Why does an inspector note frost on the underside of roof sheathing in cold weather?

Frost on the underside of roof sheathing is condensation that froze, and it forms only where warm moist air reaches a surface below freezing. In a vented attic the sheathing is deliberately cold, so the reportable half of the equation is the warm moist air, and it is arriving from the house through breaches in the ceiling plane — an unsealed attic hatch, recessed lights, top plates, plumbing and wiring penetrations, or a bath fan discharging into the attic rather than outdoors. Inadequate or unbalanced ventilation compounds it by failing to carry the moisture away. The consequence appears on the thaw: the frost melts, wets the sheathing and the insulation below, and repeated cycles produce staining, delamination and mould. The inspector reports the frost, looks for the air leakage paths, and checks that exhaust fans terminate outdoors.

Frost forms where warm moist indoor air reaches a cold surface, so its presence means air is leaking from the house into the attic and that ventilation is not carrying the moisture away.

Attic frost results from indoor air leaking through the ceiling plane and condensing on cold sheathing. Contributing factors include unsealed penetrations, exhaust fans terminating in the attic, and inadequate or unbalanced ventilation.

Frost Is Indoor Air That Got Out. The cold deck is doing its job — the leak in the ceiling is not.

Check where bath and kitchen exhausts terminate. A fan discharging into the attic delivers moisture directly and is among the most common causes.

• •Reading attic frost as a roof covering failure
• •Addressing ventilation without sealing the air leakage paths
• •Missing an exhaust fan terminating inside the attic