Closed-cell spray polyurethane foam insulation has several desirable characteristics. It’s an excellent air barrier, an excellent vapor retarder, and it has a high insulating value per inch (about R-6). Unfortunately, it’s also expensive.
Insulation contractors who want to obtain at least some of the desirable performance characteristics of closed-cell spray foam at a lower cost than a full spray-foam job sometimes promote the “flash-and-batt” approach. This involves spraying one or more inches of closed-cell foam against the interior side of the sheathing and filling the rest of the framing bay with a fiberglass batt.
A variation of this method is called “flash-and-fill” or “flash-and-blow”; these terms refer to an insulation job that combines closed-cell spray foam with a blown-in insulation material like cellulose or blown-in fiberglass.
The flash-and-batt approach can be used for walls, floors, or ceilings.
Some insulation contractors wonder whether they should compress fiberglass batts that are installed in flash-and-batt cavities. The answer is simple: Compressing fiberglass batts is a good thing to do, since (a) compressed batts have a higher R-value per inch than uncompressed batts, and (b) compressed batts are more likely to be in direct contact with the cured spray foam — and that type of direct contact is a code requirement for flash-and-batt roof assemblies.
A history of the flash-and-batt method
My first article on the flash-and-batt method was published in the November 2007 issue of Energy Design Update. At that time, the flash-and-batt method was still a little-used approach. I contacted a range of insulation manufacturers — both spray foam manufacturers and fiberglass insulation manufactures — and none of these manufacturers were willing to endorse the flash-and-batt approach.
Even though flash-and-batt wasn’t widely known in 2007, a few spray foam contractors claimed to have been using the method for a long time. Matt Momper, president…
Get building science and energy efficiency advice, plus special offers, in your inbox.