Q: What are the pros and cons of open-cell and closed-cell foam insulations? And where are the best places to use each?

A: BLOWN-IN-PLACE POLYURETHANE foams are a blend of isocyanates and urethane resins that are mixed in a spray nozzle, then blown into a wall or ceiling cavity. A blowing agent makes the foam expand and fill the space.

Open-cell and closed-cell foams use different blowing agents. They also differ in hardness, insulating value, vapor permeability, and cost.

AIR AND GAS The most familiar brands of open-cell foams are Icynene and Demilec. These use water as a blowing agent and insulate by trapping air in microscopic cavities that are connected to one another like the voids in a sponge.

Closed-cell foams are blown with gas, which also acts as the insulator. As the foam expands, it traps the gas in a matrix of tiny spheres, each about the diameter of a human hair—a kind of microscopic, three-dimensional bubble wrap. Unlike the air in an open-cell product, very little of the gas trapped in these bubbles leaks out. Insulation manufacturers buy the blowing agent, package it with the urethane mixture, then sell the system to insulation contractors.

Until recently, the blowing agents used in closed-cell foams included chlorine-based chemicals, which deplete the earth's ozone layer, but manufacturers are switching to more environmentally benign products. For instance, Honeywell's new Enovate gas (HFC-245 fa), which is used by some of the biggest insulation manufacturers, is a hydro-fluorocarbon gas that includes non-ozone-depleting fluorine.

COMPOSITION AND DENSITY Open- and closed-cell foams have different compositions and densities. Open-cell foams are soft and pliable, with densities of ½ to ¾ pounds per cubic foot. “They're like a cushion, or the packaging material molded inside a plastic bag to fit a fragile object being shipped,” says Steve Riddle, vice president of sales with North Carolina Foam Industries (www.ncfi.com), a nationwide installer of open- and closed-cell foams.

Closed-cell foam, by contrast, has a density of around 2 pounds per cubic foot; in fact, it's stiff enough to walk on without crushing it. Says Riddle: “The bubbles in a closed-cell foam are strong enough to take a lot of pressure, like the inflated tires that hold up an automobile.”

Riddle's installers find that closed-cell foam, because it's so rigid, will actually stiffen wall assemblies by helping lock a home's framing members together, making it the insulation to use in high-wind areas. This field experience has been confirmed by laboratory research.

“There has been a lot of testing done on the racking strength provided by different types of spray foams,” says Tom Kenny, of the NAHB's Research Center, in Upper Marlboro, Maryland. “And the strength provided by closed-cell foams can be substantial.”