Ice Dam Prevention
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Problem
When a roof must shed snow and shelter the house below, the warmth rising from heated rooms creates a hidden battle: heat escaping into the attic melts snow on the roof deck, water runs down to the cold eaves where no heat reaches, and there it freezes into a dam that backs water under shingles and into walls. The very act of keeping warm destroys the roof that keeps you dry.
Evidence and Discussion
The physics are simple and unforgiving. Interior heat escapes through the ceiling — through gaps around light fixtures, attic hatches, plumbing stacks, and electrical penetrations. This warm air rises into the attic space, heating the roof deck above the living area. Snow on that section melts. The meltwater flows downhill toward the eaves, which project beyond the heated envelope and remain at outdoor temperature. At -15°C, the water refreezes instantly, building a ridge of ice at the roof edge. As more melt arrives, it pools behind this dam. Water finds its way under shingles, through nail holes, into sheathing. By spring, you find stained ceilings, rotted fascia, mold in wall cavities.
The Canada Mortgage and Housing Corporation identifies three conditions that must coincide for ice dams: interior heat reaching the roof deck, snow on the roof providing meltwater, and eaves cold enough to refreeze that water. Remove any one, and the dam cannot form. In practice, the first condition — heat reaching the roof deck — is the one builders can control. CMHC's research into cold-roof assemblies, developed through the Building Envelope Performance program, establishes that maintaining the roof deck within 5°C of outdoor temperature eliminates the differential that drives melting. This requires both thermal separation (insulation) and temperature equalization (ventilation).
The Building America research program, operated by the U.S. Department of Energy through Oak Ridge National Laboratory, measured ice dam formation across roof types in Minnesota and found that air leakage, not conduction through insulation, accounts for the majority of heat reaching attic spaces. Sealing the 50 to 100 penetrations in a typical ceiling — the "attic bypass" locations — reduces heat transfer to the roof deck more effectively than adding insulation alone. The University of Minnesota Extension documented that homes with air-sealed attics and R-50 insulation showed no ice dam formation even in winters with sustained -25°C temperatures, while neighboring homes with R-38 insulation but unsealed bypasses showed significant damming.
Ventilation completes the system. A continuous intake at the soffit and exhaust at the ridge creates airflow that purges any heat that does reach the attic. The Canadian Wood Council specifies a minimum of 1 square foot of net free ventilation area per 150 square feet of attic floor, balanced between intake and exhaust. In Edmonton's climate, with 5,200 heating degree days and frequent temperature swings across the freeze-thaw boundary, this ventilation must remain clear of snow and frost accumulation — baffles at the soffit vents keep insulation from blocking intake, and ridge vents must be designed for snow shedding rather than snow trapping.
Therefore
Build every heated roof as a cold roof. Seal every penetration through the ceiling plane — lights, hatches, plumbing, wiring — with fire-rated caulk or spray foam before insulation is installed. Insulate to a minimum of R-50, with no gaps or compression at the eaves. Install continuous soffit vents with baffles to maintain a 2-inch air channel from soffit to ridge, and provide balanced exhaust ventilation at the ridge. Test the ceiling plane for air leakage: on a cold day with the attic at outdoor temperature, the roof deck should show no warm spots in thermal imaging, and snow should melt evenly across the entire roof surface or not at all.