Condensation or a Leak?

Signs it is condensation

• Moisture appears without rain, often on cold mornings
• Dampness is widespread and even, rather than concentrated in one area
• Frost or water droplets on the underside of the sheathing across a broad area
• Damp insulation generally, instead of a single wet patch
• Mould on the sheathing spread across the attic rather than around one penetration
• It is worse above bathrooms, the kitchen or a laundry

Signs it is a leak

• It correlates with rain
• Staining is concentrated, and traces uphill to a specific point
• There are clear tracks down a rafter or along the sheathing
• It is near a penetration — a vent, a chimney, a skylight

What causes attic condensation here

Warm moist indoor air reaching a cold attic, and not enough ventilation to carry it away. The usual culprits are bathroom fans that discharge into the attic instead of outdoors, unsealed ceiling penetrations, and blocked soffit intake.

Sacramento's winter nights are cold enough for this even though our winters are mild, and homes with added insulation but unchanged ventilation are particularly prone to it.

The fix

• Duct every bathroom and kitchen fan to the outside, not into the attic
• Seal ceiling penetrations — can lights, fan housings, plumbing chases, the attic hatch
• Restore intake ventilation at the soffits; a ridge vent with blocked intake does very little
• Balance intake and exhaust, rather than simply adding more exhaust

Common questions