Frost in the Attic: What Causes It and How to Fix It

What causes frost in an attic?

Attic frost forms when water vapor meets a surface below its dew point. In a Wisconsin winter, the roof deck and the nails poking through it are the coldest surfaces in the house. Winter air outdoors is dry, so the moisture almost always comes from inside your home.

Cooking, showers, laundry and breathing all add water vapor to indoor air. When that warm air escapes upward into the attic, its moisture condenses and freezes on contact with the cold wood and metal.

During a long stretch of cold weather the attic never warms enough to dry out. Frost can build up for weeks, which is why the problem often shows up all at once in late winter.

Where the moist air gets into your attic

The usual paths are small gaps in the ceiling below the attic, and they add up quickly. These are the places to check first.

• Bathroom and kitchen exhaust fans that dump air into the attic instead of venting outside
• Recessed can lights that are not rated and sealed for contact with insulation
• The attic hatch or pull-down stairs without weatherstripping
• Gaps where interior walls meet the attic floor, called top plates
• Holes around plumbing vents, wiring, ductwork and chimney chases

A bath fan pointed toward a roof vent but not connected to it still sends much of its moist air into the attic. The duct needs to run all the way to a roof cap or wall cap.

High indoor humidity makes every leak worse. Whole-house humidifiers set high, unvented clothes dryers and a lot of drying laundry all raise the amount of moisture that reaches the attic.

Is a little frost in the attic normal?

A thin coating on nail tips during the coldest weeks happens in many attics and usually dries off harmlessly. It becomes a problem when the frost is thick, covers the sheathing, or keeps coming back every cold snap.

Look for the signs of repeated wetting as well as the frost itself. Rusty nail tips, dark or stained sheathing, matted insulation and a musty smell all mean moisture has been building up for more than one winter.

What happens when attic frost melts

Attic frost causes damage when a warm spell or a sunny afternoon melts it quickly. The water drips onto insulation and through the ceiling, and homeowners often describe it as rain in the attic.

Melting frost can stain ceilings, drip from light fixtures and soak insulation so it insulates poorly. Over several winters, repeated wetting can lead to mold on the sheathing and soft, weakened roof decking.

Attic frost vs a roof leak vs an ice dam

Timing is the best clue. Frost drips show up during a thaw after a cold spell, often on a sunny day with no rain, and the wet spots tend to be spread across the ceiling.

A roof leak shows up during rain or snowmelt and returns to the same spot each time. An ice dam leak usually appears near the outside walls, under a ridge of ice along the eaves. Our guide to ice dams in Wisconsin explains that mechanism, and our guide to water stains on the ceiling covers how to trace a leak.

If you can see frost on the nails or the underside of the deck, condensation is part of the picture. That matters, because a roof repair alone will not stop frost from forming.

How to fix frost in the attic for good

The lasting fix works in order, from the source of the moisture to the attic itself. Ventilation helps, but it cannot dry an attic that keeps receiving moist house air.

1. Vent every bath and kitchen fan outside. Run insulated duct to a dedicated roof or wall cap, and make sure the fan actually runs during and after showers.
2. Air seal the attic floor. Seal the hatch, top plates, can lights and every penetration before adding insulation, because insulation alone does not stop air.
3. Bring insulation to depth, with baffles at the eaves. ENERGY STAR recommends about R-60 for an uninsulated attic in Wisconsin's climate, or R-49 if you already have three to four inches.
4. Balance the ventilation. Pair soffit intake with exhaust at the ridge. Our guide to ridge vents vs box vents explains how to size it.
5. Keep indoor humidity in check. Lower a whole-house humidifier setting during deep cold and run exhaust fans long enough to clear steam.

Skip powered attic fans as a frost fix. A fan that pulls air out of the attic can draw even more warm, moist air up from the house through ceiling gaps. Our comparison of turbine vents, power fans and ridge vents covers why.

For the bigger picture on how intake, exhaust and insulation work together, read attic ventilation 101 for Wisconsin homes.

What to do if you find frost right now

Leave the frost in place and avoid scraping it, since that only drops ice onto the insulation. If the frost is heavy, lay plastic sheeting over the insulation below the worst areas to catch meltwater, and keep stored items away from those spots.

Check whether your bath fans vent into the attic, and lower indoor humidity while it stays cold. Take dated photos of the frost, the fan ducts and any ceiling stains so whoever inspects the attic can see the pattern.

Not sure whether you are looking at frost, a leak or an ice dam? Call (608) 909-9109 to schedule a free on-site inspection that includes the attic.

Getting the attic fixed

Attic frost is a ventilation and air-sealing problem first, and it often shows up alongside ice dams and stained ceilings. A good inspection looks at the fans, the attic floor, the insulation depth, the soffits and the roof exhaust together, rather than treating one symptom.

Buckshot Exteriors is family-owned and has worked on roofs and exteriors since 1979, with offices in Madison, WI and Centerville, IA. Our attic ventilation work covers intake, exhaust and the fixes that come with it, and our attic repairs guide covers damaged sheathing and insulation. When the roof itself needs work, see our residential roofing services.

Call (608) 909-9109 to book a free on-site inspection, or use the contact form on any page if you would rather we call you.