Recurrent ice dams on a 1950's hip-roofed home

Q. We have insulated the attic in our 1950's hip-roofed home with ten inches of insulation and have pulled the insulation away from the soffit. Each winter the snow and cold arrives and we have ice damming problems and ice has to be removed from the gutters. We have covered the gutters to keep leaves out, but still have the ice problem. We have reached the age where we would like to find a solution to the problem and not have to get on a ladder and chop ice.
G.B. Kalamazoo.
A.We have a brick walkout home with a large overhang. Although the overhang is wonderful for protecting the house the elements, it aggravates the problems of ice dams. The usual suggestion for controlling ice dams is to insulate the attic, however, no matter how much insulation is present on the floor of the attic, some heat will always be available from chimneys, etc. D.M. Kalamazoo.

A. Ice dam problems are not easily addressed while they are occurring, so I held both of your February letters in check until now. The best time to fix the problem is in the spring or fall, when it is cool enough in an attic to do some work, and the outside tasks, if necessary, are easier to do without snow and ice around the house.

While the "usual suggestion for controlling ice dams is to insulate the attic," is indeed the usual suggestion, insulation is only one part of the picture. Ice dams are caused when heat from the house below enters the attic and warms the roof sheathing. Snow lying on top of the roof starts to melt where it touches the now-warm shingles, and the resulting meltwater runs down the roof under the snowpack. When the water reaches the roof edge at the overhang, it freezes there because there is no longer any heat from below to keep it melted. Over time the ice can build up to the point that it traps "ponds" of meltwater behind it, and that collected meltwater can flood the shingles along the roof edge from below, resulting in leakage through the roof and sometimes into the house.

The key to ice dam prevention is keeping the attic as cold as the air outside, or very nearly so. That stops the melting outside. Insulation - lots of insulation on the attic floor, and the right kind of insulation - helps. But there are many other ways heat can enter an attic that insulation cannot stop. Poorly fitting attic scuttleholes and pulldown stairways, wire chases, cracks in drywall and plaster, recessed canister lighting, electrical boxes mounted in the ceiling, cracks and gaps in the framing, vent fans that exhaust into the attic, all can contribute to the flow of heated air. Sealing those infiltration points can go a long way toward the goal of a cool attic.

Additionally, ventilation is very important in preventing ice dam formation. Soffit and high roof vents, working together, flush out heated (and moist) air that enters the attic. The warm air vents out the top, which draws cool air into the soffit vents. Ventilation channels installed between every set of rafters or trusses insure airflow from the soffit vents into the upper attic.

Finally, cellulose fiber insulation blown over the attic floor (ten inches, by the way, is only the minimum recommended depth) does a better job than fiberglass insulation in terms of keeping air from moving through the material, or through gaps in batt type products. Insulation, ventilation, and air sealing. You need all three to prevent ice dams.
