What can you do about a damp wall from a neighbour's aircon?

Neighbour aircon condensation on an HDB wall: why it happens

When the room next door runs its aircon for long hours, the shared wall or the slab between the flats gets chilled and turns cold. Warm, humid air on your side touches that cold surface, cools and leaves water on it, the way a glass of iced water gets wet on the outside. That film of water is what feeds mould.

Local weather adds to the problem. Meteorological Service Singapore says Singapore has high humidity all year round, with a mean annual relative humidity of around 82%, and that relative humidity is often above 90% in the morning before sunrise. So there is plenty of moisture on your side whenever the wall next door is cold.

This is why condensation on a wall in an HDB flat often shows up as a broad, cool, damp patch with spots of mould, on the part of the wall or ceiling you share with the air-conditioned room. For clean-up, see our guide to mould on the ceiling. Cleaning removes what you can see, but the damp keeps coming back until the cold surface or the humid air on your side changes.

Condensation or a seepage leak?

Before you ask anyone to fix anything, work out which problem you have.

Signs that point to condensation:

• It comes and goes with the hours the aircon next door runs.
• It sits on the cool face of the shared wall or ceiling.
• It eases when that aircon has been off for a while.

Signs that point to a seepage leak:

• It follows water use or rain, not aircon hours.
• It often leaves a tide mark or a stain.
• It stays wet at one spot, even when the aircon next door is off.

If the signs point to a leak, our guide to water seepage through a wall explains what to check next. A dripping indoor unit on your own side is a different problem, since the water comes from the unit itself: see aircon leaking water.

It also helps to know who looks after what. HDB's guide to ceiling leaks says that as a flat owner, you are responsible for the upkeep and maintenance of your flat. For leaks from common property, it says to contact your Town Council.

What HDB does when you give feedback

HDB received about 240 cases of feedback in 2025 about condensation and mould linked to a neighbour's aircon, MND told Parliament, as CNA also reported. HDB inspects to find the cause and advises both households, for example on ventilation and mould-resistant paint.

A pattern is more useful than a single wet day, so share your record when you give feedback:

• the dates the damp appeared and how long it lasted
• photos of the patch, taken from the same spot each time
• the times you notice the neighbour's aircon running, such as overnight or all afternoon
• anything you have already tried on your side, such as airing the room

The inspection is about finding the cause and advising both households, so it helps to be clear about your own part too. Our guide to who maintains what in an HDB flat sets out what an owner looks after inside the flat.

Talk to your neighbour

Your neighbour may not know their aircon is affecting your flat. Raise it calmly at a time that suits them, show your record and ask, without blame:

• Does the aircon run for long hours, such as overnight or all day?
• Is it used with the doors and windows shut for most of that time?
• Is it set at a very low temperature?

If the answers suggest a link, agree on a small change, such as shorter hours or a more moderate setting, and check the wall again after a week or two. Your record will show whether the patch dries out, which beats arguing over a damp wall from a neighbour's aircon.

If talking does not help, you can give feedback to HDB. CNA reported that HDB "can consider providing an advisory" to the neighbours on the proper use of aircon, such as the length of use or temperature settings.

Reduce the damp and repaint on your side

You cannot change the wall next door, but you can make your side drier:

• Keep the room aired: open the windows when you can and run a fan to move the air.
• Keep wardrobes, beds and shelves a little away from the cold wall so air can reach it.
• Wipe water off the surface early, before it soaks in or mould takes hold.

HDB's care guide for walls lists facilitating cross-ventilation in the flat and re-painting the walls regularly among its wall care tips.

Repainting comes last. Paint put on a damp wall does not hold well, and mould can grow back under it, so wait until the damp has stopped and the surface is dry. When you do repaint, consider mould-resistant paint, which MND told Parliament it recommends flat owners consider as a preventive measure. Our guide to painting over mould covers how to prepare the surface.

Before you repaint, finish recording the problem. Your photos and dates are the evidence behind any HDB mould complaint about a neighbour, and a fresh coat of paint covers the patch you have been tracking.

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