ICE DAMS

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GLISTENING ICICLES DANGLING from the eaves may be among the beauties of winter, but they can also be a sign of trouble for your home. Icicle formation is often a symptom of ice damming, a serious condition that can ruin the roofing, the sheathing below, the rafters and joists that support the roof, the insulation, the paint on the exterior walls, and even the interior finish of your home.

Ice damming occurs when snow melts on a warm area of roof and flows downslope until it refreezes on a colder area near the eaves. Subsequent meltwater then pools above the icy obstruction and does its dirty work. Because most residential roofing systems are designed to shed water, not to seal it out, pooled water can leak up under shingles or around panel-roof seams, penetrating the sheathing and damaging non-weatherresistant materials below.

Major ice damming is seldom the result of natural, daily freeze-thaw cycles. It is almost always caused when heat lost from the house melts snow on the roof. Warmth leaks from the interior, passing through a poorly ventilated attic (or cathedral ceiling) to the roof (Fig. 1). At the same time, portions of the roof that extend beyond the living areaoverhangs, soffits, porch roofs-remain cold and thus refreeze the melt descending from above. It's the difference in surface temperature that causes the problem. Roofs that are all cold or all warm do not get ice dams.

Right Remedies

Now that we understand why ice dams happen, let's look at ways to cure them. There are three common methods of attack: Keep the entire roof cold by reducing heat loss from the house; keep the entire roof cold by venting heat loss before it can warm the roof; or keep the entire roof warm by heating the periphery. From an energy standpoint, the first approach obviously is the most attractive. (Roof ventilation has other arguable merits. In fact, it may be required by building code. But it is not a good sole solution to ice damming.) Therefore, we need to determine why so much heat is escaping from the house-and the most effective way to prevent it.

Insufficient or deteriorated attic insulation can lead to ice damming. If your house lacks up insulation in the roof, or if the R-value is below the guidelines shown in Fig. 2, by all means add insulation. You should also inspect existing insulation to ensure that it fits snugly between the ceiling joists and that it does not touch the roof rafters.