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.