You already suspect that the damp at the foot of the wall comes from the ground. Now you have to decide what to do. There are several families of treatment, none suits every wall, and the technical sources disagree on some of them. This guide explains how the diagnosis is confirmed, what each treatment does, where it fails and what to ask before accepting a quote. To tell rising damp from condensation and leaks, start with damp walls: the three types.
Updated 3 November 2026. General information, not a substitute for an assessment by a qualified specialist.
The essentials
- According to LNEC (Portugal’s national civil engineering laboratory), if the water content is high near the ground and falls with height, rising damp is likely. If it stays in summer and in winter with no other source, that also points to capillary rise.
- A moisture meter gives comparative, not absolute readings, and salts can change the reading (LNEC). Diagnosing from a meter alone is a known mistake (SPAB).
- The FEUP authors (University of Porto) write that treatment is very complex and that many techniques lack the desired effectiveness, especially in thick, uneven old walls.
- Sealing one side of the wall, for example with cement render or tiles, pushes the damp higher (FEUP).
- Before any work indoors, deal with what feeds the water: drainage, paving, pipes.
First: is it really rising damp?
LNEC (report 115/2008) proposes combining several techniques: thermography, probes, a moisture meter, water content measured on samples and salt tests.
Salts give a clue. According to LNEC, nitrates and nitrites in the plaster suggest the water may come from the ground, and nitrites, being unstable, suggest the source is still active or was until recently.
SPAB (a British conservation charity; its technical note covers buildings built before 1919) warns that rising damp is often misdiagnosed from high meter readings, which also rise because of old salts, certain materials, rain splash or condensation. It says that when rising damp exists there is usually a visible tide mark, and that it may reach only 10 to 50 mm above the floor or climb 900 mm or more, depending on outside ground, wall type, water table and evaporation.
Before treating: take the water away from the wall
LNEC mentions that water may build up in paving next to the building because of poor drainage. SPAB lists outside works: remove plants next to walls, lower the ground beside the building to at least 150 mm below the internal floor, improve drainage. It warns that a drain without inspection can block and make the damp worse, and that disturbing the ground can affect foundations. Leaking drains and pipes also feed the wall.
The families of treatment
The table follows the paper by V. P. de Freitas and A. S. Guimarães (FEUP, 2014), with other sources where stated.
| Family | What it does | What the sources say |
|---|---|---|
| Physical barrier | Sections of wall replaced with waterproof material, core drilling (Massari method) or stainless steel sheets | Effective but difficult and slow; needs small, regular masonry units; vibration and risk of instability (FEUP) |
| Chemical barrier | Holes 10 to 20 cm apart, about a third of the wall thickness deep (two thirds if drilled from one side only), product injected near ground level | Hardly effective in very thick, uneven walls (FEUP). SPAB takes a strong stance against injection in old British buildings and notes it is hard in stone walls with rubble cores |
| Air gap and inner lining | Lining about 10 cm from the wall, ventilated to the outside, waterproof base | Effective if well built, but loses floor area and hides the wall; do not vent into the house (FEUP) |
| Controlled-porosity render | A coat that eases evaporation and salt crystallisation without damage | Used when the causes cannot be tackled; not for unrendered walls; efflorescence may still appear (FEUP) |
| Wall-base ventilation | Channels beside the wall with a humidity-controlled fan | Only if the foundation sits above the water table (FEUP). Tested in a laboratory and one church, by the patent holders |
| Electro-osmosis, Knappen tubes | Counter the capillary potential; tubes to help drying | Reduced effectiveness (FEUP) |
Macroporous renders: what to expect and what not
Two points from LNEC (Gonçalves and Delgado Rodrigues, APFAC 2010 conference paper) matter here. First, how a render copes with salts is not a property of the render alone: it depends on the substrate and conditions, so they advise preliminary tests or tests on site. Second, they criticise standard EN 998-1 for defining “renovation” renders by general properties without naming the working principle. They distinguish accumulation renders, which hold the salt and protect the surface (suitable, for example, for a home), from transport renders, which carry salt to the surface. It was a laboratory test, not a study of real works.
An example of a manufacturer’s data sheet: Robbialac describes an Anti-Salitre Barrier, a hydraulic lime mortar applied at least 1 cm thick on bare masonry (old render removed to about 1 m above the crystallisation zone), and over it the Restaura Desumidificante mortar, 2 to 3 cm thick, with vapour diffusion resistance below 5 m and CE marking to EN 998-1. The sheet says to wait 2 or 3 days between coats. These are the manufacturer’s claims, not independently verified. See also salt on walls.
When to replace the plaster
SPAB notes that treatment firms usually require, as a condition of their guarantee, that the internal plaster is removed to about 1 m above the floor, and that if new plaster goes on before the wall has dried, salts left in the wall can contaminate it. LNEC says that when replacing renders is chosen, the causes must be repaired or reduced first, and compatible materials used.
Questions to ask whoever gives the quote
- Which measurements did you take, and when? Are there readings from summer and winter?
- How did you rule out a leak, a pipe fault and condensation?
- Which family of treatment do you propose, and why for this wall (thickness, material, existing render)?
- What changes outside: ground, paving, drainage?
- What is promised in writing, for how long and under what conditions? See how to compare painting quotes.
- How long must the wall dry before painting?
Paint over the top does not treat the cause: see anti-damp and anti-mould paint: does it work?.
What we could not confirm
- Prices of any treatment: we found no reliable public table. They depend on area, wall thickness, family and the outside.
- Durations of the work and of the wall drying: only Robbialac’s sheet gives times between coats.
- Success rates of each technique in Portugal: the sources give technical opinion, not statistics.
- Whether chemical injection works in ordinary Portuguese walls: FEUP and SPAB are sceptical for thick old walls, but we read no studies on modern brick walls.
- The wall-base ventilation system is patented by the paper’s authors: we read their own description, with no independent assessment.
Sources (read on 3 November 2026)
- LNEC, Departamento de Edifícios, Metodologia de diagnóstico de anomalias devidas à humidade em paredes antigas, Report 115/2008, March 2008 (sections 3.4, 4.5, 5.2, 6 and 7).
- V. P. de Freitas and A. S. Guimarães, Tratamento da humidade ascensional no património histórico, Revista ALCONPAT, vol. 4, no. 1, 2014 (read in full).
- T. D. Gonçalves and J. Delgado Rodrigues (LNEC), Argamassas de revestimento para paredes afectadas por cristalização de sais solúveis: influência do substrato, APFAC 2010 (conclusions and discussion).
- Douglas Kent, SPAB, Control of Dampness, technical note with no printed date (file dated 11 August 2020); sections 2.3, 4.3, 6.3 and 6.6. British source, on pre-1919 buildings.
- Tintas Robbialac, Barreira Anti-Salitre 148-2006 data sheet and Restaura Desumidificante 148-2016 data sheet, versions of 06/02/2021 (Barrier) and 12/10/2021 (Restaura); substrate preparation, thickness, drying, vapour diffusion resistance.