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

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 1/2-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.