What do you call the weeks between Valentine’s Day and Easter? It’s ice dam season, of course. Eastern Massachusetts is now the wet-ceiling capital of the world, but this winter, tens of thousands of homeowners from North Dakota to Maine are struggling with ice dams.
I’ve written about ice dams before. This article makes no attempt to be comprehensive; instead, it’s an introductory article to summarize basic information on ice dams. If you still have questions after reading the advice here, you should probably read “Prevent Ice Dams With Air Sealing and Insulation.”
How ice dams form
1. 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, forming an ice dam. If the water reservoir behind the ice dam is large enough, water can back up under the roof shingles and damage ceilings. (Image #2, below, depicts the complicated shape of a typical ice dam.)
2. The most common cause of ice dams, by far, is air leakage. Warm interior air leaks through through ceiling defects or recessed can lights, as well as through hidden passages in wall assemblies and roof assemblies, and this warm air contacts the underside of the roof sheathing.
3. The second-most common cause of ice dams is thin or missing insulation.
4. Bad roof design can make ice dams worse. Since ice can form in gutters, roofs with gutters (especially gutters that extend higher than the plane of the roof) are more susceptible to ice dams than roofs without any gutters. Roofs with valleys are more…