Ice dams and how to prevent them

Posted Wednesday, December 21, 2011 in Sustainable Maine

Ice dams and how to prevent them

by Paul Kando

Ice dams are caused by a chain of events. An attic is poorly sealed and insulated. Warm, moisture-laden air escapes from the rooms below through cracks and holes, warming the attic. Additional heat arrives through insufficient or poorly installed insulation. The attic temperature rises above freezing and warms the roof. Snow atop the roof melts. The melt-water cascades down the roof under the snow until it reaches the lower edge of the roof above the eave, where no heat flows from below. Here the water re-freezes into a solid block of ice, which grows as more melt-water cascades down from above. The block of ice soon forms a dam, behind which. the melt-water backs up between the roof shingles and drips into the building.

The dripping water wets the insulation on the attic floor, causing it to lose its resistance to heat (R value). Consequently even more heat escapes into the attic to melt more snow on the roof. The water drips down through the insulation. Some of it flows down inside the wall, damaging the insulation there and, over time, causing wood-rot, mold and mildew. These, in turn, negatively affect indoor air quality. Some water also seeps through the ceiling, damaging sheet rock or  plaster. Finally it drips into the room, soaking carpets, furniture, etc.

Fortunately ice dams can be prevented. The snow can be raked off the roof after every snowfall. This works, but it is also takes a lot of repetitive labor. Some people install a zig-zag of heating cables over the bottom 18 - 24" of the roof surface, creating little channels through under the ice for the melt water to run off the roof. Icicles form whose weight sometimes damages gutters. But at least no water drips down inside.  This is an expensive way to prevent ice dams because the heating cables draw quite a lot of electric power. 

One can also install a seamless band of metal roofing over the bottom two to three feet of the roof, eliminating the gaps between rows of roof shingles. Some modern roofs have a waterproof membrane installed under the shingles, covering the lowest four feet of the roof. However these methods place a vapor barrier (metal, plastic) on the cold side of the roof deck, which can trap moisture carried by warm air reaching the roof from below through air leaks through the attic floor. This moisture can cause damage to the roof deck.

So, is there a better way?  There is. A three-step process to address the root cause of ice dams: heat from the house escaping into the attic. First, seal all holes and air leaks into the attic, including around attic hatches and stairs. Second, add a permanent, continuous air/vapor barrier to prevent any moisture from permeating through the sheet-rock or plaster ceiling. This barrier may be a continuous plastic sheet installed on the upper surface of the ceiling (beneath any insulation), or the ceiling may be prime-painted with a shellac-based primer such as BIN or Kilz, which will also prevent any water stains from showing through the finish  coat of your choice. Third, install a sufficient, seamless layer of insulation on the attic floor (a total R-60 is a good value to shoot for in our climate). Don’t forget insulating the top of the attic hatch or stair and make sure the attic itself is well ventilated to keep it cool. If the attic floor is not accessible, an alternative is to insulate the underside of the roof itself with spray foam, creating a so called hot roof (the subject of another column). 

Preventing ice dams in this way will also conserve a lot of heat, so the improvements will pay for themselves in short order.

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