During periods of extended cold weather, moisture from the living space or outside sources carried by air movement can accumulate as frost on your attic’s roof sheathing or roof trusses’ underside. You may see ice inside your attic, in or on the roof vents and on or around the soffits. The degree of frost accumulation is related to the relative humidity of the house air, air movement rate into the attic and the length of the cold spell. When the temperature rises above freezing, or the temperature rises and combined with an intensely sunny day, the frost or ice begins to melt faster than the attic’s ventilation system can exhaust the accumulated moisture — this is called attic rain.
Whether or not this results in water leakage into the living space varies with circumstances. Attics are designed to manage small amounts of moisture accumulation. Typically, when the frost or ice changes to liquid water and then to water vapour, it is absorbed into the air and is exhausted harmlessly by attic ventilation to the outside. It may be absorbed by the insulation or the framing and released slowly. It may pool on the polyethylene at the ceiling and evaporate harmlessly.
If factors are present which lead to large amounts of frost or ice in the attic, it can result in leaking into and damaging the home’s interior.
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