Wood moves with the air
Wood is hygroscopic: it takes up water from humid air and gives it back to dry air. As it does, it swells and shrinks, mostly across the grain. Doors stick in summer, floorboards gap in winter and joints open when furniture moves to a drier house. Knowing what moisture content wood will reach in its final environment, and checking it before building, prevents most of these problems.
Equilibrium moisture content
Wood left long enough in air of constant temperature and relative humidity reaches its equilibrium moisture content (EMC). The calculator uses the equation from the USDA Forest Products Laboratory Wood Handbook, fitted by Simpson to measured data. At room temperature:
- 30% relative humidity: about 6%
- 50% relative humidity: about 9%
- 65% relative humidity: about 12%
- 80% relative humidity: about 16%
Temperature matters much less than humidity, and the equation applies to most species.
Measuring moisture content
Moisture content is expressed relative to the oven-dry weight of the wood:
MC = (weight now − oven-dry weight) ÷ oven-dry weight × 100
The reference method is to dry a sample at about 217 °F, or 103 °C, until its weight stops changing. Pin and pinless meters give quick readings but need corrections for species and temperature.
Example
Indoors at 70 °F and 50% relative humidity, wood settles at about 9.2%. A sample weighing 120 g that weighs 100 g after oven drying has a moisture content of 20%: it will shrink as it dries toward 9%, so it should acclimate before being used for furniture.
Target moisture content
- Interior furniture, cabinets and flooring: about 6 to 9% in most of the US, lower in the dry Southwest and higher near humid coasts.
- Exterior trim and decking: around 9 to 14%.
- Framing lumber: sold kiln-dried at 19% or less, marked KD or S-DRY.
- Firewood: below 20% to burn cleanly.
Above about 20%, wood is at risk of decay and mold, which is why moisture must be kept out of structures.