Flash and Batt Insulation vs. the Laws of Thermodynamics

The same contractor I mentioned in my previous post also informed me that “no one is using all foam anymore.”  According to him, “they’re just using it around the perimeter of each cavity and then using regular batt insulation to fill the cavity.”

There are only 1 or 2 insulation companies that would recommend this approach.  Energsmart has only used this method once and it was at the insistence of the customer. The theory behind this approach (called flash and batt) is that if you seal the exterior walls of the house such that exterior air can’t enter, then you’ll get the same results as foam.  The laws of thermodynamics clearly prove that theory wrong!  One of those laws is that cold becomes hot and hot becomes cold.  In the case of a wall cavity in the winter, one side is really cold (the exterior side) and one side is relatively warm (the drywall side).  Within the cavity itself there is a constant battle going on between hot and cold, which is where the second law comes in – warm air rises and cold air falls.

In the case of fiberglass air can freely move within it so a “convective loop” energy swap occurs all winter long.  The air toward the inside of the cavity warms up and moves to the top of the cavity.  The air toward the outside of the cavity cools down and falls to the bottom of the cavity.  This process continues all winter.  In the summer there is also a convective loop, but it moves in the opposite direction.

Flash and batt systems are definitely better than fiberglass alone from a thermal efficiency standpoint because they slow down the convective loop, but even if the cavity is perfectly air sealed, the air within the cavity will circulate and steal energy from you.  On the other hand, foam is an air barrier and there will be no convective looping.  Energy has to transfer through foam by way of conduction, which is a much slower process. 

This is sometimes difficult to understand so please don’t hesitate to call or e-mail us with questions.