Equilibrium Moisture Content: The Wood Is Always Chasing the Room

Relative humidity is nearly all of it. Temperature has a small effect, and the species has almost none — an oak board and a pine board in the same room settle to within a percentage point of each other. Rough figures worth carrying: 30% RH gives about 6% MC, 50% RH about 9%, 70% RH about 13%, 85% RH about 18%. Those are the numbers behind the seasonal swing, and they are why a heated house in winter is the harshest environment wood meets. There is a lag, and it is useful. A thick board takes weeks to follow a change in the air, so a piece does not track every wet afternoon — it tracks the season. Thin stock follows much faster, which is why veneer and thin panels can move within days.

Bring the timber inside, sticker it so air reaches every face, and leave it. Two weeks is a reasonable minimum for boards up to about 25 mm; thicker stock and bigger changes need longer. Stickering matters more than the waiting. A stack of boards face to face dries only at the edges, so the middle of the stack is still at the old moisture content when you start work — and it will move after the piece is finished. Thin sticks at 400 mm intervals, aligned vertically, let air at every surface. Weigh a sample board instead of guessing when it is ready. The wood stops losing mass when it has reached equilibrium, and that is the same dry-to-constant-mass loop the chemistry batch uses — measurement, not judgement.

A pin meter measures electrical resistance between two pins, which tracks moisture content well between about 7% and 25%. Outside that range it is guessing, and below 7% it reads badly. Set the species correction if the meter has one, and push the pins IN LINE with the grain — readings across the grain come out high. Measure at several places and at different depths: the surface of a board can be 3 points drier than its core, and the core is what will move later. The oven-dry method is the reference when it matters: weigh a small offcut, dry it until the mass stops changing, weigh again, and the moisture content is the mass lost divided by the dry mass. It destroys the sample and it is the only method that cannot be fooled.