فهم رطوبة الخشب
محتوى الرطوبة والتوازن وتغير الوزن والانكماش — العلم وراء حركة الخشب.
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What is moisture content?
Moisture content (MC) is the weight of water in wood divided by the oven-dry weight, expressed as a percentage:
MC % = (wet weight − dry weight) / dry weight × 100
Fresh-sawn wood can be 30–60% MC; kiln-dried is typically 6–10%.
Equilibrium moisture content (EMC)
Wood stops exchanging moisture with air when it reaches EMC, which depends on temperature and humidity. Typical values:
| Situation | EMC |
|---|---|
| Heated indoor | 6–9% |
| Average indoor | 9–12% |
| Covered outdoor | 12–15% |
| Exposed outdoor | 15–20% |
Weight at any moisture
Density at MC = density-at-12% × (100 + MC) / 112
Weight = volume × density
So a board that weighs 10 kg at 12% weighs about 10.7 kg at 20% MC.
How much does wood move?
Shrinkage happens almost entirely across the grain (tangential most, radial less) and barely along it:
| Direction | Change per 1 point of MC |
|---|---|
| Tangential (across width) | 0.3–0.6% |
| Radial (across thickness) | 0.2–0.3% |
| Longitudinal (along grain) | ~0.01% |
A 200 mm-wide oak board drying from 16% to 8% shrinks about 7–8 mm.
Practical tips
- Target 8–12% for indoor furniture; build at the MC the piece will live at.
- Acclimatize lumber 48+ hours before machining.
- Never fix solid wood rigidly — allow movement with figure-8 clips or floating panels.
Use the wood moisture calculator for exact weight and shrinkage numbers.
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