Damp Proofing — Rising, Penetrating and Condensation Damp

There is damp on my wall — where do I start?

Start by resisting the urge to buy a product. A damp patch is evidence, not a diagnosis. The same brown stain on the same plaster can be groundwater drawn up through a wall, rain driven through the outer leaf during a westerly gale, or water vapour from ordinary living condensing on a cold surface. Treat the wrong one and the patch comes back — same place, next winter, with the coating you applied peeling off it.

That matters, because the wrong treatment can make things worse. Sealing a wall that is trying to dry outwards traps the moisture behind the seal, and a wall that is damp because of condensation cannot be cured by any coating on its own — the water is coming out of the air in the room, not out of the ground or the weather.

The sequence is diagnosis, then remedy of the cause, then the membrane. Our guide Rising Damp vs Penetrating Damp vs Condensation walks through the differences, and What Causes Damp Walls and What is the Solution covers the common causes on existing buildings.

How do you tell rising, penetrating and condensation damp apart?

None of these tests is conclusive on its own, and buildings frequently have two of the three at once. Taken together, though, the pattern usually points somewhere.

Where it appears on the wall. Damp associated with ground moisture is a low-level phenomenon: it sits at the base of the wall and fades upward, and it does not begin halfway up. Penetrating damp appears wherever the water is getting in — around a window head, beside a downpipe, on the elevation facing the prevailing weather. Condensation is tied to neither the ground nor the weather face, but to the coldest surfaces: external corners, behind furniture against an outside wall, at reveals and lintels where the construction bridges the insulation.

Tide marks and staining. A roughly horizontal line, sometimes with a salt bloom just above it, is the classic ground-damp signature: groundwater carries dissolved salts, left behind as the water evaporates at the drying front. Penetrating damp produces a patch rather than a line, often fanning downward from an entry point, and darkens sharply after rain. Condensation leaves no tide line and no salts — it leaves surface water, black spot mould and mustiness.

Seasonality — what it does in summer versus winter. This is the most useful test, and it is free. Condensation is a winter problem: it arrives when heating goes on, windows stay shut and outdoor temperatures drop, and it largely disappears through the summer. Penetrating damp tracks the weather rather than the calendar, appearing within a day of sustained wind-driven rain and improving in dry spells, so it can show up in an August storm as readily as in February. Ground-related damp is the steadiest of the three: it changes slowly with the water table and does not clear up in summer.

Where the water is. Condensation sits on the surface — you can wipe it, and it shows on glass first, which is why windows are the giveaway. Rising and penetrating damp are in the fabric; the surface is wet because the wall behind it is.

Detailed walk-throughs sit in Rising Damp: How to Spot It and Stop It, How to Stop Penetrating Damp in Brickwork and Condensation on Walls: Causes and How to Stop It.

What goes on the wall for rising and ground damp?

Once the external causes have been dealt with — ground levels lowered, gutters repaired, bridged cavities cleared — the fabric still needs a barrier between ground moisture and the finished surface. That is what a damp proof membrane is for. The traditional answer is a plastic sheet that has to be lapped, taped and dressed around every upstand, then survive the follow-on trades intact.

Waterproof Blue Liquid DPM (10 kg, roller applied)

Waterproof Blue is a polymer-modified liquid damp proof membrane applied by roller or brush. It cures to a seamless barrier with no joints and no weak points, which removes the need for corner reinforcement — detail and field are the same material rather than a sheet plus a tape plus a mastic. It bridges cracks and stays flexible when cured, so it moves with the structure instead of debonding from it, and it is specified on foundation walls, concrete slabs, basement floors and external surfaces, horizontally and vertically. Its free Installation and QA Record sign-off sheet, IM-QA-09, is on our QA downloads page.

Waterproof Blue Liquid DPM (5 kg, brush applied)

The 5 kg brush grade is the same system in a detailing pack, for reveals, upstands and junctions where a sheet membrane would need cutting and lapping. On most jobs the two are bought together: roller grade for the field, brush grade for everything the roller cannot reach accurately.

The method, coat by coat, is set out in How to Stop Rising Damp with a Liquid DPM. Where the structure is below ground and under water pressure rather than simply damp, the specification steps up from damp proofing to tanking — see our waterproofing page and NHBC Standards 2023: Waterproof Membranes as a Tanking System.

What stops penetrating damp through masonry?

Penetrating damp is a weather problem: wind-driven rain hits an exposed elevation, the masonry absorbs it faster than it can shed it, and the wall wets through to the inside face. Repointing, sill and render repairs come first — they are the actual defect. On a sound wall that is still wetting through, the remaining question is how to reduce absorption without sealing the wall shut.

Passivhaus Brickseal

Passivhaus Brickseal is a cream-based water-repellent treatment for brick, concrete, sandstone and mortar. The distinction that matters is where it ends up: rather than forming a film on the face of the masonry, it penetrates and cures within the masonry itself. It dries clear and is invisible in place, so the appearance of the brickwork is unchanged — which is why it is used on heritage and retrofit work as well as new build. It is breathable, maintaining vapour permeability so moisture already in the wall is not trapped behind it, and it is stated to give protection for 20+ years. Its QA sign-off sheet is IM-QA-11.

Breathability is where a lot of masonry treatments go wrong. A vapour-tight coating on the outer face of a solid wall stops rain getting in and also stops moisture getting out, so whatever is already in the wall has nowhere to go. Our article Damp Walls? Two Breathable Solutions That Actually Work covers this, and Will Airtight Paint Trap Moisture? explains why airtight and vapour-tight are not the same property.

Where the elevation is being overclad, insulated or re-rendered rather than left exposed, Passive Purple External is the alternative: water permeability Class W3 to BS EN 1062-3 with water absorption of 0.004 ml/cm², while remaining vapour open at sd 0.62 m to EN ISO 12572 under Kiwa BDA Agrément BAW-21-214-P-A-UK. It is classified B-s1,d0 to EN 13501-1 (PAVUS PK1-01-20-065-E-1) and crack-bridges at A3 at 23 °C and A4 at −10 °C. Coverage is 1.0–1.2 kg/m² on blockwork and brickwork.

What actually stops condensation damp?

Here an honest answer is more useful than a product pitch. Condensation happens when moist air meets a surface below its dew point, so there are only three levers: reduce the moisture produced, ventilate it away, or raise the temperature of the cold surface. A coating on a wall is not, on its own, any of those three.

What a coating can do is control two conditions that make the problem worse. Uncontrolled air leakage keeps surfaces cold and drives the draughts that make occupants close vents and switch off extract fans; and warm, moist indoor air leaking into cold construction condenses inside the build-up, unseen, until the damage is done. Passive Purple is an airtightness and vapour control layer in one product, so the sealed surface is also the layer limiting how much moisture enters the construction.

The measured figures are for air permeability, not for damp. Passive Purple is certified Class A (phA) by the Passivhaus Institut, with air permeability of 0.02 (±0.002) m³/(h·m²) at 50 Pa to EN 12114, tested as a system including its connections rather than as a coating on a flat panel. The phA limit is ≤0.10, so the tested result is one fifth of it; Ghent confirmed 0.019. Coverage is 0.8–1.0 kg/m² on OSB and plasterboard. Details of the internal application sit on our airtightness page.

Background reading: What Causes Condensation in Buildings and How to Stop Damp on Internal Walls.

Is damp proofing the same as waterproofing?

No, and the difference decides the specification. Damp proofing resists moisture that is present but not under pressure. Waterproofing, or tanking, resists water under hydrostatic pressure — what a basement below the water table is dealing with. A damp proofing specification applied to a genuine waterproofing problem will fail, at the point where the pressure is greatest.

The practical test is whether water would stand against the structure. A ground floor slab, a foundation wall above the water table, a solid wall wetting from outside — damp proofing. A habitable basement, a lift pit, anything below the water table — waterproofing, with the grades set out on our waterproofing page. The full comparison is in Damp Proofing vs Waterproofing: What is the Difference?.

How much material should we allow?

Existing walls are absorbent, dusty and mixed, so priming is normal on damp proofing work rather than exceptional.

Coverage for Waterproof Blue and Passivhaus Brickseal is not quoted here because we do not have a confirmed published figure for either — ask us for the current technical data sheet before ordering quantities. The rest of the range is on our application and coverage rates page.

Has it been proved on real buildings?

Only measured results are quoted here, each a recorded air test on a real building rather than a laboratory figure. These are airtightness results — evidence that the membrane performs as a continuous layer on real construction, not a measurement of damp.

Our best result on a school is 0.078 ACH at Wester Hailes High School, Edinburgh, on McLaughlin & Harvey's Passivhaus project — a whole-building pre-test after our works, measured by EDP Building Sciences to ATTMA TSL4 in November 2025 against a 0.6 design target. On projects we install, we guarantee 0.6 ACH or lower. On brickwork, the membrane has been applied directly over the existing wall as a continuous seal — see Airtight Membranes on Brickwork.

Can we see it on site before specifying it?

The 360° tours let you walk completed applications and see how the base of walls, reveals, upstands and penetrations were actually left, rather than how they look in a photograph chosen for a brochure. The video channel shows the application sequence on site.

360° site tours Application videos Technical details and drawings

Frequently asked questions