If you have ice dams forming on your roof during the winter, it means that heat is escaping the house and leaking into your attic. Ice dams are the manifestation of energy inefficiency in a home. They are the result of poor air sealing, a lack of insulation, and inadequate ventilation in an attic.
Warm air travels upward because of its natural buoyancy. As it reaches the ceiling in the top floor, it seeks ways to rise even higher through cracks and gaps in the ceiling and walls. Some of those pathways are obvious; many others are not. Openings around and through recessed canister lights, whole-house fan installations, attic-access hatchways and pull-down stairs, and electrical boxes in the ceiling and walls all provide conduits from the house into the attic. Additionally, heat is conducted upward through the top-floor ceiling through inadequate attic floor insulation. The result of the air leaks and conducted heat is an accumulation of warm air in the attic.
When snow falls on top of a roof, it acts as insulation, protecting the roof surface from the outside cold air. The combination of heat from below and snow on top creates conditions that warm the roof sheathing and shingles.
The warm shingles melt the snow that covers them, and that water runs down the roof, under the snowpack that lies on top of the roof. As the water reaches the roof edge, there is no longer any heat from below to warm the shingles and sustain the melting process. The water freezes along the overhangs and starts to build into ice dams.
As the ice dams build up higher over the course of the winter due to the constantly melting snow on the roof, water starts to form ponds behind the dams. Eventually, if the water level gets high enough and if the roof is inadequately protected from water intrusion, it starts to seep in underneath the shingles. In the worst cases the water can penetrate into the soffit areas, get behind the siding, and even enter the house through the interior ceilings and walls. Ice dams can be very destructive and result in millions of dollars in insurance claims every year.
The root cause of ice dams is excess heat in the attic. Air sealing and upgrading insulation will help reduce the heat leakage problem. The idea is to make the attic as cold as possible -- as cold as the outside air -- to reduce or eliminate the snow melting that starts the ice dam formation process. Additional ventilation in the attic also exhausts any heat that does manage to make it up there.
The ideal ventilation scheme involves several components: soffit vents that introduce air into the attic under the eave edges; air channels; chutes that hold insulation back from the underside of the roof sheathing and direct the air upward from the soffits into the attic; and high roof or ridge vents that convey the air to the outdoors. The chutes are important because insulation lying against the underside of the roof sheathing forms a thermal bridge that allows heat from the house below to travel through the insulation directly to the sheathing. It is essential to break that thermal bridge to eliminate the direct conveyance of the heat to the sheathing and to promote the free flow of air into the rest of the attic from the soffit vents.
Attic ventilation is also needed to reduce moisture concentration in the attic environment. Air that travels into the attic from the house below carries water vapor. Unless that moisture is vented away, it can condense on the cold insulation, framing, and sheathing. If allowed to continue, the wet surroundings can create conditions conducive to mildew and mold growth, and can even rot.
Adequate attic ventilation also pays off in the summer. Air flowing through the soffit vents and up through the ridge or high roof vents exhausts heat. Venting the attic means less heat is transferred downward through the attic floor insulation and into the house below. Therefore, the A/C doesn't have to run as often, which conserves your energy dollars.
Comprehensive air sealing, insulation, and ventilation can reduce or eliminate the formation of ice dams on your house roof in the winter while paying dividends in the summer.
If you aren't sure how or where to check your attic for leaks, we'll share tips on how you can find and seal those weak areas in the next section.