[The following article is a summary of a more complete article on spray polyurethane in post-frame buildings found under the Technical Resources tab. GO TO THE TECHNICAL RESOURCES TAB HERE]
By Joseph M. Zulovich, PhD, PE
Spray polyurethane foam insulation has been used in the United States since the 1970s. Although a number of challenges accompanied its early use, SPF formulation has improved, and its use and applications are now better understood. This article discusses the basic characteristics of SPF insulation, addresses some of the controversial issues regarding its use and presents some post-frame building applications for SPF.
SPF insulation is a combination of compounds that includes methylene diphenyl diisocyanate, polyols, flame retardants, blowing agents, catalysts and surfactants. These ingredients are blended together on the job site and blown into place on the building envelope to form plastic foam insulation.
Safety procedures must be followed during work with SPF insulation. The U.S. Environmental Protection Agency provides information on the safe use and application of SPF insulation.
The two categories of SPF insulation are open-cell SPF insulation and closed-cell SPF insulation. Both categories have specific characteristics that affect the success of an SPF insulation application. The two types respond differently to moisture and water. Open-cell spray foams respond to moisture in a way very similar to the way that fiberglass and cellulose type insulations respond. A building envelope system with open-cell SPF must have a vapor barrier for protection against moisture vapor penetration. Closed-cell SPF does not need to be protected from vapor transmission and does not retain water.
Closed-cell SPF insulation has been recognized by the Federal Emergency Management Agency as an acceptable insulation for use in flood-resistant construction.
An air barrier controls the movement of air into and out of a building. Perhaps more important, an air barrier keeps warm, moist, inside air from entering into post-frame structural cavities, where it can condense and cause moisture problems. Open-cell SPF insulation can serve as an air barrier when properly applied to a minimum thickness. SPF insulation has been criticized as being an ineffective air barrier because of foam shrinkage, which results in cracks and separation from other building members. These failures have been documented but normally are a result of improper installation.
Code compliance for SPF insulation involves attention to three main areas: fire protection, moisture control and building energy use. Following the manufacturer’s recommendations for a given SFP product is a critical first step in code compliance. A thermal barrier, such as half-inch gypsum wallboard, is required between the SPF insulation and an occupied space. For open-cell SPF, a vapor barrier will be required, according to the same criteria used for fiberglass or cellulose insulation. If open-cell SPF is not protected with a vapor barrier, moisture problems occur that are similar to those occurring when a vapor barrier is not used with fiberglass or cellulose insulation. It is well recognized that SPF insulation can be used to help meet energy efficiency codes and requirements. When properly installed at adequate minimum thicknesses, SPF insulation can provide both thermal resistance and air barrier capabilities to a building envelope assembly. A final check with local code authorities will help ensure that the proposed materials and application methods are approved.
Open-cell SPF can provide another option for building envelope insulation in post-frame systems. Open-cell SPF has thermal resistance similar to that of fiberglass- and cellulose-based options. Also, open-cell SPF can be incorporated into a post-frame building envelope to provide a reliable air barrier in an exterior wall system. The open-cell SPF must be properly applied, and care must be taken by the applicator to fill all cracks and gaps in the post-frame structure. The open-cell SPF should have the necessary flexibility to remain as an intact air barrier when it is part of a wall system. The interior wall liner should have the needed fire-protection characteristics to minimize concerns about code compliance issues. Closed-cell SPF insulation can also provide the needed insulation in the lower portion of a wall section when flood-resistant construction is required or when the lower part of a wall is expected to be exposed to wet conditions in the proposed use. The closed-cell SPF insulation is water and moisture resistant, as discussed earlier. The base of an exterior wall in a post-frame building typically does not have significant deflection due to loading conditions, so cracking or breaking the relatively stiff closed-cell SPF insulation should not be a major concern.
Joseph M. Zulovich, PhD, PE, is extension agricultural engineer at the University of Missouri, Columbia, Missouri. He can be reached at firstname.lastname@example.org.