Professional Contractor

Overview of Spray Polyurethane Foam (SPF)

There are several “kinds” of SPF that a contractor might encounter at a site. Although the term “spray polyurethane foam” can apply to several different kinds of products, much of this “SPF contractor/building and construction professional” website section will focus on two component high pressure SPF. It is often used in professional interior or exterior SPF insulation and roofing application during new construction or major renovations.

For information about one component foam, visit the Do-It-Yourself section of the website. For information about two component low pressure kits/systems, visit the Weatherization section.

The foam itself is made by mixing batches of chemicals that are provided in separate drums. The chemicals must be kept separate until they are mixed and the chemical reaction can begin. The two separate drums or canisters have different chemicals in them, and are referred to as the “A” side and “B” side.

  • A-side: The A-side is typically a 50/50 mixture of methylene diphenyl diisocyanate (MDI) and polymeric methylene diphenyl diisocyanate (pMDI), or MDI-based diisocyanate.
  • B-side: The B-side is a polyol resin blend — the principal ingredient being polyol — and smaller amounts of amine and/or metal catalyst, blowing agent, surfactant and flame retardant.

The drums may be color-coded red or black for the A-side, and blue or another color for the B-side. The degree of hazard (risk) presented from the chemical in the mix varies. There is a difference, for example, in the amount of MDI available prior to mixing the chemicals; after they are mixed and begin reacting; and again after the reaction has been fully completed and the foam hardens. Extreme care must be taken when working with the SPF pre-mix chemicals and throughout every stage of the installation. Improper application by unprotected workers could result in overexposure to the SPF chemicals, which can pose potential health hazards.

For more information about the typical composition of the A-side and/or B-side chemicals used in SPF systems, refer to the ACC CPI Health and Safety Product Stewardship Workbook for High-Pressure Application of SPF. Refer to SPF Insulation Basics for general information about differences between closed-cell and open-cell foam, how SPF is made and types of SPF insulation and how it’s applied.