We own a small two-bedroom apartment. The main bedroom has an en suite bathroom which we thought was a lovely feature when we purchased the property. However, the en suite is an interior room and does not have its own window. There is a fan on the ceiling but it isn’t very effective, judging by the level of condensation. What would you recommend we do to eliminate this condensation and protect against mould? There is an attic above the en suite.
Condensation and mould in bathrooms can be both frustrating and worrying, especially where there is no window to provide natural ventilation. Moisture build-up can damage finishes and fittings, lead to persistent mould and create unhealthy indoor conditions. It is a very common issue in apartments or houses with internal bathrooms. It requires the right mix of ventilation, heating, insulation and construction detailing.
Where condensation is getting worse, it is possible the existing extractor fan is underperforming or not suitable for the space. A common problem – especially where attic space is above the bathroom – is that ducting from the fan may not be connected properly or has become dislodged. In some cases, ducts vent into the attic rather than to the outside, which can severely reduce performance and create new moisture problems in the roof space.
Given the location of the en suite, below an attic, it may be possible to upgrade the fan and replace or reroute ducting to ensure it vents directly to the exterior.
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Management company approval may be needed if the works affect the common elements of the structure, such as the roof or external wall where the ducting exits the building.
Under Technical Guidance Document (TGD) Part F – Ventilation (2019), bathrooms without a window must have either continuous mechanical extract ventilation or an intermittent extractor fan. In an en suite that includes a toilet, the fan should be controlled by the lighting circuit and have a minimum 15-minute overrun (continues to operate after the light is switched off). The fan should be capable of extracting 15 litres per second and discharge to the outside.
The fan ductwork should be properly sealed and insulated to prevent condensation inside the pipe. Moisture-laden air travelling through cold, uninsulated ducts can condense before it ever reaches the external outlet. Water from condensation could potentially fall back through the duct toward the fan and ceiling. If there are bends or blockages in the duct, this can cause condensing water to build up inside the duct further affecting performance with an increased risk of leakage.
It is also worth checking for hidden moisture sources. Even a minor leak from a pipe joint, shower tray or bath seal can elevate humidity in a confined space. Although it is unlikely to be related to your issue, a roof leak above the en suite could also be contributing. These issues can sometimes go unnoticed until mould or damage appears.
Construction elements also matter. Ceilings above bathrooms should include a vapour-control layer, typically in the form of foil-backed plasterboard, which limits the passage of warm, moist air into colder roof voids. Insulation above should be continuous and tightly fitted, with no gaps or voids to reduce the risk of cold bridging.
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Recessed lights or loose attic hatches can also allow vapour into the roof space, where it may condense and cause longer-term problems. The roof space above the insulation layer should also be ventilated, typically via vents in the soffit or via proprietary vent tiles within the roof covering.
Electrical and plumbing systems should always be handled by qualified contractors, such as a registered electrician for extractor fan upgrades or a plumber to investigate possible leaks.
If, after improving ventilation and addressing any obvious defects, the issue persists, consider hiring a chartered building surveyor. A surveyor can carry out a full inspection, take temperature and humidity readings, assess insulation and ventilation and provide a detailed diagnosis with recommendations tailored to your property.
With the right attention to ventilation, insulation and moisture sources, most internal condensation issues can be successfully resolved.
Damian King is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland
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