Winter in New England is a great time to observe a classic building problem: ice dams. And no winter (in my recent memory) has been better for ice dams than this one. We have received an immense amount of snow so far this year (over 70 inches in the Boson area), which has left the eaves of many roofs lined with ice dams and homeowners scrambling to prevent roof leaks. As you can see from the picture, I’ve had my share of ice dams too.
Like many homeowners with ice dams, our first priority has been to remove the snow pack and thereby remove the source of the ice dams. But shoveling roofs doesn’t make much practical or financial sense as a long-term strategy for dealing with ice dams, especially if you have very high roofs like we do. And, of course, we can’t control the weather.
But we can minimize the damage that dense snow pack can do to our homes by insulating and air sealing our attics. Insulating and air sealing, you ask? Aren’t these merely green measures, which help us conserve energy? Of course, insulation and air sealing are green measures. But as the problem of ice dams illustrates, they are much more than green. They can also make your home more durable and cut down on your maintenance costs. In short, they are as good for your home as they are green.
So how do ice dams form in the first place?
Ice dams can form when heat escapes from the conditioned part of a home and warms the surface of the roof. Once the surface temperature of the roof rises above the freezing point, snow that is touching the roof begins to melt, flowing down the roof towards the eaves. Assuming the outdoor air is below freezing, this water will refreeze once it reaches the eaves, which are not warmed by indoor heat loss. If outdoor temperatures stay below freezing for long enough and indoor heat loss continues to melt snow touching the roof, then an ice dam can form at the eaves of the roof and trap melted snow behind it. If there is enough water trapped behind an ice dam it can leak through the roof into the attic and walls of the home.
Many roofers will tell you that the solution to this problem is to apply ice and water shield underneath your roofing, which would prevent any trapped water from penetrating the surface of the roof. But as Martin Holladay claims in a blog post on Green Building Advisor, this approach doesn’t get to the root of the problem, which is indoor heat loss. As Holladay puts it, simply slapping some ice & water shield under your roofing shingles is the “the equivalent of waving a white flag and admitting defeat.” The most effective solution to the problem is to create an airtight thermal boundary between the conditioned (i.e., heated and cooled) space of your home and the outdoors. By stopping indoor heat loss, you stop (or at least greatly reduce) snow melt along the surface of your roof.
All of which goes to show that sometimes, green building practices are really just good building practices. At least these practices should be considered good and not green, insofar green implies something extra, something that isn’t necessary, something that goes beyond standard practice. Proper air sealing and insulation should be business as usual in home construction—and not just because they save energy, but also because they are good for homes. And by extension they’re good for the people who live in them.
From the homeowner perspective, finding a contractor who can provide an airtight thermal boundary isn’t always easy. But as every homeowner has suffered through an ice dam and the requisite repairs knows, it’s worth it.