Q. I'm going to be removing the existing fiberglass batts from the attic of a moderately leaky older home, doing some air-sealing, and re-insulating with 16 inches of loose-fill cellulose. According to the homeowner, a dusting of fine powdery snow sometimes covers the batts after snowstorms, apparently because it sifts through the ridge cap of the metal roof. This hasn't seemed to hurt the batts any, but because cellulose is organic, I'm wondering if these occasional dustings of snow could lead to rot or mold.

A. Bill Hustrunk, technical manager of cellulose manufacturer National Fiber, in Belchertown, Mass., responds: The amount of snow you describe won't do any harm. Cellulose is highly hygroscopic, meaning it can disperse moisture over a very large internal surface area and then dry back out to reestablish its moisture equilibrium. Because of this, there are no localized wet areas that might give mold or other microorganisms the moisture they need to multiply. The borate-based fire retardants used in cellulose help, too, because they have antimicrobial properties.

The attic you'll be working in sounds well-ventilated, but cellulose's ability to accommodate moisture also allows it to work in unvented dense-packed roof assemblies. Its density prevents airflow - which we now know is the major transport mechanism for moisture - while moisture that enters through diffusion transport is managed by its hygroscopic properties. But it's important to note that this requires an assembly that's able to dry to at least the inside or outside, and preferably both.

During my nine years running the weatherization program in central Vermont, of the thousands of unvented roof assemblies that we dense-packed, I saw only one failure - in a home where a large tree limb fell on a roof during a rainstorm.