Condensate Leak and Ventilation Complaint in a New Build
A client in a newly built home reports persistent condensation on bedroom windows and a musty smell in the main bathroom. The house has a ducted reverse-cycle system with a return air grille in the hallway, two bathroom exhaust fans ducted to the roof space, and a continuous-flow hot water unit in a cupboard. The plumber notes the exhaust fan ducts terminate in the roof space rather than through the roof, the bathroom has no window, and the reverse-cycle condensate drain runs to a tundish inside the cupboard. The client has not used the exhaust fans consistently. Winter outdoor temperatures are around 5°C and indoor humidity is high. The plumber has a hygrometer, a manometer, and access to the roof space.
1. Identify the contributing causes of the condensation and musty smell, distinguishing building, mechanical, and occupant factors.
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Causes split into three groups:
Building/installation defects
- Exhaust fan ducts terminating in the roof space discharge moist air into the roof cavity, which can migrate back into the home and cause musty smells and possible roof damage.
- A bathroom with no window relies entirely on mechanical exhaust, so a non-compliant duct is a serious defect.
Mechanical
- The reverse-cycle return air in the hallway may draw moist air from the bathroom area rather than exhausting it.
- The condensate drain to a tundish inside the cupboard may be leaking or blocked, adding moisture.
Occupant
- Inconsistent use of exhaust fans during showers.
Climate
- 5°C outside and high indoor humidity create a strong dew-point risk on single-glazed windows.
The musty smell is most consistent with roof-space moisture and possibly damp building materials.
2. Recommend the corrective actions and how you would verify them.
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Corrective actions:
- Re-route both exhaust ducts through the roof with appropriate weatherproof terminations, insulated where required, and short, well-supported runs.
- Verify fan airflow with a manometer or anemometer against the manufacturer's specification and the required air change rate for a windowless bathroom.
- Inspect the condensate drain for leaks or blockage, correct the fall, and confirm discharge to an appropriate point.
- Check the return air path so bathroom air is not recirculated into bedrooms.
- Advise the client on consistent fan use and ventilation habits.
Verification:
- Measure indoor humidity with a hygrometer before and after; target a reasonable indoor RH.
- Confirm ducts terminate externally and are sealed.
- Re-check after a week of normal use.
Document all findings and provide a report to the client and builder.