WORKBENCH GUIDE
Moisture content and acclimatising wood
Wood gains or loses water until it matches the surrounding air. Measure it, know the target and let the stock settle before you cut.
What moisture content means
Moisture content is the mass of water in a piece of wood divided by its oven-dry mass, expressed as a percentage. Green wood can hold more water than wood substance, but only the water bound in the cell walls affects its size. The Wood Handbook uses about 30% as the fibre saturation point when it has not been measured; shrinkage starts below it.
Below that point, wood shrinks as it dries and swells as it takes up moisture. Dried lumber does not stop there: it keeps exchanging water with the air until it reaches its equilibrium moisture content (EMC), which depends mainly on relative humidity and, to a lesser degree, on temperature.
Equilibrium moisture content
The Wood Handbook tabulates EMC by temperature and relative humidity. At room temperature (21 °C), wood settles near 6% in air at 30% relative humidity, near 9% at 50% and near 16% at 80%. The decimals only show trends: actual EMC varies between species and pieces, and wood that is drying settles slightly higher than wood that is taking up moisture (hysteresis).
Air-conditioned rooms at 45% to 55% relative humidity give wood an EMC of about 8% to 10%, while winter heating in cold regions can bring indoor wood down to around 4% (Wood Handbook, chapter 10). Outdoors under cover, wood in most of the United States cycles around 12% to 14%.
| Relative humidity | Equilibrium moisture content |
|---|---|
| 30% | 6.2% |
| 40% | 7.7% |
| 50% | 9.2% |
| 60% | 11.0% |
| 70% | 13.1% |
| 80% | 16.0% |
| 90% | 20.5% |
Target moisture content
Match the wood to the conditions where it will be used. For interior woodwork, flooring, furniture and trim, Wood Handbook Table 13-2 recommends an average of 8% (individual pieces 6–10%) for most of the United States, 6% (4–9%) for the dry south-west and 11% (8–13%) for damp, warm coastal areas. For exterior siding, trim and laminated timbers it gives 12% (9–14%), and 9% in the dry south-west.
Hardwood lumber for furniture, flooring, cabinets and millwork is usually dried to 6% to 8%, while softwood framing lumber is dried to an average of 15%, not exceeding 19% (Wood Handbook, chapter 13). Framing lumber therefore keeps drying after it is built in, and cabinet stock can pick up moisture in a damp workshop.
Acclimatising in practice
A moisture meter gives a quick reading; the oven-drying method described in the Wood Handbook is the reference. Read several boards, not one: Table 13-2 asks for at least 10% of the pieces to be tested for a realistic average, and a lot is probably satisfactory when its average is within one percentage point of the target and every piece falls within the individual range.
Store the stock where it will be used, so that air reaches every face, and measure again before cutting. When the readings stop changing and fall within the target range, the wood has acclimatised; cutting earlier carries the remaining movement into your finished parts.
A red oak board at 12% is brought into a room at 21 °C and 40% relative humidity, where the table gives an EMC of 7.7%. The drop of 4.3 points narrows a 200 mm flat-sawn board by about 200 × 8.6% × 4.3 / 30 ≈ 2.5 mm.
Sources & references
- USDA FPL · Wood Handbook (2021), ch. 4: Moisture relations and physical properties of wood
- USDA FPL · Wood Handbook (2021), ch. 13: Drying and control of moisture content and dimensional changes
- USDA FPL · Wood Handbook (2021), ch. 10: Adhesives with wood materials
- USDA FPL · Wood Handbook (2021), ch. 16: Finishing of wood