Spray Foam: The Dream App

When I was a working contractor, I remember how daunting it was to walk into a large project, bare studs for walls, knowing that I had to work (with a crew of course) to stuff each bay with fiberglass batts. I remember thinking — like a boy wishing he could fly — if we could only liquefy the insulation and spray it in place the job would go much faster. It would also get the insulation into every nook and cranny and save me from using duct tape in an attempt to remove the itchy glass from my arms and neck.

That was in the 1970s, and little did I know that by the time I got out of the blue collar side of building, just such a thing would be invented in the form of spray foam. I mean, let’s face it, you couldn’t design a better dream product. The stuff flows on as a liquid, fills up every crack and what’s best, teams of guys in white jumpsuits apply it in one day’s time. Heaven.

It’s also very green. The problems with propellants have been largely worked out, and there now are water- and soy-based products as well as isocyanate-free, formaldehyde-free alternatives to suit the greenest customer. Once in place it doesn’t leak or off-gas harmful emissions, and all foams are good at reducing noise transfer and sound penetration. What’s not to like?

Here’s how you differentiate among the two different types of foam, and the attributes for each product category.

OC-SPF. Open-cell spray polyurethane foam cures in place, stays soft and is designed to slow convection (the transfer of heat through the movement of warm matter like air) by leaving irregular bubbles that fill with trapped, ambient air. Weighing in at 0.5 lb. per cu. ft. and scoring around R-3.5 per in., the foam tends to have an R-value close to captured, still air. Although it has less mass (and is therefore less expensive) OC-SPF isn’t a good vapor barrier, and it gets knocked for this.

CC-SPF. Closed-cell spray polyurethane foam cures in place and stays rigid. But unlike open-cell products, its bubbles aren’t irregular or open. The bubbles form by trapping “reaction gas,” which offers insulation properties. With the higher-density foam you get higher R-value around 7 per inch. CC-SPF weighs in at 2.0 lb. per cu. ft. and partially because of its greater mass, it costs more per R than open-cell foam. Unlike its open-cell cousin, CC-SPF is a good vapor barrier and is often code-approved as such. What’s more, CC-SPF can add to the sheer strength of a wall, and in recent tests when sprayed under a roof decking it can help defeat roof uplift (Google search University of Florida, Dr. Prevatt, foam).

By the way, the polyurethane foams you see in molding are of even higher density than CC-SPF, weighing 30 to 40 lb. per cu. ft.

Even after you compare cost and vapor barrier performance, note that some foams are inappropriate in certain applications. Open-cell foam should not be used below grade because it can absorb water. Since water is a poor insulator (relative to air), foam that is soaking wet has dramatically compromised thermal performance. You’ll also want to spec closed-cell foam in tight areas or around utilities where you can’t get the bulk and thickness required by an open-cell product to achieve the designed R-value.