Key points
- Frost forms when warm, moist air from the house reaches the cold roof deck.
- When it melts, it drips onto insulation and ceilings and can look like a roof leak.
- Common causes: bathroom fans venting into the attic, air leaks around lights and hatches, and blocked ventilation.
- Repeated wetting leads to dark staining and mold on sheathing — see mold in the attic.
How it happens
On cold Utah nights the roof deck gets very cold. Warm, moist air from cooking, showers and breathing rises into the attic through gaps and fans. When it touches the cold wood and nails, it freezes into frost. On a sunny day it melts and drips.
Signs to look for
- Water spots on the ceiling of the top floor after a cold snap, with no rain or snowmelt.
- Rusty nail tips poking through the roof sheathing.
- Damp or matted insulation.
- Dark staining on the underside of the roof deck.
Common causes
- Bath and kitchen fans that vent into the attic instead of outside — see bathroom exhaust fan mold prevention.
- Air leaks around can lights, attic hatches, plumbing and wiring.
- Blocked or too little ventilation at soffits or ridge.
- Furnace or humidifier settings keeping indoor humidity high — see furnace humidifier leaks.
Ice dams are a related winter problem at the roof edge — see ice dams and attic mold.
What we do
We find wet insulation and ceilings with a FLIR thermal camera and moisture meters, dry or remove what’s wet, and remediate any mold under containment. Fixing the cause — venting, air sealing, ventilation — is the job of a roofer, insulation contractor or HVAC contractor. In Provo, see mold remediation in Provo.
Common questions
Is a little frost in the attic normal?
A light frost on nail tips during a hard cold snap can happen. Heavy frost on the sheathing, or melting that drips onto insulation, points to too much warm, moist air getting into the attic.
Will it go away in spring?
The frost will, but the wetting it caused may not. In spring, check the underside of the roof deck for dark staining, look for damp or flattened insulation, and watch the top-floor ceilings for stains.
Can a furnace humidifier cause attic frost?
It can contribute when it is set high during very cold weather. Turn it down when windows start to fog — that is a sign the air is wetter than the house can handle. See ideal indoor humidity.
Stains on the top-floor ceiling?
Call (385) 250-2825 — answered live 24/7 by the Vault crew.
Mold remediation in Provo · Ceiling water damage · Contact us