Q. I’ve Been Told To Use a Hybrid Insulation Method With a Combination of Foam and Fiberglass, Are There Any Concerns With This System?
A. Many insulation companies in the Northeast have been promoting hybrid insulation systems, often referred to as “flash and batt,” which incorporate a thin layer of closed-cell foam followed by a layer of traditional fiberglass batt insulation. We advise builders and homeowners to proceed with caution when selecting one of these hybrid systems. The potential exists for moisture and condensation problems if the system is not designed and installed correctly.
Foam and fiberglass each relate to the properties of thermal conductivity, convection, water, vapor, and air in very different manners. The way they interact with these physical properties necessitates different design approaches for each type of insulation. But when combining the two forms of insulation, one must consider a whole host of variables and how they will interact – effective R-values, vapor barriers, vapor permeability, dew points, air-permeability, longevity, etc. The design and proper installation of such a system can often be confusing and the desired results elusive to achieve.
Since fiberglass is air permeable, warm indoor air will pass through the fiberglass and come into contact with the thin layer of foam. If the foam is not thick enough, and does not have adequate thermal resistance (R-value), a dew point may occur on the surface of the foam and condensation will form. Moreover, since closed-cell foam is a vapor retarder, hybrid systems naturally place the vapor retarder on the wrong side of the wall (vapor retarders are supposed to be placed on the warm side of the wall). Worse yet, if kraft-faced or foil-faced fiberglass is used, the result is a moisture sandwich or double vapor barrier assembly. While a proper flash-and-batt system can be designed, the execution meeting these design parameters is nearly impossible to achieve. Either the foam is applied too thin or too thick. Too thin and the risk for condensation occurs, too thick and the fiberglass batt must be compressed to fit into the cavity (thus rendering much of the fiberglass useless). These are just a few of the potential drawbacks of the flash-and-batt system. Please call for more information.