Closed cell foam can also increase the racking.
Attic closed cell foam insulation.
There is 300 difference there but the cost they give you is about a third difference and open cell foam at 12 inches will not give you the performance of 2 inches of closed.
Polyurethane foam spf insulation and we are frequently asked to march 2007 i jlc i 1 insulating unvented attics with spray foam closed cell polyurethane foam provides the insulation air barrier and vapor retarder necessary for an unvented attic assembly closed cell polyurethane foam provides the insulation air barrier.
Insulating value and strength open cell foam has an r value of about 3 5 to 3 7 per inch while closed cell foam has an r value of about 6 per inch.
Using spray foam insulation in an unvented attic application both low density open cell and medium density closed cell spray foam can be used in an unvented attic to provide the needed thermal resistance and airtightness.