What the pattern can mean
Water near an exhaust duct may come from warm moist air condensing inside a cold duct, a disconnected joint, poor exterior termination or roof leakage at the cap.
Why timing matters
Typical trigger: Bath use during cold weather or rain at the roof cap.
Moist exhaust air can cool below dew point and drain back through low points or failed joints.
Evidence that increases the fit
- The symptom follows showers in cold weather
- The duct sags or lacks insulation
- Moisture is inside the duct or at a joint
Evidence that weakens it
- Rain alone reproduces the leak
- The fan does not operate
- The duct is dry while roofing around the cap is wet
Safe first checks
- Run the fan and verify airflow at the exterior outlet if safely accessible
- Record shower duration and outdoor temperature
- Inspect visible duct slope and joint condition from a safe platform
Important boundary
Do not vent moist air into the attic or detach a powered fan without isolation.
Professional handoff: A ventilation or roofing professional may be needed when the duct route and roof termination both require correction.
Primary references for this topic family
These references ground the general building-science or safety context. They do not diagnose a specific house, replace local requirements or certify a repair.
US Department of Energy · government retrofit guidanceAttic Air Sealing GuideAttic bypasses, air leakage and retrofit sequencing.Verified 2026-07-20 · review due 2027-01-20Open source ↗US Environmental Protection Agency · government guidanceMoisture Control Guidance for Building Design, Construction and MaintenanceMoisture sources, transport, design, construction and maintenance controls.Verified 2026-07-20 · review due 2027-01-20Open source ↗
Observe→Compare triggers→Repair source→Verify outcome
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