By Mark J. Donovan
Ice dams form due to warm moist air that is trapped in the attic and the constant warming and cooling of the roof during the cold winter days and month. There are two main components that need to be addressed to prevent ice dams, insulation and ventilation.
It is imperative that there is adequate and properly installed attic insulation to prevent warm moist area in the living areas below from filtering up into the attic space. Warm air in the attic can cause the back side of the roof sheathing to become warm and the snow just above it to melt. As it melts it drips towards the roof eaves. Then, as evening approaches and temperatures cool the water freezes up. This daily repeated process of thawing and cooling causes ice dams to form along the roof eaves. Eventually the ice dams build up to the point that melted snow works its way up under the lower courses of shingles. Once the water gets to the back sides of the shingles it inevitably finds a way into the attic and eventually into the lower living areas of the home.
The other major component that needs to be addressed to prevent ice dams is adequate and proper attic ventilation. Any warm air that does filter up into the attic needs to be quickly expelled to the outside so that it does not have time to warm up the underside roof sheathing.
Attic ventilation should include soffit vents and roof vents, such as ridge vent. The rule of thumb is to have approximately 1 square feet of attic ventilation for every 300 square feet of attic ceiling area with a vapor barrier, or every 150 square feet of attic ceiling area without a vapor barrier. Typically there should be around a 50/50 split between soffit vent and roof vent.
So by making sure you have adequate and proper attic insulation and ventilation you can prevent ice dams this winter.
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