What causes moisture in an attic?
Short answer. Most attic moisture comes from inside the house: humid air leaking through ceiling gaps, bath and kitchen fans that vent into the attic, and leaky ductwork. In Georgia summers, cold air conditioning ducts and air handlers in the attic can also sweat and drip, and the rest comes from roof leaks and poor ventilation that lets moisture linger.
Winter sources
- Air leaks from the living space.
- Exhaust fans that end in the attic.
- Indoor humidity from cooking, showers and humidifiers.
Summer sources, common in Georgia
- Sweating ducts: supply ducts carry air around 55 degrees, and when attic air is humid and the duct insulation is thin, torn or compressed, water condenses on the outside and drips onto the ceiling.
- Air handler and drain pan: a clogged condensate line or rusted pan overflows into the attic.
- Disconnected supply ducts: cold air blowing into the attic makes nearby surfaces sweat.
Roof related sources
- Leaks at pipe boots, flashing, skylights and damaged shingles.
- Wind-driven rain through vents.
- Poor ventilation that lets humidity sit against the deck.
Read the location
Signs include a musty smell, darkened or spotted sheathing, rusty nails, matted insulation and ceiling spots. Moisture around ducts points to HVAC, moisture under a vent or pipe points to the roof, and dampness across a whole slope points to ventilation and air leaks.
What to do
Fix the source first, then ventilation. Have an HVAC technician check duct insulation and the condensate system, and a roofer check penetrations and vents. Read heat, humidity and roof life and common causes of roof leaks.