of a wall studs every 2' connecting inner and outer walls with insulation laid between them, SIPs use a sandwich of foam insulation between "slices" of wood, with the wood facing the inside and outside areas. Building the walls this way, while using roughly the same materials, reduces the number of "thermal bridges" (the wall studs) that provide heat a shortcut around the insulation. Although nominally the same R-value as an insulated traditional wall of the same thickness, there are fewer areas that are far below the rated R-value, yielding a true overall resistance to heat flow that can be 4 times better than the traditional construction. Think of it as averaging fewer zeroes for missed assignments (i.e. the wall studs) into your otherwise good Grade Point Average (the R-value of the insulation), and you can see why it works well. About the only downside seems to be that you _really_ need to keep moisture out, or it ruins the foam/wood bond. Experienced construction people should know how to do this.

Winter: downside of a radiant barrier?

One thought that has been on my mind lately, with our cold weather fast approaching, is whether the radiant barrier will make it harder to heat my house throughout the winter. One nice thing about living in central Texas has been that, even on the coldest winter days, the sun has enough power to warm the house pretty significantly by afternoon if the day is sunny.
But the radiant barrier rather effectively rejects that radiant energy. This, of course, is a bad thing during the winter, but a really great thing during the summer. The balancing factor during the winter is the fact that a lot of heat that would typically radiate out of the warm attic during the night will be reflected by the barrier back down into the house, keeping the house from cooling as quickly during the night as it otherwise would.
So some questions arise:

1. Does the radiant barrier help me or hurt me overall during the winter months?
2. If it hurts me, does it cost me more energy during the winter months than it saves me during the summer months?
3. How can I find out the answers to questions 1 and 2?

As it turns out, my home is heated by natural gas, and the company was willing to provide my natural gas usage since 2005. So I will be able to perform some analysis.
Just as there is Cooling Degree Day data out there on the web, there is Heating Degree Day data as well.
When this winter is over, I plan to perform a natural gas usage vs. Heating Degree Day analysis, similar to the kWh usage vs. Cooling Degree Day analysis I performed for the cooling system of the house. The graphs generated from that data should help answer questions 1 and 2.
As usual, there will be complicating factors. First, when I finally completed the radiant barrier this spring, I blew in a bunch of extra insulation. That will certainly affect my results, but I don't have a very good way to account for it. Second, there are really only about 3 months of significant natural gas usage in the house: December, January, and February. That is not very many monthly data points when compared to the 5-6 months of significant cooling usage in the summer, meaning that it may be harder to identify trends. Finally, my cooling energy is measured in kWh, but my heating energy (burning natural gas) is measured in "ccf" which is 100 cubic feet, a volume of natural gas (presumably at atmospheric pressure), so we'll need a conversion factor between the two which may carry its own complications.
Despite these difficulties, we should be able to determine in general whether my cooling-focused improvements have helped or hurt my house's heating-related performance.
Stay tuned until after the gas meter is read in February to find out!

Trees and Geezers

Let's take a quick moment here to point out a basic point about this blog so far. Despite technical discussions about heat transfer and complicated graphs with curves and lines on them, the actual things we're doing to the house are pretty simple. Other than replacing the broken air conditioner, every significant improvement I've made to the house is "geezer tech": something that your grandfather could well have done 50 years ago. This includes things like sealing up ducts, considering attic airflow for cooling, and for the imaginative granddads, perhaps even putting up reflective material to bounce the sun's heat back out. (My own father actually did this many years ago with part of his house, using kitchen aluminum foil ... it must run in the family).

The other thing to point out is that there are other ways of achieving energy savings than actually doing something to your house. One significant factor which I haven't really captured is the effect of the slowly growing shade trees around my house. We started in this house with tiny oaks on the west side of the house, but they had no effect until about 5 years in, when they had grown enough to provide shade over the west-facing kitchen windows in the hot afternoons. The kitchen cooled dramatically that year (this was before 2005, when I started taking data; I shudder to think what _those_ graphs would have looked like). There must have indeed been some good energy savings; shade trees are Nature's radiant barrier.

Another way to save energy, if you're designing or planning your own house, is to use passive solar design, something that was done rather poorly with the layout of my house. Since the sun travels further north (in this hemisphere anyway) in the summer, and further south in the winter, you can put your maximum window area facing south, where the incoming solar energy helps you the most in winter, and hurts you the least in summer. Of course, in my "energy efficient" home, the north-facing wall has 5 times the window area of the south-facing wall, meaning that I've got the exact opposite of what I should for this area. In addition, as it turns out, the positioning of my eastern neighbor's trees gives me great shade in the winter, when I don't want it, and only partial shade in the summer, when I could really use it.
I am still considering planting another tree that would give my east-facing back porch some nice shade in the summer, but who's shadow would miss the house in the winter; a sort of passive solar tree design. Unfortunately, that same tree will likely shade some of my northern neighbor's south-facing windows during the winter, so I need to be careful with the placement.

Anyway, the point of this little post is to remind folks that this isn't rocket science, it's really not all that technical in terms of the actions we're taking, and it's really not all that difficult. A 50% reduction in energy usage doing things that your grandfather probably knew how to do really seems like, well, a no-brainer. Energy efficiency waits for no man! What are you waiting for?

The mysterious number 2.6

When performing the accumulated Cooling Degree Days analysis, which the mindful reader will recall incorporated the memory (a.k.a. hysteresis) of the system to try to get a better handle on the effectiveness of the improvements, we ran across an interesting, nay fascinating result.
It turned out that a pretty good linear model for the response of energy usage to varying cooling degree days (CDD's) took into account the temperature of the previous month as well as the temperature of the current month. The fascinating part of this was that, when determining how much energy my house was going to use, the temperature of the previous month mattered 2.6 times more than the current outside temperature.
This flies in the face of everything that I had learned about homes and insulation theory, at least before I started down the path of Energy Efficiency enlightenment. How is that? Well, like many of you out there, I had learned that the way to keep your buildings cool was to keep that hot air out, and keep a good thermal wall between the cool inside air and the hot outside air, in the form of things like double-pane windows, thick walls, and thick insulation. This would slow the flow of heat from the hot outside air into the interior airspace. And to be sure, my house is now well sealed and has decent windows. However, if that were the 100% correct approach, then why does my real world data show that some factor other than the current outside air temperature matters more? In other words, if the problem is really hot outside air, why doesn't the current outside air temperature dominate the model? If the traditional insulation theory is valid in my case, why does the temperature of air from last month that's not even around any more account for some 72% of the correlation, while the supposed main problem, hot air currently surrounding the house, accounts for only 28%?

Something is definitely amiss with our theory.

Ruminating on this problem leads us to an inescapable conclusion: in my area, during my cooling season, the main heat transfer mechanism into my house must not be direct convection from hot outside air. The traditional "double-pane window, lots of insulation" approach which attacks the convection problem is quite literally missing 72% of the target.
So how is most of the heat entering my house? Well, we have exactly one clue to help us find the culprit. Since we've seen that greatest determinant of the energy usage in June, for example, is the temperature in May, it must have to do with the heat being stored over time in someplace other than the outside air. Possible culprits:

1. Heat stored in the ground -- During the course of the typical dry summer here in central Texas, the ground becomes parched and unable to cool itself through the mechanism of evaporation, since there is no water to evaporate. Much of the vegetation also goes dormant to survive, not performing its usual transpiration which might also effect some cooling. The ground is in thermal contact with my house's foundation, and heat could certainly conduct into the concrete foundation, and once there, into the interior airspace via conduction or radiation.
   
2. Heat stored in all the thermal masses around my house -- Neighboring houses (all of which have nice heat-storing brick sides), sidewalks, and streets all store heat during the day, and radiate so much of it that it is noticeable even to the casual observer walking by after sunset. That radiated energy will be coming into my house from low angles and thus will unfortunately avoid my under-the-rafters radiant barrier.
   
3. Heat stored in the thermal mass of my own house -- The bricks on the outside walls certainly store a lot of heat, and that heat can conduct or radiate into the interior of the house. The attic itself consists of a lot of wood and some metal as well as insulation, all of which can and do heat up and store that heat over time, again conducting or radiating to become a problem.
   

As far as which of these is the most significant, it is difficult to say. I have not seen any indication from other web sources that heat conduction from the ground into the slab is a big problem; it may well be that down at the bottom of my slab, several feet underground, the ground is not particularly hot. (Some data on soil temperature variation with depth in my area would be nice, if anyone can point me to it). However, all that concrete would certainly store a LOT of heat. Unfortunately, now that the house is built, I don't have any good way to insulate between the foundation and the ground, or between the foundation and the interior space. I have heard of foundation insulation during construction, but all the examples I have seen so far are in much colder climates, attempting to reduce heat flow out of the house rather than into it. Furthermore, I have no low-energy means to cool the foundation itself.
To address #2 and #3, having a radiant barrier in all the walls and windows would go a long ways towards reducing the heat gain there. Even my own bricks could be separated via a radiant barrier from the interior space, keeping their significant heat gain from being a problem on the inside of the house. I would be interested to learn how much heat gain is radiating in through my windows, and how much through the walls. There are "low-e" (emissivity) coatings that can be added to existing windows, and I suspect those could be a significant help to me.
Unfortunately for #3 (the heat stored in the thermal mass of my own house) since my house is already built, it would be prohibitively difficult and expensive to rip open the walls and install foil barrier there.
So with this analysis we have identified 3 possible culprits for the main predictor of the heat load coming into the house, and only one of them can be partially addressed as a retrofit project: the radiation of heat in through my windows from nearby sources. The others needed to be addressed in construction, and it is now too late for that.
Well, now that the attic is pretty well taken care of, our analysis of that mysterious 2.6 factor seems to be pointing us in the direction of windows. And so let us begin to follow this new trail; let us examine and look into the efficiency of windows, and how to improve it, soon...

Summer 2009: Vaulted Ceilings Saga continues

So the weekend before the roofing crew was to show up, I needed to create roughly 16 pieces of 4'x8' decking, covered with radiant barrier foil and properly spaces baffles, to be ready for the job the following week.
As luck would have it, illness struck that weekend, but even in those early days, I knew that Energy Efficiency would wait for no man(!), and I toughed out the job of moving those rather awkward pieces of decking around and working on them. The job was made easier by the fact that the radiant barrier was already 4' wide, making it a matter of a single measure and cut operation to get the barrier on each piece. To cover 8' of length with baffles required 4 baffles per decking piece, but again, that was fairly simple and required mostly eyeballing with a little measuring.
Finally, it was the next week, and the crew arrived. I couldn't sit inside and just hope that the new decking was installed properly; I had to go out and see it. Although they wouldn't let me up on the roof, I was able to assist with a few things like cutting decking for odd-shaped areas near the corners of the roof, and creating 2 or 3 more pieces of radiant barrier decking since the original estimate had been a little low.

Preparing to staple baffles to a new piece of decking

The roofing crew was quite competent, and despite the July heat they completed the job in a single day. My hat is off to them for their professionalism in dealing with a job that had a few more components to it than usual. Here is a shot of one of the last pieces going on to the back roof. The baffles and radiant barrier are facing down, of course, but they are there. You might be able to tell that we had to go two 4' widths in from the outside wall to make certain we had enough length to cover the vaulted ceiling portions, as well as to get above the level of insulation in the attic so that the open end of the baffles inside the attic would be clear.

Later that night, it was clear that the new barrier had helped, although the change was not as dramatic as adding the initial barrier; after all, we were adding roughly 600 square feet to an already existing 1500 or so square feet. Significant, but not as much as going from 0 to 1500 square feet the first time.
This improvement yielded us the following benefits:

1. 600 square feet of radiant barrier
2. Far better barrier coverage over the master bedroom (from 70% to 100%)
   
3. Significant improvements to the intake ventilation
   
4. 600 square feet of new decking to replace worn (admittedly still functional) decking
   

The overall cost was around $700, $540 for the plywood decking and around $150 in materials (baffles and foil barrier) making the roughly $1.20/square foot cost of this improvement the most expensive per-square-foot change that I'd made. Again, I balked at doing it initially due to the cost, but I thought that I would regret it over the next 15? years until we got our next new roof if I didn't do it. The improved airflow and ventilation coverage seemed worth it.
The benefits from this job are already pretty well accounted for in the analysis of the 2009 energy data here since this change occurred in July of 2009. I am considering trying to examine 2009 May and June, before the change, with July-September, after the change, but that might not prove very conclusive given the short timescales.

In the meantime, the number 2.6 has been on my mind, and its significance might require some more discussion. Until next time!

Summer 2009: Finally addressing vaulted ceilings!

After a few posts on analysis, we are returning to the narrative of what changes I made to my inefficient house to achieve dramatic savings and comfort.
After the early spring completion of the radiant barrier and the subsequent installation of a good chunk of blow-in insulation, my attic efforts were largely complete. The only remaining concern nagging at me was the fact that around the border of roughly half of the house, I had vaulted ceilings that were inaccessible from the attic as well as from the outside. There was no way, short of removing roof shingles and decking and attacking from the top, to install radiant barrier in these areas.
This was somewhat unfortunate, because this was a good 4 feet of ceiling along the outside wall that I could not cover, extending around half the house. Furthermore, two runs of vaulted ceiling intersected over the master bedroom, making that room at best 70% coverable with radiant barrier, unless one were to do something dramatic like replace the roof.
Well! As it happened, a (rather convenient) massive, record-breaking hailstorm occurred in March. The damage from that particular storm damaged roofs in my area severely enough that crews are _still_ in our neighborhood replacing roofs, lo these 7 months later. In fact, they are all over this part of town. (And, in a development near and dear to all of our Energy Efficient hearts, a lot of the new roofs I see are getting ridge vents installed. I do not know if they have investigated clearing their soffit vents or not: perhaps some of them will read this blog and know what to do.)
So, although the insurance deductible payment was not trivial, this turned out to be the year to get a new roof. And of course, since this roof replacement event happens less than once a decade, I wanted to take the opportunity to try to make my changes to the decking over the vaulted ceiling areas.
From the beginning I learned that this was unlikely to be as cost-effective as the other improvements that I had made, mainly because I would be forced to replace existing material in the house rather than simply add to it as I had until this point. The material I had to replace was roof decking; there was no practical way to bring up the old decking without tearing it up. In effect, the extra cost I was paying was because I was giving up decking that probably had another 10 or 15 years of life on it.
The deal I reached with the roofer, who was quite helpful, was this: The new decking was going to cost me roughly $1/square foot. Since he couldn't find radiant barrier decking in the exact thickness to match my existing decking, he was going to supply me with the new decking the weekend before the job. Since I still had plenty of radiant barrier left over, I would construct homemade radiant barrier decking by stapling sheets of radiant barrier along one side of the 4' x 8' pieces of decking. Then the crew would remove the existing non-radiant-barrier decking from the parts of the house that I showed them, and replace it with the new homemade radiant barrier decking. The only cost to me would be the cost of the new decking, which I felt was more than fair.
Another helpful thing about replacing the roof decking is that it would help me resolve another problem that had dogged my attic from the beginning: lack of intake ventilation. As you may recall, I had unclogged all the soffit vents that I could reach from the attic. However, there were many places, all of them over vaulted ceiling, where the vents were still blocked due to the impossibility of getting my body in front of the area to work. Replacing the old decking gave me access from outside, allowing a chance to install baffles on the bottom of the new decking to create airflow where there had been none before.

4'x8' decking with baffles and foil barrier

You can see from the picture above that the combination of radiant barrier and baffles works quite well! I had first stapled radiant barrier to each piece of decking (except the last couple of inches on either end, which were going to be nailed to rafters anyway), then stapled baffles over the top of these, spacing the baffles to where the parts flush to the decking would coincide with the rafters.
To review our radiant barrier theory, the radiant barrier needs an airspace adjoining it to work; having it touch insulation or anything else destroys its reflective properties (recommended is at least a 3/4" space; these baffles gave about 1"). The baffle serves to create that needed airspace. In addition, the airspace is creating a channel to allow outside airflow into the attic from the soffit vents, cooling the attic during the evening hours when the barrier isn't helping. Thus, these cheap Styrofoam baffles perform double duty, allowing the barrier to reflect heat out before it causes trouble, and cooling the heat that does get through by enhancing attic airflow. They are truly an excellent value to the discriminating Efficiency Enthusiast!
The only downside is apparent from the photo: each baffle has a couple of inches running down the edge and through the middle that touch the barrier. These are necessary to give you a place to staple the baffle as well as to provide structural support, but along these runs, the radiant barrier will not work due to lack of adjoining airspace. So for each of these pieces of decking with barrier and baffles, I am probably getting only about 80% of the reflective power of the barrier. Still quite worth doing. I even considered trying to install small strips of radiant barrier along the top of the problematic portions of the baffle, but that would have been a good bit of work for very little square footage of reflection, plus, depending on how much the insulation bent under the pressure of the baffle, these sections might have contacted insulation anyway after installation, making them nonreflective. So I left the decking as you see it in the photos.

Take 2: Characterizing Hysteresis

In my last post, one of the caveats about my results in determining cooling effectiveness was that the real-world data shows hysteresis, or memory, which makes determining the effectiveness of cooling difficult if it the calculation is based only on the temperature outside.
I have attempted to take into account some of this memory by replotting the energy use (kWh) vs. Cooling Degree Days (CDD) data, but with the following change: the CDD numbers on the X axis are the sum of the current CDD value added to a factor times the previous month's CDD value. For for a given month on this plot,

Accumulated CDD = (Factor * previous CDD) + CDD for this month

I utilized the Standard Error function in Excel, STEYX(), to assist me in determining the value of "Factor" by trial and error. I chose a value for "Factor" that minimzed the total standard error of the function; i.e. the sum of the errors of Y as they can be predicted by X. In another intriguiging twist, this yielded a value of greater than 1; in fact, a value around 2.6 actually worked best.
What does this mean? This means that last month's weather matters a lot more (2.6 times more) than this month's weather in determining how much heat is going to entering my home. Another fascinating result, suggesting that once again, simple air convection or conduction (represented by this month's CDD) through the exterior of the house has a lot less to do with cooling load than radiation of the hot environment around me (represented by last month's CDD). Another argument for full-house radiant barriers, if anyone is listening! In fact, the low-emissivity ("low-e") coating on modern windows helps perform exactly that radiant barrier function, but on the sides of the house, somewhere that my attic-based radiant barrier doesn't cover, and something I hope to learn more about in the future. So far, replacing windows has been off of my list due to cost constraints (my entire radiant barrier cost a lot less than 1 window), but there are efficiencies to be gained there as well. But I digress...
I actually also tried this model with a second factor multiplied by the CDD from 2 months ago, but trial and error yielded almost no impact: the factor was less than 0.1. This indicates that the main factors are (in order of importance) last month's temperature, then this month's temperature.



You'll notice a few things right off the bat:

1. The "best fit" lines look a little high. The reason: I made them fit starting at 500 accumulated CDD; I considered everything to the left of that as noise. So the slopes should match the hottest (right hand) part of the curve well, even if their level looks high. The slope is what I'm after; we're going to ignore the intercept (height) of the lines.
2. If you compare to the previous 2005 graph, you'll see that the effect of using accumulated CDDs has been to turn the large loop of 2005 data into a double loop, with a crossing in the middle. Intuitively that makes sense: that minimizes the error between a line drawn through the middle of the loop and the loop itself. The standard error for the raw 2005 CDD data was 564; the standard error for accumulated CDD data is 473, a significant improvement, meaning that a line fits this data better (although clearly still not great!)
   
3. If you compare to the previous 2009 graph, you'll see that the effect of using accumulated CDDs has been to turn the small loop of 2009 data into almost an exact line. The fact that it's a nice fit for the line means that the house's response to temperature has become much more linear; the "loopy" nonlinear part only appears far to the right in the hottest areas. The standard error for the raw CDD data for 2009 was 141; for the accumulated data, it drops to 95. Looking at the graph, you can see that the best-fit line matches quite well.

So now that we've got a better model, particularly for the 2009 data, how do the slopes compare? We've got a slope of:
2005: 0.99 kWh / accumulated CDD
2009: 0.27 kWh / accumulated CDD

Taking reciprocals to convert to effectiveness:
2005: 1.01 accumulated CDD cooled per kWh spent
2009: 3.73 accumulated CDD cooled per kWh spent
Effectiveness ratio: 3.7 to 1
Conclusion: when taking the hysteresis of the system into account, our home improvements look even better. Rather than a 70% improvement in effectiveness based on the immediate CDD method, I may well be looking at a 73% improvement based on a more accurate model.
While not a massive change, this analysis makes me feel a bit better in that I've now accounted for the rather obvious hysteresis in the data, and come out with pretty similar numbers.
There remains the lingering question of the nonlinearity all through the 2005 data and at the very far right of the 2009 data. It would appear that when things get hotter than the house can handle (which seems to happen immediately in 2005, but not until about x=2000 accumulated CDDs in 2009), energy use gets bumped up nonlinearly. There are probably good physics reasons for this nonlinearity, but I will leave it to my readers to write in a let me know what they might be and how I might model them - or even better, prevent them!
In the meantime, this diversion into analysis was fun, but we still have a little bit more story to tell about the home energy efficiency projects completed in 2009. Stay tuned!

Cooling effectiveness: check!

I will repeat from my last post:

The energy cost of cooling my home 1 degree has dropped 70% due to my efforts.

Energy efficiency enthusiasts, be enthusiastic! This is an incredible number. Every kWh of energy I run through the air conditioner cools me 3.4 times more effectively than it did in 2005 when I embarked on this journey. And, in a big Energy Efficiency Man plus, almost none of the improvements that brought me these savings will break, wear out, or require maintenance. Ridge vent? No moving parts. Baffles and soffit ventilation improvements? No moving parts. Additional insulation? Will last the lifetime of the building barring roof leaks. Radiant barrier? It's foil hanging from the rafters, folks. The business end is the reflective side facing down, and it won't even get dusty after decades up there.
Yes, the more efficient A/C unit will wear out, but I have little choice but to have an A/C unit of some type here. It is definitely the weak link, and will undoubtedly give me trouble, but it's awfully nice to have when it's 100 degrees in September.

Caveats:

1. This estimate is based on a linear analysis with a simple "slope-intercept" fit. A better analysis would take the hysteresis of the system into account.
2. Your mileage may vary: these improvements work quite well in my 5-month cooling season-dominated climate. Effectiveness of all improvements depend on your climate and house situation.

The above notwithstanding, the data (and energy bills!) clearly show much less dependence of the house on the outside temperature as was true only 4 years ago. Truly, the house is cooler to live in as well as easier to cool through most of the year.

Analysis: Effectiveness of Ventilation + Barrier

I will continue the analysis of the cooling effectiveness of my home, skipping forward to 2009. The improvements in 2007 (related to ventilation) have been enhanced by the addition of a foil radiant barrier and a 14 SEER air conditioner (to replace the failed 11 SEER unit).
Here is a graph of energy usage vs. Cooling Degree Days (CDD's, see previous posts for explanation) for 2005 and 2009. Since 2009 isn't over yet, I have substituted 2008 data for both November and December, but neither of those months is real significant for cooling costs.

1. Again, as in the previous post, the hysteresis of the system is quite apparent.
   
2. As expected from personal experience, the 2009 loop extends almost 100 CDDs further to the right than the 2005 loop, indicating the record breaking HOT summer here.
3. The curve for 2009, while showing some positive slope, looks almost unfazed by the increasing heat.

So at a glance, it appears that adding the radiant barrier and more efficient air conditioner to the already improved 2007 system has helped. But how much? Let's perform our linear best fit again and look at the results:


Including the numbers from our last post covering 2007, here are the final results of this analysis method:
2005 Slope: 2.8 kWh/CDD
2007 Slope: 1.1 kWh/CDD
2009 Slope: 0.84 kWh / CDD
So, by 2009, the effectiveness of my cooling system has increased again. Stated as "kWh of energy to cool 1 CDD", the effectiveness has gone up from:
(1/2.8)= 0.35 CDD per kWh expended to
(1/0.84)=1.19 CDD per kWh expended.
Finally: something meaningful to compare. Stated as simply as I can, based on linear best-fit analysis to remove weather dependencies:

The energy cost of cooling my home 1 degree has dropped 70% due to my efforts.

The Analysis Continues: Effectiveness of Ventilation

So given that I have energy usage data, and "cooling degree day" (CDD) data for the summers since 2005, I should be able to determine a correlation between the two, and figure out how much my improvements have helped me while eliminating the variations in the weather. I want to know: how much of my energy savings in 2007 was due to the cooler weather, and how much was due to fixing the ductwork and improving the ventilation?
To answer that question, let's take a look at a graph that plots what I'm interested in (energy usage), vs. the thing that I thing most influences it (CDD's) for both my starting year 2005, and the year we're analyzing, 2007. The graph below has been smoothed by Excel to make the curves easier to follow.

What a fascinating result! The larger, blue "loop" is the 2005 data, and the smaller "green" loop below it is the 2007 data. A few things jump out immediately:

1. The relationship between kWh and CDDs is not as simple as might be hoped
   
2. As expected, the green loop does not extend as far to the right, since a cooler summer caused there to be fewer CDDs in 2007
3. As expected, the green loop does not extend as high as the blue loop, since we know we used less energy in 2007
4. The graph indicates a hysteresis, or memory, is at work in our system. We know this because for any given X value, there tends to be more than 1 Y value, and you would need to know more about the system to determine which Y value to use.

In fact, intrepid readers, we have already learned something about my house as a physical system: it has a large amount of memory/hysteresis with regards to outside temperature and energy use. The reason for this seems pretty simple: the house itself, along with everything around it, has heated up more at the end of the year than at the beginning. Thus, even for the same number the CDDs, I must expend more energy to remove all the heat from the house at the end of the summer than at the beginning. Restated, it costs me a lot more to cool the house on a 100 degree day in August than it does on that same 100 degree day in May.
We learned all that from simply looking at the graph! Truly, a picture is worth at least 1000 words.
To verify that the hysteresis is caused by what I think it is, I should be able to look at the raw data and determine that for each year, the lower half of the curve occurs in January-June, and the upper half in July-December. Wondrous to see, I have looked, and it does indeed. If you follow the data points making up each year, you start at the left in January, move across the bottom of the loop going rightward until about the peak is reached in July or August, then move back across the top of the loop during the last few months of the year.
One thing that this complex graph tells me is that it won't be fully accurate to just estimate a slope for each of those loops and compare those slopes to determine the effectiveness of my home improvements to 2007. Nevertheless, I can do it, so I will. Again using simple features in Excel, I come up with the following "best fit" lines for each loop.

Looking at the slope of each of these "best fit" lines will give me a rough idea how well my improvements in 2007 had improved my situation regardless of the weather changes. The numbers?
2005 Slope: 2.8 kWh/CDD
2007 Slope: 1.1 kWh/CDD

This is yet another fascinating result! To the extent that it is accurate, I had already increased the energy-effectiveness of the cooling of my house by some 60% in 2007, yet my energy usage had dropped only some 43% over the same time. Perhaps that makes some sense: I had only improved the cooling-related energy use, and not any other aspect of energy use. Since the cooling-related energy use was reduced due to the cloudy summer in 2007, and that's the only part that I optimized, perhaps the overall savings should be less than 60%.

However, this analysis remains unsatisfying. Looking at the 2nd graph above, the linear fit is just a really poor estimate of the actual function we're graphing. Clearly, the memory at play in the system is significant, and perhaps I can find a way to account for it.

In the meantime, I need to finish the analysis to account for the radiant barrier added later in 2007 and 2008. Read on to fulfill your curiosity!

Analyzing Usage and Weather

I located some logged online Cooling Degree Day data nicely packaged by these folks.
To review, one Cooling Degree Day (CDD) recorded at a base of 65 degrees, for example, is a day where the temperature averaged over a day was 66 degrees. If the temperature for that day averaged 85 degrees, the CDD would be 20 for that day. Negative CDDs are ignored - actually, they would be counted as Heating Degree Days, which I am currently not using in my analysis (I may at some point in the future when I look at my natural gas usage, which heats my house).
The first question was which weather station to use; there are many in my area, and their numbers are all different. Not _too_ different, but somewhat. I decided to use the airport data since that data has the fewest gaps, even though the airport is a good half-hour drive away, and is well outside the urban heat island that I live on. One of the issues in dealing with long term archived data is dealing with the gaps. In my case, only a few months were missing, and I filled those in with data from another source.
I have the vague idea that my energy usage is mostly driven by air conditioner usage, supported by the fact well known to many Texans that the electric bills are largest in July and August, and are still not fun in June and September. Of course, I'm also using energy for other things such as lighting, computers, televisions, refrigerator, etc. but those loads should not vary as much seasonally, although, in yet another complication, lighting usage varies seasonally as the days get shorter.
So we have some known issues with our analysis, which we can hope will not matter too much:

1. Location difference between airport (out of town) and house (in town, downwind from downtown in the "heat island")
2. Small gap in the CDD data filled by data from a different source
3. Many things sum to make energy usage; air conditioning is only one, albeit a big one
4. CDD's measurements themselves can be taken different ways. Measuring the temperature every hour, and summing those results over a day, yields a different number than just looking at the (highest - lowest)/2 value that some data providers might use.
5. CDD's at base 65 might not be the wrong "baseline" for my house. Perhaps my air conditioner does not kick on until the daily average is over 70, for example.
6. CDD's do not consider sunlight, which delivers far more heat than convected air, particularly on cooler days. It would not surprise me to see some air conditioners running on a day with 0 CDD's but is sunny, and not run on a day with a few CDD's but is cloudy. However, installing a radiant barrier should have reduced this problem for me; I have far less sunny heat gain than before.
7. CDD's do not consider the non-air environment, except as it affects air temperature. What do I mean by this? Well, a lot of us who live here have seen 100 degree days in May. Although unwelcome, those days never seem so bad as 100 degree days in August. Why? For one thing, there is still moisture in the soil in May. Grass is green and growing, plants are moist and lush, and the ground hasn't been baked for months on end to a nice shade of brown. All of these things will reduce the heat radiating around the area and hitting me and my house, even in high air temperature. In August, on the other hand, the grass is dormant and not evaporating water, cooling the ground. The streets, sidewalks, and bricks in the houses are storing a lot of heat built up over the summer that they didn't have in May. All of that heat is radiated and hits me and my house in August, causing more cooling load, even though very little of it affects the air temperature (particularly the air temperature at the airport, which is out of town).

One way that I hope to discover whether some of these factors matter, and perhaps how much they matter, is to simply graphically look at the data. Do the data make sense? If I graph my monthly consumption in kWh vs. the CDD's for that month, a relationship should emerge if there is one. In a nice, pretty world, it would be a linear relationship, with a slope showing how much energy I need to expend to handle one CDD, but we'll see if that is the case.

Here is a look at the Cooling Degree Days at the airport since 2005. You can clearly see the seasonality reflected, and the amazingly cool summer of 2007 right in the middle of the graph. You can also see how our January's have been getting warmer every year fairly consistently, even while the summers fluctuate, and you can see that the summer of 2009 was all-record-breaking in terms of heat. The graph has been 5-point smoothed (each point has been averaged with the 2 before it and the 2 after it) to make it look nicer.

Attic Efficiency: Check!

It was February 2009, and I had just finished the satisfying job of adding insulation.
After a quick look around the attic, I finally felt a sense of completion. I had finally covered the 3 major areas that my internet-based education had told me were the major causes of inefficient attics: lack of ventilation, lack of a barrier to radiated heat, and insufficient traditional insulation. It took several seasons of projects, a little bit of money, and the help of some neighbors, but I had finally managed to get my attic into some semblance of energy efficiency.
I am now tempted to attempt to perform a better analysis of how well this has worked for me. Since the only numbers I have used so far have been my electricity usage, I have been unable to account for some known large differences in the weather over the course of my improvements. Without this accounting, the numbers look quite good, as you can see from my previous posts on the subject.
But I have been doing some research into how one accounts for the known temperature differences from one year to the next in ascertaining the performance of one's home. For example, was my energy usage drop from 2006 to 2007 entirely because of the well-known fact that in my area, 2007 was a cloudy and cool summer? Is the fact that my 2008 usage is less than my 2007 usage even more impressive since 2008 was so much hotter?
It turns out that there are some ways to account for this. The concept of "degree-days" is used in conjunction with either heating or cooling. The base for "degree days" seems to be 65 degrees Fahrenheit. The concept is deceptively simple: for cooling, you look at how much hotter the daytime average was than the base of 65 degrees, and you add that number to your total. You continue adding (T - 65) for each of the days you are interested in, until you get a total the represents how much hotter it was outside than inside over the span of time you're studying.
There are complexities about how to best measure the degrees on a given day (the average is not all that accurate; you'd ideally want a measurement every hour or more), and inaccuracies in that heat flow is not always linear (i.e. a 20 degree difference from outside to inside might well be expected to use more than 2x the energy of a 10 degree difference) and there is a baseload entirely unrelated to heating or cooling. Nevertheless, using this method should be an improvement over using no method at all.
As it turns out, there are sources of data out there for Heating Degree Days and Cooling Degree Days for a lot of places out there. There can be holes in the data, which require filling from other sources that may not agree exactly, etc. In addition, there are many schools of thought on how best to utilize the data. When looking at energy usage, one it looking at a sum of many variables, so when looking at energy usage vs. Cooling Degree Days, one will still have other factors unrelated to cooling influence the outcome.
Nonetheless, performing some of this analysis might be instructive. Gird up your brains, faithful readers, for the upcoming analysis is not for the faint of math.

Insulation School

As luck would have it, the day in late February 2009 that I chose to blow in the additional fiberglass insulation turned out to be the hottest day in months. It was 82 degrees and sunny, yielding an attic that, while cooler than it would have been with no radiant barrier, was definite cause for sweating.
Using the insulation blower was fairly simple, but certainly required two people for any reasonable amount of insulation. My volunteer neighbor stayed on the ground in the garage, wearing an insulation mask and feeding bags of compressed insulation into the blower. The blower, which had a set of rotating paddles inside it, tore up the compressed insulation into small enough bits that they would be blown out through the long hose. I stood in the attic with the other end of the hose, blowing a fine snow of expanded fiberglass insulation a good 15 or 20 feet out in front of me.
We quickly learned a few things:

1. It takes a _long_ time to feed in a bag of insulation and get it fully blown out into the attic. We had 12 bags, and it took us roughly 8 hours to get the job done. There was some break time to cool down, clear out the machine, etc. but the job was very slow. Perhaps some machines are faster, but ours was pretty slow.
   
2. You want to plan your route as if you were a painter; that is, work from the farthest edges back towards your attic entrance, so you aren't "painted into a corner". This helps keep the hose out of the way as well as keeps you from having to traverse over, and compress (which reduces the R-value) the newly blown insulation.
3. You might also want to plan the various positions in the attic where you are going to stand. As you might expect, the insulation shoots out in a nice arc through the air slowing as it gets further away, with the insulation landing in a nice linear pile under the arc, with most of it landing on the half further away from you. Therefore, you want to fill places that are _not_ right next to you, but are further away. If you point the hose at the floor less than about 10 feet away, you'll just be blowing insulation out of the way with the force of the air, reducing the insulation coverage instead of adding to it. Instead, you want to shoot the air _over_ the area you want to fill, and let the insulation drift down into the area. If you don't understand this, don't worry - you'll learn it pretty quickly as you work.
4. Since communication is nearly impossible between the hose operator and the machine operator due to distance and machine noise, you'll want to coordinate so that you know when to move to your different blowing positions to get an even distribution. I would ask my neighbor to cut off the machine after 2 more bags, for example, so I would know that I had blown enough insulation in that particular area.
5. Operating the hose is actually peaceful, almost trance-inducing work. There is a certain beauty (at least to those schooled in the ways of Energy Efficiency) to watching the myriad fiberglass snowflakes dancing down, slowly gathering into giant drifts of heat-flow reducing goodness.
   

After one afternoon and a brief morning of work, we were done. Another hour or so to clean up all the insulation detritus that drifter around in the garage, and the rental machine was returned. Project complete in less than 1 day!

Completion of the barrier(!) and adventures in insulation

So as we come to this point in the winter of 2008-2009, I have realized that my attic has a newly found problem.
Problem: Insufficient depth of insulation for current building codes
Solution: Simply add insulation!

At first, I began by buying a couple of rolls of fiberglass insulation. However, after the very time-consuming process of removing the blown-in insulation from an area, measuring the space, cutting the insulation roll, placing the cut piece, then covering that with the old blown-in insulation, I realized I might not complete both the insulation job and the radiant barrier job during the nice, attic-work-friendly cold weather. The installation of the insulation, if you will, was taking too long.

New Problem: Installation of insulation pieces taking too long
New Solution: Rent an insulation blower and blow in more insulation. However, due to the sheer volume of insulation that I planned to add, this needed to wait until I was pretty well done with everything else in the attic.

I completed the installation of the barrier at long last in January 2009. There was much rejoicing in Energy Efficiency Land after that relatively inexpensive but long-lasting project. The total cost of the amount of barrier foil that I ended up using was under $200.00, but the amount of labor was a goodly number of (uncounted) hours over the course of a couple of winters.
After rewarding myself with a few weeks of break from breathing the attic air, in late February I decided to go ahead and try to blow in the new insulation.
The process seemed simple enough:

1. Estimate the total volume of insulation I would need. I could do this by multiplying the attic floor area by the depth of insulation that I wanted to add. This would only be an estimate, but blowing in insulation is far from an exact science. The bags of insulation at the store should tell me their total volume.
   
2. Rent a blower at the hardware store where I would buy the insulation
3. Arrange the borrowing of a neighbor's pickup truck to carry the blower and the insulation
4. Arrange the time of another helpful neighbor to feed the insulation into the blower in the garage, while I waved the hose around the attic to distribute it appropriately.

But how well would this go? How long would it take? What new things would I learn?
As usual, read on to find out!

The important Science of Insulation

As most readers of this blog probably already know, traditional insulation is only a part of reducing heat flow into and out of a house's living area, and should be looked in concert with other means of blocking heat flow such as radiant barriers, and cooling mechanisms like attic ventilation.
Nevertheless, insulation is an important warrior in the heat flow reduction battle whose role should not be taken lightly. In fact, good insulation greatly reduces heat flow from the mechanisms of both conduction and convection, and perhaps even a small extent by radiation as well.
The heat flow reduction properties of insulation are quantified by its "R-value", with higher R-values being more able to reduce heat flow. There's a good layman's description (with full equations and a nifty table of materials) here on wikipedia.
The recommended total R-value for my home's attic when it was built in 1996 was R-30. In terms of my fiberglass blown-in insulation which has an R-value of about 2.5 per inch of thickness, that would mean I should have had 30 / 2.5 = 12 inches of insulation. That actually would have covered up most of the wooden joists on the floor of the attic, but in fact, it did not. The depth of insulation varied, of course, but my best guess as to the average depth was about 9 inches, yielding an actual R-value of 9 x 2.5 = R-22.
So not only was my insulation insufficient for the building codes in 1996, it was now 2009, and we as a society have realized (to some small extent) that we're underinsulating our buildings. The new code seems to be R-49 for my area, which would equate to 49 / 2.5 = about 20 inches of insulation.
So I needed at least 20 inches of insulation, but I had less than half that. What would I do about this discrepancy? And would it keep me from finishing my radiant barrier for yet another summer?
Next: Find out!

Winter of '08- '09: Mission creep part 2

After experiencing the dramatic cooling capability of even a partial radiant barrier installation, I raised the priority of finishing the barrier over the coming winter. As the weather started to cool, and the fierce Texas sun began to travel south for the winter, leading the Canadian geese and Mexican free-tail bats to warmer climes, I began to spend longer sessions in the attic in an attempt to finish the work.
As I continued to work in the attic installing radiant barrier, one thing that I noticed was that looking around the attic, I could see a lot of wood sticking up out of the insulation, wood that made up the joists for the ceilings below me. This can be a bad sign; typically, as I understand it, one should have enough insulation depth that the wood is pretty well entirely covered. This reduces heat flow from the attic air into the wood, which is important because the wood is in very direct thermal contact with your ceiling. Plus, unless your ceilings joists are _really_ thick, the fact that the wood is showing through means that your insulation is not really very thick.
Now, perhaps some of you are thinking, "what does this have to do with installing radiant barrier?" And you would be right to ask that question. In fact, I had distracted myself once again from the primary mission through another bout of mission creep, this time, to improve the insulation while I was in each particular area of the attic.
Next: read on, to determine what this little diversion into the fascinating science of insulation has to teach us, and learn what efficiencies are in store!

Energy saved by 40% of a Barrier?

Before we look at the numbers, I should note that it may not be entirely useful to look at the 2008 energy use vs. the 2007 energy use as a measure of radiant barrier effectiveness, simply because I had only installed the barrier over (a bit less than) half the house's total area.
It may well be that installing 40% of a radiant barrier (which is my best guess for the percentage I had put in) does not give you 40% of the performance of a full radiant barrier. Why? Well, for one thing, the radiant barrier concept relies on rejecting heat before it enters the serious thermal mass of your insulation. Having a huge gap in the barrier allowed my insulation to heat up greatly during the day. Why would that be a problem? Because at night, when everything continues to radiate heat in all directions, a lot of the heat radiated by the insulation is going to bounce off the radiant barrier above it and back down into the insulation, rather than eventually working its way out of the house. Thus, a 40% radiant barrier coverage may not give you 40% of the full radiant barrier benefit.
But I have the numbers, so we might as well look at them.

Partial radiant barrier comparison:
-----------------------------------------------
Total electricity usage for 2007: 8056 kWh
Total electricity usage for 2008: 6982 kWh
Energy savings: about 13%

Unfortunately for our accuracy in using these numbers, this was not a controlled experiment, but instead was affected by at least a couple of major changes in 2008. First, the careful reader will recall that the summer of 2007 in my area was the coolest and cloudiest in quite some time, probably depressing the usage numbers for that year. 2008, on the other hand, had a much more "normal" summer with a lot of sunshine and many 100+ degree days.
Second, in late July my old relatively inefficient (11 SEER) air conditioner finally kicked the bucket. Fortunately, I replaced it with a more efficient 14 SEER unit. Unfortunately, the old unit died in a way that made it run more and more frequently over a period of days, working harder and burning more energy to cool less and less.

So considering the difference in weather, the electricity savings of my partial barrier probably saved a good bit more than 13%.
But, considering the fact that the air conditioner got upgraded about halfway through the summer, the electricity savings must be at least partly due to the more efficient air conditioner, reducing the apparent savings from the barrier. Mitigating the air conditioner factor somewhat was the fact that the slow failure of the old unit burned a lot more electricity over a period of days than would normally happen.
It is impossible me to say for certain which of these factors was larger. Suffice it to say that I was impressed enough with the barrier's performance to stiffen my own resolve to spend more "quality time" in the attic over the winter to complete the job.
I will include a look at my total electricity usage for 2005-2008, with 2008 on the left, in case you can't tell :)

2008: The summer of comparison

Our narrative now winds into the summer of 2008. I had just completed radiant barrier coverage in the attic of a little less than half the house, including a solid section over the east-facing and mostly unshaded master bedroom suite, giving me a very good opportunity to determine whether this formerly hottest area of the house would make a noticeable difference as the fierce Texas summer came on. The other barrier-covered section was west-facing in the attic over the garage.
The first thing that I noticed, as the sun began to warm the house in the summer mornings, was that the air conditioner did not start as early in the morning. In fact, it hardly ran at all before 10 am. This unusual development seemed to indicate that the east-facing "artificial shade" of the radiant barrier was working.
Next, from about 10 am to about 3 pm, the house heated up rapidly. The air conditioner would start to run more and more frequently, as expected, as the barrier located at the eastern and western edges of the house, didn't do much good for blocking the almost directly overhead sun angles. In the late afternoon and early evening I could not tell much difference as the roof and attic were fully heated up, and the air condition ran a lot, although certainly not constantly. The west-facing section may not have helped me as noticeably, first because it was over the garage, and second because I do have partial shade already on the west side of the house.
But how was the feel of the master bedroom, which for more than a decade has been the room that we would avoid in the summer until the last possible moment, due to the heat buildup every single hot day?
The difference was nothing short of dramatic. The hottest area of the house immediately became the coolest area due to the radiant barrier. In fact, for the first few weeks, without ever planning it, we found ourselves retreating to the master bedroom after dinner to read or play on the computer because it was the most comfortable place to be.
Even more amazing was the fact that as the summer wore on and the days got even hotter, we had to RAISE the thermostat setting a degree because the master bedroom was getting too cold. Since the thermostat itself was in an un-barrier-covered part of the house, it was exposed to the full heat load and ran the A/C accordingly. However, the parts of the house not exposed to the full heat load due to the apparently incredible heat-rejection properties of the barrier still received their full measure of cold air from the A/C, dropping the temperature dramatically. I recall at least a 6 or 7 degree difference from the central (uncovered) area to the master bedroom.
These minor miracles were made even more miraculous by the fact that the master bedroom area has two of its four sides as vaulted ceilings, which were inaccessible for installation of the radiant barrier. In other words, this remarkable turnaround from hottest area to coldest area happened with only about 60% radiant barrier coverage of the room. (The only way to get radiant barrier over the sloped part of the vaulted ceilings would be to replace the roof decking with decking that has radiant barrier along the bottom of it, and the only time that it makes any sense at all to do that is when you're replacing a roof (which I in fact did in 2009: foreshadowing!))
In short, the half-done barrier worked incredibly well. The comfort level was immediately noticeable, and resulted in our actually setting the thermostat a degree warmer for that summer. The uneven heating of the house was quite noticeable and caused me a good degree of regret at my late start that winter, and renewed my resolve to complete the barrier over the entire house once the summer heat was gone and the attic was once again safe to work in.

Foil that heat gain!

After doing my own research, the most effective products looked to me like the industrial-grade foil radiant barriers. The paints seemed a little dicey to me; there were a lot of questions about their effectiveness, and the very best I found looked like they blocked about 70% of radiant energy. In contrast, the foil products were rated well over 90%, with the one I chose at 97% reflectivity.
The questions: how much did I need, and how could I get it installed?
To examine the first question, I first had to answer the second one, because there are 2 accepted ways of installing the barrier.
Method 1: roll the aluminum foil out across the attic floor, on top of the existing insulation. In this case, I would need exactly the (estimated) square footage of the attic in foil.
Method 2: staple the aluminum foil to the underside of the rafters. In this case, I would need a bit of extra foil, to account for the slope of the roof and the inevitable sagging of the material between the rafters.

I estimated the area covered by my attic, including the garage, and multiplied by a factor slightly greater than 1 to account for the pitch of my roof. My roof is a 5/12 roof; that is; 5 inches of rise for every 12 inches of horizontal length. That, if my math was correct, required about 2200 square feet of barrier, assuming I could physically access everywhere I wanted to put it.

The radiant barrier itself, which I purchased from these good folks , came in 1000 square foot rolls, with each roll 4' x 250'. Some vendors also sell them in 2' wide rolls. At times I liked having the 4' width, but at other times, I had to cut the foil width-wise because I needed narrower sections.

Installing the barrier was a long-term process for me. Since I didn't make the decision to start until late February 2008, and we often start getting good sunshine and heat in March or April, I didn't have that many weekends to get it done. Plus, to be honest, some weeks I just couldn't handle the thought of going into the attic again, so I wasn't the most diligent.

Add to that the fact that there is a good bit of setup time for me; I don't enjoy breathing insulation fibers, or feeling them against my skin for that matter, every time I worked in the attic I put on a paper "bunny suit" (painters often use these; you can get them at Home Depot or other similar stores) as well as an insulation-grade mask. Note: in my experience regular masks, like you might use for lawn care or other basic dust applications are not good enough. Wearing a mask rated for insulation use made a noticeable difference in how much distress I felt in my nose and throat after a session in the attic.

Add to that the fact that, since I used the garage as a staging area for rolling out and cutting sheets of radiant barrier as I needed them, I swept the garage pretty much every time I started a session. Why? Because dust stuck to the radiant barrier reduces its reflectiveness.

With all that, you can see that it took a good while to get started each day. Plus, there is cleanup time to put away the roll, change out of the suit, and shower and rinse out your mouth and throat (if you know how to do neti, do so after leaving the attic for the day - it will help knock out a lot more of those unhealthy fibers.)

So, with the late start in the season, and the overhead of each session in the attic, I was able to complete about half of the attic in my first effort. The parts I chose to cover were over the very sunny east-facing side of the house, which included the master bedroom suite - long recognized as the hottest area of the house in the summer - and a portion of the westward-facing area over the kitchen and garage. The entire middle of the house was left uncovered for the oncoming summer.

However, this did give me the chance to experience a summer of comparison; I could compare the feel of the house "under the barrier" vs. the feel of the area without it. How did that go? Read on, and see!

A whole new but familiar concept

For me, the discovery that radiant energy was a big part of the picture was something big and exciting, to say the least. I was starting to gain an intellectual understanding of something that my body had understood at a basic level for a very long time. I had, of course, felt radiated heat before, and have since learned to recognize it. All of us, from the simplest animals to human beings, know to stay out of the sun when it's hot. Why? Because we heat up in the sun. But few of us actually understand why that is. Why do we heat up in the sun? Because we are absorbing radiant energy. It is not because the air is any hotter in the sun; the air in the direct sunlight, and the air in the shade right next to it, are almost the exact same temperature. Instead of the air molecules bumping into us and heating us up (as most of us learn about in elementary science), a bombardment of photons, a.k.a. radiant energy from the sun, is impacting our bodies and causing them to heat up.
What this means for energy efficiency in the summertime in a sunny climate is this: if you're addressing only the hot outside air, which can affect your home only through the mechanisms of convection and conduction, you are acting like the lone cow out in the field standing in the sun on a 100 degree day, while the rest of the herd is standing in the shade: that is, you are ignoring the most important part of keeping cool.
So wouldn't it be great if there were a way to bounce that bombardment of photons right back into the sky where it came from? Wouldn't that significantly reduce the amount of energy being absorbed into a house, reducing the amount of electricity an air conditioner has to expend to remove that energy once it strikes something and becomes heat? Yes, and most definitively, yes! Enter the radiant barrier.
Officially, a radiant barrier is defined as any material that reflects 90% or more of the energy striking it, which implies that it absorbs less than 10%. There are different types of radiant barriers that you can place at various spots between yourself and the sun, including items as simple as lighter colored shingles, with the lighter color reflecting more of the sunlight than darker colors (although these don't tend to reach the 90% standard to be considered true radiant barriers) to silvered roof decking, to reflective paints, to large amounts of industrial-grade foil.

Onward, adventurers. Physics awaits!

After that brief detour, where we reconfirmed that energy efficiency is still the way to go, at least until the payoffs for our projects get much lower, let us continue our quest for improvements in the home.
Late in 2007, in my travels on the Web researching energy efficiency opinions, I began to run across a lot of information about something called a "radiant barrier". This was something new to me: a type of insulation that wasn't traditional insulation, and said to stop heat flow in a somehow different, and possibly better way than traditional insulation.
To understand the radiant barriers, we need to upgrade our knowledge about how heat actually flows into a home in the summer, or out of a home in the winter. As I understand the mechanics of the situation (nice explanation here), there are 3 ways that heat flows from a hot area to a cooler area: conduction, convection, and radiation.
Conduction refers to heat flowing through a solid material (such as from the outside face of a brick wall to the inside face), and can be reduced by using materials such as fiberglass insulation that conduct heat poorly; that is, heat only moves slowly through them.
Convection refers to heat moving due to the material (a gas or liquid) flowing through space. This type of heat movement can be reduced by trapping the gas or liquid so that it can't move. In fact, part of how fiberglass insulation works is through trapping air in tiny pockets.
Bear with me, intrepid readers: here is the good part. Radiation refers to the transmission of energy through space by means of photons. Those photons, given off by all hot objects in all directions, travel through the air until they hit something solid, whereupon that solid object absorbs their energy and heats up. If you have ever stood in the sun on a clear day, you have felt the radiation of the sun. If you have ever sat outside at a restaurant with heat lamps, you have felt the radiation of the lamps. That radiation was absorbed into your skin, causing your skin to heat up pleasantly.
At this point, an aside may be in order. For the purposes of Energy Efficiency we are talking about infrared radiation, a certain range of wavelengths of radiation, which is a different range of wavelengths than ultraviolet radiation, the harmful-to-your-skin variety of radiation also put out by our sun.
The savvy reader will have already guessed that "radiant barrier" must refer somehow to the blocking of the radiation mechanism of heat transfer. The savvy reader would, in fact, be correct. A radiant barrier is simply a way of reflecting the energy being radiated by photons through the air, rather than absorbing it.
"But how important can that really be?", the questioning reader might ask. "How much energy entering a house in the summer, or leaving it in the winter, is in the form of photons radiating through space?"
The answer, according to the radiant barrier folks, is a lot. Their numbers: over 90% of the heat gain in the summer comes through as radiant heat. Over 50% of the heat loss in the winter goes out as radiant heat. In short, if you aren't doing something about the radiant heat, you are missing most of the ballgame.
Perhaps at this point, you the intelligent reader is asking, "but isn't my insulation already catching those photons? Don't they travel until they hit something solid, like my house, and then become heat inside a material subject to the rules of conduction?"

And again, you would be correct. In the summer, the radiated energy of the sun is striking the roof of your house. For most houses, only a small percentage is reflected; the rest of the energy is absorbed and becomes heat trapped in your roof (shingles, etc.) At that point, your roof is also emitting photons since it is hot, and about half of those travel down through your attic airspace and strike your insulation, imparting their energy to it, which means, heating it up. At that point, the insulation does its conductive-resistance magic to keep the heat flowing more slowly into your ceiling, but make no mistake: that heat is indeed flowing into your ceiling. From there, some of it is convecting into the inside air, and some of it is being radiated as photons which will strike you, heating you up, and strike objects in your home, heating them up.
In the winter, this picture is reversed. Warm objects in your house (yourself, your furniture, your floor, your walls) radiate photons in all directions, removing energy (heat) from yourself and your surroundings and taking that energy outward in all directions, including up. Some of the ones hitting the walls and ceiling will reflect back, reducing the heat loss everywhere except at the windows; by design, the photons mostly go right through glass. The photons that go up and are not reflected strike your ceiling, which they heat up, and the heat begins to flow via conduction through the ceiling, through the insulation, and into the cold attic where it will either be lost to convection with the cold attic air, or radiation up to the roof and through that, again via conduction, to the outside face of the roof, and from there, radiated out into space.
Think well on these scenarios, adventurers, and be rewarded with a whole new arena of possible energy savings...

How much energy is that, in solar terms?

Having determined that I cannot afford to hire 20 or 30 people to spend most of their waking lives pedaling bikes to power my house (20 people for 12 hours at $8.00/hr = almost $2000 per day!!!) , I wondered if there might not be other, non-power-grid ways of getting my home powered. After all, Texas is blessed with pretty good sunshine; not as good as the desert Southwest of this country, but well above average. Perhaps I can power my home with solar panels, using some of that energy to my advantage, rather than just fighting it, to reduce its effect of heating my house up through the summer months.

I've got roughly 500 square feet of roof space that might be exposed to enough sunlight for panels to make sense. (This is my estimate; I'd have to get a real professional to look at it for a final number). According to a nice solar system calculator at findSolar, which takes into account the measured sunlight in my area, I might be able to just fit a system into my roof space.
So my electricity needs are solvable with solar, since I seem to have about the required roof space. (I realize that the sun doesn't shine at night; but my house is grid-tied, so I can pull from the grid at night or on cloudy days. I would effectively put energy back into the grid on sunny days where I didn't need all of it; the proposed system is sized to cover my needs as averaged throughout the year, so my net draw from the grid over a year would be 0.)

The solar calculator estimates that the system would cost me around $20,000 (including various rebates and credits). While that's a chunk of change, once I spend it, it covers my energy costs for quite some time. Compared to paying people $2000/day to pedal bikes, this justifies itself after just 10 days. And since panels last for decades (although some of the ancillary equipment won't), this is looking like not too bad a bargain.
So the solar option is clearly better than paying people to pedal. But a more subtle question then arises: is it better for me to spend money buying solar panels, or is it better for me to continue to reduce my energy usage with efficiency measures?
And so we start down the road that most people considering these sorts of things travel: how much do I spend up front vs. how much do I save over time? If I consider $20,000 an investment, what effective interest rate does it pay me? In fact, to know this, I need to know what is the future cost of the electricity it is offsetting, which is, of course, a guess. Like any investment, this "calculation" involves guesswork about the future of the investment, the future of the market, etc.
Looking at what I spent on electricity in 2007, roughly $700.00, the panels might take $20,000 / $700 = about 28 years to pay off in electricity savings, at current electricity prices. That's a return of roughly 3.5%, which is far lower than my efficiency investments have returned. That rate of return for solar will go up as electricity prices go up; however, so will the rate of return on my efficiency investments. So at this point, it's still looking to me like efficiency is the way to go, until I've gotten to the point that the estimated rate of return on those drops to below 5%, or until solar panels undergo another dramatic price drop.