Ice dams cause millions of dollars of property damage across the snowbelt every year. In Minneapolis, we certainly get our share of problems, enough so that a few years ago I decided to add a sideline to my remodeling business - removing ice dams and retrofitting homes with proper air-sealing and insulation to keep them from coming back. It's proven to be a sound strategy: Although our main business is home renovation, removing ice dams has led to a lot of remodeling work. We've done 40 to 50 ice dam retrofits in the past six years and are about to start a $250,000 remodel that grew out of an ice dam job.
We make a point of presenting clients with a few solutions at different price points. For instance, one option might include sealing attic bypasses, installing spray foam insulation, and cutting in new continuous ridge venting, while a second option might be tweaking the existing roof ventilation to improve performance and adding fiberglass batts where they might help. For each alternative we include reasonable expectations in terms of thermal performance and the likelihood of ice damming in the future.
We never claim to prevent ice dams; instead, we tell clients that we will greatly minimize their likelihood. Although we write a custom contract for each retrofit job, we always include a stipulation about the importance of keeping snow off problematic areas.
The author's ice dam service runs the gamut from roof shoveling and ice removal to comprehensive retrofits involving air-sealing, improved insulation, and new roofing.
Understanding the Problem
Ice dams form when heat from the house escapes into the attic and warms the roof enough to melt snow on top. (For simplicity's sake, I'm using "attic" to refer to any area between the heated interior and the roof deck, including traditional storage attics, roof truss cavities, and spaces in rafter bays above cathedral ceilings.) Because hot air rises, higher areas on the roof tend to be warmer than lower areas. Snow in contact with roof surfaces with temperatures above 32°F will melt. When that snowmelt flows downslope to a colder part of the roof - typically the eaves, but not always - it refreezes. As this freeze-thaw cycle recurs, a ridge of ice develops and traps water on the roof. That water frequently leaks into the home, damaging walls, floors, ceilings, insulation - not to mention the homeowner's sense of tranquility.
Although ice dams are the result of a complex interaction between the snow cover on the roof, the outdoor temperatures, and the heat-loss characteristics of the house, a typical recipe might look like this: Moderate to heavy snow is followed by daytime temperatures of 15°F to 25°F. Mix in a dash of rain or dramatic temperature fluctuations and you have the perfect storm.
Why 15°F to 25°F? Because when it gets much colder than that, the average home, at least in our area, has enough insulation to prevent the roof deck from warming to the point where snow melts. Conversely, when temperatures reach into the 30s, the snow on the roof melts away too quickly for it to refreeze.
In theory, preventing ice dams is simple: Keep the roof surface cold. That means good air-sealing to prevent warm indoor air from leaking into the attic; enough insulation to prevent conductive heat losses; and adequate ventilation, to allow any heat that does make its way into the attic to mix with outdoor air before it warms the roof sheathing.