Lowering Wood Moisture Content

A woodworker has some lumber at 12% moisture content that he wants to dry down to 9%. Here are some quick and cheap ways to accomplish that. October 19, 2013

(WOODWEB Member):
I have some walnut that's been cut for years and stored in a barn. I've pulled some hoping to use it in a pinch, but it's at approximately 11-12%MC. I've considered equilibrium in my shop to be 9% for years, and a quick tour around with my meter confirms that. Is there any way I can hasten a move into the 8-9% range?

The rest of the story is that I simply don't have time (my clients don't have time) to allow the wood to acclimate slowly. If I have to buy what little I need it'll just gut my already small profit on this particular job.

The pieces in question would be app 2x2x40 (4) and 1x6x50 (2), so we're not talking about a lot of lumber here.

Forum Responses
(Sawing and Drying Forum)
From contributor K:
I have a fair amount of air dried lumber around from my little mill that I like to take on down before using. I have a temporary table back in a corner of my shop, which I use to finish off the drying for projects. I sticker the boards, just like they were green, then suspend a 1" thick foam board a few inches above the stack on props. Then I drape a tarp over that, so it hangs down a couple of feet on all sides. In one end, I cut a hole in the tarp, the same size as those little space heaters which can be bought in any good hardware stores. The last one is on its own oscillating base, which is great for keeping the air mixed up better when there are several stacks. The heater is taking in outside air, heating it, then some escapes out from the far end, but it doesn't satisfy its thermostat, which monitors the outside air coming in its back.

Of course, you will need to leave a couple of feet of clear space in front of the heater to avoid a fire hazard.

I believe Dr. Wengert has stated that you need to get the ambient air temperature ~ 25 higher, regardless of whether it is winter or summer, to reach your target. This is the cheapest way I know to get there. The heater is in the $40 range, and you probably already have a tarp.

From contributor D:

Put your walnut in the attic. 1-2 weeks should do it around here.

From the original questioner:
Thanks. It makes sense, and I have all the stuff on hand. Space to do it will be another story. I wonder how long it'd take to drop the MC that last 2-3 points? I know you can't really say, but an educated guess would suffice.

From contributor G:
I followed an example from Finewoodworking. Made a box out of blue board insulation. Cut small hole in side bottom, placed heater there and blew in the heat. Can't remember exact temp, but didn't hit the 160 I was looking for. Problem is you have this large box hanging around (you could undo it - I staggered taped it with duct tape).

From the original questioner:
Any idea as to what issue of FWW you saw that in?

From contributor K:
Just a couple of days may be enough, but it is easy to just lift the tarp, and meter it any time. Harder woods will be slower than soft.

From Professor Gene Wengert, forum technical advisor:
All you need is a room (trailer, box, temporary room, etc.) that you safely heat to 110 F. To kill insects go at least to 140 for a few days. An attic works well if you do not overload the rafters, beams, etc. For cedar, the room may be 80 or 85 F. Heat makes the process go faster. Add enough heat to get an average humidity of 25-30% RH.

From contributor T:
Dehumidifier under a tarp with wood. It takes more than just heat depending on the area. As Doc said, 25-30% RH.

From contributor G:
The author's names was Beesvort or something. He placed the box over sawhorses to dry some boards. I agree - a tarp would work. I saw the article and figured this would work. Luckily I have enough room to place this over my plywood dryer. I needed something smaller to dry the pine I use for drawer sides and backs. Need to set the pitch.

From contributor D:
My response to the poster was he had 6 boards to dry down a few percentage points. Why reinvent the wheel when most folks have the resource right there - the attic? Around here, the attic temps is going to be 130* or so, the increase in temp is going to drop down the RH to where within a week or less the poster should be at his desired MC or lower.