Posted By Rafi on 15 Apr 2015 12:24 PM
@billnaegli: When you sprayfoam under the roof, do you usually go with the open cell or closed cell? Closed cell is so extremely expensive. Our city requires R-38 value. With the open cell foam we would need appr. 8 to 9 inch of open cell spray foam thickness. That is a lot, the estimates came back with 5.5 inch, which is the thickness of a 2x6. What is common with residential homes these days?
Open Cell per inch = R 3.5 avg
Closed Cell per inch = R 6.0 avg
OPEN CELL is vapor OPEN or permeable while CLOSED CELL is vapor CLOSED or impermeable (based on total approx thickness of course).
Code calls for R-38 minimum so that would be
10" of open cell or
6" of closed cell in order to pass inspection code. That is the bare minimum in order to pass code but most say that R-49 should be the minimum in an energy efficient home. The 2012 IECC requires R-49 minimum in the roof/ceiling in Zones 4+.
If you only do 6" of open cell you would be at around R-21 and the county/city inspection would fail it. Closed cell at 6" would bring it close to code at R-36 and close enough to R-38 that it would most likely pass inspection. Not to mention that an R-21 ceiling would contribute to higher energy bills.
Open cell carries a slight risk because if the ceiling is not done 100% correctly, any air leaks can cause water vapor drives inside the home to rise and when it hits the roof sheathing, it can condense and create moisture/rot issues.
I believe closed cell is the better product but it all depends on the climate location, roof sheathing, house design, vapor barriers, etc.