This winter has been one of the worst in recent memory for leaks caused by ice dams. The majority of roofs in Ontario are covered in a deep layer of snow, some with ice and icicles hanging off the eaves. The snow alone, however, is not the problem. Normally you need three conditions for ice dams to form: snow, heat to melt the snow and cold to refreeze the melted snow into solid ice.
Most days this winter have been cold enough that snow on roofs has not melted much, if at all. However deeper snow on the roof acts as an insulator, trapping more indoor heat beneath the roof deck and warming the roof sheathing. And every time we get a warm spell, we have even better conditions for ice damming. When air temperatures warm up closer to the freezing point, the surface of the roof warms up as well. The heat that builds up within the attic as a result of radiant heat from the sun or heat loss from the home rises to the higher points of the roof, causing the snow cover to melt in those areas. This melting snow runs down the roof surface below the snow, where it contacts the colder areas of the roof along the eave or overhangs, where it re-freezes. The continuous freezing of melting snow forms a ridge of ice or "ice dam". As more water from continuing snow melt arrives at the ice dam, it is blocked and begins backing up under, and through, the roofing materials.
This is a diagram of the cause of ice damming in action. Water that's trapped above the ice has nowhere to go but down through the roof, into the house.
- Stop warm air from leaking into the attic from the house. As the primary cause of ice damming, this should be addressed first. This can’t be stressed enough. Seal all holes in the floor of the attic for wiring and plumbing runs, and gaps at intersections of interior walls and ceiling. Seal all leaks around ceiling fixtures, bathroom exhaust fans, attic hatches, etc. Almost any house over 20 years old will have “chases” or passages for chimneys, gas appliance vents, and vent systems, which allow large volumes of warm air to escape. Often “bulkheads” above kitchen cabinets, or lowered ceilings in showers are open to the attic. If you want to seal these air leaks yourself, consult with a home inspector about using Infrared Thermography to highlight them, as they are mostly hidden under the insulation and can be very difficult to find. Simply adding more insulation won’t help, as most commonly used materials do not do much to stop the airflow (that’s why fiberglass is used in furnace filters: it offers little resistance to airflow).
- Ensure adequate levels of insulation. A layer of insulation, applied evenly with no gaps, equivalent to R-32 is the bare minimum required. R-40 or more is better. It is common to find only a couple of inches of insulation, if any, above the top of exterior walls. There is little room for insulation below the roof framing here, and this area is hard for installers to reach. Adding insulation to fill these areas will reduce heat loss here, and ice damming. The best approach is to use an insulation which can be packed tightly in place, like cellulose, or use a material with a higher R-value per inch – specifically, closed-cell spray foam insulation. It’s not necessary to have the entire attic lid insulated with spray foam, but having it installed at the eaves is a great way to help compensate for the lack of space here. The only downside to having this type of insulation installed is that it’s more expensive than other types of insulation, and the work should be performed by an experienced spray-foam insulation installer. It’s not a DIY project.
- Provide sufficient passive ventilation to keep roof sheathing cool. A combination of soffit and peak or ridge vents of adequate size will exhaust any heat which does accumulate in the attic. Be sure soffit vents are not blocked by insulation, and that the roof sheathing below peak vents has been cut open properly. The old formula to determine adequate ventilation was for 1 square foot of vent area for every 300 square feet of attic floor, to be divided equally between soffit and peak or ridge venting. This is a minimum requirement, more is obviously better. Gable and turbine vents used on older houses are simply not adequate, and should be augmented with soffit and peak or ridge vents.
If you see any signs of potential damage from ice dams, consider consulting a home inspector who performs infrared scans, an energy auditor, or an insulation contractor for solutions. It’s a good bet that money spent preventing ice dams will be considerably less than the cost of repairing roof or interior damage caused by water leaks.
Here are two photos of the "chase" around an exhaust vent for the furnace, open all the way from the basement (left) to the attic (right), creating a constant supply of heat into the attic.
The photo on the right shows heat loss around a plumbing vent stack, again into the attic.