During snowy winters, many northern homes are plagued by ice dams. If your house suffers from wet ceilings during the winter, you may be ready to call up a contractor. Be careful, though: since most contractors don’t understand the causes of ice dams, they often suggest the wrong solution.
Ice dams form when a home’s escaping heat warms the roof sheathing and melts the underside of the snow layer on the roof. Water trickles down the roof until it reaches the cold roofing over the eaves, where it freezes. After a while, the ice at the eaves gets thicker and thicker, forming an ice dam. Eventually, water backs up behind the ice dam. If the water reservoir is large enough, it can back up under the roof shingles and damage ceilings. (Image #4, below, depicts the complicated shape of a typical ice dam more accurately than the simplified drawings in Images #2 and #3.)
The four possible solutions to ice damming are:
- Sealing air leaks between the warm interior and the attic or cathedral ceiling.
- Adding more ceiling insulation.
- Improving ventilation between the top of the insulation and the roof sheathing.
- Installing a rubberized membrane under the roofing.
While the first two of these solutions can reduce or eliminate the problem, the last two solutions are the equivalent of waving a white flag and admitting defeat.
Step one: seal the air leaks
Most ice dams are caused by flaws in a home’s air barrier. If escaping indoor air finds its way to the underside of the roof sheathing during the winter, the heated air raises the temperature of the sheathing. That’s bad.
One energy expert who made a career of correcting ice-dam problems was the late Tony Woods, a Canadian physicist and founder of CanAm Building Envelope Specialists in…