What's the Best Way to Deal With an Ice Dam?

Ice dams are an unfortunate fact of life for many houses in snow country, especially older homes where attic insulation is skimpy and air leaks abundant. Drive down any back road in rural New England and you're likely to find a handsome old home or two whose eaves are heavy with icicles. The problem is not unique to old houses. We live in a house just five years old where ice 6 inches thick builds up in roof valleys over the winter.

Ice dams begin when snow on the roof melts, runs toward the eaves and refreezes. In time, water can pond behind the ice and work its way under roof shingles where it damages roof sheathing, insulation and finished ceilings inside.

There are a couple of short term fixes, including the use of electric cables at the eaves or loading an old stocking with calcium chloride and placing it over the ice. Either one should melt a channel for water to escape.

The real solution is to address the underlying problem by increasing the amount of insulation in the roof, eliminating leaks that allow air to warm the bottom of the roof deck, and insulating and sealing any warm air ducts that run through unconditioned attics.

The goal of all of these strategies is to keep the underside of the roof deck cold so snow won't melt in the first place. That's the point of building a vented roof in which soffit and ridge vents work together to keep a layer of cold air beneath the roof sheathing. Baffles hold insulation far enough away to provide a clear channel for air.

If cold roofs were once the norm, unvented "hot" roofs are becoming increasingly common. In this scenario, a thick layer of insulation is packed into rafter bays, the idea being that enough insulation keeps the bottom of the roof deck cold enough to prevent ice dams from forming.

Closed-cell polyurethane foam is often the insulation of choice in a hot roof. Unlike open-cell foam, it prevents the migration of water vapor so condensed moisture won't collect on the back of the sheathing. It's not, however, always a successful approach, and there are hidden dangers.

First, without an additional layer of rigid foam insulation to act as a thermal break, the closed-cell foam in the rafter bays may not provide high enough R-values. Wood has an R-value of about 1 per inch, so even when closed cell foam, with an R-value of 6 or 6.5 per inch, fills the rafter bay, the rafters themselves aren't doing much to block the flow of heat.

Second, the foam can trap any water that gets through the finished roofing. Roofing contractors sometimes cover the entire roof deck with a peel-and-stick membrane like Grace Ice and Water Shield, which is designed to form a completely water impermeable layer. While this should eliminate the risk of a leak, sometimes it doesn't. If the membrane is applied incorrectly, or some other flashing detail isn't right, water can still get past the membrane. If there's a solid layer of closed-cell foam below the sheathing, there's no place for the water to go. The water just sits there, unable to dry to the inside or the outside, and slowly turns the roof sheathing to mush.

This is one of those instances where modern building materials and techniques can backfire, leading to a more insidious problem than ordinary ice dams that plague older homes.