Northern Thermal Comfort
2379 Rux Rd.
Arbor Vitae, WI 54568

The Insulation Lab

Flash and Batt – Risky in Cold Climates

Jeff Aderholdt - Wednesday, July 27, 2011

In recent years, insulation science and technology has grown by leaps and bounds. New products and new techniques are constantly hitting the market. Added to this, is the fact that we live in an information age. Information about the latest and greatest ways to insulate your new home is at our fingertips. A few clicks of a computer mouse, or a few buttons on the television remote control, and we have a banquet of ideas, all courtesy of our favorite do-it-yourself show or website. This is a good thing. Knowledge is power. Homeowners are empowered to be able to make informed decisions in regards to how their home is to be built. But not always.

An inherent danger of the information age is that much of the time we have a lot of incomplete information. We see this in the news broadcasts where much of the time, all we get is the headlines. This happens, at least in part, because of of time constraints. Too much information but too little time. The same is true with the insulation products and techniques that are demonstrated on our favorite do-it-yourself show. They just do not have the time to explain all the pros and cons. We just get the headlines. A example of this is the use of the insulation technique called “Flash and Batt”.

Flash and Batt

Flash and batt is a hybrid application of 2 insulation materials, fiber glass and spray foam. Fiber glass has the advantage of being easily available and relatively inexpensive. The biggest disadvantage of fiber glass is that it does not control air movement, within it and through it. At best, it only slows the airflow, never stopping it. On the other hand, spray foam has the advantages of excelling in controlling air movement, as well as a high insulation value. The biggest problem with spray foam is the cost. Spray foam is very, very expensive. In order to try to maximize the benefits and minimize the cost, flash and batt was born.

Flash and batt consists of two steps. The first step is coating the exterior sheeting of the wall stud cavities with a layer of spray foam, usually 1” - 2” thick. This greatly reduces the amount of spray foam that is used (Did I mention that spray foam is very, very expensive?). The remaining space of the cavity is then filled with less expensive fiber glass. The result is that the strengths of both materials are used and the cost is kept down. A first glance, this seems like a good application. But appearances can be deceiving. Let's look beyond the headlines.

Flash and Batt – the Risks

When flash and batt started to become more prevalent, and some of my competitors starting using it, something about it just didn't seem right to me. It seemed to be flouting the basic laws of physics. The biggest concern I had was the potential created for moisture problems.

In an any insulation system, moisture is controlled by managing vapor movement and air movement. Both of these need to be controlled for the best results. Vapor movement is controlled by a vapor barrier, and air movement is controlled by an air barrier. It is important, not only for these to be managed, but also for them to be properly placed in the insulation system. This is the problem with flash and batt in cold weather applications. Both the air and vapor barriers are in the wrong place!

Flash and Batt Condesation RisksIn a flash and batt application, both the vapor barrier and the primary air barrier are on the exterior half of the insulation area. In a cold weather climate, they need to be to the inside, in fact the more inside the better. The best application is that both the vapor barrier and the primary air tight barrier be in conjunction with the interior wall material so that all the insulation is to the exterior. The reason for this is if moisture can get into the insulation, in cold weather, it will find a cold spot and condense. The more moisture, the greater the chance for moisture problems, such as mold and structural deterioration. In a flash and batt system, the greatest risk for condensation is at the weakest point, where the spray foam meets the wall stud.

These concerns were raised in a recent article that appeared in the March 2011 edition of “Fine Homebuilding” magazine in the article entitled “Why Flash and Batt Makes Sense”. Climate Zone Map It brought out that in southern areas where cooling is the main concern, “...flash and batt can be a good choice”. But it had this to say about flash and batt in cold climates: “The colder the climate, the riskier is to use the flash-and-batt technique...” , “in cold zones, … there is far more danger of condensation causing problems ...”, and “in climate zones 7 or 8 …. dense-pack cellulose might be better ...”.

Yes,we live in the age of information. This is a good thing, as long as we have ALL the information. If you are building a home in our area and would like more information on insulation options, give us at NTC a call. We will make sure you have the information you need to make NTC your choice for insulation.

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