Flash and Batt – The Oddly Illicit Sounding Insulation System

by Erik North on June 18, 2012

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One thing I’ve noticed over the years is that construction doesn’t change quickly and when it does, the change is apt to be along the lines of current practices. It makes sense. It’d be near impossible to run a business changing your technique and product five times a week.

Flash and batt insulation is one example from the housing industry. Flash and batt is a hybrid insulation approach combining fiberglass and closed cell spray foam. Fiberglass devoured the insulation industry in the ’70s and 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, easy!

However, fiberglass’ shortcomings are just as well known … Fiberglass isn’t an air barrier or vapor barrier, performs poorly in very cold climates, and is less green than cellulose. For these reasons, fiberglass was paired with spray foam to improve performance without fundamentally changing the construction process. Any contractor building a standard wall frame could improve its thermal performance and air tightness without fundamentally altering the project.

What is Flash and Batt?

Flash and batt insulation takes the standard wall construction and augments it 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, dry wall interior finish, OSB or plywood exterior sheathing, Tyvek or a comparable water resistant barrier and siding.

The cavity insulation is normally 6 inch fiberglass batts. This is replaced in flash and batt systems with 2 inches of closed cell foam and 4 inches of either fiberglass or occasionally cellulose (if insulated with cellulose or loose fiberglass, the system is sometimes called ‘flash and fill’…still sounds oddly illicit). This creates a higher R-value assembly but doesn’t fundamentally change the wall framing. It’s not a SIP (Structural Insulated Panel) for example, which would be an entirely different building concept.


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 (around R-3.5 per inch) with 2 inches of closed cell spray foam (an aged R-value of around 6) 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 being the call to a spray foam sub-contractor. While other systems like SIPs or advanced framing can be installed more quickly and easily with experienced crews, flash and batt approaches 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 or wet sprayed) 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 with moisture control. The two inches of spray foam will help maintain a higher temp in the wall assembly, keeping surfaces over the dew point. However, some overzealous builders install a poly plastic vapor barrier on the interior. With a vapor retardant layer on both sides, the wall would have limited drying potential. If the assembly were to get wet, the moisture would likely remain for an extended period.


Thermal Bridging – One of the big knocks on flash and batt is that it doesn’t address thermal bridging. The spray foam bumps up overall R-value but the heat radiating through the uninsulated wood studs compromises this. 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 is the same cost as standard framing plus the expensive application of spray foam. Unlike SIPs, whose simplicity can allow faster construction, flash and batt actually take longer (requiring more labor) as time allowance must be made for spraying and curing. And unlike advanced framing, there’s no material savings with the added spray foam.

Green – Flash and batt runs into that old energy auditor chestnut of embodied energy vs. green vs. energy saved. I mean, you get into these kind of arguments all the time, right? Right? The whole assembly will conserve more energy than a standard wall. However, the spray foam has a much higher embodied energy than the fiberglass it is replacing. Spray foam’s propellants are often nasty non-green stuff. Finally, the vast majority of spray foams are petroleum based. There are variants that address all these issues, but they’re considerably less common.

Flash and batt or flash and fill insulation systems gained popularity for their plug and play simplicity. Same wall as before, just add spray foam. However, they inherit many of the shortcomings of standard framing as well.

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