[turbo6bar]

If you already have the foam, by all means, use it, especially if it's closed cell foam (high R value/in). I would caulk the joint between the foam and wall studs. Then again, is this for walls or other? You say studs, implying walls, but 2x4 walls won't accept four inches of foam.

Another alternative is sprayed cellulose for walls and ceilings. I recently used parpac membrane with dense-pack cellulose (not wet spray). The labor is intense, but the cost is very low. Air infiltration is minimal when coupled with foam and caulk. Additionally, cellulose is made from waste products. jurgen