Can an energy retrofit trap moisture?
I know we’ve looked at this many times, this question addresses a specific point for clarification. Energy retrofits can be expensive. If we are going to make a difference in climate change we need to make retrofits more attractive to home-owners by making them more affordable.
I have a project in NH designing an energy retrofit for a typical colonial home in zone 5, 7,000 HDD. It is 10 years old with a newer 3rd floor expansion. It has 2×6 stud walls w/ R-19 F.G. batts, craft faced in the original, poly under the drywall in the expansion. The sheathing is probably OSB. The owner and I want to get rid of the vinyl. We’re designing an energy retrofit with some sort of foam insulation over the existing vinyl siding (thick enough to avoid moisture accumulation) – either Larsen trusses with spray foam or rigid sheets with vertical strapping. We don’t want to remove the drywall and incur the added expense of all new interior finishes.
My question has to do with moisture. Does EPS, XPS or open cell spray foam have a high enough perm rating to not trap moisture in the wall? My experience tells me not to do it. But the manufacturers tell me these foams are vapor semi-permeable materials.
Dave Ely, AIA
Windy Hill Associates
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