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Thread: Moisture content confuses me.

  1. #1
    Join Date
    Apr 2009
    Location
    Columbus Ohio
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    394

    Moisture content confuses me.

    So this is what confuses me. Why do we kiln dry lumber down to 6-8% when most areas of the country have an equilibrium moisture content above 8%? I understand that a lot of homes have conditioned space with EMC near 8% but probably not as low as 6%.

    I live in Central Ohio where my EMC is low to mid teens. I have a nonconditioned home. So is using kiln dry lumber more of a risk for movement than air dried lumber for me?

  2. #2
    Join Date
    Jun 2014
    Location
    Western PA
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    486
    Cant speak to why the standard is what it is coming out of the kiln, but mid teens sounds high to me. In either event, as long as you allow lumber to acclimate in your shop prior to working with it, i dont think it makes one bit of difference whether it was air or kiln dried. Most lumberyards dont store their stock in conditioned spaces, so im guessing the kiln dried material that is available to you i already acclimated to similar conditions as your unconditioned home/shop. If you have a moisture meter, keep a scrap piece in your home/shop and compare it to the new material you bring in.

  3. #3
    Join Date
    Dec 2010
    Location
    WNY
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    6,123
    You do have heat in your home, yes? So it's not really unconditioned, mostly like just no AC. Your reference to the EMC being in the mid-teens where you live is likely true enough - outdoors. But indoors in mid Winter it's more likely 6 - 8%, almost certainly so if your home is not super tight and you don't use a humidifier. My house got down to less than 30% RH (6% MC) last Winter before I added a humidifier. That's why you want to use KD wood. Of course, in the Summer KD wood will gain moisture in your a house with no AC, but the associated expansion with that generally will cause fewer and more easily managed problems than building something in the Summer with AD wood and having it shrink when Winter comes.

    Both KD and AD wood move when the RH changes, no getting around that. And it's certainly possible to build furniture with AD wood if you plan for and manage the shrinkage that's going to happen in the dry season.

    John

  4. #4
    Join Date
    Oct 2008
    Location
    Columbus, OH
    Posts
    1,644
    Definitely not an expert on this, and I've not had the opportunity to work with AD lumber, but my understanding (from reading) of kiln drying wood to 6% results in a more stable board as stored water in the cell walls is forced out and that water will not be replaced even if overall MC of the board rises back to 8% with moisture from the environment. Air dried lumber will only release the free water between cells. I've also read that kiln drying softens the lignin allowing internal stress to relax before hardening when it cools down.

    Kiln dried wood though can be problematic if it is dried too quickly at high temps. This results in case hardening, where the surface of the board is much drier than the center. A case hardened board can wiggle all over place when ripping. It can get exciting at the TS when you bump into one of those boards.
    Brian

    "Any intelligent fool can make things bigger or more complicated...it takes a touch of genius and a lot of courage to move in the opposite direction." - E.F. Schumacher

  5. #5
    Drying the wood initially from it's natural juices will stabilize the wood. It's different than it drawing moisture from the air. That moisture won't create as much trouble especially since it will find an equilibrium with the environment. I'm sure you've heard most people say it takes a year for every inch thickness to air dry wood. It doesn't take that long to dry wood from humidity. You could take a kiln dried board that has been in an extremely damp place and move it to where it was climate controlled place and it would dry in a couple weeks.

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