Inadequate Attic Insulation Causes Ice Dams. Ice dams occur when snow melts on sloped roofs and then refreezes again. They are most often caused by attics and roofs with inadequate insulation. Two circumstances combine to make an ice dam.
If your home does not have proper and effective attic insulation, heat can escape from the living space during the cold winter months. Heat is transferred from the living space and also generated from recessed light fixtures, air ducts, furnaces and other appliances and equipment located in the attic. This heat raises the roof temperature above the freezing point, causing the snow and ice on the roof to melt and flow downward.
This area is often just inside the perimeter of the outside walls. Once the melted snow, or water, meets back up with the colder part of the roof (overhang) that hangs over and outside of the interior walls (just above the soffit vent area), it begins to freeze again, expand and back up. This dam that forms is where this effect gets its name from; ice dam.
Have you ever noticed one house that is completely covered with snow on the roof? Then you see a house with no snow, or with areas that have completely melted away. Contrary to one’s belief, the house with all the snow on its roof, typically resembles the better insulated one. You may also notice huge, long icicles hanging down off the roof of one house, but not on another. These are all signs of an inadequately insulated roof and/or attic space. See our section on SPF Attic Insulation and Ventilation.
Backed up water can get under the shingles and through the roof deck. It can drip on the ceiling insulation or run down the underside of the deck to the connection between the roof and the walls. It then makes its way into the building in the form of damaging leaks. It can also lead to mold and mildew.
Melt water under the unmelted snow can decrease the friction between the snow and the roof and cause a dangerous snow slide, like an avalanche. Melt water often refreezes as icicles hanging from the gutters or edge of the roof. These icicles eventually break off when they get too heavy. Both snow slides and falling icicles endanger passersby.