The Flash and Batt insulation system is when Closed-cell Spray Foam and Fiberglass are both used to insulate a space. This hybrid insulation system involves a layer of Closed-cell spray foam sprayed and then overlapped by a layer of standard Fiberglass insulation. This is an effective system that is better than just Fiberglass and less expensive than all Closed-cell spray foam.
Batts, or Batt insulation is the traditional type of fiberglass insulation you usually see in homes. It's popularity comes from being extremely easy to install as well as inexpensive.
While it's the easiest and most common form of insulation, it's the least effective overall. Fiberglass batts alone aren't an air barrier or vapor barrier and it doesn't perform well in colder climates.
Spray foam is an expandable foam cellulose product that's sprayed in between the wood joists. After expanding, it solidifies in the wall cavity. It comes in two variants: open-cell and closed-cell. For a Flash and Batt system, open-cell spray foam is used.
Spray foam prevents pests such as mice, rats, and insects from finding their way into your home. It also lasts longer than fiberglass. Foam insulation is a bit more expensive than traditional insulation, but it's significantly more effective.
While the improvement isn't as drastic as a complete switch to spray foam, using spray foam and fiberglass batt insulation together improves the efficiency significantly. Fiberglass has an r-value of 3.5 per square inch, while closed-cell spray foam has a value of r-6. Replacing even just a few inches depth makes a definite difference.
The layer of spray foam prevents air from leaking into your home through standard framing. Standard fiberglass and cellulose are air-permeable and require both an air and vapor barrier. Spray foam, particularly closed-cell, isn't permeable at all.
Spray foam also keeps the temperatures in the wall assembly above dew point. So not only can new damp not get in through the solidified foam, but the moisture in the existing air won't condense in your walls (if the spray foam layer is thick enough).
The best time to install a flash and batt system is during wall construction as you're building your home. While spray foam alone can be sprayed through small holes bored in the wall, the addition of the fiberglass makes it slightly more time consuming.
First, a thin layer of closed-cell spray foam is applied between the wall studs, at least 2 inches thick. Colder climates should replace more of the fiberglass with spray foam. After the foam has cured, install the fiberglass insulation and finish building the walls.
While this insulation system is both cost effective and efficient, there is a potential for issues when the insulation isn't installed properly. Make sure your insulation team has experience installing this type of insulation system!
One common problem that can occur after a Flash and Batt insulation system is installed is the chance of condensation building up between the layers of insulation materials. Condensation primarily develops if the Closed-cell spray foam layer is too thin.
When closed-cell foam layer is too thin, it doesn't provide enough insulation. Condensation develops between the two layers when the dew point is reached, which shouldn't happen if there's enough closed cell foam. If the problem is not taken care of right away, than the homeowner risks mildew and mold developing.
It's important to make sure you have at least 2 inches of spray foam insulation in your flash and batt system. If you don't, you won't get enough benefit from the foam, and condensation can build up. In colder climates, you will want more than 2 inches of the foam insulation to keep your home warm.
The other issue that can occur without enough closed-cell spray foam is that your insulation won't seal. If there's no air seal, condensation and water can get between the spray foam and the fiberglass insulation, causing mold, mildew, and other issues.