Hybrid Spray Foam insulation method – Flash and Batt
Flash and batt insulation is one example of a new technique being implemented by insulation contractors. Flash and batt is a hybrid insulation approach combining fiberglass insulation and closed-cell spray foam.
Fiberglass devoured the insulation industry in the 1970s, for obvious reasons. It is cheap, simple to install (if not easy to install well), and readily available. Any weekend warrior with a stapler and utility knife can install the stuff. Sure, there’s all kinds of compromising gaps and holes but, hey — it’s easy!
However, fiberglass’s shortcomings are just as well known. Fiberglass isn’t an air barrier or vapor barrier, it performs poorly in very cold climates, and it is less green than cellulose.
For these reasons, fiberglass has been paired with spray foam to improve its performance without fundamentally changing the construction process. By using the flash-and-batt method, any contractor building a standard wall frame can improve the wall’s thermal performance and airtightness without fundamentally altering the project.
What is flash and batt?
Flash and batt insulation is a method of insulating standard stud walls by augmenting fiberglass batts with 2 inches of closed-cell spray foam applied to the interior of the building sheathing. The standard framing includes 2×6 vertical studs spaced 16 inches on center, drywall interior finish, OSB or plywood exterior sheathing, Tyvek or a comparable water-resistant barrier (WRB), and siding.
The cavity insulation normally consists of 6-inch fiberglass batts. In a flash-and-batt installation, part of the fiberglass is replaced with closed-cell spray foam and the remainder is either fiberglass or (occasionally) cellulose. The depth of the spray foam depends on the climate zone, with colder zones requiring thicker foam to keep the wall cavity above the dew point during the winter. If the stud bays are insulated with cellulose or loose fiberglass, the system is sometimes called “flash and fill” — an expression that still sounds oddly illicit. This system creates an assembly with a slightly higher R-value than insulating with batts alone, but it doesn’t fundamentally change the wall framing. It’s not (for example) a SIP (structural insulated panel) wall, which would be an entirely different building concept.
Advantages of flash and batt
R-Value: While it isn’t a radical improvement over standard walls, flash and batt systems do have a higher R-value. Replacing 2 inches of fiberglass (at around R-3.5 per inch) with 2 inches of closed-cell spray foam (with an aged R-value of around R-6 per inch) will improve the wall’s thermal resistance. And who doesn’t want that?
Ease of installation: Of all your higher performing wall systems, the flash-and-batt system is by far the easiest for most contractors to install. All of the framing elements are standard; the only change is the call to a spray foam subcontractor. While other systems like SIPs or advanced framing usually require an experienced crew, a flash-and-batt project can be installed by any contractor.
Air leakage: The addition of a layer of spray foam makes these building assemblies much tighter than standard framing.
Normal fiberglass and cellulose are air-permeable (even dense-packed cellulose and wet-sprayed cellulose aren’t quite airtight) and must be paired with an air barrier. The 2 inches of spray foam acts as an air barrier, stopping air movement through the exterior sheathing.
Moisture: Flash-and-batt systems present something of a mixed bag when it comes to moisture control. The 2 inches of spray foam will help maintain a higher temperature in the wall assembly, keeping surfaces above the dew point during the winter. However, some overzealous builders install a polyethylene (plastic) vapor barrier on the interior. If the wall has a vapor retardant layer on both sides, it has limited drying potential. If the assembly were to ever get wet, the moisture would likely remain for an extended period.
Disadvantages of flash and batt
Thermal bridging: One of the big knocks on flash and batt is that it doesn’t address thermal bridging. The spray foam provides a little more R-value than fiberglass, but the heat flowing through the uninsulated wood studs still compromises the wall assembly’s overall R-value. Like standard insulation and framing, the whole-wall R-value is considerably lower than the advertised R-value.
Cost: Another negative of flash-and-batt installations is the added cost. It costs the same as a standard fiberglass-batt job — plus the expensive application of spray foam. Unlike SIPs, whose simplicity can allow for faster construction, a flash-and-batt job actually takes longer (and requires more labor), since a time allowance must be made for spraying and curing. And unlike advanced framing, there’s no material savings with the added spray foam.
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