Attic Ventilation & Fungal Growth: Why PNW Bath Exhaust Fans Can Rot Your Roof
In wet climates, the attic is an exhaust system. When warm indoor moisture enters the attic and cannot escape, it condenses on cold plywood roof decks, turning framing black with fungal growth.
Zachary Bradley
Licensed Home Inspector · WA Lic #26014533
Key Inspector Takeaways
- ✓Bathroom exhaust fans venting into attic insulation instead of dedicated roof caps cause massive roof rot.
- ✓Balanced ventilation requires equal intake airflow at soffit eaves and exhaust airflow at the roof ridge.
- ✓Insulation baffles (accu-vents) must keep blown-in fiberglass from blocking perimeter soffit intake vents.
- ✓Blackened roof sheathing around roofing nails indicates chronic condensation during winter months.
The Thermal Physics of a Pacific Northwest Attic
During winter, your home is heated to 70°F with elevated indoor humidity from cooking, showers, and breathing. Warm air rises through ceiling penetrations into an unheated attic where outdoor temperatures hover near freezing.
When moist air contacts cold plywood or OSB sheathing, water condenses into droplets. Over several winters, this continuous dampness rots roof sheathing and spawns toxic mold colonies.
The Infamous 'Dump into Attic' Bath Fan
One of the most frequent defects inspectors find is flexible vinyl or foil bathroom fan ducting terminated directly beneath attic insulation or pointed toward a passive roof vent without a mechanical seal. Every hot shower pumps 2 cups of water vapor directly into the attic structure.
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