Wet Attic in Winter: Condensation or a Roof Leak?

What attic condensation looks like

On cold nights, the underside of roof decking gets colder than the air in the attic. If that air carries moisture from the house, it condenses on the decking and on nail tips poking through. You may see frost on the nails on a cold morning, then drips onto insulation as it warms. Over time you'll see dark staining spread across the decking, rusty nails and damp insulation — often over a wide area, not in a single line from one spot.

How it differs from a roof leak

• Condensation: widespread, worst on the north side, shows up on cold mornings even without rain, rusty nail tips everywhere.
• Roof leak: localized, follows a path downhill from a specific point, tied to rain and wind.

Where the moisture comes from

Usually from the living space: bath fans and dryer vents that dump into the attic, gaps around recessed lights, attic hatches and plumbing chases, and high indoor humidity. Damp crawl spaces add moisture to the whole house. Too little attic ventilation — or soffit vents blocked by insulation — lets that moisture stay.

Fixes

• Vent bath fans and dryers to the outdoors.
• Seal air leaks from the house into the attic.
• Balance intake and exhaust ventilation. See ridge vents vs box vents and our attic ventilation service.
• Keep soffit vents clear with baffles.
• Address crawl space moisture. See crawl space encapsulation vs vents.

When decking is already damaged

Long-term condensation can delaminate or rot decking just like a leak. If the decking is soft, that section has to be replaced — see rotted roof decking repair.

A roof inspection that includes the attic will tell you whether you're dealing with a leak, condensation or both.

Frequently Asked Questions

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