What’s the best way to insulate the underside of a 10×15-foot sunroom?

The purpose of a vapour barrier is to prevent the movement of air through insulated structures like walls and floors, but vapour barriers are only necessary when the insulation itself is unable to prevent this unwanted air movement. The best insulation for your floor (and for many other applications) is the kind of spray foam that’s capable of blocking air infiltration all on its own. Given the amount of space you’ve got under your sunroom, the application of spray foam onto the underside of the floor and between the floor joists is a fast, easy and highly effective way to create a very warm floor. Besides preventing the passage of air through the insulation by osmosis, it also seals cracks and air leaks. The only drawback is the high cost of spray foam, but I have a suggestion.

To create a properly insulated floor, you really should have at least R15 worth of insulation, and that’s about three inches worth of foam to achieve this. More is better. Instead of building up this much spray foam alone, you’ll save quite a bit of money by cutting two-inch-thick pieces of rigid foam to fit between the floor joists, then applying spray foam over top to seal everything. You can easily spray this yourself, using one of the two-part spray foam systems designed for do-it-yourself application. I’ve had good results with Tiger Foam (888.844.3736).