However, Kenny points out that this strength isn't acknowledged by building codes. In other words, while closed-cell foam can help stiffen a structure in a high-wind zone, you can't reduce the structural bracing as a trade-off.

INSULATING VALUE

The insulating value of any foam depends on the gas trapped inside of it. Because a gas like Enovate is a better insulator than air, closed-cell foams give you more R-value per inch. Some open-cell manufacturers like to differentiate their product by pointing out that it has a stable R-value, while gas leaking from the closed-cell product degrades its effectiveness over time. Riddle says this is misleading. Closed-cell foams do lose some gas soon after installation, but they stabilize over time, and their long-term R-value is still impressive. In fact, his company advertises its closed-cell foam's long-term R-value of 6.8 per inch, which is nearly twice that of open-cell's R-value of 3.5 per inch.

AIR AND VAPOR SEALING

Both foam types resist the passage of water vapor better than fiberglass or cellulose, but closed-cell foams do it best. In fact, some closed-cell foams have a perm rating of less than 1.0, the standard perm rating for a vapor retarder.

OPEN OR SHUT CASE

OPEN OR SHUT CASE

Under normal circumstances this may not mean much: Condensation in walls and ceilings is usually caused by leakage of moist air, not differences in vapor pressure, and even low-density foams do a good job of blocking air movement. However, Riddle recommends using closed-cell foam where outside water is a concern: on the coast, in flood-prone regions, in crawl spaces, and in attics. (His company even uses it as a flat roof covering. It sprays it on the roof surface, then covers it with gravel.) It's also a good choice where you need to get a lot of insulating value into a small space.

THE BOTTOM LINE

One disadvantage of closed-cell foam is it costs more per inch than open-cell, since it's denser and requires more material. Cost per R-value can vary in different parts of the country, so you'll have to check with your installer.

The choice of foam should be based on the requirements for strength and vapor control, available space, and so on. “We select a foam system depending on the particular requirements of our clients' projects,” says Riddle. The table on this page includes some suggested uses.

Charles Wardell is a freelance writer based in Vineyard Haven, Mass.

SOURCE: NORTH CAROLINA FOAM INDUSTRIES