Do Radiant Barrier Systems Really Work?
by Ken Sheinkopf, a communications specialist with the American Solar Energy Society (www.ases.org)
This article appeared in the York Sunday News on August 3, 2008
Q. I recently heard about some new cooling strategy called "radiant barriers," in which heat from the house is directed up to the roof. Do you know if this really works, and if reflecting the heat out through the roof can negatively affect the roof shingles?
A. Those are good questions for this time of year, since one of the best ways to keep your home's living space cooler is by keeping your attic cooler.
First, let me point out that the use of radiant-barrier systems in a home is far from anything new. Researchers have been studying this strategy - and people have been using it - for more than 25 years that I know about, and homebuilders have actually used foil in homes for decades.
Note that I used the word "system" when talking about this cooling strategy, though, since it is more than just a product - it's actually the way the product is used.
It's really a very simple concept. The basic product is a type of aluminum foil, though there are many different types of builders' foil on the market today that hold up better than your typical kitchen foil for a use like this.
This material is placed in the attic, often stapled to the underside of the top chord of the roof trusses or to the underside of the roof decking so that there will be an airspace below it to the attic floor. The radiant barrier will block the radiant heat transfer between a surface that is giving off heat, such as a hot roof, and a surface that can absorb this heat, such as conventional attic insulation.
Without something like this to block the heat coming into the attic through the roof, the heat gets radiated to the insulation which transfers it to the material it touches, primarily your home's ceiling. Now you've got a hotter house, making you uncomfortable and causing the air conditioner to run longer to get rid of the heat.
Research has found that a radiant-barrier system can block 95 percent of the heat that gets radiated downward by the roof before it gets to teh insulation.
Even if your attic is well-ventilated, open its access door this summer and you'll feel a blast of hot air, since temperatures in a typical attic on a hot summer day can easily reach or exceed 150 degrees F. That's why you need to get this heat out of your attic before it gets into your living space.
Now that you understand what a radiant barrier does, let me answer your question about it effects on roofing materials. It is pretty unlikely that these systems can cause any damage to roof shingles, even on the hottest days.
Keep in mind that roofing materials are obviously made to withstand very high temperatures, and research has found that, depending on the color of the shingles, a radiant barrier only increases their peak temperature by five degrees or less. That's not very significantwhen you consider that peak temperature on the shingles can easily reach 160 to 190 degrees without causing any damage.
If you're unsure of what effect this strategy might have on your roof warranty, check with your roofing manufacturer or contractor to make sure there is no working in the warranty that might possibly cause concern.
In the more than 20 years that I have been working with researchers in the energy field and talking with literally thousands of consumers, I have never met anyone who said a radiant-barrier system had caused any shingle damage or other problems.
Radiant barrier systems are fairly easy to install, since there doesn't need to be a continuous barrier such as insulation needs to be effective.
The hardest part of the installation is for you to withstand the high temperatures when you are doing the installation in the attic, so it is best to install the foil in cooler temperatures. Actually, it's best to have it installed when a house is being build, since builders can easily put the material in place before the attic is finished.
You also need to face the foil downward so dust won't accumulate on its surface. And you can't just roll the foil on your attic floor - it needs the airspace below it to do its job.
There also are radiant-barrier paints on the market that can be easily applied in place of stapling the foil material. Just be sure you buy paint specially made for this purpose, since it will have a low emissivity (it reflects thermal radiation very well while emitting very little heat).
Check out some Web sites or local contractors and building supply stores, and you'll find all kinds of foil material available.
There's some great material available from the Floriday Solor Energy Center (www.fsec.ucf.edu) with more information on this cooling strategy and good directions for installing it yourself.
Those of us at K'nA thought this article was very intersting. We have some thoughts on the subject that you can read now.
Let me follow up with an important fact - there needs to be an air space between the radiant barrier and the roof decking. It is this air space that keeps the radiant heat from getting into the attic. Also, by having soffit and ridge vents the trapped air is automatically expelled.
Now let me explain the products that can be used based on the construction of the attic.
If your attic is built using trusses, the product to use is Low-E Tab. This product is designed to be installed between the trusses. On each edge, there is a 3/4" flange. The flange is used to staple the radiant barrier to the inside of each truss. You do not push the radiant barrier all the way to the decking. When the staple flange is even with the underside of the truss you stop and staple.
When your attic is built using rafters, you have a few choices of material. The products you can use are Micro-E and Low-E. All of these products are applied the same way. The radiant barrier is run out perpendicular to the rafters. At each rafter, you staple the product to the rafter. After everything is covered and stapled you should go back and add nails with caps to hold the material in place. This extra step will keep the material from pulling through the staples.
One other item that is necessary when applying any of these Low-E Radiant Barriers. Any place where two pieces of materials are butted together the seam needs to be taped. The reason - so the radiant heat can not pass through the joint. The tape to use is any Aluminum HVAC tape.
Using these products and following the application rules will give you a Radiant Barrier System that really work