I’d like to insulate the floor of my attic with spray foam. How do I know what type of foam to use?
I live in Iowa and my attic insulation requirement is R-49. I currently do not have enough insulation and I’d like to take it up. I have been told to use fiberglass or cellulose depending on who I’m talking to. I’d prefer to use cellulose, but I have concerns about too much weight on the ceiling that would cause it to sag. I would like to foam the floor of the attic and put blown-in cellulose over top. I think this would provide the best air sealing and insulation performance. I’m confused about what type of foam to use. I understand that open cell foam is vapor permeable and would allow moisture to pass through which might be beneficial for letting the building dry? Versus the closed cell is a vapor retarder which only allows moisture to pass very slowly. Would I have to worry about the wood underneath closed cell foam molding or rotting because it retards vapors/moisture from passing through? Wouldn’t that trap the moisture in the attic floor? Thank you.
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Replies
Jason,
Q. "How do I know what type of foam to use?"
A. If you end up using spray foam, either open-cell foam or closed-cell foam will work. To learn more about spray foam, see Spray Foam Insulation: Open and Closed Cell in the GBA Encyclopedia.
Q. "I'd prefer to use cellulose, but I have concerns about too much weight on the ceiling that would cause it to sag."
A. Unless your ceiling is unusual, there is no reason to worry that cellulose will cause sagging. Call up a cellulose insulation contractor if you have more questions on this issue.
Q. "I'm confused about what type of foam to use. I understand that open-cell foam is vapor permeable and would allow moisture to pass through which might be beneficial for letting the building dry? Versus the closed cell is a vapor retarder which only allows moisture to pass very slowly. Would I have to worry about the wood underneath closed cell foam molding or rotting because it retards vapors/moisture from passing through? Wouldn't that trap the moisture in the attic floor?"
A. Either type of foam will work well. As long as the foam is installed at an adequate thickness -- at least 2 inches of closed-cell foam or 5 inches of open-cell foam -- and as long as the spray-foam installer is skilled, either foam will create a good air barrier. Creating an air barrier is the most important way to prevent moisture from entering your attic. There is no reason to worry about vapor diffusion in this location.
Martin,
concerning your 3rd answer....
Since the purpose of the foam is to connect the air barrier materials (framing & gyp-board)....
Can you explain why 5 inches of open-cell foam is required?
Surely it does not take 5 inches of open cell to connect an air barrier.
John,
You're right -- five inches is conservative, to account for inconsistent installation. [Later edit: this information might be wrong. See my next post.]
I know for a fact that 2 inches of open-cell foam won't make an air barrier. If I were using open-cell foam for an air barrier, I would want at least 5 inches.
Anyone have any testing results they want to share?
John,
More information: maybe I should have said that you need at least 6 inches of open-cell foam to create an air barrier. Here's some information from a spray-foam website -- it needs to be confirmed by finding more info:
http://www.spartaninsulation.com/?page_id=22
"Open Cell foam is not a Code Approved Vapor Barrier with a permeability rating of over 400 ng and is not an approved air barrier until a 6″ thickness is achieved."