Full article here: Insulating a Slab With Spray Foam
How long has closed cell spray foam been around? No worries about it deteriorating under the slab and leaving a big void in 10 years?Why SPF? In cold climates, insulating under a concrete floor is just common sense. But it’s fair to ask why SPF should be used instead of rigid foam board. Perhaps the greatest advantage SPF offers is that it provides a monolithic layer of insulation, with no seams or other imperfections in the thermal barrier. Because it bonds aggressively and completely to just about anything it contacts, the foam creates a superior vapor barrier around plumbing and other slab penetrations
Detailing foam board to provide an equally tight barrier takes time and great care. Getting the substrate sufficiently smooth and flat to evenly support the board takes a lot of effort too. SPF, by contrast, is totally forgiving of irregularities in the substrate. And as soon as the SPF cures — within 15 minutes — it can be walked on without concern for punching through into a concealed void (Figure 2). There’s also far less waste involved with SPF; there are no off-cuts or other discards. We can spray the exact amount required. “Extra” foam goes to the next job instead of into the dumpster.
Cost comparison. The material cost for spray foam is higher, but once the labor to properly detail rigid foam board is taken into account, the installed price becomes competitive. Our installed cost for a 2-inch layer of closed-cell foam runs about $2.20 per square foot. Two-inch polystyrene board costs around $1.80 a square foot installed, but it has a lower insulating value of R-10. Also, the bigger the slab area, the more labor is required to detail foam board around the perimeter and penetrations. On slabs 2,500 square feet and greater, we often reduce our price slightly, which ends up making SPF the less costly route. And because of the time saved, the slab pour can be moved up in the schedule.