Rising Damp - Damp - Faulty Cavity - Insulation Removal
Rising damp is water from the ground soaking up through the bottom of a wall, the way a sponge draws up water. It usually shows as a damp band or tidemark along the lower part of a ground floor wall, rarely more than about a metre high, with white salty marks and spoiled skirting boards.
In this post we explain what causes it, the signs to look for, what is often mistaken for it, and what you can safely check yourself. A damp wall needs a proper diagnosis before anyone drills into it or strips the plaster, so that the treatment fixes the real cause.
What rising damp actually is
Brick, stone and mortar are porous, which means they are full of tiny pores that water can travel through. Water in the ground creeps upwards through these pores by capillary action, the same pull that lets a paper towel soak up a spill.
To stop this, walls have a damp proof course, or DPC. This is a waterproof layer built into the wall just above ground level, usually slate, bitumen felt or plastic. Solid floors have a damp proof membrane, a waterproof sheet under the concrete.
Rising damp happens when the DPC is missing, has failed, or is being bypassed. Homes built before the late 1800s often have no DPC at all, and that matters in Wales, which has the oldest housing in the UK. Around a quarter to a third of Welsh homes were built before 1919. A wet wall also loses heat faster than a dry one, which is why a room with damp walls often feels cold.
The signs of rising damp
Genuine rising damp has a recognisable pattern:
a damp band or tidemark along the bottom of the wall, usually below about a metre
white, powdery salt deposits on the plaster or brickwork
paint or wallpaper bubbling and peeling low down
skirting boards that are stained, soft or rotting
plaster near the floor that crumbles or sounds hollow when tapped
It stays fairly steady through the year and is worst on ground floor walls. Black mould is not a typical sign. The salts that come up with ground water make the wall a poor place for mould to grow, so mould usually points to condensation instead.
What is often mistaken for rising damp
Rising damp is often blamed for damp that has a different cause. These are the most common lookalikes:
Condensation. Moisture in the air settles on cold surfaces, leaving black mould in corners, behind furniture and around windows. It is worst in winter.
Penetrating damp. Rain gets in sideways through cracked pointing, a leaking gutter or downpipe, or failed render. Patches can appear at any height and get worse after wet weather.
Bridging. Something lets moisture skip past the DPC, such as soil or a patio built up against the wall, render or plaster carried down below the DPC, or rubble sitting at the bottom of a cavity.
Wet cavity wall insulation. In a cavity wall, insulation that has soaked up rain can carry water across to the inner wall, often low down where the water collects. It looks like rising damp, but a new DPC will not fix it. The answer is often removing the wet cavity wall insulation.
Old salts. A wall that was damp in the past can stay salty. The salts draw moisture from the air, so the plaster can look damp long after the source has gone.
Our guide to identifying the signs of damp in your home covers these other kinds of damp in more detail.
Why a moisture meter alone cannot prove it
Handheld moisture meters measure how easily electricity passes through the surface. Salts, foil-backed plasterboard, metal and some wallpapers all push the reading up, even when the wall is dry. The British Standard on diagnosing rising damp, BS 6576, says surface readings alone cannot prove it.
A proper diagnosis looks at where the damp sits and how high it goes, the outside of the building and the ground levels. Where needed, it also tests small samples taken from the wall itself.
Safe checks you can make yourself
You can learn a lot from the ground, without ladders or tools:
Walk round the outside and find the DPC. It is often a thin line of slate or black felt in a mortar joint, about two bricks above the ground.
Check that soil, paths, patios and flower beds sit below it. If they are level with or above it, the DPC can be bridged.
Watch the gutters and downpipes in heavy rain for leaks or overflows splashing the bottom of the wall.
Make sure the air bricks low on the wall are not blocked.
Inside, note how high the damp reaches and whether it changes with the weather. Photos taken a few weeks apart help.
Leave digging near the foundations, stripping plaster and removing skirting boards until the cause is known. Avoid sealing the wall with damp-proof paint, which traps moisture and moves the problem elsewhere.
How rising damp is treated
The right treatment depends on the cause. Where the DPC is being bridged, removing the bridge, such as lowering the ground outside or cutting back render, can be enough.
Where the DPC is missing or has failed, the usual answer is a new chemical DPC. A water-repellent cream is injected into a line of holes drilled along a mortar joint, forming a new barrier. The salt-damaged plaster is then replaced with a plaster that resists salts. Older solid walls often need breathable, lime-based materials so they can still dry out.
How we look at a damp wall
At Cavi Clear Ltd we start by finding where the moisture is coming from, then recommend the treatment that suits the wall. Because we also work inside cavity walls, we check for wet insulation and bridging rather than assuming every low damp patch is rising damp.
If you have a damp tidemark along the bottom of your walls, Cavi Clear Ltd can find out what is causing it before any work is planned. From our base in Neath we cover South Wales and Bristol. You can book a damp survey through our damp proofing and treatment service.
Author: Cavi Clear Ltd
Published date: 2026-11-20
Featured image: choose from the stock library: a freshly replastered and painted ground floor wall with a new white skirting board in a UK terraced house, dry and clean, with no logos or readable text. (No customer photo of damp proofing work is on record, and the photo must not show a damp or mouldy wall.)
