Open cell foam on roof deck
We’re in the thick of our home construction (zone 4/Tennessee) and I met with the spray foam installer today. We’re using open cell foam on the roof deck and shooting for minimum R38. The installer uses Carlisle open cell foam (R3.7@1″).
The installer typically applies 5″ on roof decks in our area. We’re shooting for a PGH and using the energy star regional guidelines as our performance minimum measures. Energy Star requires R38 in our region. We don’t have building codes here..at all. The foam installer stated that foam installation thickness R-value measurement is doubled for code compliance (Rvalue/in x Thickness x 2). For example he sprays just over 5″ to meet a nearby local code of R38 (3.7 x 5.1 x 2).
I can do some math, and it seems likely that if we want an actual R38, we will need 10.27″ of open cell foam @R3.7/in. The installer insists that this is a complete waste of money and he has never installed anywhere near that much foam in a house. I should mention that spray foam installers are pretty rare in this area and this installer is the #1 recommended by everyone we’ve talked to.
Is this doubled R-value measurement a real thing? Or do I need to push for the thicker (two pass) installation?
Any thoughts or advice will be greatly appreciated.
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