Proper Roof Ventilation, Preventing Air Leakage Can Stop Ice Formation

Q. I’ve had ice damming on my roof in the past, and I’m afraid it may happen again. What causes it and what can I do to prevent it?

A. Ice damming occurs when ice accumulates on top of the eaves of your roof. The roof is usually colder at the eaves. When snow higher up on your roof repeatedly thaws and freezes, water runs down to the eaves and produces an ice build-up when it freezes again. Repeated cycles of thawing and freezing can accumulate up to a foot of ice at the eaves. It can be hard to detect if new snowfall hides it.

During warmer weather, the remaining snow on the roof can melt rapidly. The water dams up behind the ice and can actually run back under the shingles and into the attic. This wets your insulation, reducing its effectiveness. It may also result in water damage inside your house.

Three main conditions contribute to ice damming:

During cold weather, ventilation protects a home by keeping the roof cold so snow doesn’t melt. Problems occur if a roof or attic lacks sufficient ventilation, which allows outside air to flow from the eaves to the peak of the roof.

Air leaking from inside the house to the attic allows warm moist air to escape into the attic and warm up the roof. Moisture in this air can condense on the cold underside of the roof deck and freeze. When it gets a chance to thaw, it rains down on the ceiling and insulation below. The resulting damage can be the same as ice dams but may have you scratching your head if no visible ice dams are on the eaves.

If the attic or roof lacks sufficient insulation, heat loss can be substantial enough to warm the roof enough to melt snow.

Preventing ice dams

Make sure that your roof structure meets the minimum ventilation level required under the Uniform Building Code (one square foot of net free area for every 150 square feet of attic area). This will help ensure that the roof remains as cold as possible.

Eliminate air leaks into your attic and roof. Do this before adding insulation. Common sources of air leaks include recessed electrical light fixtures and exhaust fans. Never vent an exhaust fan directly into the attic space. Seal between the fixture and the ceiling.

Large quantities of warm air can escape into the attic from where the chimney penetrates the ceiling. This space around the chimney must be sealed with an approved fire resistive material. Also any vertical duct chase or places where piping enters the attic from the wall below should be sealed.

If you have little or no insulation in the attic, consider increasing the level to R-38 or more. Remember, insulation will do little good if you have major air leaks. Don’t allow insulation to block ventilation near the eaves.

Temporary measures include shoveling or raking snow off the roof and eaves to keep ice from accumulating. You can try de-ice chemicals to melt channels in the ice to prevent water from backing up during a fast thaw cycle. Electric heat tape on the eaves will keep ice from forming. However, there are operating costs and the tape must be installed before the problem occurs.

For more information, call the Energy Hotline at (800) 962-9731 or 324-7980. Ask for these free factsheets: “Reducing Home Air Leakage,” “Home Insulation” and “Attic and Crawlspace Ventilation.”

MEMO: Send your questions to Washington State Energy Office Education and Information Network, 1212 N. Washington, Room 106, Spokane, WA 99201-2401; or call 324-7980 or (800) 962-9731 (in Washington), weekdays.