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February 7, 2010 - 11:23pm

Ice dams and attic condensation

posted by Howard B. Owens in Sponsored Post, Mark Lewis Agency.


Two types of attic water damage are common in cold climates: ice dams and condensation of water vapor on cold surfaces in the attic.

Ice Dams

gutterdamning.jpgIce dams sometimes occur on sloping roofs in climates with freezing temperatures. When the temperature in your attic is above freezing, it causes snow on the roof to melt and run down the sloping roof. When the snowmelt runs down the roof and hits the colder eaves, it refreezes.

If this cycle repeats over several days, the freezing snowmelt builds up and forms a dam of ice, behind which water ponds. The ponding water can back up under the roof covering and leak into the attic or along exterior walls.The right weather conditions for ice dams is usually when outside air temperatures are in the low 20s (°F) for several days with several inches of snow on the roof.

Research shows keeping the attic air temperature below freezing when the outside air temperature is in the low 20s can reduce the occurrence of ice dams. Research has also shown sun exposure in the winter has little effect on attic air temperature. Warm air from living spaces below penetrating into the attic is usually the culprit in the formation of ice dams.

Attic Condensation

icedam1.gifCondensation of water vapor on cold surfaces in attics can cause attic wood products to rot, which can lead to costly repairs. Condensation typically occurs when warm, moist air migrates into the attic from living spaces below. Research indicates unusually high humidity levels in the home's living spaces is strongly associated with attic condensation problems.

Building codes have some requirements that attempt to prevent the problems of ice dams and attic condensation. But codes do not address all the issues, and many houses are built without following building codes. It is the builder or designer's job to understand the relationship of humidity and air movement when designing and constructing the house so these problems don't occur.


  • Prevent warm, moist air in living spaces from infiltrating into the attic with a good air barrier and appropriate water vapor control at the base of the attic.
  • Provide good attic ventilation to replace warm air in the attic with cold outside air.
  • Provide adequate attic insulation to reduce the transmission of heat into the attic from living spaces below.
  • Consult a professional for the best way to avoid ice dams and water damage in your home.
  • Tips on saving energy and more at home.

What not to Do

  • Do not routinely remove snow from the roof. It will likely lead to shingle damage.
  • Do not attempt to "chip away" the ice of an ice dam. It will likely lead to shingle damage.
  • Do not install mechanical equipment or water heaters in attics, especially in cold climates.
  • Do not use salt or calcium chloride to melt snow on a roof. These chemicals are very corrosive and can shorten the life of metal gutters, downspouts and flashings. Runoff that contains high concentrations of these chemicals can damage nearby grass and plants.
  • Keeping the gutters clean of leaves will not reduce the occurrence of ice dams. However, clean gutters can keep them from overflowing and spilling rainwater next to the house. This can cause moisture problems in the house.

If ice dams are causing water damage

The following are short-term fixes. In order to avoid the same problem in the future, a qualified contractor should assess the amount of attic insulation and attic ventilation (especially around where the ice dam formed), and look for holes in the air barrier between the living spaces and the attic.

  • If you see water staining at an exterior wall or ceiling where there is snow on the roof above it, act quickly to avoid extensive damage.
  • Hire a contractor to carefully remove most of the snow from the roof above the ice dam. The contractor should avoid touching the roof with the removal equipment or even walking on the roof if possible. Cold roofs are more prone to damage because they are more brittle than they are in the summer.
  • Have the contractor create one or two grooves in the ice dam to allow the ponding water to drain off. These grooves should not be taken down to the roofing as this may damage the roof. If necessary, the contractor can use heat tape in the groove on a temporary basis to keep the groove open.

State Farm® believes this information is reliable and accurate. We cannot, however, guarantee the performance of all items demonstrated or described in all situations. Always consult an experienced contractor or other expert to determine the best application of these ideas or products in your home.

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