The term ‘relative humidity’ (RH) is a measure of the quantity of moisture in the air, expressed as a percentage of the maximum amount it could hold at the same temperature.
Since cold air can hold less moisture than warm air, the RH in a room will increase as the temperature falls.
Organic materials are sensitive to RH, and gain or lose moisture as it changes. Too dry an atmosphere desiccates materials like wood, parchment and ivory, leaving them shrunken, cracked and embrittled.
In too damp an atmosphere, they become swollen and distorted, prone to mildew and mould attack, insects flourish and metals may begin to corrode.
Because almost all house contents are far more sensitive to RH than they are to temperature, the latter can be used to regulate the former. This is known as ‘conservation heating’.
An RH of 50–65% is a good compromise for most materials. Levels within this range can nearly always be maintained by ‘conservation heating’ – seldom raising the indoor temperature by more than about 5°C above the temperature outdoors.
This is why radiators may sometimes be on in summer if the weather is humid, whereas on a chilly, dry day in winter, after the Trust’s houses have been closed for the season, a visitor would find the places far too cold for comfort.
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