Foam home: sprayed-in-place insulation combines exceptional R-values with an air barrier and vapor control

Building Products, May-June, 2004 by David Frane

Most residential structures are insulated with fiberglass batts because they are a cheap source of added R-value. But there's more to insulation than R-value.

For best results, batts must be accurately cut to fit the joist or stud cavities, and an effective air barrier is needed to keep unconditioned outdoor air from penetrating the insulation. In most climates, a poly or kraft-paper vapor retarder also is needed to limit the flow of moisture-laden air and prevent condensation from forming within the insulation.

Unfortunately, insulation installations typically are subbed out to the lowest bidder, and vapor retarders, air barriers, and insulation are often thrown into place with little regard to quality.

When quality is a more important consideration than price, spray-applied polyurethane foam is the first choice of a growing number of builders. Although it costs up to several times as much as its competitors--an R-11 application of low-density foam is at least $1 per square foot of wall, compared to about 65cents for spray cellulose and 25cents to 55cents for fiberglass batts--foam eliminates many installation headaches.

WHY FOAM?

First, foam has exceptional air-sealing ability. When sprayed or injected into a framing cavity, it sticks tight to the sheathing and framing and rapidly expands to fill every crack and opening in the exterior shell. This is especially valuable around rim joists and other difficult-to-seal areas.

Second, some types of foam also are effective vapor retarders, so it's often possible to omit the separate poly or kraft-paper vapor retarder. Finally, because dense varieties offer a lot of insulating value per inch of thickness, it's often possible to size studs and rafters based on structural loads rather than the amount of space needed for insulation.

The many foam brands vary widely in density and insulating power, but most residential products weigh between .5 and 2 pounds per cubic foot.

With most common building materials, lower density translates into higher insulating value. But the opposite is true of foam. A .5-pound foam, for example, has an R-value of about 3.5 per inch--roughly the same as fiberglass batts or loose-fill cellulose.

A denser, 1.8-pound foam, on the other hand, has an R-value of about 7. But because the 1.8-pound foam contains nearly four times the amount of chemicals per unit of volume as the .5-pound material, the square-foot cost is substantially higher.

High-density foams usually are applied to a total thickness that's significantly less than the depth of the framing. Low-density foams, by contrast, expand much more and usually bulge out beyond the framing. The excess material must be trimmed off.

With low-density foam, as with fiberglass batts or cellulose, the dimensions of the framing are driven more by the insulation value required than by structural considerations. For example, 2x6 wall studs are used on many residential jobs because they are deep enough to accommodate R-19 fiberglass batts. Because the R-value of low-density foam is comparable to that of fiberglass, the framing requirements are similar.

But when a denser foam is used, it's possible to pack more R-value into a shallower bay. With 1.8-pound foam, you can frame walls with 2x4s and still achieve an R-value of 24. Another option is to flame with 2x6s and fill the cavities only partially, leaving an open space for running pipes or wires.

In walls or ceilings insulated with porous insulating materials such as fiberglass, a poly or kraft-paper vapor retarder usually is installed on the warm side of the insulation (that is, on the inside in heating climates and on the outside in cooling climates) to prevent condensed moisture from wetting the insulation. But because foam is resistant to water vapor, it may be possible to omit this added step. The question of whether to install a separate vapor retarder will depend partly on the specific foam you choose and partly on your local building inspector.

Dense foams have what's known as a closed-cell structure, which means that the gas bubbles that form during the application process remain permanently locked into the cured foam. Because there are no interconnections between individual bubbles, the foam absorbs little water and resists the passage of water vapor. According to most building codes, a vapor retarder must have a perm rating of less than 1.0, and some dense foams meet this standard.

Low-density open-cell foams, on the other hand, have a structure more like a fine-grained sponge. These open cells are too small to permit the passage of much air, but they are more permeable to water vapor than closed-cell foams. Unless there's an exceptional amount of vapor drive, though, that isn't usually a problem. Some building inspectors will allow you to omit the vapor retarder even if the foam's perm rating is above the required minimum value.

IDEAL APPLICATIONS

Cathedral ceilings are notoriously difficult to insulate effectively. Unlike walls, ceilings don't have air barriers and are usually vented to maintain a cool roof surface and to prevent ice dams. But venting makes it easier for cold air to infiltrate batt insulation, which reduces its effective R-value. Recessed lights also are common sources of air leakage.