EMC and Wood Storage: Why Your Lumber Rack Has a Hygrometer
Equilibrium moisture content, the Hailwood-Horrobin sorption curve, and how to keep your stock at the MC you dried it to.
EMC is the MC your wood wants to be at
If you store lumber in a 25 °C room at 60% RH, it will eventually reach 11% MC — that is the equilibrium moisture content (EMC) for those conditions. EMC depends only on temperature and relative humidity, not on species. The EMC tool uses the four-constant form of the Hailwood-Horrobin sorption equation (Simpson 1973) which matches the Wood Handbook 2021 Table 4-2 to within half a percentage point.
The desorption curve sits above the adsorption curve
Wood that is drying (desorption) holds more water at the same RH than wood that is absorbing (adsorption). The gap is 1–2 percentage points MC. For kiln schedules and storage in a heated shop you should use the desorption curve; for long-term storage in a humid warehouse the adsorption curve applies. The EMC tool defaults to desorption; switch to adsorption if you are conditioning dried stock back up.
Indoor EMC is 6–10% in most of the world
Central-US homes in winter (20 °C, 30% RH) sit at 6% EMC. UK summer (22 °C, 65% RH) is 12%. SE Asia outdoor (30 °C, 80% RH) is 16% — that is why tropical hardwood furniture sold to the US must be dried below the SE-Asia outdoor EMC or it will keep losing water in the customer's living room. Run EMC for your destination climate before you commit to a build MC.
A storage shed without climate control is not a kiln
Stock piled in an unconditioned shed will move toward outdoor EMC — 12–18% in most temperate climates, up to 20% in humid tropics. If you dried to 8% MC and parked the lumber in an outdoor shed, in two months the outer 25 mm of every board is back to 14–16% MC. Wrap stacks in plastic, store in a closed room with a dehumidifier, or sell the stock fast. The reverse mode of the EMC tool tells you the RH you need at a given temperature to hit a target MC.
Use a thermo-hygrometer, not your hand
A $15 digital hygrometer is more reliable than 'feels dry'. Place it next to the stack, not the wall, and let it sit for 30 min to equilibrate. If the EMC tool says the room is at 12% EMC and your moisture meter reads 8% on the board surface, the board will pick up another 3–4 pp before it stabilises. Either sell the board now, or store it in a drier room.