The types of insulation that historically have made our homes comfortable and energy efficient are truly a motley crew. In the old days, sawdust was sometimes dumped inside walls to slow heat loss. Then came vermiculite — porous lightweight pebbles with the look and feel of kitty litter.
Today we still rely on an insulation veteran, fiberglass batt insulation that's made from spun glass fibers. However, if you're looking for a standout with qualities that other types of insulation can't touch, you need spray foam. More specifically, we're talking about closed-cell polyurethane spray foam.
Poly spray foam has a higher R-value per inch than any other commonly used insulation —R-6.2 or higher, depending on the formulation.
Air barrier: Spray foam air-seals and insulates in one step. Its ability to expand and fill gaps and cracks make it an ideal air-sealing material, and it stays in place because of its super-adhesive qualities. Spray foam is often spot-applied as an air-sealing treatment prior to filling an attic with blow-in insulation. But when applied in more generous amounts, it can eliminate the need for other types of insulation.
Waterproof: Moisture will soak its way through fiberglass and cellulose insulation, but polyurethane spray foam stops it cold. Its closed cell structure means that water can't migrate from one tiny cell or bubble to a neighboring one.
Foolproof: Did you know that a void (missing insulation) of just 5% of an insulated area can diminish overall insulation performance by as much as 50%? It's very difficult to install fiberglass insulation perfectly, so voids are not uncommon. But spray foam's expanding, super-sticky characteristics just about eliminate the possibility of voids. And once the foam cures, it's not going to settle or shift like fiberglass or cellulose can.