Closed-cell polyurethane spray foam is amazing stuff. It’s a great insulator, with a higher R-value per inch than almost any other commonly available material. (Polyisocyanurate foam can be equivalent, but it varies with product and temperature.) When installed at least an inch or two thick, it essentially blocks water vapor and air movement. It even has some structural benefits: It’s good at adhering sheathing to framing, and it also enhances racking resistance. Thanks to these qualities and more, including strong marketing from the industry, closed-cell spray foam is only getting more popular. Unfortunately, it also has some serious issues that aren’t typically discussed outside of green building circles. Understanding these drawbacks may convince you to limit its use to those special situations in which it makes the most sense, or even to avoid it entirely.
Polyurethane spray foam starts as two containers of liquid chemicals, referred to as the “A” side (isocyanates, the primary component of the solid foam) and the “B” side (a blend of resins, catalysts, a blowing agent, a flame retardant, and other compounds). When combined during installation, the chemicals react to create a polymer filled with tiny bubbles. The bubbles, or cells, are filled with a blowing agent and provide the insulating value of the material, while the polymer forms the cell walls.
The ingredient that makes closed-cell foam special is the blowing agent—typically HFC-245fa—but that’s also its biggest drawback.
HFC-245fa is a hydrofluorocarbon refrigerant and a persistent greenhouse gas; it goes up into the atmosphere and stays there, preventing heat from escaping the earth. Carbon dioxide is commonly used as a measure of the damage a greenhouse gas can do, and HFC-245fa is considered 1030 times worse than CO2. The only common building product more potent is the…