Ice dams are a common problem that form on buildings in areas that receive significant snowfall. It causes significant ice and icicle build-up on the edge of the roof which could cause water leakage and dangerous ice conditions in the case of falling icicles. Ice dams can occur on any building where temperatures reach below freezing for at least one full month per year, but are less likely to occur in very cold climates. There are many ways for ice dams to form, but in general, they are caused by snow melting on the upper part of the roof then refreezing near the edge to create an icy buildup and icicles.
Icicles may be pretty to look at but they can be extremely dangerous for passerby, especially if near or above common walking areas such as sidewalks or above doors.
There are several causes of ice dams, including:
- Lack of sufficient insulation
- Air leakage in proximity to the roof membrane
- An uninsulated heat source near the roof
- A difference in snow thickness along the roof
In all three of these cases, the snow on the roof acts as a thermal insulator. This causes the temperature close to to the roof to increase, thereby melting the snow. As this melted water runs down the roof, it accumulates at the edge of the roof where it refreezes due to cold air currents and the lack of insulation.
In the first case, of insufficient insulation, the formation of the ice dam is accelerated due to excessive heat rising through gaps in the insulation, especially at the corner of the wall and ceiling. This causes warm air from the building to reach the roof where it speeds up the melting process of snow in contact with the roof. To counter this, the best solution in most cases is to simply add more insulation. The amount of insulation required will depend on the climate, but it is recommended to have at least R30 for ventilated attics, R35 for ventilated cathedral ceilings, and R40 for unventilated cathedral ceilings. You should consult a professional insulation installer to determine the correct amount of insulation for you.
Another technique may be used if sufficient insulation is already installed. Cool air from the outside can be vented in along the edge of the roof to further reduce the temperature on the underside of the roof membrane. Remember that this should be in addition to, not a replacement for, proper insulation.
Air leakage, which is the major cause of heat loss in a home, can also be a major cause of ice dams. Air leaking through your ceiling causes the roof to heat up, melting snow, and it can also cause condensation to build up on the underside of the roof. This could eventually lead to water leakage through the ceiling. Some of the largest causes of air leakage are components such as lights, vents, and fans installed into the ceiling. The solution here is proper air-tight sealing of the ceiling and around fixtures. There are many ways to create an effective air barrier, including caulking, drywall, polyethylene sheets, and expanding polyurethane foam or spray-foam insulation.
Do to poor building practices, some buildings have heat sources in the attic or spaces near the roof. These are things like ducts or other units that emit heat. The best solution would be to remove or relocate these elements. If that’s not possible, make sure that they are air-tight and cover them with insulation to avoid heating the roof as much as possible. In addition, venting the roof as explained above may help to remove any excess heat and cool the roof cavity.
The last cause is the hardest to prevent. Differences in snow thickness along the roof are caused naturally by wind and the slope of the roof. The severity will depend on the climate, weather, roof shape and roof material. Exposed tiles can increase temperature significantly from the sun, melting the surrounding snow, which then flows into areas of thicker insulation from the snow, eventually coming down to potentially form an ice dam. The best solution here is to use all of the above recommendations to reduce heat coming from below, as well as to make sure you have a waterproof membrane to avoid water leakage in the event that there is meltwater buildup above an ice dam that has formed.
If you have noticed ice dams forming on your home or building, they may be an indication of improper insulation, which means that there is heat leaking out and a potential for increasing your home’s energy efficiency. To prevent further heat leakage or potential water leakage, we recommend that you have a professional inspect the building to determine the cause so that it may be fixed. Getting rid of ice dams is just one more way to prevent damage and increase your home’s efficiency.
If you are building a new home in an area which receives snowfall for a month or more out of the year, be sure that you have proper insulation which creates an air barrier, as well as no heating elements in the roof area. Also, provide ventilation of outside air to the roof area to further reduce the temperature.
Keep a lookout for ice dams and stay safe this winter by not walking under icicles!