Open-cell insulation foam is composed of many air bubbles held within a polyurethane matrix. It is these trapped air bubbles that lessen the transfer of heat through the foam. Applied through a spray nozzle to an open wall cavity, open foam insulation quickly expands to fill the space. It adheres to anything it touches, making it idea for irregular wall shapes.
It has an average R value of 3.5 per inch, so it does not match the insulating capabilities of closed-cell foam, but it still has useful applications. Open-cell insulation foam does not block vapor but instead allows it to pass through the foam, meaning it is particularly effective in hot climates where loads are primarily cooling. It is also cheaper than closed-cell foam, and less dense. When installed in wall cavities, the foam may overfill the cavity with the excess later trimmed away.