Last edited 19 Nov 2018

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BRE Buzz Researcher Website

Damp in buildings

Mould growth in building.jpg

Contents

[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 damp is correctly diagnosed. The complexity of existing buildings means that damp is very often misdiagnosed, leading to future 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 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.

See Condensation and Interstitial condensation for more information.

[edit] Rising damp

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 1 m 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.

[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 a number of 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 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.

[edit] Laboratory techniques

Drilled samples and moisture contents can be subjected to techniques such as a carbide meter, oven drying and soluble salt analysis. These can be used to confirm the results of thermal imaging and thermo hygrometry.

[edit] Temperature and humidity measurement

Recording the relative humidity (RH) and temperatures in a series of rooms and outside using half hourly sampling can be an effective way of diagnosing damp.

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 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.

--BRE Buzz

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