air drying lumber in midwest

[Randy88]

This has never been an issue before, we've always had enough lumber to choose from, but now we're needing some furniture lumber we ran out of and I've been doing some reading on air drying lumber.     

Most tables gives the days to dry lumber down to 20 percent, not below 10 percent that I'm needing, also are there tables out there I can't find that give this by species and thickness to air dry to 5-8 percent moisture stacked outside under a tin roof with ample air movement?

I'm wondering about cottonwood and basswood right now, but also red oak as well.      I know I can buy a moisture meter, but I'm curious if this will take six months, or two years or what to expect for time before I could even think about using this newly sawn lumber for furniture.   Thanks in advance for any information or where to send me to look.   

[Don P]

You won't find that table for good reason. Wood will not get to those moisture contents by air drying outside. The wood will reach equilibrium with outdoor relative humidity and will dry no further. In IA this will bottom out between 12.4 and 14.9% depending on time of year. To continue drying you will need to provide heat and lower humidity.

This is a table listing the emc's for wood dried under cover at outdoor locations.
http://www.woodweb.com/knowledge_base_images/zp/equilibrium_moisture_content.pdf

The answer to when depends on how aggressive those artificial methods are. A kiln operates at the maximum rate of drying without degrade. Less aggressive conditions take longer. Someone posted about trying to dry green lumber by a heater indoors which would be more aggressive. Although the wood would dry very fast it would also suffer very badly from checking. When the conditions are uncontrolled, about all you can say is the wood will be dry when it is dry and without being able to measure the moisture content you will have only a vague notion of how dry that is. For some uses that is good enough, as the use becomes more critical it takes better environmental control and measurement. For my own crude methods I can bring a little 1" thick lumber that has air dried all summer inside in the winter and it'll run on down to around 8% within a couple of months. If I were to bring green lumber into that same environment right now it would be dried enough but checked enough to throw into the woodstove at about the same time, not what you want!

[Randy88]

Thanks Don, I absolutely never took into account relative humidity in the air.     

So for those using a kiln, what moisture levels does it come out of the kiln?

[scsmith42]

Randy, it depends upon the kiln operator but typically DH, conventional or vacuum kilns will easily dry down to the industry standard of 6% - 8% MC.  Solar kiln will achieve this target in the summer, but not always in the winter.  Thick species will take more time to dry.

Cottonwood and Basswood will dry a lot quicker - and easier - than red oak.

[Randy88]

With relative moisture higher than that, where do people store their lumber to prevent it from picking moisture back up again once out of the kiln, in a heated shed, shop or house basement?

[Ianab]

in a heated shed, shop or house basement?

Basically yes. If you want the wood to stay around 8% MC, it needs to be stored in an environment that stays around 40% RH. If you stack it in the outside air where the RH stays around 60%, it's going to suck moisture out of the air, and it's MC will soon go up to ~11%.

We had 82% humidity here yesterday, that equates to 16% MC, although the average is more like 70% RH / 14% EMC.

As an experiment I oven dried a small sample board from my shed (microwave oven method). By weighing it on my digital scales, working out how much lighter it get, I was able to calculate it's initial MC at 13%, which is about would you would expect.  Then I left the board on my desk, and weighed it again each day. After 2 weeks, it was back up to around 13% again. Now this was a piece of cedar, which will dry (and regain moisture) quickly, so other species can take longer to acclimate. But we are talking how many weeks, not years.  Basically I did this experiment to confirm what the books say about EMC / air drying / etc, and it confirmed that they were right, to within the accuracy of my scales at least  :D

In your situation, I'd suggest you get an average quality moisture meter so you can confirm what is going on. Then bring some of your air dried wood inside (attic, spare room, heated garage etc) Sticker it and point some cheap fans at it, and wait a few weeks. Heck a quick test is to weigh a sample board on some reasonably accurate scales. Weigh it again every few days, and you will actually see it losing weight (water). When it stops losing weight, it's "dry", for the environment that it's currently in. You might not know exactly what that number is, but leaving it longer isn't going to change anything.  You aren't sure if it's 5% or 10%, although you can guess from your humidity. But you know it's acclimated to that environment.

[Randy88]

Thanks for the information and explanation.     

All we've ever done was air dry lumber, first stacked with stickers outside with a tin roof over it, then into a shed with a cement floor near a door, then into the back of the shed to forget about it for a few more years, then into my planer shed, then once its cleaned up and close to the thickness we need, then into the insulated garage, then when room allows, into a heated basement to acclimate a while, then used and replaced as the whole process progress's. 

So this whole kiln drying thing is new to me, not to mention we usually have enough lumber on hand, to not worry about any newly sawn lumber for maybe five years or more, this is the first time we ever ran out of drawer sides and furniture back material.     

Have no idea anything about moisture meters, what's an average quality unit cost and what brands, models and etc, and how do they work.

[GeneWengert-WoodDoc]

A kiln is a quick drying method that uses heat and low humidities near the end of drying to quickly achieve low final MCs.  You can also use a heated room, 80 F and 25% RH, such as might occur in a heated basement, to achieve a low final MC.  For small quantities, putting the lumber in an attic will also work.

As you and others have indicated, storing 7% MC lumber at around 38%RH is an important step after any low MC drying process to avoid moisture gain. DRYING HARDWOOD LUMBER has a practical chapter on storage.