Finishing the Job Indoors

Why outdoor drying stops short

A roof and a breeze will take lumber from green to about 20 percent, then slowly toward the outdoor equilibrium of 13 to 15 percent in Madison. They can’t go lower: that’s what the outdoor air is in balance with ¹. Furniture and interior woodwork need an average of about 8 percent, with pieces between 6 and 10 ¹. Build with 14-percent lumber in a heated house and it will shrink after it’s built, opening joints, loosening tenons, and cracking glued-up panels.

Let the house finish it

The simplest kiln in Wisconsin is a heated building in winter. Heated indoor air there often runs 20 to 35 percent RH, an EMC of about 5 to 7 percent. Restickered lumber in a heated shop or basement keeps drying toward that.

How to do it:

• Sticker it again indoors, with the same care as outdoors: level supports, aligned stickers, weight on top. Don’t solid-stack lumber that’s still drying; the faces in contact can’t dry, and you get mold and uneven moisture.
• Give it time. One-inch lumber usually settles over several weeks to a couple of months; 8/4 needs several months. A small fan moving air through the stack speeds it up.
• Don’t overshoot. A dry, hot shop in February can take thin stock below 6 percent. That’s not a disaster, it will regain moisture in summer, but very dry wood is brittle and its joints get fit at an unusually small size.
• Check it with a meter, at a quarter of the board’s thickness, on several boards and away from the ends (the measuring module has the details). The goal is the same moisture through the thickness, around 6 to 9 percent.

A dehumidifier in a closed room with the lumber stickered and a fan running is a step up, and the same idea as a dehumidification kiln at lower temperature.

Check the core, not just the surface

After indoor drying, the surface is often several points drier than the core. Saw a cross-section an inch long from a few boards, a foot or more from the end, and measure the center versus the outer quarter. Or split that section into a shell strip and a core strip and dry them separately in an oven (the oven-dry method in the measuring module). A board with a wet core will move as you cut into it.

Insects: air-dried lumber isn’t sterilized

Kiln-dried lumber leaves the kiln sterilized: the FPL hardwood manual states that kiln drying above 130 °F kills insects and their eggs ². Air-dried lumber has had no such treatment, and powderpost beetles can emerge from it years later inside your shop or your furniture.

The international standard for heat-treating wood packaging (ISPM 15) requires holding the core of the wood at 56 °C (133 °F) for 30 minutes. A USDA Forest Service study found that schedule killed most but not all wood borers; holding the core at 60 °C (140 °F) for 30 minutes killed all of them ³. The key word is core: thick stock takes hours to heat through. Options for a small shop:

• Pay a commercial kiln to run a sterilization cycle.
• Use a solar or dehumidification kiln that reaches 140 °F and hold it there until the core of the thickest piece gets there (a probe thermometer in a drilled hole tells you).
• Inspect regularly. Fresh, light-colored powder below small round holes means active beetles.

Mill in two stages

Even well-dried lumber holds some stress and a slight moisture gradient. Cutting into it releases both, so boards can bow or cup as you work. The standard practice:

1. Rough mill: crosscut and rip parts oversize, joint and plane them about 1/8 inch fat.
2. Let them rest stickered for a few days, so released stress and newly exposed wetter wood can settle.
3. Final mill to exact size.

This is especially important for lumber dried quickly or dried indoors in a short time.

Store it right

Store dried lumber where the humidity matches its future use: a heated, dry space. Lumber at 7 percent stored in a damp unheated barn climbs back toward 12 to 15 percent within months. Keep dried stock separate from green or air-dried stock and from debris that can harbor insects ². Stand or rack it so it can’t sag, and solid-stack it only once it’s fully dry.

What to take from this

Outdoor air stops at 13 to 15 percent; a heated Wisconsin shop in winter, with the lumber restickered and checked by meter at quarter-thickness, carries it to 6 to 9 percent in weeks to months. Air-dried lumber isn’t sterilized: insects die only when the core holds 60 °C (140 °F) for 30 minutes. Mill in two stages, and store dried lumber in heated, dry space.