Envelope Efficiency to Address Ice Dams
Ice dams are caused when snow packed on a roof begins to melt, runs down, and re-freezes when it reaches the roof edge (eave). If melting and refreezing persists, a barrier of ice forms at the eave which stops any meltwater from running off the roof. The meltwater then backs up behind the ice dam, penetrates under the shingles, and leaks into the building. To address the melting problem, a two-part approach is used.
The first step of the approach is to ensure adequate ventilation of the attic space within the home. Air is meant to enter the attic space through soffit vents and exit through vents higher on the roofline. This constant flow of air keeps the interior surface of the roof at a temperature consistent with outside conditions, reducing the likelihood that snow will melt off of the roofs surface while the eaves are still cold enough to refreeze the meltwater. The second step of the approach is to minimize heat transfer between the conditioned space of the home and the attic space. Any heat transfer will warm the attic space and cause the roof temperature to exceed the exterior temperature, which produces melting. To ensure minimal heat transfer to the attic space, the attic should be air sealed around all penetrations and along the perimeter to stop any hot air from rising out of the conditioned space into the attic, and the insulation levels in the attic should be consistent with current Energy Star guidelines by climate region to limit heat conduction from the interior space.
If attic insulation levels are high enough, the attic space is air sealed, and adequate ventilation is provided through the attic from the outside, the effect of ice dams should be minimized. Air sealing the attic and providing sufficient insulation levels will also reduce the heating load of the building during the winter, and the cooling load of the building in the summer. Air sealing combined with increasing attic insulation levels can see a heating and cooling energy cost reduction of 19-20% depending on climate region, according to Energy Star.
Click here to see the Energy Star estimated savings for air sealing and insulating a typical US house: