The neighbour’s aircon is cooling your wall below the dew point

Black mould on a Singapore bedroom wall is often condensation, not a dirty wipe. A neighbour’s 18°C aircon cools the shared slab. The warm humid air in your room meets that cold face and drops water. MDF wardrobe backs and a leaky bathroom exhaust do the same. Bleach and a coat of paint fail in about three weeks. A spore kill, an anti-condensation primer and a drier room are S$180 to S$750.

Air at 80 percent relative humidity has a dew point close to the temperature you are living in. If a shared bedroom wall or a floor slab is cooled by a neighbour holding 18°C, that face drops below the dew point. Water appears in the paint pores. Aspergillus and Cladosporium use it. The mould is black or dark green, often behind the wardrobe where air does not move. People call it a leak. A leak is wet when the neighbour showers, and it fails the meter and tissue tests in the leak guide. Condensation is wet on humid afternoons and on the cold face only. Feel it. A slab leak and a dew-point face are different repairs, and painting both the same way wastes the paint.

Unsealed MDF wardrobe backs are a second machine. MDF wicks the condensate and holds it against the wall. A false ceiling near a bathroom exhaust that has come loose does the same with steamy air. The fix starts by emptying the cupboard, finding whether the exhaust actually leaves the flat, and sealing or replacing the MDF back so it is not a sponge. Chlorine bleach on a porous surface kills the surface colour and leaves the root. In three weeks the spot is back, and the bleach has damaged the next coat of paint. The sequence that holds is a quaternary ammonium spore kill, drying, a thermal anti-condensation primer, and then paint. The room needs a dehumidifier or enough aircon runtime, and the aircon trunking that sweats needs insulation so it is not a second cold face above the wardrobe.

If the moisture meter says the wall is wet from inside the slab, stop the mould treatment and open the leak guide. Biocide on a leak is perfume. Homey Furnishing treats the condensation case and will say when the photos look like a leak instead. Move the wardrobe 50mm off the wall when it goes back. A cupboard screwed tight to a cold face will grow the same mould in the same season.

Homey Furnishing Pte. Ltd. (ACRA UEN 202018148K), 27 Mandai Estate, #06-10 Innovation Place, Singapore 729931, carries the permit-exempt portion of this work. V On Construction Pte. Ltd. (ACRA UEN 201611708H), listed on the HDB Directory of Renovation Contractors, carries any scope that needs an HDB renovation permit. Those two invoices are not interchangeable. Biocides and primers are used as labelled. This is not a renovation permit unless the wall is opened. Inter-floor water is the leak guide, including GRA and BMSMA section 101.

Spend ten minutes with a torch, your phone camera, and a tape before you buy a product or book a hack. Write down what you see. The safe checks below do not ask you to open a live wire, cut a pressurised pipe, or lean out of a window. A cheap humidity reading and a hand on the wall are the start. Note the time of day and whether the neighbour’s aircon is on. If you have a moisture meter, compare the mouldy face with an internal wall.

Stop at the red-flag column. Stop if the wall is saturated from a leak, if bleach fumes are already strong in a small room, or if the plan is paint over a soft MDF back. The reason DIY fails is almost never a lack of effort. It is that the tool or chemical makes the next professional repair harder, or it hides a leak, a live conductor, or a deposit line until the damage is larger than S$180 to S$750 for a room. A leak behind the mould is a different budget..

This is the sequence a careful technician uses so you can tell a shortcut from a finished job. Tools stay in the van. Readings and photos go on the same worksheet as the price. Quaternary ammonium wash, drying time, thermal anti-condensation primer, paint, and insulation for sweating trunking. The wardrobe back is sealed or replaced. Masks and ventilation stay on during the wash.

1. Step 1. Make the area safe and confirm the symptom still matches the photos. For The neighbour’s aircon is cooling your wall below the dew point, that means Stop if the wall is saturated from a leak, if bleach fumes are already strong in a small room, or if the plan is paint over a soft MDF back.
2. Step 2. Measure before touching the finished surface. A cheap humidity reading and a hand on the wall are the start. Note the time of day and whether the neighbour’s aircon is on. If you have a moisture meter, compare the mouldy face with an internal wall.
3. Step 3. Set protection and lay out the tools that belong to this material. Quaternary ammonium wash, drying time, thermal anti-condensation primer, paint, and insulation for sweating trunking. The wardrobe back is sealed or replaced. Masks and ventilation stay on during the wash.
4. Step 4. Carry out the repair in the order the material allows. bleach and paint are used on live mould, the wardrobe goes back tight to the cold face, or a leak is treated as condensation is the failure this order is written to avoid.
5. Step 5. Test in place. The spots are killed and primed, the back board is not a wick, the cupboard stands off the wall, and you know whether to keep a dehumidifier running. A leak, if found, is written as not treated.
6. Step 6. Clean, label the photographs against the invoice lines, and leave the statutory openings, weep holes, access panels and labels visible.

The order matters because bleach and paint are used on live mould, the wardrobe goes back tight to the cold face, or a leak is treated as condensation is what happens when a step is rushed. Handover is a measurement, not a handshake. The spots are killed and primed, the back board is not a wick, the cupboard stands off the wall, and you know whether to keep a dehumidifier running. A leak, if found, is written as not treated. If the reading or the water test fails, the visit is not closed and a second line is priced before anyone leaves. Permit work, if the diagnosis has moved into hacking, waterproofing, or a new layout, stops here and is repriced through V On Construction Pte. Ltd. UEN 201611708H.

A single wall is the small band. A wardrobe back and a ceiling are the larger one. A leak repair is not in these bands. Prices below are 2026 Singapore bands for the described scope. They are not a coupon and they are not a guess after an unopened door. A second trade, such as a PUB licensed plumber or an EMA licensed electrician, is named when the law requires it rather than buried inside the labour line. Compare rows by what they include. The planning-life column belongs to that row only.

2026 SGD scope bands

Brand, material and lifespan

If the sofa against the same wall is mouldy, use the leather guide after the wall is dry.

Three worked jobs show how the same symptom is priced differently in a new BTO, an older resale flat, and a private condo or landed house. The totals are the lines a household actually paid, not a national average. Permit-exempt invoices are issued by Homey Furnishing Pte. Ltd. UEN 202018148K.

Case 1: 4-room BTO bedroom in Tengah

The shared wall behind the wardrobe was cold in the afternoon and dry to a meter. Spore kill, primer and a new sealed back were S$420. The cupboard was reset 50mm off the wall. Three weeks later the paint was still sound. Bleach had not been used.

Case 2: resale flat in Bedok

Bleach had been applied twice. The wash had to neutralise that surface before primer. S$380. The owner was told another bleach would start the clock again. A small dehumidifier went in the closed room.

Case 3: condo in Queenstown

The stain followed showers, not the aircon hour. Condensation treatment was refused. The leak guide’s meter test led to the upper bathroom. The lower ceiling was not painted until the membrane was addressed.

Send a photo of the spot and say whether it is wet after showers or after the neighbour’s aircon. Send the photos and the measurements from the ten-minute check. The reply prices the band on this page against what the photos actually show.

• Check it is not a leak
• If the sofa is mouldy too
• Bathroom mould