Attic Condensation

Attic condensation is water that forms on the cold underside of a roof deck, nails, framing or underlayment when moist air contacts surfaces below its dew point. Over time it can cause frost build-up, dripping, stained ceilings, mould, rotting sheathing and rusted fasteners. It is one of the most common moisture problems in pitched roofs in cold and mixed climates.

Where the moisture comes from

Building science research identifies air movement, not vapour diffusion, as the main way moisture reaches attics. Warm, humid indoor air leaks through gaps in the ceiling: around light fixtures, attic hatches, plumbing and wiring penetrations, and leaky ductwork. Bathroom and kitchen fans that vent into the attic rather than outdoors add large amounts of moisture.

Night sky radiation

Roof surfaces radiate heat to the clear night sky and can cool below the outdoor air temperature, especially metal, membrane and shingle roofs in dry or high-altitude locations. The underside of the deck can then fall below the dew point of the attic air, causing condensation even when indoor moisture levels are moderate.

Ventilated attics

In cold climates, attic ventilation mainly keeps the roof deck cold (which also helps prevent ice dams) and removes moisture that enters from the house. Building codes in North America specify net free vent area as a ratio of the insulated ceiling area, ranging from 1:150 to 1:600; 1:300 is the most common and is typically allowed where there is an air barrier at the ceiling. Ventilation does not cure air leakage; sealing the ceiling is the primary control.

Unvented roof assemblies

Unvented (conditioned) attics are increasingly used, especially where ducts or equipment are in the attic. To avoid condensation, they keep the roof deck warm, for example with rigid insulation above the deck or air-impermeable spray foam against its underside.

Signs and remedies

Signs include frost on nail tips in winter, damp or blackened sheathing, and water stains after cold snaps. Remedies typically involve:

1. Air sealing ceiling penetrations and duct leaks.
2. Venting exhaust fans and dryers outdoors.
3. Reducing indoor humidity where it is high.
4. Correcting blocked or unbalanced attic ventilation.
5. Adding insulation above the deck in unvented designs.