Knowing how to treat damp is essential, as is understanding how to prevent it, what causes it, the signs that you have a problem and getting to grips with the best ways of removing any mould that might have formed because of it.
Damp can damage much more than a building’s appearance. It may lead to the deterioration of plaster and masonry, promote timber decay and create unhealthy conditions for occupants. Unfortunately, inappropriate treatments for damp commonly cause greater harm to old buildings than centuries of other kinds of degradation.
Over-reliance on electrical moisture meters frequently leads to unnecessary expense and damage through the retrospective installation in walls of horizontal damp proof barriers (damp proof courses or DPCs). Equally the harm done by modern solutions that aim to seal old walls rather than improve their ability to breathe is underestimated. An appreciation of how the basic construction of old buildings differs from that of new ones will help you avoid such misguided remedies.
In this article, Douglas Kent, technical director at the Society for the Protection of Ancient Buildings (SPAB), explains the very best ways of both preventing and treating damp.
The importance of breathable walls
Old buildings must be allowed to breathe. Whereas modern buildings rely on keeping water out with a system of barriers, buildings that pre-date the mid-19th century are usually constructed of absorbent materials that allow any moisture that enters to evaporate back out.
Because most old buildings were constructed with solid walls without damp proof courses and originally had no roofing felt, rain or below ground moisture could both enter. This did not, however, mean dampness was inevitable. Before central heating was commonplace, heat from open fires drew in large quantities of air through loosely fitting windows and doors. This high rate of ventilation would have quickly evaporated moisture from permeable internal surfaces while the wind dried out any damp roof timbers or permeable external wall surfaces.
An equilibrium was therefore established whereby the moisture being absorbed was equal to that evaporating. When upgrading an old building, you must maintain this equilibrium for the building to work as intended and remain dry. In fact, unsympathetic upgrades to old buildings is one of the leading causes of damp.
The main causes of damp
So, exactly what causes damp? The main causes are:
- Energy-saving measures that reduce ventilation in old buildings: Upgrades such as double-glazing increase relative humidity. Humidity is also raised by modern lifestyles that generate large quantities of water vapour, from bathing, cooking and washing. Condensation will occur on any surface below the dew point (i.e. temperature at which saturated air releases surplus moisture vapour). Interstitial condensation within the pores of materials reduces thermal insulation and further increases the risk of condensation.
- Defective roofs: Roofs, chimneys, parapets and other exposed parts of a building are most susceptible to rain penetration, especially where access for maintenance is difficult. Junctions in roofs are potential trouble spots, with water exploiting defective lead flashings, mortar fillets, ridges or hips.
- Poor maintenance: Concentrated and prolonged wetting of walls and external joinery arises from poorly maintained rainwater goods, and leaks from parapet and valley gutters can cause significant damage to structural roof timbers.
- The wrong materials: Hairline cracks in pointing and render invariably admit moisture where cement mortar has been used for repair, rather than lime.
- Leaks: Overflowing baths or showers, burst pipes, the gradual breakdown of pipe joints, leaks from washing machines or dishwashers, and accidental damage are all causes of damp.
How to spot damp
If you are unsure whether your house is suffering from a damp problem, be on the lookout for any of the following:
- Damaged/blocked roofs and rainwater fittings: Inspect your roof during wet and windy weather to decide if a damp ceiling patch is due to roof leakage and/or condensation. Debris on the ground (broken slates, tiles and so on) or daylight seen inside lofts indicate possible roof problems. Defective rainwater fittings may be most obvious during heavy rain, but stains on walls and plant growth provide further clues. Don’t forget to check gulleys at ground level.
- Condensation: This is diagnosed from diffuse areas of damp, beads of water droplets on hard shiny surfaces and mould growth on internal finishes. It is intermittent, like penetrating damp, but unrelated to wet weather.
- Wet patches on walls: Penetrating damp typically shows up as well-defined patches after heavy rain on south- and west-facing walls. Anticipate moisture ingress through hairline cracks in unsuitable hard, modern cement pointing or rendering.
- Tidemarks on walls: Below ground moisture causing rising damp can extend up to 900mm above floor level, sometimes with a classic tidemark on finishes. Salts appear as white deposits but mould growth is rare.
- Rising water bills: Unusually high water bills (if metered) or a constantly refilling tank may suggest leakage.
Damp proof courses explained
There are several different types of damp proof course (DPC), as well as some alternative systems, all of which are suitable for different types of houses and locations:
- Physical DPC: These are inserted by cutting in or during rebuilding. While they can cure rising damp, this drastic method is usually inappropriate. Drawbacks include possible major structural problems and potential damage to historic finishes internally. A new physical DPC is also unsuitable for randomly coursed walls, homes with access difficulties and can sometimes cause deterioration of masonry below the damp proof course where moisture is concentrated.
- Chemical solutions: In this case, walls impregnated with chemical solution through holes at bottom to create waterproof barrier. Although this method is widely used today, it is not always appropriate for old buildings. You also need to be aware that drilling holes is inadvisable in flint and granite and it is hard to form a proper barrier in rubble walls with voids. In addition, holes can look unsightly and sometimes cause deterioration of masonry below damp proof course. This type of DPC typically costs £195/m (including replastering).
- Ceramic tubes: With this method, holes are drilled to receive porous siphons approximately 50mm in diameter that absorb damp and evaporate it from each tube. While a popular method, problems may occur in practice. Drawbacks include salt accumulation in the tubes that may increase moisture and air-flow is sometimes inadequate. In addition, the tubes are commonly set in hard cement mortar which is unsightly. Costs are typically 125/m.
- Electro-osmosis: This system uses electrical potential aimed at reducing capillary rise using electrodes bedded in the wall. While cheap, there is a dearth of evidence that electro-osmosis is effective and the system is rarely used today due to the adjustment of current needed to match variations in damp along a wall usually proving impractical.
- Magnetic field: An Austrian product presently under trial in the UK claims to inhibit the passage of water up a wall by inducing a local magnetic field. This is achieved non-invasively with a unit plugged into mains, typically in the loft. The estimated cost is around £3,000/unit (one unit covers an average-sized house).
Testing for damp
Scientific analysis can be an essential aid for accurately diagnosing a damp problem but the importance of your sight, feel and smell should not be undervalued. Tests include the use of electrical resistance and capacitance meters, on-site moisture testers, hygrometers and salt analysis.
Bear in mind though, that care must be taken when interpreting results. A frequent mistake is to diagnose rising damp purely on the basis of high electrical moisture meter readings. Elevated readings may indicate the presence of salts from past dampness or, if there are no salts, an on-going condensation or possible penetrating damp problem. Continued monitoring is often needed to establish the true cause of a damp problem.
Surveyors have a legal duty to follow a trail of suspicion. Regrettably, many still simply note the occurrence of high meter readings and pass on all responsibility for further investigation to remedial treatment contractors. These contractors have a vested commercial interest, encouraging over-specification. Should a mortgage company insist on work you believe is misguided, challenge this and consider obtaining a written report from an independent surveyor or architect.
Getting rid of damp
Effective remedial measures depend on accurate diagnosis, but applying staged remedies can also be part of understanding the cause of a damp problem. Before deciding on more extensive work, the first remedy may involve nothing more than basic maintenance such as clearing a blocked rainwater gulley. Remedies will either cure dampness by addressing the cause (for example, improving drainage) or will manage it by treating the symptoms (changing washing or cooking habits, for instance).
Be sceptical of written guarantees, which are often loaded with get-out clauses and may have no insurance backing. The right approach from your contractor coupled with good workmanship is your best guarantee.
When it comes to how to stop condensation, reducing air humidity or keeping surfaces above dew point temperature are both important. Humidity is reduced by cutting the amount of moisture available or increasing ventilation by opening windows, etc. Tumble dryers should be vented to the outside if not of the condenser type, and clothes drying indoors is best avoided.
Temperatures are maintained above dew point with suitable heating. The permanent use of dehumidifiers is a poor substitute for efficient heating and adequate ventilation.
Condensation in chimney flues can be eliminated with proper linings. Redundant flues that have been sealed should be fitted with ventilation grilles or re-opened. Lofts should be well insulated and ventilated but make sure insulation does not restrict ventilation at the eaves.
How to control penetrating damp
In order to get a penetrating damp problem under control you should start by looking at the roof.
Reinstate dislodged and missing slates and tiles before damage occurs to roof timbers or plaster ceilings. SPAB, the Society for the Protection of Ancient Buildings, recommend that renovators avoid spray-on roof foams for the underside of roofs, or external bitumen coatings although other experts disagree. SPABs view is that they prevent proper inspection, hinder the re-use of slates or tiles and, by reducing ventilation, increase the risk of decay.
Brush moss off roofs since it can block gutters and retain moisture, which may damage certain roof coverings in frosty weather. Also, clear gutters and rainwater pipes regularly, particularly if your building is surrounded by trees or perched on by pigeons. Parapet and valley gutters need to be cleared of snow to prevent melt-water rising above them and causing damp internally.
Next you should look at repointing old walls. Whilst cement is fine for modern buildings, it is important to use a lime:sand mix (preferably without cement) for most buildings pre-dating about 1900. Localised re-pointing is generally all that is required. Daub, lime mortar or oakum (ships caulking) are useful for closing gaps that may develop around the edges of panel infillings in timber-framed buildings.
Where rain penetrates an exposed south- or west-facing wall, limewash, lime render and slate or tile hanging are traditional solutions although these cannot be employed without changing the external appearance of the wall. Sometimes installation of a ventilated dry lining system internally is appropriate. The use of colourless water-repellent treatments or plastic-based paints on old masonry is strongly inadvisable.
Treating rising damp
Rising damp is neither as widespread as commonly thought nor a total myth, as sometimes now claimed. Floors become damp where the evaporation of moisture from below is inhibited by vinyl sheet, rubber-backed carpets or other impervious coverings.
New concrete floors or impervious coverings also drive excess moisture into the bases of nearby walls (including chimney stacks), where it rises by capillary action. Damp proof courses were not compulsory in walls prior to 1875 but this is only likely to become a problem where breathability is compromised.
In addition to rising damp, below ground moisture can result in problems where ground levels around your building rise unduly.The best solution to rising damp may well be to take measures that help your building breathe. Replacing hard cement render or pointing using a more suitable lime-based mortar often improves a damp wall and enables rising damp to dry out. Conversely, the application of waterproof renders and bituminous coatings tends to create or exacerbate damp problems.
Where a floor has a modern damp-proof membrane (horizontal barrier or DPM) that is displacing moisture to the bottoms of walls, it may be sensible to replace this completely with a breathable construction or to at least provide a breathing zone for evaporation around the perimeter of the room. When underfloor heating is being installed, there are many situations where DPM-insertion can be avoided by employing materials such as lime concrete and expanded clay insulation.
Reducing or removing the source of moisture may also help alleviate rising damp. Installing a French drain can be an effective and relatively inexpensive answer but it is preferable not to site them directly against walls and rodding points must be provided. Otherwise, blockages can effectively convert them into a sump and increase dampness. Consider also the structural and archaeological implications.
Although retrofit DPCs can sometimes be appropriate, with an old building always consider first whether rising damp is actually too minor to matter and, if it is significant, whether more sympathetic ways exist of dealing with it. Where any timber is at risk of decay, for example, you might be able to simply isolate it. Similarly, the eradication of any contributing moisture from other sources such as rainsplash off closely abutting patios could obviate the need for more extensive remedial treatment.
Damp can be particularly troublesome in cellars but increased ventilation (including opening up redundant flues), re-pointing and lowering the water table locally can be effective. Failing this, it may be worth considering a dry lining system. Tanking (applying waterproof linings to walls and floors) is not recommended in old buildings.
Treating damp on internal walls
To minimise the risk of future problems, lime plaster should usually be used for any replastering rather than the anti-sulphate or renovating plasters favoured by many treatment companies.
Decoration with paints such as limewash and soft distemper, where possible, will maximise breathability.
Old items of joinery removed during work should be carefully repaired and reinstated, not automatically replaced.
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Douglas Kent is a chartered building surveyor specialising in building conservation and is technical and research director at the Society for the Protection of Ancient (SPAB), where he has worked since 2000.
He has held surveying roles in the private and public sectors, including the military.
Douglas publishes and lectures regularly on building conservation and has contributed to various radio and television programmes. He has also renovated a listed property.