Spray polyurethane foam (SPF) is a two-component thermoset plastic insulation that is mixed on site and sprayed directly onto building surfaces. Unlike batt or board insulation, SPF expands to fill the cavity completely, adheres to the substrate, and creates a continuous air barrier. The material was first developed by Otto Bayer in Germany in 1937, but its widespread use in building insulation began during the 1970s energy crisis when demand grew for high-performance building envelopes.
Two-Component Chemistry
SPF is created by reacting two liquid components carried to the job site in separate pressurized containers. The A-side, or iso side, is polymeric methylene diphenyl diisocyanate (pMDI), a reactive chemical that forms the backbone of the polyurethane polymer. The B-side, or resin blend, contains polyol resins, amine or metal catalysts that control reaction speed, silicone surfactants that stabilize the cell structure during expansion, flame retardants, and a physical blowing agent that creates the cellular structure.
When the A and B components mix at the spray gun in a roughly 1:1 volume ratio, the isocyanate reacts with the polyol to form polyurethane polymer chains. Simultaneously, the isocyanate reacts with water in the B-side to generate carbon dioxide gas. Together with the physical blowing agent, these gases expand the liquid into foam that can reach 30 to 100 times its original liquid volume. The reaction is exothermic, generating heat that drives expansion and accelerates curing. Full cure typically occurs within 24 hours, though the foam can be trimmed within minutes.
Open-Cell SPF
Open-cell SPF has a density of approximately 0.5 lb per cubic foot (8 kg per cubic metre) and an R-value of R-3.5 per inch. The cells in the foam are interconnected, giving the material a sponge-like structure. Water serves as the primary blowing agent, reacting with the isocyanate to produce carbon dioxide. Because the cells are open, this foam is permeable to water vapor, which is desirable in climate zones where wall assemblies need to dry to the interior. Open-cell SPF expands roughly 100 times its liquid volume, filling cavities effectively. It also provides good acoustic dampening due to its soft, porous structure.
Closed-Cell SPF
Closed-cell SPF has a density of approximately 2 lb per cubic foot (32 kg per cubic metre) and an R-value of R-6.5 per inch, nearly double that of open-cell. The cells are discrete sealed spheres that trap the blowing agent gas within, contributing to the higher R-value. At 2 inches thickness, closed-cell SPF functions as a Class II vapor retarder. Its compressive strength, typically 15 to 25 psi, allows it to be walked on in attic applications and contributes structural racking strength to wall assemblies.
Blowing Agent Evolution and GWP
The blowing agent used in closed-cell SPF has been the subject of significant regulatory attention. Historically, HFC-245fa was the dominant blowing agent with a 100-year global warming potential (GWP) of approximately 1030. Under the Kigali Amendment to the Montreal Protocol and EPA SNAP regulations, the industry has transitioned to hydrofluoroolefin (HFO) blowing agents with a GWP of approximately 1. This transition represents the most consequential technical shift in the SPF industry in decades. Modern closed-cell SPF products predominantly use HFO-based formulations that deliver the same R-6.5 per inch performance without the high environmental cost.
Installation Requirements
Substrate temperature is critical for proper SPF installation. Most manufacturers specify a substrate temperature range of 5 to 38 degrees Celsius. Cold substrates slow the chemical reaction, resulting in poor cell structure, reduced yield, and compromised adhesion. In cold weather, chemical drums must be stored and maintained at 21 to 32 degrees Celsius using heated drum blankets. Each spray pass should be 0.5 to 1.5 inches thick for closed-cell foam; thicker passes risk excessive exothermic heat that can scorch the foam core.
Occupant safety during application is paramount. The Spray Polyurethane Foam Alliance recommends that building occupants vacate during spraying and for a re-entry period of 24 to 72 hours afterward to allow full curing and off-gassing of residual chemicals. Installers wear full-face respirators with organic vapor cartridges, protective suits, and nitrile gloves.
Major Brands
Leading SPF manufacturers include Icynene (now part of Huntsman Building Solutions), known for its open-cell Lite and closed-cell ProSeal product lines; Demilec, a North American manufacturer offering both open-cell Sealection and closed-cell Heatlok products; and Lapolla, whose Foam-Lok closed-cell formulations are widely used in residential and commercial applications. Each brand offers HFO-blown versions alongside legacy HFC products, and selection typically depends on regional contractor availability and project specifications.