
Q.Assuming an attic is leaking some heat that reaches the roof sheathing, causing an ice dam, does the snow on the roof tend to act as insulation, trapping the heat near the sheathing? Will the melting of the roof snow be worse when there are 12 inches of fluffy snow on the roof than when there are 4 inches of snow?

A.Corresponding Editor Paul Fisette responds: The simple answer is yes. Snow is an insulator. Its R-value varies, depending on moisture content and density of the snow granules; but on average snow has an R-value of 1 per inch — about the same as wood. Twelve inches of snow have roughly the same insulating value as a 2x4 wall filled with fiberglass insulation.

The worst ice dams occur when there is deep snow followed by a period of very cold weather. To prevent ice dams, the most important and least expensive step is to seal all air leaks connecting the house to the attic. It is also important to reduce conductive heat loss from the house with deep layers of carefully installed insulation.

Once you have minimized heat flow into the attic, install an effective roof venting system. Continuous soffit vents that communicate effectively with continuous ridge vents are a good choice for most houses. I think ridge vents that have an external baffle are best. They provide more reliable suction because the air stream jumps over the baffle regardless of wind direction, creating negative pressure over the vent.