Solving Ice Damming
Although sometimes thought of as a problem with roofing or attic ventilation, ice dams are actually caused by the presence of warm air in the attic, combined with snow on the roof and the right weather conditions. When heat leaks into the attic it melts the underside of the snow on the roof, which then flows down the roof surface until it reaches a cold spot (such as the eaves or soffit) where it forms a frozen dam. The ice build-up can back up under the shingles, damaging them and allowing water to leak to the ceilings and walls below. The source of ice dams: attic air leaks
Warm air leaking from the house into the attic is the primary cause of ice dams. Anywhere there is a penetration into the attic space (around wires, plumbing vents, light fixtures, chimneys, knee walls) there is the potential for air leaks. Even homes that are only a few years old may not be properly sealed. To avoid these types of problems and eliminate most ice dams, attic air leaks must be sealed with caulking or expanding spray foam. Inadequate insulation, especially near the eaves, is also a contributing cause of ice dams.
Minnesota Department of Commerce
Division of Energy Resources
On the inside of the attic, the warm moist air condensates on the underside of the roof. Ice and water will create dry rot on the sub-roof wood. In the photo to the right ice has formed under the roof as warm moist air leaks up from the house. The photo on the top left shows the ice dam that has formed and is commonly the cause of roof leaks, damaged gutters and facia.
Spray foam insulation sprayed to the underside of the roof will create both an air barrier and insulation between the warm moist air and the roof surface. Condensation can no longer occur. The photo on the left shows 5-6" of spray foam insulation sprayed to the underside of a roof.