NHIE exam practice question · Roofing
A roof-mounted exhaust termination carries a backdraft damper for two reasons: to stop conditioned air escaping when the fan is off, and to stop outside air, rain and snow entering the duct. A flap stuck open defeats both. The consequence that matters to the building is what happens inside the duct: warm humid air from the bathroom meets cold duct wall and outside air, condenses, and the resulting water runs back down the duct toward the fan housing and the ceiling below. Over a season that produces staining, insulation wetting where the duct passes through the attic, and in cold climates ice accumulation in the duct itself. The inspector reports the inoperative damper as a moisture-entry condition; it is visible from the roof surface or by observing the termination, so it is within a visual inspection's reach.
With the flap held open, outside air and precipitation enter the duct freely, and moisture condensing inside runs back toward the fan and the ceiling. Preventing that reverse flow is the damper's function, so an open flap is reported as a moisture-entry defect.
Bathroom exhaust must discharge to the exterior, not into an attic or soffit, and terminations include a backdraft damper. Warm moist air condensing inside a cold duct is a common source of ceiling staining and attic moisture in cold climates.
The Flap Is a One-Way Door: moisture out, weather stays out. Stuck open, the door swings both ways and the weather walks in.
Check where the duct discharges as well as whether the damper works. Terminating into an attic or soffit is a more serious and very commonly tested defect.
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