SPF compounds are comprised of isocyanates and polyols that have been in use for over 30 years. It is applied in liquid, two-part states. As the two materials mix, they expand, typically at an average ratio of 30:1 (or more). During the process of the two parts mixing, a chemical reaction generates an internal heat. If installed improperly, the foam can generate an internal heat high enough to start a fire inside the core of the foam. Spray formulations are fast-reacting to allow vertical and overhead applications. Additional layers can be applied almost immediately in consecutive passes.
Usually pale yellow in color, polyurethane foam resists water, mildew, and fungus; does not shrink or settle; contains no urea formaldehyde; is 80 percent cured in a few minutes; and is 95 percent cured in 24 hours. Polyurethane foam adheres to most surfaces, insulates hard-to-reach areas, is odorless, and resists mildew and fungus. Polyurethane foam also has no nutritional value that would attract insects or other pests
As discussed in previous chapters, intrusion of air into the stud wall cavity causes convection (movement of warm air to the top of the cavity and cold to the bottom). Like other foamed-in-place insulation products, polyurethane foam contains no seams or joints, thereby eliminating air leakage. After the insulation is installed properly, there should not be any air circulation from one side of the stud space to the other. Polyurethane foams reduce the temperature variance and the effect of convective looping. Any structural cracks or breaks in the system will reduce the effectiveness of the insulation, however.
Polyurethane foam (closed-cell, 1V2 to 2 lb/ft3 density) has a structural quality that actually increases the twist and rack resistance of framed walls. The National Association of Home Builders Research Center (NAHB) conducted tests on 45 different wall designs, with and without foam. These tests proved that foam walls are three times as strong as walls without foam. (Additional testing of SPF by the NAHB Research Center between metal studs produced similar results.) Polyurethane foam is tack-free within seconds of the application, and 80 percent of the physical properties are present after 12 hours.10