interior wall damp proofing succeeds or fails on one question: where does the water come from? So answer that before a board, a membrane or a tin of paint comes near the room. An internal wall takes water from the ground beneath it, from a leak close by, or from the air in the room.

Each source calls for a different answer, and interior wall damp proofing means answering the right one. Otherwise the work looks fine for a season, then the same mark returns.

What interior wall damp proofing has to deal with

Water from the ground draws a low band, roughly within a metre of the floor, and its top edge runs fairly level. It climbs through autumn, then eases in summer.

A leak behaves differently. The stain stays where it is, it ignores the seasons, and it usually sits near a pipe run or under a window.

Water forming on cold plaster keeps to the coldest surfaces: window reveals, the corner shared with an outside wall, the strip behind a wardrobe. Our article Condensation or rising damp: how to tell the difference sets the three signs side by side, so an hour spent there saves months of interior wall damp proofing.

Why the plaster keeps crumbling

Brick, stone and lime mortar are porous, and their pores form microscopic channels. Water travels up them with nothing pushing behind, an effect known as capillary action. Our article ATE: anti-damp device for damp walls and rising damp treatment follows that climb from the footings up.

At the surface the water evaporates and leaves its salts in the plaster. They crystallise, they swell, then they break the render apart from behind. So interior wall damp proofing that leaves those salts in place stays cosmetic.

Salt crystals also take up moisture from the air. That is why a heavily salted wall still feels damp long after the climb has eased, as our article Buying a house with damp walls: what to check explains.

The masonry layer comes first

So interior wall damp proofing begins at the masonry. The ATE is an electromagnetic polarity inverter. It plugs into an ordinary socket and sends very low frequency electrical pulses through the wall, 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 climb and favour the return of the water towards the footings.

That is a design principle, and we present it as nothing more. Nothing is drawn out and nothing is heated, because the wall dries by ordinary evaporation. So visible results come between 18 and 24 months.

The ATG works on the same principle as a geomagnetic polarity inverter, with no supply at all. Both units make interior wall damp proofing a matter of the wall itself rather than of its covering. Our guide Persistent musty smell: causes and useful actions compares the two ranges.

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.

Boards, membranes and the finishes that follow

Plasterboard on battens hides the mark within a day, while the water carries on climbing behind it. The battens then sit in the damp zone for years, so interior wall damp proofing of that sort stores up a bigger repair.

An internal waterproof membrane behaves much the same way. It pushes the wet face deeper into the wall, and it often sends the water up into the floor above.

Once the masonry has dried, interior wall damp proofing carries on with layers that stay vapour-open: lime plaster, mineral paint, a breathable render. Our guide Blistering paint on a wall: understand the issue before repainting runs through the choices, while Damp wall after renovation: why the problem comes back sets out the route that leaves the room in one piece.

Watch the outside as well, because interior wall damp proofing counts for little while the ground keeps feeding the wall. A path raised above the floor level inside, a blocked air brick or a downpipe discharging at the foot of the wall will beat any treatment.

Frequently asked questions

Does interior wall damp proofing work if the ground outside sits higher than the floor?

Not while that stays true. Lower the ground or cut a gravel margin along the wall, since the masonry drinks from whatever is piled against it.

Can I insulate a wall that has been damp?

Insulate once the masonry has dried, and choose materials that let vapour through. Insulation fixed over a wet wall keeps the water inside and cools the plaster behind it.

How far from other equipment does the unit go?

Keep 1.50 m between the unit and any other electrical equipment. Its back has to touch the masonry, so plasterboard and insulation are both ruled out.

What if the foot of the wall carries a cement render?

Cut that render back over ten centimetres or so, because it seals the water in. The masonry then reaches the open air and evaporates freely.

How long does interior wall damp proofing take with an electronic unit?

Visible results come between 18 and 24 months, since the wall dries by evaporation. A thick stone wall sits at the slower end of that range.

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