Ice Dams

Ice Dams

Conditions

Wisconsin winters can bring the problem of ice buildup on roof eaves, often accompanied by icicles along the roof edge. You'll typically see them when:

  • Outside temperatures remain below freezing for a week or two.
  • We've received several inches of snow.

Cause

  • Heat in the attic melts the snow on the roof.
  • The water runs down the roof slope to the cold overhang where it refreezes.
  • The ice starts to build up, holding water which can then back up the roof under the shingles.

Solution

  1. Be sure to maintain adequate attic ventilation.
  2. Stop warm air from leaking into your cold attic by sealing attic bypasses, the small holes and paths by which the heated air flows into the attic. Some typical warm air leak sources include:
    • Uninsulated or poorly insulated heat ducts in the attic.
    • Electrical boxes.
    • Exhaust fans.
    • Lighting fixtures including recessed lights and wire penetrations.
    • Plumbing and heating penetrations (e.g., chimneys and flue pipes).
    • Attic hatches or entrances.

Just adding fiberglass or cellulose insulation over the top won't often fix the problem. You need to find the small holes and stop the air leaks first. Detecting and sealing air leaks is best accomplished by a contractor using a blower door. This device simulates a 20-mph wind on all sides of the house at once. It allows contractors to find and seal air leaks more effectively. A Home Performance with ENERGY STAR® evaluation includes this service.

Note: MGE does not provide blower door or home performance evaluations. Contact Focus on Energy to find a list of participating contractors at 1-800-762-7077.

For more information

Call our Home Energy Line at 608-252-7117 during business hours.
Addressing Ice Damming [812 kB PDF]
How to Insulate Your Attic [781 kB PDF]
Ice dams fact sheet from Focus on Energy [181 kB PDF]
The Interconnected House Ice Dam Video