Attic Frost and Condensation in Winter

Wet windows are the condensation you can see. The attic is where the same moist house air can do damage out of sight. Canadian government guides treat the two as one problem: indoor moisture, carried by air leaks, landing on cold surfaces.

How house air ends up as attic frost

NRCan’s Moisture Problems booklet gives the numbers. Air at 20 °C and 50% RH that leaks into a winter attic reaches 100% RH once it cools to about 9 °C, and then water or frost forms on the nearest cold surface, usually roof sheathing and nail tips. CMHC compares a heated house to a chimney: all winter it pushes warm air up through the top-floor ceiling.

NRCan’s Keeping the Heat In lists the usual leak points: bathroom and kitchen vents through the ceiling, plumbing stacks, chimney chases, wiring penetrations, and pot lights. Discoloured insulation is a tell-tale sign of an air leak below it.

How much is normal

NRCan says to check the attic during or just after a cold snap. Some frost is expected. Heavy buildup of about 10 mm (3/8 in.) or more means you should confirm the attic ventilation is present and not blocked by insulation. Checking again during or after a rainstorm helps separate roof leaks from indoor moisture.

What fixes it

1. Seal air leaks in the top-floor ceiling first. CMHC is explicit: adding ventilation alone can draw more moist house air into the attic and make it worse. Once air movement from the house is stopped, existing ventilation is usually enough.
2. Weatherstrip and seal the attic hatch.
3. Duct bathroom fans outdoors. CMHC says attic runs should be solid metal, insulated, and sloped to the outside, never dumping into the attic.
4. Use sealed pot lights, or avoid them on the top floor. Safety rules prevent sealing many standard fixtures.
5. Keep indoor humidity in range. The same habits that keep windows dry, like exhaust fans and a winter RH at or below CMHC’s 45% guideline, reduce the moisture available to leak upward.
6. Keep soffit vents clear of insulation.

If you see dripping, wet insulation, or black mould on the sheathing, get a qualified contractor to assess the attic. Don’t disturb vermiculite insulation, which NRCan flags as a health and safety issue.

Sources: NRCan, Keeping The Heat In: Section 5 Roofs and attics; NRCan, Moisture Problems; CMHC, About Your House: attic venting and ice damming; CMHC, Moisture and Air.

Originally published at https://dry-glass-notes.surge.sh/articles/attic-frost-condensation-winter.html (AI-assisted, sources cited). If this helped, zaps go to hollisvarrow3753@coinos.io.