Equilibrium Moisture Content: Wood Follows the Air

Wood is always trading water with the air

Wood is hygroscopic: it takes up water vapor from moist air and gives it up to dry air. Below fiber saturation, a piece of wood keeps exchanging bound water with the air around it until the two are in balance. The moisture content where that balance lands is the equilibrium moisture content (EMC) ¹. Leave a board in one place long enough and it ends up at that place’s EMC, no matter how wet or dry it started.

This is why drying is never “done.” Kiln-dried wood stored in a damp barn picks moisture back up; a tabletop shrinks every January and swells every August.

Humidity sets the balance

EMC depends on two things: relative humidity (RH) and temperature. Relative humidity matters far more. At 70 °F, the Wood Handbook gives ¹:

Temperature shifts these values only a little at room conditions: at 50 percent RH, wood settles at 9.5 percent at 40 °F and 9.1 percent at 80 °F. Temperature matters much more at kiln temperatures, which is why kilns can push EMC very low.

The handbook is blunt that the decimal place in its table is for showing trends only. Real wood varies by a percentage point or more from these values depending on species, extractives, and history. Wood also lands slightly lower when it is picking up moisture than when it is losing it, an effect called hysteresis. Treat the table as a very good guide, not a law.

Wisconsin outdoors: 13 to 15 percent

The Forest Products Laboratory, which happens to be in Madison, publishes average outdoor EMC by month for U.S. cities from decades of weather records. For Madison ²:

Two consequences follow directly. First, lumber air-drying outdoors in Wisconsin, even under a good roof, bottoms out around 13 to 15 percent; thoroughly air-dried lumber in most of the country reaches 12 to 15 percent and no lower ². Second, it is driest in late spring, not summer: May and June bring warm, breezy, comparatively dry air, which is also when drying goes fastest.

Wisconsin indoors: winter swings it low

Inside, the story is about heating. Cold winter air holds very little water; heat it to 70 °F and its relative humidity collapses. A heated Wisconsin house without a humidifier often runs 20 to 35 percent RH from December to March, which puts wood at about 4.5 to 7 percent. In a humid July without air conditioning, indoor RH of 55 to 65 percent puts wood at 10 to 12 percent.

That annual swing of around five points is the moisture change you design furniture and floors around. It is also why the Forest Products Laboratory recommends drying interior wood to an average of 8 percent for most of the country: it sits near the middle of the indoor range, so the wood moves both ways around the size you built it at ².

How fast wood catches up

Wood reaches equilibrium from the outside in. Veneer and thin stock follow the air within days. A 1-inch board takes weeks to settle through its thickness; an 8/4 board, months. Finishes slow the exchange but do not stop it: every practical finish lets some water vapor through, so a finished tabletop still moves with the seasons, just more slowly and with smaller swings from short humid spells. Finishing all faces equally, including the underside, keeps the two faces gaining and losing moisture at the same rate, which prevents cupping.

What to take from this

Wood always drifts toward the equilibrium moisture content of the air around it, and relative humidity sets that equilibrium. Outdoors in Madison it is 12.6 to 15.4 percent depending on the month, so air drying alone can’t produce furniture wood. Indoors, Wisconsin heating seasons pull wood down to about 5 to 7 percent and humid summers push it up to 10 to 12, which is the movement every piece built here has to accommodate.