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Last edited 26 Sep 2017

Damp in buildings


[edit] Introduction

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:

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:

[edit] Condensation

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.

See Condensation and Interstitial condensation for more information.

[edit] 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.

[edit] Penetrating damp

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.

[edit] Identifying damp

When identifying damp and its causes, architects, surveyors and project managers need to consider the current condition and the expected post-construction condition of the building.

There are four established methods for diagnosing damp.

[edit] Observation

Manual viewing is the least costly, but potentially least reliable method of diagnosis.

[edit] 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.

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.

[edit] Temperature and humidity measurement

Recording the relative humidity and temperatures in a series of rooms and outside using half hourly sampling is the most effective way of diagnosing damp.

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.

The 'data' from surveys can help to inform the project manager on the time scale, for not only the current condition of the building, but how long it could take to dry down.

This should inform the project on the materials and strategies that the design team should consider prior to beginning on site.

--BRE Buzz

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