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With a cold roof, the roof temperature stays the same as the temperature of the outside air. Cold roofing prevents the formation of ice dams, which can be a regular hazard with warm roofs. Ice dams are formed when snow melts on the roof and then refreezes on the roof overhang. As more snow melts, the water is caught by the ice dams, causing moisture to seep through the roof into the building, which can lead to structural damage. With cold roofs, as the snow doesn't melt, there is no question of ice dam formation and all the problems that go with it.
The traditional cold roof construction consists of a well-ventilated attic space with a pitched roof. Insulation is placed horizontally between the attic joists and the ceiling of the floor below. Any gaps in the ceiling are sealed to keep out moist air from the building. The roof rafters are fitted with a bitumen roof underlay that cannot be penetrated by air and moisture.
The space between the pitched rafters and the insulation is ventilated. The warm inside air rises up and escapes through the vents and the inside space fills up with the cold outside air. The temperature between the outside and the inside soon equalizes. There is some risk of condensation in these types of cold roofs.
There is also a cold roof design with an unventilated attic space. Here, the cold roof insulation is placed in the same manner as the ventilated cold roof and the ceiling is completely sealed to prevent entry of moist air into the attic space. A polypropylene breathable membrane is used as a roof underlay; this allows the inside moist air to diffuse through it, but keeps outside moisture from seeping inside. There are no vents and the moisture in the space exits through the breather membrane.
The risk of condensation is low with unventilated cold roofs. If there are any water pipes passing through the space, however, there may be a chance of them freezing over. This problem can be resolved by covering the pipes with insulation.
Apart from preventing ice dams, cold process roofing is good for maintaining the building ceiling insulation in a proper condition. By well-ventilating the attic space and getting rid of inside moisture, a cold roof reduces winter humidity inside the building. When the weather turns warmer, a cold roof makes itself useful by keeping the building cool.