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Building Science

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Spray Foam Facts

Polyurethane Basics

Polyurethane is widely used for new homes and retrofitting non-insulated houses. It has no off-gases that could be potentially harmful in living spaces. Vapors may be present while the foam is actually being applied, but the cured material is nontoxic.

Density and R-value

There are many brands of proprietary foams on the market, and they vary widely in density and insulating power. Commercial flat roofs, for example, are often insulated with a high-density material that weighs about 3 pounds per cubic foot, which makes it hard and strong enough to walk on without damage. But most residential foam insulation weighs between .5 and 2.0 pounds per cubic foot.

With most common building materials, lower density translates into higher insulating value. That’s why fiberglass batts insulate better than wood and wood insulates better than concrete. But the opposite is true of foam. A 1/2- pound foam such as Icynene, for example, has an R-value of about 3.5 per inch — roughly the same as fiberglass batts or loose-fill cellulose.

Framing dimensions

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, the 2x6 wall studs used on so many residential jobs are overkill from the standpoint of supporting the weight of the building. The real reason for using them is that they provide stud bays 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 also 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 (see Figure 1). Another option is to frame with 2x6s and fill the cavities only partially, leaving an open space for running pipes or wires.

Going with foam can provide added flexibility in designing a framing package: Because dense varieties of foam 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.

Moisture Control

In walls or ceilings insulated with porous insulating materials such as fiberglass, a poly or kraft-paper vapor retarder is usually 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 itself 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.

Proponents of foam

It’s an ideal insulating material for mixed climates, where the warm and cold sides of the building envelope reverse during the year. During the heating season, the vapor retarder belongs on the inside of the wall, but when the air conditioning kicks on during the summer, it belongs on the outside. This is a practical impossibility with permeable insulating materials. But because foam is uniformly solid, it resists the passage of vapor equally well in either direction.

Roofs and Attics

Cathedral ceilings are notoriously difficult to insulate effectively. Unlike walls, ceilings don’t have air barriers like Tyvek and are usually vented to maintain a cool roof surface and prevent ice dams. But venting makes it easier for cold air to infiltrate batt insulation, which reduces its effective R-value. Ceiling penetrations like recessed lights are also common sources of air leakage. Cold roofs and foam. One way to deal with these sorts of troublesome leaks is to fill the ceiling with spray foam.

Foam and structural strength

According to Craig DeWitt of RLC Engineering in Clemson, S.C., Clemson University has performed extensive testing to evaluate the structural value of foam. Racking tests showed that walls filled with sprayed-in-place foam were stiffer than walls filled with fiberglass batts. Tests also showed that spray foam significantly strengthened the bond between rafters and sheathing, which is a plus in high-wind areas.

Other Applications

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 like rim joists and other areas that are difficult or impossible to insulate with batts.

topics: Education, Icynene, Incentives


I had no idea that any one product could do so much for our homes as well as benefit the clients. We were able to reduce our energy consumption dramatically, which reduced our energy bills.