Spray-Foam Problems

Fine Homebuilding

September 22, 2016

Mr. Paul Bennett, principal engineer, published "Spray-Foam Problems," in the October/November 2016 issue of Fine Homebuilding.

Just what is spray foam? Generally speaking, there are two types of spray polyurethane foam insulation (SPFI): open cell and closed cell. Open-cell SPFI is fairly compressible, has an R-value of R-3 to R-5 per in., is liquid and vapor permeable because of its open-cell structure, and is less expensive than closed-cell foam. Closed- cell SPFI is rigid and has an R-value of R-6 to R-7 per in. With no open path through its cell structure, closed-cell SPFI can retard moisture-vapor transmission through walls and roofs. It is often chosen for this feature. Both types of SPFI are comprised of two components that are mixed on-site. (There are single-component foams, but they're typically sold in aerosol cans for small air- sealing tasks.)

Specialized skills on the part of the installer are required to feed the two components together into a mixing system that sprays and forms the SPFI. Component A is fairly standard across the industry and consists of monomers or prepolymers whose molecules end with reactive groups of isocyanates. Component B is more likely to vary among manufacturers but contains one or more polyols (which can be petroleum based or plant based) as well as proprietary blends of minor components, such as the blowing agents to promote foaming, a catalyst to accelerate the reaction, surfactants to control the consistency and foam cell size, a flame retardant, and coloring agents.

Once the parts are mixed, a chemical reaction permanently combines, or "cures," the isocyanates and polyols into polyurethane during a rapid foaming process accompanied by a release of heat.

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