Spray Foam Insulation

Spray foam insulation is a two-part liquid sprayed onto a surface, where it reacts, expands, and cures into a solid foam in seconds, filling gaps and bonding to the framing so it insulates and air-seals in one step. Unlike batts or blown insulation, which lay material into a cavity but leave gaps at odd framing angles and do nothing to stop air leaks, spray foam does both jobs at once: it slows heat flow and stops the air leaks that drag down a home's efficiency.

Spray foam comes in two types. Open-cell foam is lighter and softer, runs about R-3.5 to R-3.8 per inch, and is a good air barrier at thickness. Closed-cell foam is denser, hits roughly R-6 to R-7 per inch, blocks moisture, and adds stiffness. The PNNL Building America Solution Center favors closed-cell foam for unvented assemblies because spray foams "effectively air seal complex assemblies" that batts cannot. The trade-off is cost: foam is more expensive than fiberglass or cellulose for the same R-value.

For example, on a Cave Creek custom home with HVAC in the attic, spraying closed-cell foam to the underside of the roof deck creates a sealed, unvented attic. The ducts now sit inside the cooled envelope instead of baking in 140-degree vented-attic air, which can meaningfully cut summer cooling loss. The foam also seals the countless small gaps where hot desert air would otherwise leak in.

Spray foam is not a cure-all. Open-cell foam in an unvented attic can trap moisture if the assembly is not detailed for the climate, so the design has to match Arizona's conditions. Foam also needs an ignition or thermal barrier where code requires it. Used correctly, it gives a high R-value and a tight air seal at once, which a blower-door test can confirm.