3.6 per inch

Only if your attic is used to service your mechanical equipment. If you have a walk up attic, you may be required to used a Thermal barrier or drywall to cover the foam.

1/2-lb SPF refers to generic spray polyurethane foam weighing between 0.4 and 0.6 lb/ft3 when fully cured. Spray-applied to a substrate, it expands about 150 times its original volume to form a semi-rigid, nonstructural plastic. 2-lb SPF, on the other hand, weighs between 1.5 and 2 lb/ft3 when fully cured. The material is used in interior applications, spray-applied to a substrate, before expanding about 35 to 50 times its original volume, and forming a rigid plastic with a compressive strength between 15 and 25 psi.

The advantages of closed-cell foam compared to open-cell foam include its strength, higher R-value, and its greater resistance to the leakage of air or water vapor. The disadvantage of the closed-cell foam is that it is denser, requires more material, and therefore, is more expensive. Even though it has a better R-value, typically the cost per R is still higher than open-cell foam. The choice of foam can also be based on the requirements for the other performance or application specific characteristics such as strength, vapor control, available space, etc. Open-cell SPF has an R-value around 3.81 per inch and typically uses water as the blowing agent. Closed-cell SPF has an R-value of around 6.6 per inch (aged R-value) and uses high R-value blowing agents.

Icynene 1/2lb. foam and Bayer 2 lb. foam.