Wood is hygroscopic, which means its moisture content (MC) will fluctuate based on the relative humidity (RH) of the surrounding air. As humidity increases, the MC increases, and the wood expands, and as the humidity decreases, MC decreases, and the wood shrinks. This relationship is referred to as equilibrium moisture content (EMC), and can be accurately predicted.
Understanding Equilibrium Moisture Content
The moisture content of wood is tied directly to the relative humidity of the surrounding air. The higher the relative humidity, the higher the MC of the wood. Period. If you’re installing wood that’s recently been transported, or installed on a job, it might take a little while for the material to reach its EMC with the air—in other words, for the wood to accommodate to the humidity level for the climate around the wood: the wood may take on more moisture or it may dry out. For example, if wood at 10% MC is exposed to 25% RH, the wood will dry to 5% MC (and shrink as it dries).
The EMC helps us understand the response wood will have to relative humidity, whether it will shrink or expand. For woodworkers and carpenters, the EMC is more helpful than RH. The simplified chart above shows the EMC values of wood when stored at the humidity and temperatures indicated.
Regional Equilibrium Moisture Content
Knowing the regional EMC for exterior wood application in your area of the country—and the time of year of your installation, is also critical if you want to ensure durable joinery and long-lasting woodwork. Click on/download the following chart! Find the nearest location to your area and identify the time of year you’re installing exterior material.
If you can’t acclimate your materials to the suggested EMC, at the very least you’ll be able to predict the movement after the material is installed. Using that prediction, you can calculate exactly how much to space your material for future expansion.
(This Toolbox App article is abbreviated from Understanding Moisture Content and Wood Movement.)