Air drying: using the yard to remove the easy water

Why air dry at all

Lumber is commonly air-dried to 20 to 25 % moisture content before kiln drying (Wood Handbook, North America). Air drying is the cheapest way to remove the first, easy water, all of it above the FSP. Each percentage point removed outdoors saves roughly 50 to 85 Btu per board foot of later kiln heat (McMillen and Wengert, 1978, USA), and the yard reduces the kiln capacity needed.

The yard is not for everyone. Swedish Wood reports that about 95 % of Swedish sawn wood is kiln-dried at sawmills and about 2 % is seasoned in stacks (Europe). A high-throughput softwood mill cannot afford months of stacked stock. The case is strongest for valuable, thick or slow-drying boards.

Two limits set by the weather

• The EMC floor. A board never dries below the EMC of the surrounding air (Lesson 3). Thoroughly air-dried lumber in much of the US reaches 12 to 15 % at best, and a humid maritime yard cannot take boards much below 15 to 18 % in autumn and winter.
• The fungal threshold. Blue stain and wood-destroying fungi cannot grow at 20 % or less (FPL, North America). The yard must get boards through the 30 to 20 % band before fungi win.

Drying time: thickness and season

FPL-GTR-117 states that drying time rises roughly with thickness to the power 1.5, so 50 mm lumber needs about three times as long as 25 mm (2 to the power 1.5 is 2.83). For 25 mm lumber to 20 % moisture, US sites range widely, for example northern red oak 70 to 200 days and Sitka spruce 40 to 150 days.

Season matters as much as species. Simpson and Hart (2001, USA) list days to 20 % for 4/4 red oak at Madison, Wisconsin:

The paper describes a critical stacking date in late summer or early autumn, after which drying time rises quickly, more so farther north. Summer sun is the crack season; winter is the stall-and-stain season.

Worked example 1: scaling time

A mill air-dries 25 mm oak in 60 days under its own conditions. For 50 mm stock the estimate is 60 x 2^1.5 = 60 x 2.83 = about 170 days. If the same lot were stacked in October instead of May, the Madison data suggest a factor of 188 / 65 = about 2.9. These are US figures; no European series of comparable air-drying times was opened, so calibrate against your own yard.

Worked example 2: a European trial

A Polish trial (Kowal and co-authors, 2013, 28 mm beech) air-dried for five months to about 30 % average moisture. Kilning then took 222 hours from 32 %, while green beech loaded at 42 % needed 279 hours. The saving is 279 - 222 = 57 hours, or 57 / 279 = about 20 % of the green-batch cycle. The paper reports no final moisture content or energy, so no energy conclusion follows.

Stacking and yard checklist (US sources)

1. Stack within 2 to 3 days of sawing (Wisconsin DNR).
2. Keep foundations off the ground, about 460 to 610 mm high for hand-built piles (FPL-GTR-117).
3. Use dry stickers about 20 mm thick, spaced 12 to 24 inches (300 to 600 mm; closer for valuable or warp-prone lumber), aligned vertically.
4. Apply a thick moisture-resistant coating to freshly trimmed green ends before checking starts.
5. Add a roof, projecting about 0.3 m at the ends, and shade against direct sun for oak and beech.
6. Orient stacks parallel to the prevailing wind, with weight on top to restrain warp.

Key points

• Air drying removes water above the FSP cheaply; a point removed outdoors saves roughly 50 to 85 Btu per board foot of later kiln heat (USA, 1978).
• The local EMC sets the floor, and fungi cannot grow at 20 % or less.
• Drying time rises with thickness to the power 1.5, about 2.83 times for a doubling.
• Stacking date matters: Madison red oak took 65 days from May but 188 from October.
• Air drying a Polish beech batch to about 30 % cut the kiln cycle from 279 to 222 hours; the evidence is from one trial.