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.