Attic Mold Treatment

Black staining across the north-facing roof sheathing, caused by bath fans venting straight into the attic.

About the project

The owners noticed black staining across the roof sheathing while retrieving Christmas decorations. It covered most of the north-facing plane and stopped abruptly at the ridge — a pattern that tells you almost everything before you take a single reading.

The challenge

Attic mold is rarely an attic problem. It is a house problem that shows up in the attic. Warm, moist air from the living space rises, finds every gap in the ceiling plane, and condenses on the coldest surface it reaches, which in a Wisconsin January is the underside of the north-facing roof deck.

Here there were two contributors. Both bathroom exhaust fans terminated inside the attic rather than through the roof — dumping several hundred litres of shower moisture up there every week — and the attic hatch, recessed lights and top plates were all unsealed.

The result

We contained the attic access, HEPA-vacuumed and abrasively cleaned the affected sheathing, and applied a fungicidal treatment followed by a clear encapsulant over the cleaned deck.

Then the actual fix: both bath fans re-ducted in insulated pipe to proper roof terminations, the attic hatch weatherstripped and insulated, recessed lights swapped for airtight-rated fittings, and top plates and penetrations air-sealed before insulation was topped up to R-60.

We went back the following March to look. The deck was clean and dry, with no new staining after a full winter.