Ice Dam Prevention
Ice Dam prevention is different for different climate zones. This is why here so many conflicting solutions to this problem. Also why the new trade “whole house contracting” is so important. The roofer insists it is all about roof ventilation. He talks you into adding a ridge vent. The building performance contractor understands that more insulation and air sealing the attic floor are important factors. But both trades are missing the real solution. You must address roof venting, air sealing and insulation. Walk through neighborhoods in the wintertime and you will notice distinct patterns in the ice dams, depending on when the house was built.
We know that ice dams form when there is lots of snow, and winter temperatures are mild; and that heat escaping from the top floor of a house into the attic causes snow to melt, move down the roof, and refreeze at the eave. But walk through neighborhoods in the wintertime and you will notice distinct patterns in the ice dams, depending on when the house was built. There is no doubt that poor construction techniques—a disregard for good air sealing and thorough insulation—are largely responsible for the formation of ice dams. But what about poor design?
The fact that many pre-WWI homes have steeply pitched roofs, or that their occupants kept the inside temperature of their homes lower than they do today, may offer a clue as to why these homes seem less prone to severe damage in the living space from ice dams. There is no denying that homes built after WWII—sprawling bungalows with can lights and ductwork in the attic—are much more susceptible to ice damming.
What about attic ventilation? Is this another factor in the development of ice dams? Attic ventilation is, in effect, an acknowledgment on the part of both builders and code officials that poor insulation, and poor air sealing of the attic floor, are facts of life in new construction. The purpose of attic ventilation is—ideally— to ensure that the warm, moist air that leaks up through the top of the house and into the attic convects out the ridge vent, causing cool, dry air to come in through the eave vents, thereby compensating for the sin of sloppy construction. Even though studies from Sweden and more recently from Canada Mortgage and Housing Corporation have shown that “identical attics, one unvented and the other vented to code, have much the same humidity and temperature,” ventilation codes have not changed and building codes still require extensive ventilation. When concerns about unvented roofs arise, shingle manufacturers warn us that shingles will fail prematurely. Yet studies have shown that shingle life may be degraded by at most 10% reduction in lifespan when comparing a vented versus unvented roof; that ventilating a roof reduced temperature by 0.5ºC; and that shingle color, direction, and geography has more effect on attic temperature than adding ventilation, even with wind assistance.
Snow cover on a roof affects the inside temperature of an attic in three ways; snow is a great insulator; snow may block vent air flow; and snow reflects the sun’s heat. Because snow is a good insulator, the greater the snow load on a roof, the easier it is for a warm roof deck to melt the snow in contact with the shingles. Normally, hot air in an attic could dissipate out through the vents, but if the vents are covered with snow, the heat will be trapped in the attic. The trapped heat will raise the temperature of the sheathing, causing the snow on the roof to melt. The melt water refreezes at the eaves to create ice dams. Once a section of roof is bare, the sun can beat down on its usually dark asphalt surface, raising the temperature of the air in the attic. At this point even north-facing slopes are not immune to ice damming, thanks in part to the sun and shingle color.