Heat balance

Relative humidity is a function of both quantity of vapour, measured as vapour pressure, and temperature.

Excessive humidity results from insufficient ventilation, poor air circulation and a cold surface.

Relative humidity is a measure of how much vapour is in the air compared to air’s capacity to hold vapour. It is a function of vapour pressure (quality of vapour) and temperature. As temperature rises, air can hold more vapour.

Conversely as temperature drops, air holds less vapour until it meets the dew point at 100%RH, when dew or condensation forms.

Surveyor tip

  1. The ideal property from a relative humidity perspective is:
    • where heat is evenly distributed throughout the property, without significant temperature differences.
    • and throughout the day and night, without significant differences.
  2. Four tools to help achieve this:
    • Insulate cold surfaces, especially lofts, evenly and into the eaves.
    • Use a laser thermometer to measure temperatures of radiators when on, nearby walls and the thermostat,
    • Turn the valves up or down to even out the distribution of heat,
    • Set up the thermostat so that variation is minimal, and never drops below the dew point, normally about 10°C at night in winter.
  3. A modern thermostat such as as Nest or Hive has multiple temperature settings.
  4. If you have an old fashioned thermostat, with-only one temperature setting, then bring on the heating for say 10 minutes every hour.
  5. Make sure that the thermostat is not too close to a radiator, and reflects the average temperature.
  6. If you want to leave a section of property unheated, then either leave a window partially open (on the safety lock) or trickle vent, or use a dehumidifier regularly decanted or fed to a sink.

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