How frost forms
Warm interior air carries moisture: cooking, showering, laundry, breathing, even houseplants contribute. In Calgary winter that warm moist air rises and seeks any path into the attic. When it hits the cold underside of the roof sheathing, it condenses. Below freezing, it forms frost.
Where the air actually leaks
Bypasses around plumbing stacks, recessed light fixtures that aren't airtight-rated, an uninsulated attic hatch, top plates of interior walls, bath fans venting into the attic instead of through the roof, and unsealed wiring penetrations. These add up to many small holes that move surprisingly large volumes of moist air.
Normal vs warning
A light dusting of frost on roofing nails on extreme cold days is normal and harmless. Widespread frost across the sheathing is a warning. When the next thaw arrives — often during a Chinook — that frost melts. Where it goes is the problem: it stains ceilings, soaks insulation, and over years can rot sheathing.
Why more ventilation isn't the answer
Adding more attic ventilation doesn't address the source; it only carries away some moisture if the ratio is right. The right fix is air-sealing the ceiling plane — caulk and foam at top plates, airtight-rated and properly sealed recessed lights, weatherstripped and insulated attic hatch, bath and dryer vents ducted through the roof or wall, and sealed wiring/plumbing penetrations.
Humidity management
Secondary causes are interior humidity that's too high and inadequate ventilation of moisture-generating rooms. Target indoor humidity in Calgary winter is 30–35%. A $20 hygrometer pays for itself many times over. Bath fans should run 20–30 minutes after showers; consider a humidity-sensing fan.
What inspection documents
Attic frost is documented along with the suspected source bypasses. Recommendations distinguish between immediate air-sealing items, longer-term envelope improvements, and humidity management. A construction-informed inspector frames the fix in terms of root cause, not symptoms.
