EMC (Equilibrium Moisture Content)
Equilibrium moisture content is the moisture level at which wood neither gains nor loses moisture to its surrounding environment. EMC is determined by the ambient temperature and relative humidity. Wood in service will constantly move toward its EMC, expanding in humid conditions and shrinking in dry conditions.
In-Depth Explanation
Understanding EMC is fundamental to successful woodworking and construction because it predicts how wood will behave after installation. Every combination of temperature and relative humidity produces a specific EMC — for example, at 70 degrees Fahrenheit and 40 percent relative humidity (typical heated interior in winter in northern climates), the EMC is approximately 7.7 percent. At 70 degrees and 70 percent relative humidity (typical summer conditions in the southeastern US), the EMC rises to about 13 percent. This means wood installed at 7 percent MC in a heated home in January will absorb moisture and swell when summer humidity rises, and will return to its lower MC when heating resumes in fall. Designers and woodworkers must account for this seasonal movement by building in expansion gaps, using appropriate joinery that accommodates movement, and acclimating lumber to the expected EMC of its installation environment before final machining. The EMC concept also explains why kiln-dried lumber should not be stored in damp, uncontrolled environments — it will re-absorb moisture until it reaches the EMC of the storage location.
Usage Example
The cabinetmaker acclimated the cherry lumber in the client's home for two weeks before final milling, allowing it to reach the equilibrium moisture content of the space.