Relative humidity

Air holds vapour.

The warmer it is, the more vapour can be held in air.
The colder it gets, the less vapour can be held in air.

What is relative humidity?

Relative humidity is the measured or calculated amount of vapour (or vapour pressure) expressed as a percentage of air’s capacity of hold vapour. Relative humidity is a measure of both vapour quantity and temperature.

Dew point

  • The dew point is reach when the temperature is so cold, that air can no longer hold all the vapour.
  • The relative humidity at the dew point is 100%RH.
  • Condensation takes place below the dew point temperature.
  • Water evaporates above the dew point.

The dew point in summer can be 19°C, causing dew to form on the grass at night.

In winter the typical dew point in homes is about 10°C.
An excessively humid house, or house with cold walls is at risk of condensation.


Condensation forms when relative humidity reaches 100%RH. Condensation is commonly found at the base of a wall, surrounding doors and windows and on other cold surfaces.

Rising damp

It is also common for condensation to become absorbed deep into a wall.
Wet bricks are poor insulators, reducing the temperature increasing the risk of condensation. Water is drawn down by gravity, this is why condensation often appears to be rising up and why condensation is often mistaken for rising damp. Read more…

Interstitial condensaiton

Interstitial condensation forms where relative humidity within a wall or ceiling reaches 100%RH. The most common occurrence of interstitial condensation is surrounding Roll Steel Joists (RSJs”), that is supporting steel beams or pillars. Read more…


Mould is a sign of poor ventilation and or poor heating or insulation. Mould is common to most houses and is benign to most people. Dust mites grow in similar environments to mould. Dust mites can cause allergies. The solution is to improve ventilation and control heat.

Normal relative humidity can sometimes cause condensation and mould

  1. The relative humidity in the centre of most homes is around 60%RH, but the surface relative humidity of external walls can be much higher.
  2. Vapour quantity and temperature are both functions of relative humidity:
    • Condensation forms at 60%RH if a wall is 8°C colder than ambient air.
    • Mould grows where relative humidity exceeds 85%RH for 6 or more hours.
      • Mould can form at 60%RH if a wall is 6°C colder than ambient air.
  3. So improve ventilation and control temperature to reduce the risk of condensation and mould.

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