06/16/2026
There is one bathroom fan shortcut that quietly rots the roof from the inside.
The International Residential Code (M1505.3) requires bathroom exhaust to terminate outside the building, not in an attic, soffit, or crawlspace. The reason is simple. A standard 8-minute shower releases close to a quart of water as v***r. That v***r rises through the fan, hits cold attic air, and condenses on the underside of the roof deck. Within a season, you get dark staining on the sheathing. Within two, you get soft spots and the start of black mold.
Repair numbers from restoration contractors land in the same range every time. Mold remediation for a 200-square-foot attic section runs $1,500 to $3,500. Replacing rotted sheathing adds $2,000 to $4,000. Most homeowners insurance policies exclude this damage outright, because insurers classify long-term moisture as a maintenance issue, not a covered loss.
The fix is a rigid 4-inch metal duct that runs in a continuous downward slope to a wall cap or roof cap rated for exhaust (not a passive attic vent). Seal the joints with UL-181 foil tape, not cloth duct tape. Slope the duct 1/4 inch per foot toward the exterior so condensation drains out instead of pooling.
If you can already see frost on attic nails in winter or a damp patch around the fan housing on the ceiling, the moisture cycle has started. Pull the duct out of the attic and reroute it to the exterior before next summer. Waiting until you smell mildew means waiting too long. [2SXXF]
