Understanding damp

Excess damp found in properties is largely as a result of changes to the original design, location or use. Properties are built to absorb rain and evaporate moisture without excessive damp inside. Lifestyles have changed over the years, such as taking showers more often. The resultant high humidity means most properties are at increased risks of condensation. Damp is not inherently dangerous. However, it can spoil decoration and encourage rot, mould and insect infestation. Rot is omnipresent and starts when wood cells rupture above 28{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} moisture content with a constant source of water. Brown rots, such as dry rot proliferates in poorly vented humid voids. Wood boring beetles are attracted to moist wood. Mould requires humidity on the surface of over 85{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} relative humidity (RH) to grow.


Rising damp can spoil decorative surfaces. However, there is insufficient moisture in rising damp to cause wood to rot. Ground water contains nitrates, that inhibit mould growth. Rising damp needs a constant source of water, such as a high water-table within 100mm of the ground. Stop the source of water and rising damp will dissipate. According to Thames Water, London’s water-table is low, below the lowest tube-line.  Rising damp results from the high relative force of attraction of silicone (found in sand, bricks, glass etc.), a phenomenon unhelpfully described as capillarity. The attractive force of silicone spreads water through connected pores in all directions. Plaster can be particularly absorbent. Water spreads downwards first through the additional force of gravity, until lower pores become saturated.


Condensation is caused by humid air condensing on cold surfaces, condensation starts when a wall’s temperature falls below the “dew point”. The dew point increases as humidity rises. There is often a line within a wall where the temperature is below the dew point, this is called the dew point line. Walls are designed to absorb and evaporate moisture daily.

Damp is often cumulative. For example, condensation is more likely to form near a wall that is damp from penetrating rainwater. Likewise, rain will not evaporate as quickly if the wall surface is already humid through condensation. Furthermore, wet external walls are poor thermal insulators. North, North-Eastern and North-Western walls receive minimal warmth from the winter sun. Some damp only occurs infrequently, once every few years, resulting from persistent rain and wind. Damp detection depends on conditions during the survey.