Equilibrium moisture content: how the air sets the target

Wood follows the air

Wood is hygroscopic. Held in air of constant relative humidity (RH) and temperature, it gains or loses water until it reaches an equilibrium moisture content (EMC). RH is the dominant control. A kiln or a yard never sets the moisture content directly: it sets an air state, and the wood moves toward the matching EMC. A board will not air-dry below the EMC of the air around it.

The EMC table (North America, about 21 °C)

The Wood Handbook table for 21.1 °C (70 °F) was derived primarily for Sitka spruce but is said to apply to wood of any species for most practical purposes:

At 65 % RH the same table gives 12.1 % at 15.6 °C and 12.0 % at 21.1 °C, so a five-degree change barely moves the equilibrium. Rule (Conners): EMC is more sensitive to RH than to temperature.

In a kiln at 150 °F dry bulb and 130 °F wet bulb (a depression of 20 °F, North America) the air is at 57 % RH and the EMC is 8 %. The larger the depression, the lower the EMC.

Worked example 1: EMC lookup

A joinery shop is held at about 21 °C and 50 % RH. The table gives an EMC of 9.2 %. Stock delivered at 7.0 % will gain 9.2 - 7.0 = 2.2 points, and stock delivered at 12 % will lose 12 - 9.2 = 2.8 points.

A damp store at 80 % RH instead of a dry one at 40 %: 16.0 - 7.7 = 8.3 points of difference in EMC.

A yard example (Europe)

Swedish Wood gives an example: in Malmo in November, outdoor RH of about 89 % corresponds to about 20 % moisture content, whereas heated indoor air at about 32 % RH corresponds to about 7 %. A yard in a humid maritime climate cannot take boards much below 15 to 18 % in autumn and winter.

Hysteresis: the same air, two answers

EMC depends on history. Wood that approaches a given RH by losing water (desorption) ends up more moist than wood that approaches it by gaining water (adsorption). The ratio of adsorption to desorption EMC averages about 0.8 near room temperature, and the very first drying of green wood gives a higher EMC than later cycles. The published table is a practical compromise when the direction of approach is unknown.

Worked example 2: hysteresis

If drying wood settles at 16.0 % in air of 80 % RH, wood re-wetted in the same air settles at roughly 0.8 x 16.0 = 12.8 %. The ratio is an average.

Targets depend on climate and use

Bergman's chapter (Wood Handbook, North America) recommends these average moisture contents, with the range for individual pieces in brackets:

Typical kiln targets from the same chapter: framing softwood averaging 15 % and not above 19 %, appearance softwood 10 to 12 %, hardwood furniture 6 to 8 %.

In Europe, Eurocode 5 service classes (EN 1995-1-1, as reproduced in a University of Galway handbook) link softwood moisture to climate: class 1 (20 °C, RH above 65 % only a few weeks a year) up to 12 %; class 2 (RH above 85 % only a few weeks a year) up to 20 %; class 3 above 20 %. A European delivery must be specified from the product standard and the contract, not from the US table.

Key points

• EMC is the moisture content wood reaches in air of given RH and temperature; RH matters more than temperature.
• At about 21 °C the Wood Handbook table gives 7.7 % at 40 % RH, 9.2 % at 50 %, 16.0 % at 80 % (North America).
• Hysteresis: adsorption EMC is about 0.8 of desorption EMC.
• Set the target for the destination climate and use; US interior averages are 6 to 11 %, Eurocode 5 class 1 allows up to 12 % for softwoods.
• Stock delivered away from the service EMC will move after delivery.