Architect spec'd 6" of open cell foam in rafter cavities directly below roof. At what thickness does OC foam become enough?
Architect also spec'd bituthene on roof deck.
R38 or higher is the value I always try to get to on a roof.
- Open cell foam has an R value of about 3.5 per inch.
- So 6 inches of open cell foam yields only R19.
If your rafters are not that deep I would recommend considering closed cell foam instead which has double the R value per inch.
For more information:
Read "I am putting spray foam insulation in my attic. What is the better option: open or closed cell?" a Q&A answered by Polly Osborne.