Windowless bathroom always damp? An exhaust fan done right

The fix for a permanently damp windowless bathroom is an exhaust fan that pushes air all the way to the outside of the building — sized for the room, fitted with a backdraft shutter, and left running 15–20 minutes after every shower. The part most people get wrong is the “outside” bit: a fan that simply blows into the ceiling void does not remove the damp, it relocates it into your roof space, where it does quieter and more expensive damage.

Middle bathrooms are everywhere in our service area. Intermediate terrace houses in Serdang and Seri Kembangan, and most apartment layouts in Balakong, Bangi and Puchong, put at least one bathroom deep in the floor plan with no external wall. The only ventilation is a small grille above the door — enough to satisfy the building plan, nowhere near enough to dry a tiled room that gets hosed down twice a day in our climate.

How to tell the room never dries

• Black mould keeps returning to the ceiling corner above the shower no matter how hard you scrub.
• Paint peels in sheets near the ceiling while the lower walls look fine.
• The bottom of the door swells and rots — we replace far more doors on windowless bathrooms than on ones with a window.
• A towel hung up overnight is still damp the next morning.

All four are the same problem: warm shower moisture with no way out, condensing on the coolest surfaces day after day.

The ceiling-void shortcut, and why it backfires

The cheapest installation is a fan in the plaster ceiling blowing straight up into the void with no ducting, and it feels effective for a few months. But when we open up ceilings for other jobs, we can tell within seconds which house has an unducted bathroom fan: the roof timbers above it are stained dark and the back of the plasterboard has gone soft. The moisture never left the house — it just condensed one floor up. Insist on ducting, rigid or flexible, that runs to an external wall grille or an eave vent, with a shutter that closes when the fan is off. That little flap matters more than people think: an open duct is a highway for mosquitoes and other people’s cooking smells to come straight back in.

Sizing and choosing the fan

Aim to change the room’s air about ten times an hour. A typical terrace-house bathroom of 1.5 m × 2 m with a 3 m ceiling holds roughly 9 m³, so you want a fan rated at least 90 m³ per hour — and one size up if the duct run is longer than about 3 m or has bends, because a fan’s rated airflow drops quickly once it has to push through ducting.

On top of the fan itself there is the wiring — a switched connection, often a new point in the ceiling. A bathroom is a wet zone, so this is not the place to tap a live feed off the lighting circuit yourself. All wiring work we arrange is carried out with our licensed partner contractor, Suruhanjaya Tenaga compliant, and typical electrical jobs in this range run RM 80–600 depending on the switching and cable run.

Running it so it actually works

• Switch it on when the shower starts and leave it running 15–20 minutes after — a run-on timer module does this automatically and is worth the small extra cost.
• Give it make-up air. A fan can only push out what can leak back in; if the door seals tightly and the over-door grille was painted shut years ago, the fan just hums without moving much. Leave the door slightly ajar after showering.
• Run it after mopping too, and leave the bathroom door open whenever the room is not in use.

If mould, peeling paint or a rotted door has already taken hold, fix the ventilation first and redecorate after — otherwise you are repainting on a schedule. An exhaust fan install pairs naturally with a repaint or door replacement in a single visit; see our services and price guide for where each job sits.