Overview: How to avoid ice dams
Ice dams cause millions of dollars of property damage across the snowbelt every year. In Minneapolis, where we work, we certainly get our share of problems, enough so that a few years ago (21 to be exact) I decided to add a sideline to my custom remodeling business — removing ice dams and retrofitting homes with proper insulation to keep them from coming back.
Why ice dams happen?
Ice dams are the result of a complex interaction between a particular home and the environmental conditions around it. Ice dams form when heat from the house escapes, passing through the ceiling into the attic where it warms the roof enough to melt the snow on top. The recipe for a good ice dam season is simple: Moderate to heavy snow followed by daytime temperatures of 18˚F to 25˚F. Mix in a dash of rain or some dramatic temperature fluctuations and you have the perfect storm. Why 18F to 25F? 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 thirties, the snow on the roof melts away too quickly to allow for it to refreeze.
Theory on preventing ice dams
The way to prevent ice damming is simple, in theory. Keep the roof cold. That means properly detailed insulation, ventilation, and air sealing. It may also mean modifying your lifestyle a bit. If you prefer 42 percent indoor humidity and 84 degree temperatures, you are asking for big problems. Big ice dam problems and big attic condensation problems. Logically it follows that keeping your home a little cooler and dryer will help minimize the likelihood of such problems.
Tips on preventing ice dams
Air Sealing First
Although a surprising number of contractors neglect air sealing, it’s perhaps the most crucial step in preventing ice dams. Warm air currents will always find a way to get through the ceiling plane and into the attic. These leaks are called thermal bypasses, and the goal of proper air sealing is to minimize them; if you don’t, all the insulation in the world won’t prevent ice dams.
The good news is that most bypasses can be sealed with a can of spray foam and a little elbow grease.First you have to find them. The ultimate method to detect bypasses – and indeed to figure out the overall thermal behavior of the house – is to conduct a heat loss study. This can involve blower door tests, infrared cameras, and other specialized inspection equipment. For the client with an adequate budget, a blower door test is a great first step.
Insulation and Ventilation
Citing by most contractors as the first place to start when you want to prevent ice dams, insulation and ventilation are the next steps to pursue after air sealing. It is quite possible and entirely common to see poorly insulated or over insulated spaces. This usually happens when an inexperienced person is making the decisions. More insulation does not equate to better building performance. It is essential to have the right combination of insulation and ventilation if your goal is to prevent ice dams from happening.