I’ve never found any issues involving off-gassing of modern spray foam once it’s hard. I think you’re okay on that front. The other issue is cost verses effectiveness. Spray foam is very effective, but it’s also expensive. A less costly alternative involves cutting pieces of 2″-thick extruded polystyrene foam to fit between the 2×4 frame you plan to put up, followed by a layer of spray foam over top. Cut each piece of foam so there’s a 3/4″ gap around the perimeter, leaving room for the spray foam to fill and seal the gap. An even easier and less expensive option involves using insulated panels instead of a 2×4 frame. The only product of this kind that I know of is Barricade Wall Panels (866.544.6879). They’re 24″ x 96″ panels with one layer of oriented strand board (OSB) factory-glued to the face of 2″-thick extruded polystyrene sheets. These panels simply tilt upright into position, anchored with screws driven into the existing wall. Drywall fastens with screws driven into the OSB. Though rated at R12, don’t be fooled by this seemingly small number. R values only measure conductive heat loss, and make no comment on energy losses by radiation and convection. This is why R12 worth of foam performs vastly better than R12 worth of batt insulation.
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