Symptoms
- Frost or ice crystals visible on the underside of the roof deck or nails
- Water dripping or staining insulation below the roof in winter
- Dark staining on roof sheathing or rafters
Likely causes
- CommonWarm, humid indoor air leaking into the atticGaps around light fixtures, the attic hatch, or ductwork let conditioned air escape into the cold attic.
- CommonInadequate attic ventilationWithout enough intake and exhaust airflow, moisture that does get into the attic has nowhere to go.
- PossibleBathroom or kitchen exhaust fans venting into the attic instead of outsideThis dumps a large amount of moisture directly into the attic space.
What to do
- On a cold day, check the attic for frost, dripping, or dark staining on the underside of the roof deck.
- Seal obvious air leaks from the living space into the attic, such as around the attic hatch, recessed lights, and duct penetrations.
- Confirm bathroom and kitchen exhaust fans vent all the way outside through a duct, not just into the attic space.
- Check that soffit vents aren't blocked by insulation and that there's a clear path for air from soffit to ridge or roof vents.
- Add or top up attic insulation after air sealing, which helps keep the attic consistently cold and dry rather than warm and moist.
- Run a whole-house or bathroom exhaust fan more during humid periods to reduce the moisture available to migrate upward.
- Have an insulation contractor address ventilation balance if frost or staining continues after these steps.
Tools and parts
- Flashlight
- Caulk or spray foam
- Dust mask
Attic hatch weatherstrippingSearch: “attic access door insulation kit”
Attic insulationSearch: “blown-in attic insulation”
When to call a pro
Call an insulation contractor to air-seal and balance ventilation if condensation or frost continues, since it can rot the roof deck over time.
Safety
Use caution
Wear a dust mask and step only on joists or a plank in the attic, never directly on the drywall ceiling below.