The delicate balance of airflow

I've been thinking a lot about attic airflow lately. In the warm, humid climate that I live in, having appropriate airflow through the attic is important in at least two ways. First, it is important because it allows cooler outside air to automatically replace the hot air in the attic. Secondly, it is important because it reduces the chance of condensation, the enemy of all homeowners. How does it do this? By allowing the air to find its way out of the attic as it cools. This particular point may we worth delving into in detail, because many a roll of insulation and many a roof has been lost to the perils of unexpected condensation.

Critical Features of Condensation

Two main principles drive condensation. The first is hopefully known to everyone who attended elementary school: hot air rises. Or more accurately, air that is warmer than the surrounding air is also less dense than the surrounding air, so it is buoyed upwards. This principle explains why we don't hold our hands over candles, why all the nice hot air in your house rests against the ceiling, and why hot air balloons work.
The second is known to anyone who has taken a passing interest in meteorology: warm air can hold more moisture in it than cold air. This little phenomenon explains why cold fronts are often led by a line of rainstorms, how fog forms, and why clouds tend to dump all their moisture when they reach the front of a mountain range. This last one is quite fascinating to me: the surface winds sweep up the mountainside, bringing warm lowland air higher and higher. As the air gets higher (via orographic lifting) it naturally loses pressure and therefore temperature (due to a process known as adiabatic cooling). Once the air gets high enough that it can no longer hold the moisture that it carried before with no problem when it was warmer, the moisture condenses out of the air and falls to the ground as rain.

Now, given these two main principles, we can apply them to attics and see why they can create moisture problems. One side of effect of human activities such as breathing, cooking, cleaning, showering, and so on carried on inside a house is the creation of warm moist air. Thanks to the principle of warm air rising, the warm air we create in our houses moves upwards. Since things like cracks, gaps, and seams exist in most houses, the warm moist air finds its way into the attic.
Once the air is there, if there is not sufficient ventilation to move it out and replace it with new outside air, the moist humid air will stay in the attic, encouraging bad things like mold growth. Furthermore, if the attic cools down quickly like it would on a nice clear fall or winter night, that trapped warm moist air would become trapped cool moist air. If the air cools enough, thanks to our second principle above, it will no longer be able to hold the moisture that it held when it was warm and the water will come out of the air and "rain" (condense) in the attic. Two factors determine the severity of this "indoor rain event". The first is the amount of moisture that was put into the air to begin with, and this can be reduced by good habits like running the vent fan when showering. The second is how far the air cools: the cooler it gets, the more water will be squeezed out of the air. The way to ameliorate this is to have enough ventilation so that as much of the air as possible can escape before it cools down enough to drop its moisture.
If these "indoor rain events" occur often enough in the attic, you will begin to see water damage to your house.

Seasonal Balance

So if condensation is bad and ventilation can defeat it, why don't we just get as much ventilation in place as possible? Why don't we just cut vents in all 4 attic walls, stick screens over them, and be done with it! The mindful reader will recall that the title of this post implies that there is a balance to be achieved here. That balance, like most balances one can achieve, depends on one's place in life. Or more correctly, the place that one lives.
If you live in an area that is almost never cooler than you want the interior of your house, putting in as much attic ventilation as humanly possible is probably the way to go.
If you live in an area like I do, where most of the year is spent with the outside world a lot hotter than I want my house to be, but a good 3 months is also spent with the outside world a lot colder than I want my house to be, I want enough ventilation that I get some good cooling in the summer and good protection from condensation year-round, but I don't want so much ventilation that my attic loses all it's heat-trapping potential for the winter. This is particularly important in my house because a lot of water pipes run through my attic, and an attic that freezes solid in my few freezing winter nights would freeze and burst water pipes over my living space.
I would guess that if you live somewhere that's cooler outside most of the year than you want your house, and rarely much hotter than you want your house, you would want just enough ventilation to avoid condensation problems, but not more than that.

Flow Balance

So have just discussed one axis of balance for our ventilation. There is another axis as well: the amount of space dedicated to air leaving your attic (net free area outflow) should be balanced as closely as possible by the amount of space dedicated to air entering your attic (net free area inflow). Why does this matter? In fact, why do you need any intake at all? Isn't the point just to get that hot air out?
If you've ever tried to pour milk out of a gallon jug, you might have noticed that it doesn't work well if you just tip the jug over so far that the milk just slops out of the jug in series of bursts, interrupted by gasps when you can hear the air rushing back into the jug, almost as if it were taking a breath before dispensing another sloppy burst of milk. Why is it so difficult for the milk to get out? What's slowing it down?
The answer, of course, is lack of replacement air. When a certain volume of milk leaves the jug, it leaves a vaccuum, or more accurately, the air inside the jug is left at lower pressure than the air outside. This causes the air outside to force its way in, interrupting the smooth flow of milk out of the jug, and reducing the speed at which the jug can be emptied. In fact, the best way to really move that milk out of the jug is to punch a hole in the bottom just before we turn it over, to allow the air to come in from the top as the milk empties out the bottom.
Now: if we turn the whole thing upside down, replace the jug with our attic, the milk with hot air, and the opening in the jug with attic roof vents, the analogy makes perfect sense. Bear with me as I explain.
If you just had roof vents in your attic, and did not have any soffit vents, gable vents, or other air intakes, your attic would be just like that milk jug turned upside down before we punched a hole in the bottom. The air would slosh out of those vents as it heated up and tried to leave, but that would create a lower air pressure region inside the attic. This would tend to pull air back in the vents (the only openings), dramatically slowing the airflow out of the attic, just as the inrushing air slowed the milk from leaving the jug.
The solution, of course, is to punch a hole in the bottom of the attic just as we punched a hole in the bottom of the milk jug. That is what soffit vents and other air intakes are: a way to get air to "backfill" for the hot air that leaves through the vents in the top of your attic. The fact that the backfilling air can come in through a different path than the air leaving means that you can get a nice, uninterrupted flow of ventilation through your attic. Plus, well designed intakes will pull replacement air from cool, shady spaces like the underside of the eaves.
Now you might begin to get a feel why the outflow area should equal the intake area. If the intake vents can't supply enough air, the air to make up the difference will try to come through the outflow vents, and the more it does that, the more you mess up your nice smooth outflow, and the less the attic air will be able to escape.
Indeed, attic airflow is a delicate balance.

Payoffs by category

After talking to some other energy efficiency enthusiasts, I have learned that there seems to be a lot of interest in the financial rate of return for the different Energy Efficiency improvements that I've made. Although I've discussed these things before, it behooves me to say again that my methods are not as rigorous as a true scientific experiment; rather, they are the methods of someone who wanted to improve his Energy Efficiency and has taken some basic notice of the costs and cost savings.

I will include numbers for transportation energy use (which I don't typically discuss here), because transportation by car is a giant user of energy; likely at least roughly equivalent in scale to the energy use of my house. This is why there is much discussion of plug-in hybrid cars being used to feed energy to (and take energy from) houses in the future; they might well carry enough battery storage to be significant in the home environment.

You should be able to click to enlarge the table.



From that table, it looks like the attic efficiency work is paying off rather well at 28%, and probably would be paying off spectacularly well had I not muddied the waters by paying for a bunch of roof decking plywood to be replaced in 2009 so that I could get radiant barrier into some areas inaccessible to me from the attic. The cost of that plywood per square foot was roughly 10 times the cost per square foot of the radiant barrier that I ended up putting on it, which is why that rate of return drops so dramatically :( But I was getting the roof replaced at the time; my next shot at getting the sun blocked from that accursed vaulted ceiling would likely have been 15 years later, so I bit the bullet and replaced that plywood with radiant-barrier (and baffle-)covered plywood, improving radiant barrier and ventilation in one fell swoop.
Some might also judge, from that table, that the radiant barrier (completed in 2009) might not pay off as well as the attic ventilation (completed in 2007, which returns a whopping 51% and has already paid for itself). But you might be wrong in that analysis partially because of the plywood cost that's thrown in to the mix. My gut feel is that the payoff of ventilation (plus duct sealing) vs. radiant barrier tilts slightly in the direction of ventilation, but they're both incredibly efficient uses of your money to save energy. And the comfort improvement with radiant barrier is outstanding.
In fact, with rates of returns this high, you literally should be taking a loan at any interest rate less than 20% and applying it to any of these techniques and making money by doing so. But I will not belabor that point; this is a blog about Energy Efficiency, not Making Money... although since these numbers show we can clearly do both at the same time, why aren't we?

Chronically underestimating efficiency?

Listen, dear readers, to something that is confusing to Energy Efficiency Man. I often read about energy efficiency measures in the press. Most of the articles written by reporters and policymakers talk about incentives or various combinations of steps homeowners can take, and if they mention numbers at all, they mention numbers like "9% energy reduction" or "20% savings". In fact, I can't recall a single press or government-authored article that I've read recently that mentions a number more than 30%. This New York Times article discussing recently released White House paper on "Cash for caulkers" goes so far as to estimate 28%. Now, as nice as 28% is as a savings, it simply doesn't grab the attention or change the terms of the energy discussion like a number greater than 50%.
However, as readers of my blog over the past few months have learned, I've personally experienced electric energy usage reduction of nearly 60% (in the literally hottest summer ever in Central Texas, mind you), and I've read articles by people who have actually done these things themselves, and the energy savings they typically mention are 50-75%.
My question: why are the people actually saving energy in the real world saving 50-75%, but the folks putting out most of the articles only think you can get 20%? To me, a 50% energy reduction is a game changer. With reductions like that, the current debate in my municipality about what to do with our stake in a local coal plant (which provides only 30% of our energy) becomes entirely moot. What to do about the coal plant? Shut it down - we wouldn't need it, and we don't want the 70% of our emissions that it produces. We could shut it down and still have an additional 50%-30%=20% spare capacity for population growth. (By the way, that is NOT one of the options under consideration by our city council - instead, we will likely spend tens of millions of dollars investing in making the coal plant "cleaner").
Are 50% savings like that too radical to make the press? If all energy users (homeowners, businesses, and governments) could achieve the savings the real-world Energy Efficiency enthusiasts have, we wouldn't need to build another coal plant, ever. We could achieve these energy savings, as I have in the very real world, by investing money borrowed at a typical rate of 5-10% for a rate of return between 20% and 40% according to my own experience. In effect, we could make money by saving energy. And of course, our rate of financial return would go up as the price of energy goes up.
Some of you may be familiar with Factor Four by Ernst Ulrich Weizsäcker, Amory B. Lovins, and L. Hunter Lovins. Energy efficiency enthusiasts will recognize Amory B. Lovins of the Rocky Mountain Institute, a policy think tank on energy issues. The book has been around for over a decade, and in it, the authors argue that based on real-world experience with industry and manufacturers, we are using energy at about 1/4th the efficiency that we could be. In effect, we could be doing 4x as much work as we are now with the same energy, or twice as much work using half the energy we use now, or the same amount of work using 25% of the energy that we use now, based on making improvements that pay for themselves in a manner timely enough to pay off the loan to make them. I would have to say that my own experience makes me think he is likely correct. There are still a few things that I'd like to do to my house that might get me to that magic 75% reduction, and although I've probably gathered most of the low-hanging fruit, there is more fruit to be gathered (probably mostly in replacing windows that are currently inexplicably surrounded by one of the most heat-conductive materials available, a fact which I still plan to discuss in a future post).
So why are we reading about all this work we have to do to save 20% at best, but folks actually doing it are saving 50%-75%? Comments welcome!

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.