There are two general types of spray foam that are commonly used; closed cell and open cell foam. Closed cell spray foam insulation is mainly used with sub-floors, and is applied at a two inch average. The average estimated cost for this application is usually around $1.75 per square foot.Closed cell spray foam insulation is also very suitable with metal roofing and metal walls. Open cell spray foam insulation is the most common used of the two, it is generally used in walls and roof lines for wood structure homes.The average estimated cost for this application is usually around $1.15 per square foot for wall cavities and is applied at three inches.oof lines are around $1.50 per square foot and is applied at a five inch average to the underside of the roof deck, not the ceiling floor. With that said, in some cases open cell spray foam is applied to the bottom of a ceiling floor, but it’s not common.