Expanding Foam

What it is: Made of either open-cell or closed-cell polyurethane (a plastic), or a special cement, this insulation goes on as soft foam or foaming liquid, filling all spaces and then stiffening in place. Applied only by professionals, it's more costly than other options but it is the beste at plugging air leaks.

Open-cell polyurethane, or polyicynene, is a low-density, spongy foam. It's sprayed between exposed studs and expands to 100 times its volume in mere seconds. On finished walls, installers pour a tamer version through small holes; it expands over minutes to 60 times its volume. Closed-cell polyurethane foams to 30 times its volume and dries to a very hard shell.

Cementitious foam, which goes on like shaving cream but hardens over days into a meringue consistency, requires mesh across the studs to contain it.

Performance: Polyicynene gives about R-3.6 per inch of thickness; closed-cell polyurethane, between R-6 and R-7; and cementitious foam, R-3.9

Best used: When you can look beyond initial cost to long-term comfort.

Issues: Polyicynene may crack existing walls or leak out and stain a floor. Polyurethane is not stable when exposed to UV rays. Both closed-cell polyurethane and cementitious foam aren't flexible, so as studs expand and contract, gaps may open.

Cost*: Polyicynene and polyurethane are about $1.50 per square foot, including labor, if the wall is open, and $2.25 per square foot for existing walls. Cementitious foam costs about $1.40 to $2 per square foot. Installers can also spray a thin layer of foam to seal leaks, then fill in with less expensive insulation.

*Note: All prices are approximate for 1 square foot in a 2x4 wall
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