Flash and Batt Insulation

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Flash and Batt Insulation

This oddly illicit-sounding insulation system is becoming more common

Posted on Jan 22 2013 by Erik North

One thing I’ve noticed over the years is that construction methods don't change quickly — and when they do change, the change is apt to be along the lines of current practices. That makes sense. It’d be near impossible to run a business if you were changing your technique and product five times a week.

Flash and batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. 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 barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. 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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . 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.


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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-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. than insulating with batts alone, but it doesn’t fundamentally change the wall framing. It’s not (for example) a SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. (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 framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope. 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 bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. : 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.

Greenness: Flash and batt runs into that old energy auditor issue: embodied energyEnergy that goes into making a product; includes energy required for growth, extraction, and transportation of the raw material as well as manufacture, packaging, and transportation of the finished product. Embodied energy is often used to measure ecological cost. vs. greenness 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 some products with different ingredients that address these issues to some extent, but they’re considerably less common than the nasty stuff.

Flash-and-batt or flash-and-fill insulation systems gained popularity for their plug-and-play simplicity. It's the same wall as before — just add spray foam. However, this type of wall still inherits many of the shortcomings of standard framing as well.

Erik North, the owner of Free Energy Maine, is an energy auditor and home performance specialist in Westbrook, Maine. He is also the author of the Energy Auditing Blog.

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  1. Fine Homebuilding