Attic Ventilation and Water Damage

Attic Ventilation and Water Damage

Ice dams and attic condensation can cause severe damage to homes. Attic ventilation is one of the key factors in preventing this type of damage in northern climates.

This article explains the importance of attic ventilation and other factors that can help prevent ice dams and attic condensation.

Cold Climates

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

Ice dams sometimes occur on sloping roofs in climates with extended below freezing temperatures combined with several inches of snow. When winter conditions are just right (usually outside air temperatures in the low 20s (F) for several days with several inches of snow on the roof), ice dams can form at the eaves if the air temperature in the attic rises to above freezing. The elevated attic air temperature causes snow on the roof to melt and run down the sloping roof. Once the snowmelt runs down beyond the exterior walls below, 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.

Research indicates keeping the attic air temperature below freezing when the outside air temperature is in the low 20s (F) can reduce the occurrence of ice dams. If the outside air temperature is colder than the low 20s, keeping the attic air temperature below freezing is usually not a problem. When the outside air temperature is higher than the low 20s, the snowmelt on the roof usually does not refreeze at the roof eaves. Research has shown sun exposure in the winter has little effect on attic air temperature. Warm air penetrating into the attic from living spaces below is usually the culprit in the formation of ice dams.

Attic Condensation

Condensation 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 the 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 role of the builder or designer to understand the relationship of humidity and air movement when designing and constructing the house so these problems don't occur.

Prevention