About Spray Foam Insulation
Spray polyurethane foam (SPF) is better than any other type of insulation at reducing air leakage.
With foamed-in-place insulation, it is relatively easy (though not inexpensive) to fill wall and ceiling cavities completely. Closed-cell spray foam provides a higher R-value per inch (6.5) than less expensive insulation types like cellulose and fiberglass (3.5 to 3.7).
Most spray polyurethane foam is called “two-component” foam. Two ingredients—conventionally called “A” and “B” components—are mixed on site using special equipment mounted in a trailer or truck. Heated hoses convey the components to a mixing gun that sprays the chemicals on the surfaces to be insulated.
A chemical reaction begins as soon as the chemicals are mixed. The liquid mixture foams, expands, and eventually hardens.
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Spray polyurethane foam is usually installed by a spray-foam contractor equipped with a truck or trailer to carry the necessary chemicals and spray equipment.
For smaller jobs, builders can purchase disposable tanks of two-component polyurethane foam. These tanks are sold in various sizes, and range in cost from about $200 to $500. For very small jobs, small aerosol cans of one-component (moisture-cured) polyurethane foam can be purchased at most building-supply stores for about $5 a can.
Although spray polyurethane foam has many advantages over other types of insulation, spray foam installation isn’t foolproof. Some builders have reported problems with sloppy foam insulation. For example, some installers have been known to begin spraying before the chemical components are up to temperature, which can affect component mixing and foam performance. When components are poorly mixed, or mixed in the wrong ratio or at the wrong temperature, cured foam has been known to shrink away from rafters or studs, leaving cracks. Some installers rush through their spraying, resulting in voids.
As with any type of insulation—whether fiberglass batts, cellulose, or spray foam—it’s important to choose an installer with a good reputation; to monitor the installer’s work; and to verify that the insulation work meets expectations before making the final payment on the job.
Spray foam is messy
GBA advisor Michael Chandler provided memorable advice to builders contemplating their first spray-foam job in his February 2009 Fine Homebuilding article, “Prepping for Spray Foam.”
“As the foam is sprayed, small expanding droplets of foam end up in the air. This stuff gets in your hair, on your skin and clothes, and all over any building materials or tools inside the house. I once failed to warn a homeowner of the mess. He was excited to capture the spray-foam-insulation process with his video camera. He got only a small amount of footage and never got to use the camera again. The best bet is to get everybody who’s not part of the foam crew out of the house. Have some helpers nearby to watch the installation, and be ready with drop cloths, tape, and caulk to stop or catch any drips that find their way to the exterior of the house.”
Half-pound foam, also known as open-cell foam, has a density of about 0.5 lb. per cubic foot and an R-value of 3.5 or 3.6 per inch.
Open-cell foam is relatively vapor-permeable. Three inches of open-cell foam have a permeance of 16 perms.
Some of the low-density foams are made in part from bio-based raw materials — for example, soybean oils — in place of a portion of the petrochemicals. Open-cell foams use water or carbon dioxide as the blowing agent.
Compared with closed-cell polyurethane, open-cell products use significantly less material, making them attractive from a resource-use standpoint. However, open-cell foams have a lower R-value per inch than closed-cell foams.
Open-cell foam often requires an interior vapor retarder.
When used to create a cathedralized attic in a cold climate (climate zones 5 and higher), open-cell foam should always include a vapor retarder (for example, a layer of gypsum wallboard finished with vapor-retarder paint). Recent research has shown that vapor-retarder paint is ineffective when sprayed directly onto cured foam insulation, so cold-climate builders who don’t plan to cover the spray foam with a layer of drywall should stick with closed-cell spray foam.
Open-cell foam is riskier than closed-cell foam when it is installed on the underside of roof sheathing.
Evidence is accumulating that roof sheathing can get wet when open-cell foam is sprayed directly against the underside of roof sheathing. For more information on this problem, see Open-Cell Spray Foam and Damp Roof Sheathing.
Two-pound foam, also known as closed-cell foam, has a density of about 2 lb. per cubic foot and an R-value of 6 to 6.5 per inch. Two-pound foam is significantly more expensive than half-pound foam.
Closed-cell foam is a vapor retarder. Two and a half inches of closed-cell foam have a permeance of 0.8 perm.
The blowing agents in most types of closed-cell spray polyurethane foam are hydrofluorocarbons (HFCs) with a high global warming potential. Because the global warming potential of these damaging blowing agents is 1,430 times more potent than carbon dioxide, many green builders avoid the use of closed-cell spray foam. For more information, see Calculating the Global Warming Impact of Insulation.
Some insulation contractors install a thin layer of closed-cell spray foam in conjunction with fiberglass batts.
Read more at: http://www.greenbuildingadvisor.com/green-basics/spray-foam-insulation-open-and-closed-cell
Related article: Airtightness and Spray Foam Insulation