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.