White powder on the wall: salt, chalky paint or mould?

Three likely causes

A white, powdery patch on a wall often appears near a bathroom, around a window or on an external wall. Before you scrub or paint over it, work out what it is, because each cause points to a different fix.

Three common causes are:

• Salt deposits (efflorescence). Salt left behind as water dries out of the plaster or concrete, usually as white crystals or a crusty bloom.
• Chalky old paint. Aged paint that turns powdery and rubs off on your finger.
• Mould. Some mould is white or grey and can pass for powder at first glance.

Salt and mould both send you looking for damp, so the real question is often where the water is coming from. Chalky paint is mainly a surface issue, usually fixed by cleaning and repainting. The table below sets out the signs that tell them apart.

How to tell them apart

You do not need tools for a first check. Look closely at the patch, touch it with a dry finger and note where it sits. Those three checks usually point to one cause.

If you are still unsure, wipe a small area clean with a dry cloth and check the same spot over the following weeks. Salt that returns there suggests moisture is still moving through the wall.

Salt deposits: what efflorescence means

A technical bulletin from the National Institutes of Health (NIH) Office of Research Facilities describes efflorescence as a deposit of soluble salts brought to the surface by water. In plain terms, water carries salt out of the wall and leaves it behind when it dries. The bulletin says efflorescence is not harmful in itself. The concern is what it tells you: NIH says persistent deposits may point to excess moisture or rising damp.

That is why efflorescence on wall surfaces that returns to the same spot deserves more attention than a single bloom after a leak that has since been fixed. To remove it, the NIH bulletin says to wash it off gently and avoid wire brushes. A soft, damp cloth is enough for most patches.

If the salt comes back, the next step is finding the water. Check the other side of the wall, nearby pipes, window frames and the bathroom, and read our guide on water seepage through a wall for how to trace the source.

Chalky or peeling paint

Chalky paint on wall surfaces is usually old paint breaking down. Run a dry finger across it: if a fine, even powder comes off in the paint colour, the paint is the source rather than the wall behind it.

HDB's walls guide says to repaint walls regularly, and it covers cracks among the wall issues it lists. A powdery finish is one sign that a wall is due for a fresh coat.

Also check for bubbling, flaking or peeling, especially near a bathroom or external wall. Those signs can mean moisture is getting under the paint, and a new coat may not hold until the wall is dry. Our guide on wall paint peeling explains how to deal with that.

If it is mould

White fluffy mould on wall corners, behind wardrobes or near the bathroom can look like powder from a distance. Up close, mould tends to look fuzzy or spotty, and it can be white, grey, green or black. Unlike salt, it does not feel dry and gritty.

HDB's walls guide says to keep rooms well ventilated, with cross-ventilation. In practice, that means opening windows on opposite sides when you can, running the bathroom exhaust fan if you have one and leaving a gap between furniture and walls.

For cleaning steps, see how to remove bathroom mould. If mould keeps returning after cleaning, look for a hidden leak or seepage behind it.

When it comes with cracks or bulging

White powder together with cracks, rust stains, bulging or flaking concrete is a different matter. Those signs need a professional look, because the problem may be in the concrete rather than on the surface.

HDB's home care guide on spalling says spalling is caused by carbonation and that a humid environment speeds it up. The same guide says to keep the home well ventilated and to paint regularly. If you see these signs, read our guide on spalling concrete and have the wall inspected before you repaint.

Questions