Spraying vapor barrier paint on open-cell foam
Taunton Guts| Posted inGBA Pro Helpon
Hi,
I want to use open-cell foam on a new project and am trying to meet the IRC guidelines. The insulation installer suggests spraying vapor barrier paint directly on the foam. I have read previous research on GBA that found this ineffective. However, would it also be detrimental to the performance of the foam, or would it make no difference?
Thanks,
Josh
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Replies
Josh,
I don't think it hurts the foam. It just doesn't work.
Researchers at the Building Science Corporation sprayed vapor-retarder paint on cured open-cell spray foam, and then tested the vapor permeance of the foam. The paint didn't change the vapor permance of the foam at all (because the foam is so porous that it's impossible to get enough of a film on paint on the spray foam to change the foam's vapor permeance).
Vapor retarder paint was developed for use on paper-faced drywall. It works on drywall, but not on spray foam.
Josh,
You may be thinking, "As long as the building inspector thinks it's OK, it's good enough." But depending on the type of assembly you are talking about, and your climate zone, the vapor retarder may be necessary to keep your sheathing from getting damp and rotting.
So if you need a vapor retarder, install a real vapor retarder.