What is relative humidity?
Relative humidity (RH) is the amount of water vapour present in the air, expressed as a percentage of the maximum amount the air can hold at the same temperature. Warm air can hold more water vapour than cold air. That means RH changes whenever the temperature changes, even if no moisture is added or removed.
Worked example
Air at 20 °C can hold at most about 17.3 g of water vapour per m³. If it contains 8.6 g, the RH is about 50%. Cool that same air and the RH rises. At roughly 9 °C the air is saturated (100%): that is the dew point. Any surface colder than the dew point will collect condensation.
The reverse also applies. Outdoor air at 0 °C holds at most about 4.8 g/m³. Heat it indoors to 20 °C without adding moisture and the RH drops to no more than about 28%. That is why indoor air is often dry in winter.
What level is healthy?
The Dutch public health institute RIVM gives 30 to 70% as a good indoor humidity range for homes. If it stays damper than that for long periods, mould and house dust mites thrive. For mould growth, what matters most is the RH right at the surface. A widely used design criterion (IEA Annex 14) is that the monthly mean RH at interior surfaces should stay below 80%. On a cold wall or at a thermal bridge that limit is reached far sooner than in the middle of the room.
Practical application
- Measure with a hygrometer in several places and at different times. Don’t hang it next to the shower, a window or a radiator, or you’ll only measure the extremes.
- Extract at the source: use the cooker hood when cooking, and run the extractor or open a window during and after showering.
- Tackle cold spots. Condensation that keeps appearing in the same place often points to a thermal bridge or poor insulation, not just too little ventilation.
- Don’t ventilate a cool cellar with warm, humid summer air. It cools against the cold wall and deposits condensation there.
- Fix building defects first, such as leaks or rising damp, before reaching for a dehumidifier.
Related terms
- Dew point
- Condensation
- Thermal bridge
- Vapour diffusion
- Hygroscopic
- Mechanical ventilation
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