Wood Movement

The Forest Products Laboratory calls the resulting figure equilibrium moisture content. At 70 degrees Fahrenheit, wood sits at about 4.5 percent moisture content in 20 percent relative humidity, 6.2 percent at 30 percent, 9.2 percent at 50 percent, 13.1 percent at 70 percent, and 16.0 percent at 80 percent. That row is not a straight line: 30 to 50 percent relative humidity adds three points, while 70 to 90 percent adds more than seven. The top of the range is disproportionately dangerous, which is why a bathroom punishes cabinetry in a way a dry winter never does.

Two caveats belong with those numbers. The tabulated values were derived primarily for Sitka spruce under oscillating vapour pressure desorption, chosen because it sits midway between adsorption and desorption — a practical compromise rather than a species-specific reading. And sorption hysteresis means wood arriving at a given humidity from the wet side settles higher than wood arriving from the dry side, at a ratio averaging about 0.8 near room temperature.

Above the fibre saturation point — which averages about 30 percent moisture content but varies by several points between species and pieces — wood stops changing dimension altogether. Below it, every change in moisture content is a change in size. Cabinetry lives permanently below fibre saturation, which is another way of saying a cabinet is never dimensionally at rest.

A quartersawn board is oriented so that seasonal width change follows the radial coefficient rather than the tangential one, so the prize is the ratio between the two — and that ratio varies more between species than most specifications acknowledge. In American beech and commercial red oak it is roughly 2.3 to 1, so quartering removes well over half the movement; in eastern white pine about 3 to 1; in yellow birch only about 1.3 to 1.

The two directions combined are also what distorts a board rather than merely resizing it. Because tangential shrinkage is about twice radial and the rings are curved, the Wood Handbook illustrates flat-sawn boards cupping away from the heart, squares becoming diamonds, and rounds becoming ovals. A quartersawn board carries that curvature through its thickness instead of its width, so it stays flatter as well as narrower — which is why the Architectural Woodwork Standards require the method of slicing to be specified for wood casework rather than left to the shop.

The Forest Products Laboratory tabulates equilibrium moisture content for outdoor conditions in United States cities, averaged from thirty or more years of temperature and humidity data. Fresno runs from 16.4 percent in January down to 7.8 percent in July — a swing of nearly nine points, with the dry extreme in high summer. Los Angeles runs the other way and far more gently: 12.2 percent in January, 15.0 percent in July, peaking at 15.1 in August, a total range of about three points. Conditioned interiors compress those outdoor figures, but the shape of the two curves survives, and the shape is the design input.

For installation moisture content the Forest Products Laboratory recommends an average of 8 percent for interior woodwork across most of the United States, 6 percent in the dry southwestern area, and 11 percent in the damp warm coastal area, with individual pieces held to 6 to 10, 4 to 9, and 8 to 13 percent respectively. California spans a wide band of that guidance — the reason a shop building statewide asks where a kitchen is before deciding how wide a solid panel may be.

Shrinkage begins at the fibre saturation point and proceeds fairly linearly as wood dries, so a cabinet responds to the size of the swing rather than the position of the midpoint. A house held reliably at 45 percent relative humidity and one held reliably at 35 percent will both be fine. One that alternates between them every few weeks opens joints.