Ice Dams

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.


  • 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 back up under the shingles.

Possible solutions

  1. 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:
    • Leaky and 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.
  2. Just adding fiberglass or cellulose insulation over the top won't often fix the problem. You need to find the holes and stop the air leaks first using materials such as spray foam insulation. 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® assessment includes blower door testing.
  3. Add more ceiling insulation if needed.
  4. Improve attic ventilation to cool down the underside of the roof sheathing. Ventilation is the last step to take. Sealing air leaks and adding insulation (if needed) are much more important.

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 or email Ask the Experts at

Focus on Energy has information about hiring "Trade Allies" to prevent ice dams, plus possible incentives for doing so. For more information, see Addressing Ice Dams.