Radiant Barriers Don’t Work with Spray Foam Insulation on Them
I’m teaching a HERS rater class this week, and on Monday we went out to look at a couple of houses and do a Blower Door test. The second house we visited was under construction, and the builder had used Louisiana-Pacific’s radiant barrier decking, Techshield (photo, left).
When we talked with the builder, he told us that he was planning to spray foam insulation in the roofline. Yes, right on top of the radiant barrier. Will this work? No. I wrote about how radiant barriers work not long ago, and when you put a radiant barrier in contact with other materials on both sides, heat just conducts right through it.
Now, before you go calling this builder an idiot, he actually did this intentionally. He knew that radiant barriers need an air gap, but the Techshield cost him less than a dollar a sheet more than regular decking. His intention was to put the Techshield up there, and then that would give him the option to go either way: an unconditioned attic with a radiant barrier or a conditioned attic with spray foam insulation at the roofline. After talking with contractors and doing his homework, he decided to go with spray foam.
After I published my other radiant barrier article, I got an email from someone who was having a home built for his family, and they were also about to spray foam over a radiant barrier. In his case, they didn’t do it to have more options. They just didn’t know.
So, if you’re thinking of radiant barrier roof decking and spray foam insulation, make sure you understand that radiant barrier roof decking does not work with spray foam insulation on it. It’s an either/or choice.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard.
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For summer cooling, the
For summer cooling, the radiant barrier may be better than the spray foam, most of the attic’s heat gain is through radiation in our sunny climate in Denver Colorado, a well vented attic, with a radiant barrier, helps our clients “keep their cool”, prolong their shingle life, while reducing their conditioned space. Off to D.O.E. Building America’s mtg in Denver 8/9-8/11. Hope to see see you there!
Allison, great article! Thank
Allison, great article! Thank you for helping educate the world about the PROPER use of radiant barriers. The same principle applies to putting radiant barrier BETWEEN shingles and roofing felt – it won’t work. Here is some good information on how you CAN get radiant barrier to the outside of a foam closed attic. Radiant Barrier Under Metal and Tile Roofs
The radiant barrier acts as your first line of defense against radiant heat and the foam closed attic acts as the second line of defense against conductive heat flow.
I have wondered if spray foam
I have wondered if spray foam installed directly on the sheathing will shorten the life of the roof covering if no baffling is provided. I have not found a satisfactory answer to this question.
Bill, the Florida Solar
Bill, the Florida Solar Energy Center has reported on this (http://www.fsec.ucf.edu/en/publications/html/fsec-cr-1220-00/). Several roof covering manufacturers have already made statements that their products will work with SPF on the underside of the deck. Check with them about warranty issues. A white roof with SPF is cooler than a charcoal roof without it.
Presumably, radiant barriers
Presumably, radiant barriers do not work with ANY insulation, or indeed any solid stuff, up against them?
This would then include all those foil coated polyisocyanurate foam boards and such. Does the foil, despite looking high tech, add to, or perhaps even detract from, the insulation value of the product?
Is the air gap on BOTH sides is a key thing for proper radiant barrier installation, OR is an air gap on one side enough? (Clearly my brain is still insulated from reality.)
Paul
Allison, I wanted to send you
Allison, I wanted to send you a follow up on the emails we exchanged last year. If you recall, I did a full foam encapsulated attic with a double-deck / radiant barrier “skin”. Here is an article I wrote on my blog about Combining Spray Foam Insulation With Radiant Barrier. The results have been spectacular. I cut out a small piece out of the bottom deck and embedded temperature probes to the bottom of both decks before foaming. This was done to give a reading of difference between doing a “standard” foam encapsulated attic and the double deck system. Recently, with temperatures at 100º I have had a max temperature between the top deck and the foam at about 185º and the temperature between the bottom deck and foam at about 125º. The attic stays at about 78º. So, I have effectively dropped the Delta-T coming through the roof surface from 107º to down to 47º.
I also went with Mitsubishi Muli-ports using both ductless and ducted air handlers. (The ONLY way to go on high performance construction) The air quality has been great! 2×6 advanced framing, exterior foam board to reduce thermal bridging and aRadiant Barrier House Wrap. We have a pretty good passive solar design too. My bills are about half of what my old home was and this is a larger home. If anyone wants more information on what I did, feel free to contact me at AtticFoil.com
I had radiant barrier paint
I had radiant barrier paint applied several years ago – probably about 7 years – am thinking of adding foam to the entire decking and attic walls.The radiant barrier from what I’ve heard does deteriorate with age.Is there a problem with doing that?