Attic Ventilation, Ice Dams, and Mold

Ice dams form when the upper roof is warm enough to melt snow and the eave is cold enough to refreeze it. Water backs under shingles or slate and wets the deck and the top of the wall. Separately, bathroom and cooking moisture leaks through ceiling penetrations and condenses on nails and sheathing. One leaves a localized stain. The other leaves widespread speckling, worse on the north slope.

Cleaning the speckle without changing the airflow just rents you a clean attic until next winter. Soffit vents buried under a deep insulation job are a frequent find in 'upgraded' older homes.

On a humid afternoon, open the hatch. A sharp musty smell and gray wood, with dry insulation, often means the deck is condensing or has grown and dried. Wet, matted insulation means a leak or a fan. A bath fan duct lying on the insulation is the single most satisfying thing we find, because the repair is obvious.

We clean colonized sheathing and remove only the insulation that is wet or moldy. Then we write down the fan and vent notes for you or your roofer. We are not the roofer. We will not 'kill the mold' on a deck that is still being rained on.

From the hatch, with a flashlight and a footing on a joist only if you can do it safely: where do the fans go, is the insulation wet, is growth widespread or in one bay? Do not step through a ceiling. If the roof is low and the summer attic is dangerously hot, leave it for a cooler morning or for us.

Outside, look at the eaves in winter. Icicles in a perfect row can mean a dam. Gutters packed with October leaves make both dams and overflowing water at the foundation — two mold paths, one neglected gutter.