A dark patch in a corner, paint blistering above the skirting board, a yellow stain on the ceiling after rain: people call all of these “damp”, but the water reaches the wall by different routes, and the right repair depends on the route. This guide explains the four common types (condensation, rising damp, rain penetration and plumbing leaks), the signs of each, and what you can observe yourself before calling in a specialist.
Updated 3 November 2026. General information, not a substitute for an assessment by a qualified specialist.
The essentials
- Damp in a wall almost always has one of four sources: indoor water vapour condensing on cold surfaces, ground water rising through the wall, rain getting in through a building defect, or a leak from pipes or fittings.
- Portugal’s health authority (DGS) notes that excess humidity can cause condensation on cold surfaces, especially walls, with possible mould growth.
- Where the stain is, how high it reaches, the season and how it reacts to rain are good clues. Portugal’s national civil engineering laboratory (LNEC) warns that when observation is not enough, diagnosis needs tests and measurements.
- Painting over it does not fix it. The consumer body DECO PROteste says that painting or applying anti-damp products without fixing a leak from outside can make things worse.
- On a wall with rising damp, finishes that let little vapour through push the damp higher (University of Porto engineering faculty, FEUP).
The four types side by side
| Type | Where the water comes from | Where it usually shows | When it gets worse |
|---|---|---|---|
| Condensation | Vapour produced indoors (bathing, cooking, drying clothes) | Corners, around windows, behind furniture against outside walls, bathrooms and kitchens | Winter, with windows shut and little ventilation |
| Rising damp (capillary) | Wet ground in contact with the base of the walls | Lower part of walls, starting at the floor, mostly on ground floors and in older houses | All year; it does not go away in summer |
| Rain penetration | Rain entering through cracks, roofs, gutters, window frames or joints | Top-floor ceilings, outside walls, around windows | After heavy rain, especially wind-driven rain |
| Plumbing leak | Water, waste or heating pipes, sanitary fittings | Next to bathrooms, kitchens, pipes inside walls | Persistent patch with no link to the weather |
The four types follow the English government’s guidance on damp and mould (2026) and DECO PROteste.
Condensation: the water comes from the air
Indoor air always holds water vapour. When it meets a colder surface, the vapour turns to liquid. The WHO explains that thermal bridges (such as metal window frames), poor insulation and cold water pipes can leave surfaces below the dew point of the air, and that is where damp forms.
Typical signs, according to DECO PROteste: irregular patches in corners and around windows, mould, a musty smell and surfaces that feel cold. Misted windows are another sign of over-humid air (Swiss Federal Office of Public Health, FOPH). The DGS adds that windows kept shut and sealed to save energy stop the air from being renewed and lead to condensation.
What helps: ventilate often, cut down vapour (lids on pans, extractor fan on, little laundry drying indoors), keep furniture away from outside walls and, if the wall or window itself is cold, deal with thermal bridges and insulation. DECO PROteste treats a dehumidifier as a supplement, not a cure.
Rising damp: the water comes from the ground
Walls in older buildings (stone, brick, lime mortars) are porous and draw water up like a sponge. A paper by Vasco Peixoto de Freitas and Ana Sofia Guimarães (FEUP, 2014) explains that rising damp appears when the wall is in contact with water or wet soil, the materials are highly capillary and there is no damp-proof course at the base. Water rises until evaporation balances the uptake.
The same paper shows the damp front climbs higher with very humid indoor air, on north-facing walls, in thicker walls and under finishes with low vapour permeability: the less the wall can “breathe”, the higher the damp goes.
Typical signs: patches that start at the floor and climb, paint flaking and plaster crumbling low down, and often salt deposits (white powder or crystals) left as the water evaporates at the surface. See the guide on salt on walls.
LNEC uses a simple clue: if moisture content is high near the ground and falls with height, rising damp is likely. And if the wet area stays wet in summer as well as winter, with no other water source (such as a burst pipe), the cause is probably capillary.
What helps: the fixes are building works, not paint. The FEUP paper describes physical barriers, injected chemical barriers, renders with controlled porosity, inner linings with an air gap vented to the outside, and ventilation at the base of the walls. The authors warn that many techniques do not work as well as hoped, especially in thick old walls, and that each case needs a proper study.
Rain penetration: the water comes from outside
Water gets in through a defect: a crack in the façade, a broken tile, a blocked gutter, a worn window joint.
Typical signs, according to DECO PROteste: yellow patches of varying size that darken when mould sets in, worse after heavy rain and easing in dry weather. LNEC lists leaks through roofs and wall cracks among the sources, so a stain on a top-floor ceiling calls for a roof inspection.
What helps: find and fix the way in first (cracks, gutters, joints, junctions between parts of the building) and only then treat the inside. If the water comes from the flat above or a shared part of the building, see leak from upstairs: who pays.
Plumbing leaks
DECO PROteste describes persistent patches, sometimes with mould, next to pipes and sanitary fittings, with no link to the weather. Find the leak, repair the pipe, and only then repair the wall.
Step by step: working out which type you have
- Photograph the patch with the date, on different days. Repeat after rain and in a dry spell.
- Check the height. Does it start at the floor and rise? Think rising damp. Is it on the ceiling or high on an outside wall? Think rain penetration or condensation.
- Check the season. Only in winter, with misted windows? Condensation. After rain? Penetration. All year? Rising damp or a leak.
- Measure the air humidity with a hygrometer. The Swiss FOPH recommends keeping relative humidity between 30% and 50% during the heating season.
- Ventilate and reduce vapour for a few weeks. DECO PROteste says that if a dehumidifier and ventilation do not bring humidity down, there is very probably a structural problem.
- Get a diagnosis. LNEC describes thermal imaging, moisture meters, probes and laboratory tests, and warns that a moisture meter gives comparative, not absolute, values, and that salts can distort the reading.
- Paint only afterwards, once the cause is fixed and the wall is dry. To choose the paint, see which paint for each room. To work out how much to buy, use the calculator.
Note that the types can combine. A cold wall with condensation can also have a leak. If mould appears, read the guide on mould on walls.
What we could not confirm
- Prices for diagnosis or treatment. We found no reliable public price list. Get quotes and compare what each one includes.
- The minimum air change rate required by law for homes (the REH regulation, Portaria n.º 138-I/2021, a ministerial order). Academic papers quote a figure, but we did not read the article in the Diário da República (the official gazette), so we do not give it.
- How well each commercial product works (injections, renders, electronic devices). We did not assess brands; the FEUP paper says electro-osmosis has low effectiveness.
- A reliable home test to tell condensation from damp coming through the wall. We found none described by an official source.
Sources (read on 3 November 2026)
- Direção-Geral da Saúde (DGS), Qualidade do Ar Interior (paragraphs on humidity, condensation and mould) and Fontes de poluição do ar interior (paragraph on sealed windows and condensation). Pages undated.
- LNEC, Departamento de Edifícios, Metodologia de diagnóstico de anomalias devidas à humidade em paredes antigas (Diagnosis methodology for moisture damage in old walls), Report 115/2008, March 2008 (sections 2, 3.4, 5.2, 5.3, 6 and 7).
- V. P. de Freitas and A. S. Guimarães, Tratamento da humidade ascensional no património histórico (Treating rising damp in historic buildings), Revista ALCONPAT, vol. 4, no. 1, 2014, pp. 1–13 (sections 1 and 2).
- DECO PROteste, Que tipo de humidade tenho em casa? (What kind of damp do I have at home?), 21 March 2026 (full text).
- World Health Organization, Regional Office for Europe, WHO guidelines for indoor air quality: dampness and mould, 2009 (publication page; executive summary, pp. XIV–XVI, read in PDF).
- Department of Health and Social Care (UK), Understanding and addressing the health risks of damp and mould in the home, updated 1 April 2026 (section on types of damp).
- Swiss Federal Office of Public Health (FOPH), Bolor em espaços habitáveis (Mould in living spaces), Portuguese edition, February 2021 (indoor humidity recommendation).