Equilibrium moisture content EMC

Technical paper 35: Moisture measurement in the historic environment, published by Historic Environment Scotland in 2021, defines equilibrium moisture content as: ‘The moisture content at which a material is neither gaining nor losing moisture through exchanges with the ambient environment.’

Equilibrium Moisture Content (EMC) refers to the specific state of a hygroscopic material in relation to its moisture content, and relative to its surrounding environment. It is the point at which the material is neither gaining nor losing moisture and remains in balance or equilibrium with its local environment. The value of the EMC depends on the material being described, as well as the relative humidity and the temperature of its surrounding environment or the air within which it is in contact.

For example, wood is a hygroscopic material, so its moisture levels can rise and fall relative to its local environment. In timber the EMC is most likely to be lower than its Fibre Saturation Point (FSP) which is generally around hen 30%. The exact EMC figure will vary depending on a number of variables which include the type and age of the wood, the area in the cross section of timber and the moisture and temperature of the local environment. For more information see: Fibre saturation point.

This characteristic of hygroscopic materials to help balance moisture and temperature levels can be beneficial to internal environments when left exposed internally.

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