Humidity and Temperature

Introduction

Relative humidity and temperature are essential components of a comfortable working environment. However, there can be some conflict between the needs of people and the requirements for the care of collections.

Relative humidity—RH—is the amount of water vapour contained in the air at a particular temperature compared with the total amount of water vapour the air can contain at that temperature. Relative humidity is expressed as a percentage. Various materials respond differently over a range of humidity levels and there is an optimum level of RH suitable for the display and storage of mixed materials.

Extremes of relative humidity and temperature can adversely affect the condition of objects, particularly those made of organic materials. But it is important to remember that many materials will stabilise in a particular climate, even though conditions are extreme. Once stabilised and conditioned to the climate, these materials will not necessarily be adversely affected by the constant, extreme conditions.

If the climate changes or objects are moved to a different climate, problems can arise. Changes in relative humidity and temperature—particularly rapid changes—are potentially far more damaging to most materials than are constant extremes. These changes can occur easily with sudden changes in the local weather, when air-conditioning units break down, and when objects are moved from storage boxes to open display or from one region of the country to another.

Steps can be taken to minimise fluctuations in relative humidity and temperature, and to protect valuable collections from the adverse effects of extremes of relative humidity and temperature. To do this, it is useful to understand how relative humidity and temperature are related, what causes relative humidity to fluctuate, and to know what effects they have on different types of materials.