Speaking of Ice Dams
Snow on the upper part of the roof melts, water runs down under the blanket of snow and refreezes into a band of ice at the roof’s edge, creating a dam. As snow-melt pools against the dam, it backs up between the shingles, and leaks into the building. Damage occurs. If the house is poorly insulated and air sealed, this can happen with as little as a couple of inches of snowfall followed by sub-freezing temperatures.
The upper roof surface is directly above the heated living space. Heat lost from here warms this section of the roof, melting the snow above, while the roof overhangs, not warmed by indoor heat-loss, remain at sub-freezing ambient temperatures. With more snow and cold temperature the size of the ice dam grows. Snow has an R-value of 0.5 – 1.0 per inch. It insulates the roof deck, trapping more indoor heat, warming the roof sheathing. The worst ice dams occur when deep snow accompanies cold weather. The damage caused can be considerable.
To prevent ice dams, we must maintain the entire roof surface cold enough to prevent the snow from melting. Properly built houses do this by plenty of ceiling insulation, a continuous air barrier around the heated living space, and effective roof ventilation. Here in the northern US, ceiling insulation should be at least R-38; but up to R-60 is preferable (and also saves money). The insulation must be continuous and uniformly deep. A potential problem area is above the exterior walls, where there may be insufficient room to accommodate the insulation. In existing structures, the space between the wall’s top plate and underside of the roof sheathing may have to be filled with high R value insulating foam.
In addition to insulating, we must block the flow of warm indoor air into the attic or roof area. Even small holes allow significant volumes of warm moist indoor air to pass through. Seal all leaks — electrical outlets and switches, wiring and plumbing penetrations, ceiling light fixtures, attic hatches, around chimneys, bathroom exhaust fans and the tops interior partitions – using spray-foam, caulking, or weatherstripping. Recessed lights and chimneys demand special care for fire safety reasons.
A soffit-to-ridge ventilation system is the most effective to cool roof sheathing. The vents should run continuously along the length of the house. A baffled ridge vent will exhaust attic air regardless of wind direction, sucking cold make-up air into the attic through the soffit vents. A 2 inch "air-chute" between the roof sheathing and the insulation will allow cold incoming "soffit air" to wash the underside of the roof, keeping it cool.
There are many widgets sold to prevent or cure ice dams – expensive to use electric wires come to mind and brute force ice dam removal that can do more damage — but they only treat the symptoms. If at all possible, it is better to treat the root cause – heat from below that melts the snow in the first place — as described above.