Water in wood: moisture content, fibre saturation and equilibrium

Every kiln decision starts from one number, moisture content, and from knowing what that number does and does not tell you.

Moisture content has a basis

In sawmilling, moisture content (MC) is the mass of water divided by the mass of totally dry (oven-dry) wood, times 100. This dry basis is used by the USDA Forest Products Laboratory (FPL) Dry Kiln Operator's Manual (North America) and most lumber specifications.

MC (dry basis) = (m_wet - m_dry) / m_dry x 100

Values can exceed 100%; the FPL manual gives a green MC range of roughly 30% to 200%. On a wet basis water is divided by the original mass including water (common for wood fuels). The conversions are plain arithmetic:

• wet basis = dry basis / (100 + dry basis) x 100
• dry basis = wet basis / (100 - wet basis) x 100

Worked example. A section weighs 1.80 kg when cut and 1.20 kg oven-dry. Water is 0.60 kg. Dry basis: 0.60 / 1.20 x 100 = 50%. Wet basis: 0.60 / 1.80 x 100 = 33.3%. Same piece, two numbers, so every specification and kiln log must say which basis it uses.

Green wood varies

Wood Handbook Table 4-1 (North America) gives typical averages, and stresses that variation within and between trees is considerable:

A Swedish study (Hajian et al. 2023, Europe) measured Scots pine sapwood at 107%, Norway spruce sapwood at 95% and Norway spruce heartwood at 35%. A softwood charge can start with a very wide spread, which is why sorting by sapwood share matters.

Free water, bound water and the fibre saturation point

Free water sits in the cell cavities; bound water is held in the cell walls. The fibre saturation point (FSP) is the working boundary. The Wood Handbook states it averages about 30% MC, with species and pieces differing by several points. Above FSP, losing water makes the piece lighter but does not shrink it. Below FSP, the wood shrinks and drying costs more energy: about 5% more at 15% MC and about 15% more at 6% MC than free-water evaporation (FPL manual, North America).

Treat 30% as an engineering convention with scatter, not a sharp switch. For shrinkage calculations the Wood Handbook uses 30% for most species and 22% for a few flagged ones such as western redcedar and redwood.

Equilibrium moisture content (EMC)

Wood is hygroscopic: in air of constant humidity and temperature it settles at an equilibrium moisture content (EMC). The kiln does not set MC directly. It sets an air state, and the wood moves toward the matching EMC. Relative humidity (RH) dominates; temperature matters less.

Anchor points (North America): at 21.1 degrees C and 65% RH the EMC is 12.0% (Wood Handbook). At 150 degrees F dry bulb and 130 degrees F wet bulb, a wet-bulb depression of 20 degrees F, RH is 57% and EMC is 8% (FPL manual). A larger depression means drier air and lower EMC.

Targets and thresholds

• Fungi cannot grow at 20% MC or lower. Boards at 20% or above are unseasoned; softwood below 10% is overdried (FPL manual).
• Eurocode 5 service classes (Europe): class 1, average MC of most softwoods not above 12%; class 2, not above 20%; class 3, above 20%.
• North American targets (Wood Handbook Table 13-2): interior woodwork and furniture about 8% (individual range 6 to 10%); exterior siding and trim about 12% (9 to 14%); framing about 15% average, not above 19%.

A European delivery must still be specified from the product standard and contract, not from North American tables.

Key points

• MC is normally dry basis, can exceed 100%, and must always be labelled.
• FSP averages about 30% MC but varies by several points.
• Bound-water removal below FSP causes shrinkage and costs more energy.
• The kiln sets an air state; the wood moves toward its EMC; a larger wet-bulb depression means a lower EMC.
• Targets follow end use: about 8 to 12% for North American joinery, at most 19 to 20% for structural softwood.
• Fungi do not grow at or below 20% MC.