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 65¢ for spray cellulose and 25¢ to 55¢ 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 frame 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.
One way to deal with troublesome leaks is to fill the ceiling with spray foam. According to Matt Momper, whose Indiana-based company is one of the region's largest installers of foam, fiberglass batts, and other materials, foamed cathedral ceilings should be vented if possible.
“Some roofing manufacturers won't warranty their shingles if the roof isn't vented,” he says.
Foam also is effective where codes permit unvented attics. This technique is especially popular in parts of the South, where the humidity is high and it's common to put air handlers in the attic. Spraying the underside of the sheathing and the gable-end walls turns the attic into a conditioned space and prevents humid air from entering and condensing on cold ductwork.
In addition, spray foam works well under floors because it won't sag or fall down the way batts sometimes do. This makes it a good choice for rooms over exterior porches or small additions built on elevated piers. Foam is especially useful for insulating truss-framed assemblies and other areas that are difficult or impossible to insulate with batts.
Furthermore, spray foam adheres well to masonry, including irregular stone foundations. As a result, it's becoming a popular choice for sealing and insulating the perimeter walls of crawlspaces, especially in areas where unvented crawlspaces are permitted by code.
Some insulation contractors install foam and batts in the same framing cavity in order to combine the air-sealing and vapor-resistant properties of foam with the economy of fiberglass. Momper uses this technique regularly. The framing cavities are first sprayed with a ½-inch layer of closed-cell foam before the rest of the cavity is filled with batt insulation to beef up the overall R-value.
Momper reports no problems with this approach, but the technique is controversial. Opponents refer to it as “flash and dash,” the implication being that it's shoddy workmanship. They claim that putting foam outside the fiber insulation may result in a wrong-side vapor retarder in heating climates. But proponents say the foam prevents air from infiltrating the wall and vapor usually gets into walls because of air infiltration, not because of diffusion.
There's both anecdotal and scientific evidence suggesting that spray-in-place foam also adds strength and stiffness to wood-framed buildings. Builder Joseph Jackson, of Faust Contracting in Little Silver, N.J., recalls framing a house that moved slightly every time the wind blew. Once the walls were sprayed with 2-pound foam, Jackson reports the structure felt absolutely rigid.
According to Craig DeWitt of RLC Engineering in Clemson, S.C., Clemson University has performed extensive testing of foam. Racking tests showed that walls filled with sprayed-in-place foam were stiffer than walls filled with fiberglass batts. Clemson tests also indicated that spray foam significantly strengthened the bond between rafters and sheathing, which is a plus in high-wind areas.
DeWitt cautions that building codes do not recognize sprayed foam as a structural component. But he says that engineers can include the strength of this bond in the structural calculations for engineered buildings.
–This article was reprinted with permission of THE JOURNAL OF LIGHT CONSTRUCTION. For subscription information, call 800-375-5981 or visit www.jlconline.com.