Wood EMC by Climate: Why Your Garage Dries Lumber to 11-12% and Your House Wants 6-8%

Every August I walk into the garage and something's warped. Not always badly. A drawer front that won't sit flush. A panel with a crown in it. Stock I paid good money for in February, flat as a griddle when I bought it, now doing its best interpretation of a taco shell. For years I blamed the lumberyard. Turns out I was blaming the wrong party. The garage did it.

Here's the deal nobody explains when you're starting out: wood doesn't care what month it is. It cares about relative humidity, and it will chase the humidity in whatever room it's sitting in until the two agree. When wood stops gaining or losing moisture, it's hit what's called equilibrium moisture content, or EMC. The USDA Forest Products Laboratory's Wood Handbook (Table 4-2) puts numbers on it: at 70°F, wood at 30% RH settles around 6.2% MC, at 50% RH around 9.4%, at 60% RH around 11.1%, and at 70% RH it climbs to about 13.3%. Notice the thermometer isn't in those numbers. At ordinary shop temperatures, RH is the driver, not the reading on the thermostat. The Wood Handbook even shows EMC barely moves between 40°F and 70°F at the same humidity. So stop arguing about shop heat and start arguing about water in the air.

What Your Garage Actually Holds

Here's the part that stung when I first learned it. According to THISisCarpentry, the Carl Hagstrom and Gene Wengert article on wood movement, lumber stored long-term in an unheated space anywhere in most of the U.S. drifts to roughly 11–12% MC. Not 7%. Not 8%. Eleven to twelve.

Unheated spaces swing about 10% in humidity through the year, so dry material stored out there swells and shrinks on a season-long loop. How far it goes depends entirely on where you live. Oregon State University Extension gives the cleanest example I've seen: air-dried lumber under cover reaches roughly 12–14% MC in western Oregon but only 8–10% in eastern Oregon. Same species, same sticker stack, half the state apart, four points of moisture difference. Climate sets the floor, and you don't get a vote.

When "Let It Acclimate" Is Dead Wrong

The standard advice is to let new stock sit in the shop for a couple weeks before you build. Sometimes that's right. A lot of the time it's backwards, and here's the logic.

Acclimation should converge on the EMC of the end-use environment, not the shop's. Wengert's rule, echoed by Wagner Meters, is that wood shouldn't change more than about 2% MC when it goes into service. If you're building interior furniture, the target is 6–8% MC, because that's what a heated house holds. If your garage sits at 11–12% like most unheated American garages, then "letting it sit" for a month actively drags your stock away from where it needs to be. You're seasoning it for the wrong room. If the shop's EMC is further from the house than the lumber already is, acclimating in the shop moves the wood the wrong direction.

The cost of getting this wrong is not theoretical. THISisCarpentry runs the math: flat-sawn wood changes size about 1% for every 4% change in MC, and quarter-sawn moves about half that. A 12-inch-wide flat-sawn maple board swings about a quarter inch over a typical 6-point MC change. A quarter inch is a split panel. A quarter inch is a drawer that binds every July. Material delivered at 6% MC and installed outside will climb to around 12% in humid months — that's the same 6-point swing, just pointed the other way.

So before you let anything sit, ask one question: which direction is the moisture flowing, and where does it need to end up?

Strategy #1: The Dehumidifier, and What It Actually Costs

If you want the garage to hold lumber at interior EMC, you have to pull water out of the air, and that means a dehumidifier. The EPA recommends keeping indoor humidity between 30% and 50%, and below 60% — sensible targets for a shop, too. But here's the catch: per the Wood Handbook, 50% RH puts wood at about 9.4% MC. That's fine for exterior-type work, which Wagner Meters puts at 9–14% MC, but it's still above the 6–8% you want for interior stuff. A dehumidifier set at 50% gets you most of the way there. It doesn't get you all the way.

The running cost is real but not scary, and it scales with your utility rate more than anything. Here's what HVAC Base's calculator shows at 18¢/kWh running 12 hours a day: a 30-pint unit draws about 300 W and costs $19.44/month; a 50-pint draws about 590 W and runs $38.23/month; a 70-pint at 700 W runs $45.36/month; up to $58.32 for a 110-pint unit. Over a four-month humid season, that 50-pint unit costs about $153. Year-round, about $459. And 12 hours a day is a generous assumption — most units cycle off once they hit the humidistat setpoint, so real bills land lower.

Two things move that number hard. First, your rate: HVAC Base notes the same usage costs around $26/month at North Dakota's rates versus roughly $98 in Hawaii. Pull your own bill and do the math — watts times hours times your $/kWh. Second, don't try to dehumidify the outdoors. Close the garage door, seal the obvious leaks, keep the unit's filters and coils clean, and size it so it isn't running flat-out all day.

There's a second lever in the same physics. The Forest Products Laboratory's Research Note FPL-RN-0268 includes a graph showing that heating an enclosed lumber-storage space above outdoor temperature lowers the EMC the wood settles to. Warm air at the same RH simply holds less water for the wood to chase. That's why 90°F air at 15% RH drives EMC below 4% — conditions OSU Extension says a kiln operator wouldn't even subject green lumber to. A small heated, sealed corner of the shop beats an open garage for holding stock dry, and it doesn't have to be fancy.

Strategy #2: Stickers, Airflow, and a Roll of Plastic

If a dehumidifier is more than the job needs, stacking discipline gets you a long way for free.

The OSU Extension air-drying guide (EM 8612) lays out the standard: dry, uniform stickers — 1-inch thick is common — spaced 18 to 24 inches apart for most lumber, stacked directly above one another, with a sticker at every end of every layer and a roof overhanging 2–3 inches on all sides. Penn State Extension specifies stickers about ¾-inch thick between courses so air can move through the pile. The point of all this is equalization: every surface exposed, air moving through the whole stack.

Where you stack matters too. Never over vegetation — the ground under the bottom layer breathes moist air into it. Concrete or asphalt dries faster. Raise the stack, keep it shorter and narrower, and face it into the prevailing wind. And 2-inch stock is not 1-inch stock with more patience required: OSU Extension says it takes three to four times as long to air-dry to target. Plan accordingly or buy it at thickness.

For humid spells, THISisCarpentry describes the zero-cost play: wrap stock in vapor-impermeable plastic with few gaps, and it won't pick up appreciable moisture while the air turns to soup. That's not a moisture meter, so check the stock before you trust it — but as sealed storage, plastic wrap and tight gaps is the cheapest insurance in the shop.

One more lever people forget: moisture moves roughly twice as fast for every 20°F temperature increase, and it doesn't stop below freezing — water in the cell walls can't freeze. Winter is not a pause button. It's just a slower season.

The Verdict

"Let it acclimate" is only true when your shop already matches the room the project will live in. Figure out where the wood is going, not where it is. Right tool for the job, same as always. Right environment for the lumber, same as always too. Your drawer fronts will thank you by football season.