Equilibrium Moisture Content (EMC) and Environmental Physics
Wood is a hygroscopic material, meaning it constantly absorbs and releases moisture to stay in equilibrium with the surrounding atmosphere. The moisture content of wood at equilibrium is called the Equilibrium Moisture Content (EMC). EMC is determined by the relative humidity (RH) and temperature of the air.
To model EMC mathematically, the USDA Forest Products Laboratory uses a complex equation based on Hailwood-Horrobin models: \(EMC = \frac{1800}{W} \left[ \frac{kh}{1-kh} + \frac{k_1 kh + 2 k_2 k_1 k^2 h^2}{1 + k_1 kh + k_2 k_1 k^2 h^2} \right]\), where \(h\) is relative humidity as a fraction, and \(W, k, k_1, k_2\) are temperature-dependent coefficients. Solving this formula allows woodworkers to predict wood moisture changes when moving lumber between different climates.