What causes frost in an attic?
Short answer. Attic frost forms when warm, moist air from the house leaks into the attic and meets the roof deck and nail tips after they have cooled below freezing, usually on clear winter nights. The moisture freezes as frost, then melts when the sun warms the roof, dripping onto insulation and ceilings in a way that looks like a roof leak.
Why it happens in Georgia
North Georgia winters bring regular clear nights below freezing, especially in the mountains. Roofs radiate heat to a clear sky and can get colder than the air around them. If moist air is reaching the attic, the coldest surfaces collect frost first: nail tips, then the underside of the decking. It shows up most on the north-facing slope, which stays cold longest.
Where the moisture comes from
- Bath fans venting into the attic or into a loose duct that ends near the soffit.
- Air leaks through recessed lights, the attic hatch, plumbing and wiring holes in the wall top plates, and gaps around chimneys.
- Dryer vents disconnected in the attic or garage ceiling.
- High indoor humidity from humidifiers, unvented gas heaters, a full house, or a damp crawl space.
- Weak ventilation that does not carry moisture out before it settles.
Fixing it
- Make sure every bath fan, kitchen hood and dryer vents all the way outdoors through a sealed duct.
- Air seal the attic floor: foam around penetrations, a gasket on the hatch, sealed light fixtures.
- Confirm the soffit vents are open and the exhaust is balanced.
- Lower indoor humidity in winter.
Frost once on an unusually cold morning may be minor. Frost that builds up regularly leads to stained decking, mold and rusty nails over time. Read roof ventilation basics, and for the cold weather problem that really is about the roof, ice and water shield in Georgia.