Damp in buildings
Excess water/moisture in all its forms (vapour, liquid, and flooding) is still the most common problem in housing. Damp in buildings may be apparent from:
- Damp patches.
- Mould growth, which is a cause of respiratory allergies.
- Mildew, salts, staining and 'tide marks'.
- Damage to surface finishes, such as blistering paint and bulging plaster.
- Corrosion and decay of the building fabric.
- Slip hazards.
- Frost damage.
- Poor performance of insulation.
- Damage to equipment, or electrical failure.
Successful treatment can only be achieved if the type of dampness is correctly diagnosed. The complexity of existing buildings means that dampness is very often misdiagnosed, leading to future damp problems, cost and disruption to the occupants.
The most common causes of persistent damp in buildings are:
Condensation occurs when moist air cools below its dew point, and water condenses. This can occur as surface condensation, but occurs all the time within the fabric of a building itself, as interstitial condensation. Modern surveying involves the use of accurate thermo hygrometers and thermal imaging to highlight areas of cold wall where gaseous water is diffusing, and condensing within pore spaces.
If it is not trapped by cement render, or tanking, it can evaporate. If trapped it builds up in the wall and is then described as rising damp by a voracious chemical industry which makes millions of pounds a year from fraudulent chemical treatments.
 Rising damp - discredited
Rising damp used to be supposed to be caused by incorrect placing of, faults to, or the absence of a damp-proof course. It was generally only apparent up to 1 m above ground level because it was imagined that capillary action was drawing moisture up through porous elements of the building structure. It has proven impossible to reproduce rising damp in a laboratory. Jeff Howell, a researcher at South Bank University failed in many tests to make it happen.
The phenomenon is actually a process of interstitial condensation, trapped by impervious materials on walls. When materials are removed, walls dry out. Porous materials such as lime plaster allow moisture to diffuse and walls stay dry.
See Rising damp for more information.
Penetrating damp is moisture that penetrates laterally through the fabric of a building, typically as a result of leaks to pipework, damage to the building fabric which allows water to penetrate, high ground levels, blocked drains, leaky gutters, cracked masonry, broken flashings etc.
See Penetrating damp for more information.
There are four established methods for diagnosing damp.
Manual viewing is the least costly, but potentially least reliable method of diagnosis.
 Moisture meters
Moisture meters cannot be used to confirm the observed diagnosis. They are ONLY capable of measuring moisture in freshly felled wood, and even then need calibrating for timber species. BS 5250 and BS 6576 both state that these meters cannot be used to diagnose moisture in any other building material. Chemical or gravimetric testing is the only way. The only valid reading in a moisture or conductivity meter is one of zero when applied to anything other than green timber.
Drilled samples and moisture contents can be subjected to techniques such as a carbide meter, oven drying and soluble salt analysis. These confirm results of thermal imaging and thermo hygrometry.
 Temperature and humidity measurement
Thermal imaging is used to accurately record temperature of building fabric and isolate areas that are either close to, or below dew point. These areas will suffer interstitial condensation and be damp. A thorough understanding of the relationship between temperature, relative humidity, and absolute humidity is essential. RH and Temp can vary wildly, yet moisture content of the air stays the same. Good, dry air should be around 7 grams/cubic metre.
At 12 g/m3 building fabric is susceptible to interstitial condensation and can give rise to the symptoms of the mythical 'rising damp'. This same total moisture content (TMC) can also start to raise moisture content of timber to the point where beetle attack and fungus can take hold.
 Related articles on Designing Buildings Wiki
- Cavity tray.
- Cold bridge.
- Damp-proof course.
- Damp proof membrane.
- Defects in brickwork
- Defects in stonework.
- Dew point.
- Dry rot fungus.
- Interstitial condensation.
- Penetrating damp.
- Rising damp.
- Rising damp in walls - diagnosis and treatment (DG 245).
- Understanding dampness.
- Wall insulation and moisture risk.
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