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 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 also 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 surfaces where gaseous water is diffusing, and condensing within pore spaces.
Rising damp is caused by incorrect placing of, faults to, or the absence of a damp-proof course, and is generally only apparent up to 1m above ground level because of the limits of capillary action to draw moisture up through porous elements of the building structure. Rising damp can be exacerbated by alterations to ground levels, flooding, leaks and so on.
See Rising damp for more information.
NB Jeff Howell, a researcher at South Bank University claims to have failed to reproduce rising damp in a laboratory. It is suggested that the phenomenon is actually a process of interstitial condensation, trapped by impervious materials on walls. When those materials are removed, walls dry out. Porous materials such as lime plaster allow moisture to diffuse and walls stay dry.
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 a number of established methods for diagnosing damp.
Manual viewing is the least costly, but potentially least reliable method of diagnosis.
 Moisture meters
Moisture meters can be used to confirm the observed diagnosis. In skilled hands with regular site calibration moisture meters can be a good starting point, however, depending on the complexity of the building, the materials used, its present condition and maintenance history, moisture meters can lead to misdiagnosis.
NB It is suggested that BS 5250 and BS 6576 state that these meters are only suitable for diagnosing moisture in timber. The only valid reading in a moisture or conductivity meter is one of zero when applied to anything other than green timber. Chemical or gravimetric testing is necessary for other materials.
 Laboratory techniques
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 the 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 temperature can vary wildly, yet the 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. 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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