A. You can use closed-cell spray foam that way if you want, although you should realize that the spray foam will prevent the roof sheathing from drying inward. This approach should only be used if the roof sheathing is bone dry, because it will never dry when sandwiched in this manner. Most building scientists would prefer to see fiberglass rather than closed-cell spray foam under the roof sheathing in this type of assembly.
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how much is attic spray foam
Here's a simpler, less expensive alternative: Cut some 2-inch-thick rigid-foam insulation and glue it to the subfloor between the joists or support it with nails driven partway into the joists. Then fill any gaps between the edges of the foam boards and the joists using the canned spray foam sold at home centers or hardware stores. An even easier option is to nail the foam panels against the bottom edge of the joists and seal the joints with canned foam, but you'll lose some headroom and access to any pipes and wires between the joists. As with spray-foam kits, protect yourself and the floor from the dripping globs of canned foam. A full face shield, gloves, and a hat would be a good start.
what spray foam to buy
This depends on the geographic location, insulation, and cost of electricity. Lawrence Berkeley National Laboratory (LBNL), Oak Ridge National Laboratory (ORNL), National Roofing Contractors Association (NRCA), and several roofing material manufacturers have computer programs that can estimate the savings. A growing number of energy consultants are available to provide environmental regulation compliance information. http://m.youtu.be/ggLAUsiuI_o