A damp bedroom wall has one of three sources: water climbing out of the ground, water forming on cold plaster, or a leak close by. So spend ten minutes looking before you spend anything else. Then the remedy chooses itself.

Run a finger along the skirting board, then along the window reveal. Powdery plaster low down and beads of water up high point in opposite directions, though a damp bedroom wall can carry both marks at once.

Ten minutes with a torch, reading a damp bedroom wall

Start at the skirting board and press the plaster with a thumb. If it crumbles and leaves a chalky powder, water has been travelling up through the masonry for a while.

Then move to the window reveal and to the corner shared with the outside wall. Beads of water on cold plaster belong to the air in the room instead. A damp bedroom wall of that kind eases with better airing and a gap behind the furniture.

Next, pull the bed and the wardrobe half a metre clear. Still air against cold plaster is where a damp bedroom wall most often begins.

Finally, step outside with the torch. A blocked gully, a cracked render or a flower bed piled above floor level explains many a damp bedroom wall. Our article Condensation or rising damp: how to tell the difference sorts the signs into three columns.

Ground water, salt and the low band

Brick, stone and mortar carry microscopic channels, and water travels up them with nothing pushing behind. The effect is capillary action, the same thing that carries tea into a lump of sugar. Our article ATE: anti-damp device for damp walls and rising damp treatment follows that climb from the footings.

Water fed this way draws a fairly level line across the plaster, which is how a damp bedroom wall of this kind gives itself away. The water then evaporates at the surface and the salts it carried stay behind. They crystallise, they swell, and they push the render off from within.

Salt also takes up moisture from the air on its own account. So the white bloom keeps returning for a while, as our article Buying a house with damp walls: what to check explains.

Treating the masonry behind the bed

Airing a room changes nothing on a wall fed from the ground. Neither does a dehumidifier, since it works on the air. So treating a damp bedroom wall of that kind means treating the masonry.

The ATE is an electromagnetic polarity inverter. Plugged into an ordinary socket, it sends very low frequency electrical pulses through the masonry, without a break. Its design principle comes from electro-osmosis. The pulses act on the electrical charges where the water meets the wall of the pore, so they work against the rise and favour the return of the water towards the footings.

We put that no higher than a design principle. Nothing is drawn out of the wall and nothing is heated, because the drying happens by evaporation. So results show between 18 and 24 months after fitting.

The ATG follows the same principle as a geomagnetic polarity inverter, with no supply at all. Our guide Persistent musty smell: causes and useful actions sets the two ranges side by side, while Damp wall after renovation: why the problem comes back covers the route that leaves the room intact.

Choosing the model that covers your walls

Each unit works over a radius, measured from the point where you fix it. So measure the walls you want to treat, then pick the model whose radius covers them from a central position.

Model	Range	Price
ATE LC15	7.5 m radius, 15 m across	€1,290
ATE LC30	15 m radius, 30 m across	€1,590
ATE MAX	30 m radius, 60 m across	€1,990

Every unit draws 0.75 W on a 12 V supply and measures 322 x 232 x 32 mm. Each one carries a 10-year manufacturer warranty. All three are made in France, and delivery costs nothing. The ATG follows the same design principle with no mains supply, which suits a wall with no socket within reach.

Fitting the unit yourself

Fix it to a load-bearing wall or a partition wall, between 30 cm and 1 m above the floor. Aim for the centre of the area to treat. Two wall plugs and a socket, less than an hour, no tradesman.

Never fix it to plasterboard and never to insulation. The back of the unit has to touch the masonry. Keep 1.50 m between the unit and any other electrical equipment.

If the bottom of the wall carries a waterproof cement render, cut that render back over ten centimetres or so. Then the water reaches the air and evaporates, instead of sitting trapped behind a sealed skin.

Habits that keep a bedroom dry

Open the window wide for ten minutes, morning and evening, even in January. Short and wide beats a window left ajar, because the room loses far less heat.

Then dry the washing somewhere else, and put lids on the pans next door. A loaded clothes rack releases litres of water into the house.

Leave a hand’s width behind the wardrobe on any outside wall. Air moving across the plaster does half the work of drying a damp bedroom wall.

Habits alone will not dry a wall fed from the ground, though they keep the rest in check. Decorate last, since lime plaster and mineral paint let the wall carry on releasing water. Our guide Blistering paint on a wall: understand the issue before repainting runs through the choices.

Frequently asked questions

Why is the plaster powdery near the skirting board?

Water rising through the masonry carries mineral salts to the surface, where they crystallise and break the plaster up. That powdery band is the usual sign of water coming from the ground.

Does a damp bedroom wall always come from the ground?

No. Water forming on cold plaster and small leaks each leave their own mark, and together they are more common than a climb from the ground. Height and shape tell them apart.

Where does the unit go in a bedroom?

Fix it to a load-bearing wall or a solid partition, between 30 cm and 1 m above the floor, as near the centre of the area as possible. Keep 1.50 m from other electrical equipment.

How long does a damp bedroom wall take to dry?

Where the water comes from the ground, visible results come between 18 and 24 months. A thick stone wall sits at the slower end of that range.

Should I redecorate a damp bedroom wall straight away?

Wait until the wall has spent months bare, with air moving over it. Paint applied to a wall that still holds water lifts within a season.

Before you order

The unit treats rising damp and capillary damp, and nothing else. It does nothing for a leaking gutter, for driving rain, or for poor ventilation. Those three causes together remain more common than capillary damp, so rule them out first.

Rather than guess, send us two or three photos. One of the bottom of the wall, one of the room, one of the outside face. We read them and tell you which model fits, or tell you plainly that your problem lies elsewhere.

See the ATE and ATG models · free delivery, 10-year warranty, made in France