Best Rigid Foam for Site-Built Baffles
Hi there, just wondering what rigid foam product is the best for building ventilation baffles on-site. Is it OK to use rigid foam that is foil-faced for this purpose? If so, would the foil side face out?
From what I understand using foil-faced is fine so long as the ceiling has good air-sealing, but I’m worried about moisture buildup if air does make it through the insulation (which in this case will be cellulose). I don’t have much depth to work with for the insulation so I’m looking at creating a 1″ air gap.
Here are a few products I’m looking at for baffles:
– https://www.homehardware.ca/en/1-x-4-x-8-r60-ener-air-foam-insulation/p/2719426
– https://www.homehardware.ca/en/1-x-2-x-8-type-3-foam-insulation/p/2718980?page=search-results%20page
– https://www.homedepot.ca/product/durofoam-eps-rigid-insulation-96inch-x-48inch-x-1inch/1000116769 (this is foil-faced – is this a bad idea?)
Would you advise against any of these? Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I used RMax 3.2 Foil Faced Polyiso boards that are 1/2" thick. These have foil on both sides. I put the shiny side down because I read somewhere on here that if you put the shiny side up it eventually gets dusty and does not deflect heat.
I did not cut all the way through them - I scored them enough to snap them. So for a 1" space, I would have cut my approx 30" long piece, scored at 1", then scored again at the width of rafter bay less 1" (because I have a 1/2" leg on each side), then cut the board 1" from that. Once in the attic, I sprayed water on the outside of the vertical legs and the score, then fit them up between the rafter bays. I cut them so they would fit snugly. After I fit enough of them that I could reach from one location, I spray foamed the score to the rafter to keep them in there. The water helped set the spray foam faster for those that were not quite snug enough. They held up just fine when they sprayed the cellulose.
+1 for 1/2" polyiso. The foil facer doesn't matter, because you have a vent channel to carry away any moisture that might be present. What you want is something cheap and reasonably stiff, so that you can cut it to fit and have it stay in place. You don't want it too thick though, since that just makes it harder to work with.
Note that it's not important to get a super good seal on those baffles. I usually just wedge them in place and inject some canned foam along the edges to keep them where I want them. I don't worry too much if I didn't get a perfect seal, since a small amount of air leakage will make only a very minimal effect in terms of wind washing of the insulation, which is really what those baffles are there to protect against.
Bill
Are you talking about for holding back insulation on an attic floor, or are you talking about a cathedral ceiling configuration? For the former, foil facing is fine. For the latter, I think that vapor permeability is a good thing.
Hi Charlie, I'm talking about a cathedral ceiling - sorry, I should have specified. Thanks for the reassurance, I was leaning towards something vapor permeable.
I have used 1"polyiso with foil on both sides. The 1" is stiff enough to span 24" without deflection issues from dense pack insulation. And I also cut both sides 1" and fold it. Then staple it on with large crown staples. The 1 inch fold keep the air gap. Used this method many times and works great.
Shawn
Thanks Shawn, the photo is helpful too.
I went back and found the article I learned how to do this from Fine Homebuilding. I made the little tool the same way. It works great. They even have a video of the method on the website here: https://www.finehomebuilding.com/project-guides/insulation/rigid-foam-vent-baffles
@aciarn: See also this thread:
https://www.greenbuildingadvisor.com/question/eps-xps-or-polyiso-for-exterior-rigid-foam