Damp masonry is a symptom, not a diagnosis. The same dark patch above a floor can have several causes, and each needs a different repair. A repair for the wrong cause costs money and the damp comes back. This text draws on the technical advice note Control of Dampness from the Society for the Protection of Ancient Buildings (SPAB) in the United Kingdom. It describes traditional solid-wall buildings built before 1919. SPAB notes that most buildings of that age have no damp-proof course (section 1). Houses from the 20th century may differ, for example with a horizontal damp-proof layer in the wall. Check every point against the actual wall. If you are still deciding whether to keep the house, see renovate or demolish.
Why is damp solved by diagnosis and not by a product?
SPAB says the cause of damp is often misdiagnosed, and then an unnecessary and expensive repair follows. A typical example is a retrofit barrier against rising damp in a wall that has no rising damp. Treating the cause is always better than treating the symptoms. If the damp is passing or minor, the repair may do more harm than the defect (SPAB, sections 3.1, 4.1 and 5.2).
Where does the water in the wall come from?
SPAB divides the sources into liquid water and water from the air. Liquid water includes rain, moisture from the ground and plumbing leaks. Water from the air includes condensation, high humidity and hygroscopic salts (section 2.1). Rising damp is only one possibility, and SPAB treats it as a smaller problem than people usually suppose. It usually reaches only 10 to 50 mm above the inside floor, but it can rise to 900 mm or more (section 2.3).
| Source | Typical sign according to SPAB |
|---|---|
| Rainwater penetration | Well-defined patches or leaks, often near gutters and downpipes. They return after heavy rain (section 2.2) |
| Moisture from the ground | A sharp line between wet and dry or a tidemark on the finish, a damp floor edge, salts. Lateral entry through a cellar wall under water pressure (section 2.3) |
| Plumbing leak | Local stains, damp that does not follow the weather (section 2.4) |
| Condensation | Droplets on cold hard surfaces, diffuse patches and mould on porous ones. Corners and walls behind furniture (section 2.5) |
| Hygroscopic salts | More damp in muggy weather. Salts stay after rising damp, even after it ends (section 2.6) |
Two related terms are surface condensation and the dew point. Condensation forms when air cools below its dew point on a cold surface (section 2.5).
Why does damp appear or come back in an old house?
SPAB describes five groups of causes, which often combine (section 3).
- Neglected maintenance. Leaves, moss and dirt quickly block gutters and downpipes, and the water then spills over the edge. It is worst during prolonged heavy rain (section 3.2).
- Unsuitable repairs. Hard cement renders, vapour barriers or insulation with low vapour permeability stop moisture from evaporating out of the wall. They raise the risk of interstitial condensation (section 3.3).
- Less ventilation. Old buildings need more ventilation than new ones to remove moisture from the fabric and from the occupants. Excessive draughtproofing and new windows have reduced it. SPAB advises not to cut ventilation below 0.4 to 0.5 air changes per hour (section 3.4).
- The way the house is used. Cooking, drying laundry and bathing put more moisture into rooms. When thick walls are heated intermittently, moisture from the already warmed air condenses on colder surfaces (section 3.5).
- Poor workmanship. For example, roof tile laps that are too short or gaps in insulation that form cold spots (section 3.6).
How is damp diagnosed properly?
SPAB recommends a stepwise method. First comes an inspection. During heavy rain, look at the gutters, downpipes, gullies and the ground level at the wall (section 4.2). Next come electrical moisture meters. They help to show how moisture is distributed, but their readings alone often misdiagnose rising damp. High readings also appear in old buildings that are not very damp. Causes include salts from an earlier problem and some coatings and wallpapers (section 4.3).
If that is not enough, a detailed investigation follows, including salt tests on site or in a laboratory (section 4.4). Carbide meters and the gravimetric method (drying a sample and weighing it) take time and damage the wall. They show whether the wall is damp through its thickness. Data loggers give a longer view of temperature and humidity (section 4.5). SPAB advises caution with anyone who proposes a retrofit damp-proof course based only on an electrical meter reading (section 5.4).
What helps an old wall?
SPAB advises treating the cause at the source and letting the wall dry. The order of measures follows the order of evidence: simple work around the house first, then work on the wall.
Rainwater and ground level
Faults in roof drainage should be repaired promptly (section 6.2). Water from the ground can also be reduced by groundworks: removing plants beside the wall or lowering the ground at the house (section 6.3.1). Such work has structural consequences. Changing the ground and the water table can damage foundations, and digging close to old walls can have archaeological consequences.
Drainage around the house
A French drain can, in some cases, help to lead surface and subsoil water away from foundations. It must lead the water away from the house, and it needs inspection chambers so that it can be checked every year. Otherwise it can block and, especially in clay soil, form a moat around the house that raises damp (section 6.3.1). See also perimeter drainage system.
Breathability of the masonry and plaster
In a traditional wall, the best answer to rising damp may be to replace hard cement render or pointing with lime mortar. Waterproof renders and cement-rich pointing make damp worse (section 6.3.2). Colourless water-repellent solutions should normally be avoided on old buildings (section 6.2). For old masonry, lime plaster therefore suits better.
A retrofit horizontal barrier
SPAB holds a strong presumption against a retrofit damp-proof course. In an old building it can be damaging, ineffective and an unnecessary expense. Physical and chemical barriers are not advised in earth walls. In flint and rubble-cored walls the treatment is difficult (section 6.3.4). SPAB notes that guarantees on injection often have exclusions and may have no insurance backing (section 5.2).
Internal linings and a pump in the cellar
If the cause cannot be removed, a ventilated dry lining with an air gap in front of the wall can help. It does not suit places where it would hide original features (section 6.3.3). SPAB says a cellar through which water seeps at the joints is best left to drain naturally. If damp must be controlled, drainage with a sump and a pump can reduce it. SPAB calls this more reliable than traditional tanking, which is expensive and depends on faultless workmanship. A float-operated pump needs regular checks (section 6.3.3). For the vertical layer, see vertical damp-proofing.
Renovation plaster and salts
Salts are dealt with only after the source of damp is removed. If old plaster is removed and replaced before the wall dries, the new plaster is contaminated by salts from the wall (section 6.6.1). Firms that offer a retrofit barrier usually insist on removing plaster to about 1 m and replacing it with salt-resistant plaster. SPAB says this often only hides the problem for a time (section 6.6.2).
How do you deal with condensation and high air humidity in a cellar?
With condensation, the air and the surface temperature need work, not the wall itself. According to SPAB, condensation in a building is removed by lowering relative humidity to 70 % at most. The optimum range is often taken as 40 to 60 % (section 6.5.1). See relative humidity.
- Less moisture at the source. SPAB advises first to generate less moisture and to contain it, for example when cooking and drying laundry (section 6.5.2).
- Ventilation. Ventilating to the outside helps if it is not done in an uncontrolled way that lets large amounts of warm, moist air into a room. SPAB suggests opening windows, not draughtproofing windows in kitchens and bathrooms, or using an electric extract fan (section 6.5.3).
- Heating. More heating keeps surfaces above the dew point, especially when it runs constantly at a low level and not intermittently. Radiant heat does this better than convective heat (section 6.5.4).
- The space under the floor. For high humidity, SPAB advises removing impervious floor coverings, clearing air vents and increasing subfloor ventilation (section 6.7).
Watch the timber. In a normal dry living environment, timber generally holds about 9 to 16 % moisture. Above 21 % it becomes susceptible to fungal decay (section 2.7). SPAB says whole-house ventilation systems are not the automatic answer in old buildings, because installing them can harm the historic fabric (section 6.5.6).
What is mis-sold in damp remediation?
- A barrier from one reading. A retrofit barrier recommended only from an electrical meter (section 5.4).
- A guarantee instead of a diagnosis. Guarantees often have exclusions and may have no insurance backing. SPAB says the right approach and good workmanship are the best warranty (section 5.2).
- Waterproof renders and hard cement repairs. On traditional walls they make damp worse (section 6.3.2).
- A dehumidifier as a permanent answer. SPAB calls it a poor substitute for good heating and ventilation. It suits a building after flooding or repair, while the wall dries (section 6.5.6).
- A builder with no experience of old buildings. SPAB advises choosing one who has that experience (section 5.3).
Can you insulate a damp wall?
Find and remove the cause of the damp first. SPAB also advises avoiding insulation materials with low vapour permeability, because an old wall must be able to let moisture evaporate. It gives this advice in the part on interstitial condensation (section 6.5.5). Check the moisture behaviour of the build-up with the designer before you insulate.
In what order should you proceed?
- Inspect during rain: gutters, downpipes, gullies, and the fall of the ground and paving at the house.
- Diagnose the wall: an electrical meter, a salt test and, if needed, samples for gravimetric or carbide measurement.
- Identify the source or sources, and only then choose measures.
- Repair roof drainage and ground, then drainage around the house if needed. Work on the wall and a retrofit barrier come last, and only if the diagnosis showed they are needed.
- Dry with check measurements. Plaster and finishes only after the wall has dried.
If you are still buying the house, damp diagnosis belongs among the checks before signing. See buying an old house.