Spray foam insulation is substantially more costly than fiberglass or mineral wool batts, but it does work much better. There are two reasons why:
- it never sags
- it seals structures tightly against air infiltration and drafts
One way to reduce installation costs is to do the work yourself. A product called Tiger Foam (888-844-3736) allows DIY application that may save you money. It’s still pretty expensive stuff, but way better than any kind of batt insulation. You can’t really compare the performance of expanding foam with any other kind of insulation, even if they do have the same R-rating.