Moisture Content of Wood | Timber moisture content | Wood Movement
How does moisture affect my wood? Understanding moisture content in wood and what effects it has
The moisture content level within timber ultimately dictates the subsequent movement and potential distortion when in service. Where your timber item is to be used affects the amount of movement and what happens to the wood. This handy infographic indicates the recommended moisture content for joinery and timber products used inside and outside of buildings in the UK.
Please note that for external timber exposed completely (such as in gardens, landscaping, car ports and other exterior structures) the moisture levels can be considerably higher or lower in summer. Consideration to the time of year of any given installation is needed, as the timber will affectively acclimatise to its environment.
The above is a guide only and more, in depth research and analysis, of the specific environment, temperature and humidity is recommended when planning the use of timber items in (or outside) of buildings.
Equilibrium Moisture Content of Wood
The moisture content of wood depends on the relative humidity and temperature of the air surrounding it. If wood remains long enough in air where the relative humidity and temperature remain constant, the moisture content will also become constant at a value known as the equilibrium moisture content (EMC). Thus, every combination of relative humidity and temperature has an associated EMC value. The EMC increases with increasing relative humidity and with decreasing temperature.
The United States Department of Agriculture - Forest Service have produced an excellent document - Equilibrium Moisture Content of Wood in Outdoor Locations in the United States and Worldwide
They indentify areas of the UK with results as follows- Please read in conjunction with their report in PDF;
In reality - all timber moves, when exposed to different conditions as it; ‘wants’ to be in ‘tune’ or in balance with its environment. It is hygroscopic in its nature and therefore will take on water in wet environments (expanding) and lose water when dry (shrinking) all of which causes movement, surface and even whole component damage.
This is unavoidable but can be reduced and managed by;
a) selecting the right timber for the job
b) ensuring that the moisture content at the time of processing and installation is correct.
If you purchase timber regularly or have concerns over moisture content levels in timber (this especially applies to building contractors, architects and other trade related professionals) we recommend that you invest in an accurate, quality moisture meter. It is important to obtain the correct type of meter – we strongly recommend the Wagner MMC220 (we have used one for years with great success). Having outstanding accuracy, another key advantage of these meters is that they provide measurement deep into the wood without the downside of unsightly holes that you get when using a pin-type meter. Wagner's technology allows quick scanning of your wood, and Wagner meters provide INTELLISENSETM technology with virtually ignores absorbed surface moisture that can negatively affect other meters’ accuracy. For more information click here