Unlike people, many objects are not much affected by temperature but can undergo quite dramatic changes as the relative humidity (RH) changes. These changes in relative humidity are directly related to temperature.
Warm air can hold more moisture than cold air. Therefore in the winter when heating is used, air can become drier and will absorb moisture from objects such as wood. When the heating is turned off and the temperature falls, even if no moisture is added to the air, it will become damper and objects will reabsorb this moisture.
Moisture content of air
Maximum amount of moisture that air can hold at that temp %
- 0% = very dry
- 100% = very damp
If air is too dry or damp for a material it is referred to as the wrong RH: this can mean too damp, too dry or fluctuates too much.
Causes of low RH
- Near heat source such as a radiator
- Caused by solar gain
- Caused by heat from a lamp
Causes of high RH
- Outside RH in UK normally 75-90%, unheated buildings will follow this pattern.
- Water leaks
- People’s activities such as cooking, washing floors or even lots of wet coats and umbrellas
- Lack of ventilation e.g. if the building is too well sealed, or in microclimates such as behind pictures, bookcases etc.
This occurs when the temperature drops and can no longer hold the moisture in it, so some will condense out.
The temperature at which condensation occurs depends on the amount of moisture in the air and is known as the dew point.
Damage caused by the wrong RH
Low RH (below 40%)
- Causes organic materials to contract and shrink as moisture is lost.
- Textiles fibres become brittle.
- Wood may break, veneers lift.
- Glues and adhesives can dry out and break.
High RH (above 65%)
- Organic materials swell e.g. painting canvases will sag.
- Enables mould and pests to flourish.
- Accelerates some chemical reactions such as rusting of metals.
Changes in RH
- Organic materials will be damaged by cycle of swelling and contracting.
- Objects become acclimatised to a certain RH and changes can cause “RH shock”.
- Repeated crystallisation and dissolution of salts in inorganic materials such as plaster and stone.
Appropriate levels of RH
The aim of controlling RH is to reduce fluctuations by preventing high and low levels of RH, whilst making every effort to maximise energy efficiency and use renewable energy sources where possible.
The accepted range of RH is now generally considered to be between 40 -65%.
RH in a space should be monitored for at least a year before any changes are made to the environment. This enables the RH set point to be set at a level to which the objects have acclimatised.
|Hair Hygrometers||Require regular calibration monthly|
|Paper humidity strips||Cheap and easy to use
Will only read to the nearest 10%
Best for use in small spaces e.g. display or packing cases
|Thermohygrographs||Require regular calibration monthly
Will show weekly record of temperature and RH
|Whirling hygrometers||Generally now replaced with electronic hygrometers|
|Electronic hygrometers||Require regular calibration biannually
Will produce spot readings
|Data loggers||Require annual calibration
Will record up to a years’ worth of temperature and RH data
|Sensors||Can be hard wired or telemetric
Will record continuous temperature and RH data to send back to a central data collection point
- Determine existing conditions using at least a years’ worth of monitoring data.
- Decide appropriate levels of RH for the collection.
- Decide what is appropriate for the building structure housing the collection.