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Ice Dams

What they are and how they start

The diagram above shows the formation of a typical ice dam.

Infrared image which portrays heat loss around the chimney causing snow to melt and run down the roof. The water will cool down and refreeze along the edge of the roof and cause an ice dam.

An ice dam is an accumulation of ice along the edge of a roof that does not allow water to run off the roof. Heat loss through a roof can cause snow to melt and the water to run off towards the roof overhang or the valley between two adjoining roofs. Once water hits the uninsulated and colder overhang or valley, it can refreeze. When this process happens enough times, a dam of ice builds up, and the roof has no drainage.  Subsequent water run-off hits the ice dam and backs up underneath the roof shingles. Once water is under the roof shingles, it can penetrate the roof sheathing and cause leaking.


Ice dams start with heat loss from a building’s roof. The heat loss can be attributed to several different factors:

Air and vapor leaks from the inside through recessed light fixtures, fan units, chimneys, and other openings in the building envelope. These air leaks are often started when warm air rises, which causes the upper floors to be under positive pressure. Air under positive pressure will move or ex-filtrate to areas of lower pressure. This is commonly called the stack effect.

  • Lack of thermal insulation in the roof assembly.

  • Gaps in the Air Barrier.

  • Wind washing of insulation near the soffit, which makes the insulation much less effective.

  • Air leakage from exterior walls.

  • Lack of thermal insulation where the exterior walls meet the roof assembly.

Using foam as part of the solution

The best way to prevent ice dams is to stop the heat loss that causes the snow to melt and refreeze near the roof overhang. This can be accomplished by:

  • Adding insulation to the roof assembly if necessary.

  • Using foam to fill holes (gaps) in the building envelope, often found around bathroom fans and recessed light fixtures, that can lead to the exfiltration of warm, moist air into the roof assembly.

For more information on ice dam solutions visit

More information, case studies, and reports


Bynum, Richard, 2001. Insulation Handbook, McGraw-Hill, New York, NY.

Lstiburek, Joseph and John Carmody, 1993. Moisture Control Handbook, Van Nostrand Reinhold, New York, NY.

Lstiburek, Joseph, 1998. Builders Guide: Cold Climates, Building Science Corporation, Westford, MA.

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