The degree of hotness or coldness measured in degrees centigrade (ºC).


The ratio of the amount of water vapour actually present in the air to the greatest amount possible at the same temperature. At a ratio of 100% the air would be fully saturated. If the same parcel of air is heated, the relative humidity decreases; conversely, as air cools the relative humidity increases.

Inside a home, for optimum health and comfort, values between 40% and 50% are considered to be ideal.

As values creep above 60% action should be taken to ventilate, heat or dehumidify the air.

Significantly higher values become uncomfortable and unhealthy plus condensation will occur on colder surfaces that may feed mould growth.


The dew point is the temperature to which air must be cooled to become saturated with water vapour. When further cooled, the airborne water vapour will condense to form liquid water (dew). The dew point temperature changes as relative humidity and air temperature changes.


If a surface inside your home is at or below the dew point temperature, condensation will start to occur on that surface. The greater the difference between the surface temperature and dew point, the faster condensation will build. On a porous surface such as wallpaper or an emulsioned wall the moisture will be absorbed and will not be obvious – however, mould spores will start to grow.

Dew Point

Air Temperature
Relative Humidity %

AT IAN PERKS we take this subject very seriously. Prolonged high humidity within your home will have a significant affect on health. We have developed this tool to help in the understanding of the causes of condensation and mould.

We recommend the use of inexpensive measuring tools to help monitor indoor air quality. These are freely available online – the links are below.

For measuring temperature and relative humidity: ThermoPro TP55 Digital Thermo-hygrometer or ETI Ltd Healthy living thermometer & hygrometer

For measuring the surface temperature of walls and windows where condensation occurs: URCERI IR-802 Digital Laser IR Infrared Thermometer Non Contact High Temperature Gun Instant Read

Two golden rules to follow when buying or using a dehumidifier:

  1. Cheap models are less efficient. They will be costly to run and probably not achieve the result you expect. We recommend Meaco Home Dehumidifiers
  2. Dehumidifiers should not be switched off. Leave on 24/7, they automatically measure humidity and will switch on when required.