Is Kiln Drying to 7% Worth It?

      Why kiln dry wood to 7% moisture content when it will gain moisture in storage? Here's an informative thread about proper drying targets and storage based on the intended end use of the lumber. December 31, 2013

Question (WOODWEB Member) :
I've been spending a lot of time, money and effort trying to dry all my lumber down to 7% in the core with probes and sample pieces. When itís done drying it goes in my big unconditioned pole barn (every lumber yard here in central Michigan has unconditioned lumber storage) and in a few weeks its reading 10-12%. Why bother to kiln dry to 7% in the first place? The only thing I can think of is sterilization, but I run the kiln at 140 for the last four days anyhow. The first 24 hours at 140 should do it.

Why fiddle so much with the final MC if it always sits a few months before the customer gets it anyhow. It seems like this is a lot of wasted effort. Am I missing something? My air dried lumber is close to 12% MC. Can't I just put in at 140 for a couple days and call it kiln dried and sterilized? Do most hardwood furniture grade lumber yards have conditioned storage of all their stock when it comes out of their kilns?

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
I doubt that the internal temperature of the wood will reach 133F when the air is 140F. I also doubt that you will increase the MC as much as you indicate in such a short time. You can wrap the lumber in plastic wrap and then store it for years. Air drying to 12% MC is excessively dry and is not usually done in air drying as degrade is high. A common value is 25% to 20% MC in air drying.



From the original questioner:
I sell lumber in a co-op that has unconditioned space for our lumber sales. I've taken a moisture meter and gone over the whole area. We have 4-5000 board feet in vertical racks for customers to sort through, and most of the wood (all kiln dried) is reading between 10-12%. True, Iím not sure how long it takes from out of the kiln to get there, maybe a month or so, but itís usually there by the time itís sold. I'm not sure how other lumber yards keep their stock dry unless itís air conditioned and heated which seems quite expensive, or they count on quick turn around? I still don't quite understand how it works out.

On the internal temps of 4/4 lumber reaching 133 degrees, do Nyle type dehumidification kilns not have the ability to sterilize then?, I've been told not to run them over 140 by Nyle. I've not thought about the idea that the lumber might not get to 133 even after a number of days at 140. This is a good question to figure out. Might have to call Nyle and see what they suggest.



From Contributor N:
I've seen kiln operators simply heat wet wood to that temperature and call it kiln dried. I've seen many examples of where a kiln operator will dry the wood fast, leaving the inner wood soaking wet but still calling it kiln dried as the outer shell reads 10% or so. To me itís a weak way to run a kiln. One guy I know takes his down to 6% in the kiln so he can rely on the wood being dry and stable even after it re-moistens due to post-kiln environments. I think it's a matter of what you want at the end. Sounds to me like you are making a good product, but I do not know how absolute that extra drying is.


From Contributor Z:
The answer depends on who you're selling to. Many wood sellers have conditioned warehouse space. Most (not all) also manage their inventory so it's not sitting around for a long time as well. If your customer is a manufacturer who's depending on 6%-9% then they will specify that and you'd need to provide that if you want their business. That said, there are many people who are content with 10%-12%. There are also some people who prefer air dried stock. Maybe your COOP can do some experimenting with a section of air dried wood. Then you can determine if your market will support it and you can answer your question of whether it matters.


From contributor V:
I was told the same thing by Nyle that once the chamber temp has reached 140F, hold for 24 hours to ensure the internal temp reaches 133F to kill any bugs. Have any tests been done to verify what the temp of the chamber must reach and how long it must be maintained for the wood to reach internal temp of133F on 4/4 material?


From the original questioner:
I haven't contacted of Nyle yet about kiln temps, but as far as lumber storage I've been to a fair number of high-end hardwood furniture grade pro-lumber dealers all around the Midwest and have never once seen their kiln dried lumber stored in air conditioned/heated buildings, nor have I ever seen them wrapped in plastic. Itís always in big pole buildings. I'm sure some places do but these are very large operations that deliver all over the Midwest, not mom and pop mills. I can see the merits of kiln drying to 9% but in my loads of quartersawn white oak it can take an extra four or more days to get the core to seven. I can't figure out why, unless all downstream handling is conditioned space. I don't have it, where I sell doesn't have it, and the industry as a whole around me doesn't have it. So I'm not seeing a great argument for tying up the kiln and paying to get to 7%. Maybe it's just for certain applications. All people around me want is kiln dried and always explain that it comes out at 7 but sits here in storage so goes up to 10-12% eventually.


From Gene Wengert, forum technical advisor:
It is critical for furniture, cabinets and even flooring that the wood be at or very close to 7% MC. Open sheds are often used for storing softwoods, but seldom for hardwoods. Closed sheds can be safely used for hardwoods for a month or so, especially in cold weather. There are quite a few plants that use an in-line MC meter to check the incoming MC, so wet lumber can be avoided. Oak at 10% MC will shrink about 1% in width as it dried back to 7% MC. On a 24" wide table top, this is 1/4" and will often show up as an open joint. In a 30 foot wide floor, this will be several inches of movement, often showing up as cracks between the pieces.

Of course, if the grain is not straight, some warp can also be seen. Anyone delivering 10% or wetter lumber to a furniture, cabinet or flooring plant can expect to hear back about wet lumber. There are many cases where such wet wood has resulted in lawsuits with damages exceeding $1 million in some cases.

Bottom line: You need to dry to 7% MC for hardwoods and then provide storage facilities at 35% RH (7% EMC). You should also meter at least ten pieces just before you ship, to prove that the wood is still at the correct MC and that any problems later would be because of poor shipping or storage by the customer and not by you. In fact, it would be wise to specify that all your MC is measured with a Model 2000 Delmhorst insulated pin meter (or similar Lignomat), as some customers use an inferior meter.



From the original questioner:
I understand this would be how itís done in the industry, but Iím a artesian sawmill who cuts and sells maybe 20,000 board feet per year. I sell to small cabinet shops and hobbyists. I do want to sell them the best product they can get, but having conditioned space for our lumber storage is uneconomical. The plastic wrap makes sense for small amounts, but probably hard for customers to look through.

Maybe summer is the wettest time and I can keep the lumber in a big tent in the barn and dehumidify, though our winters are quite humid too. It often takes six months or more to turn over our stock. I guess Iíll have to see how I can keep the lumber dryer in storage, but Iím still stuck with the fact that the facility that retails our lumber is unconditioned and the lumber will simply go to 10-12% over the summer months. I did talk with the Nyle guys about the heat treating, and was told they normally suggest keeping at 140 for 24 hours.



From Gene Wengert, forum technical advisor:
You can buy clear plastic wrap from a place like Costco. It wouldnít cost a lot to wrap, and even rewrap. Make sure that there is plastic on the bottom too. You can also buy big plastic bags that you could lift off to show the wood and then cover the wood again. In most places in the USA the humidity outside is the same, summer and winter.


From contributor Y:
Gene, is there any advantage of KDing to 7% if the material still comes back up a few points, to say 10%? Is there any advantage in the amount the cellular structure shrinks in the kiln that helps the material maintain some form of stability in seasonal fluctuation of RH?

I see moisture creep back in to dry lumber as quick as the original poster does. A stick dried to 8% will be about 11-12% in just a few weeks in unheated dry storage (metal roof). But bring it indoors, still unheated, and it will maintain the MC within an acceptable range.

Why is ADing to 12% too dry? Many woodworkers would kill for real seasoned AD material that was dry enough for use in furniture work. I try to dry to 15% or less before kilning (five months for 4/4 in Hawaii, most all species, 12-13% in six months).

I think harvesting, drying, storing, end use and the size of your operation all create unique circumstances that a strict scientific method cannot work with. I think many people especially in Northern N. America specify KD because of the obvious disadvantage of using wet lumber. Typical AD may only be 15% at best. Here in Hawaii many people will use 15% AD for furniture and cabinets depending on the climatic zone.

If there is no benefit of drying to target when the material will experience gain in storage, maybe the OP can only dry to 10% then custom dry for orders and get it out the door fast. It also sounds like if youíre going to sell retail then you should invest in a show room that has some form of climate control. At least make it a bit tighter.

I have a friend that I dry small custom loads for. I dry to 8%. His storage and the duration it sits there yield him 12-14% final, but he still sells it as KD. I do not believe you can even call it KD after it gains so much. For personal use I only try to dry enough for a month's work at a time.



From Gene Wengert, forum technical advisor:
Understand that for hardwoods there is no specific MC level associated with the term kiln dried. The EMC in Hawaii is 11% to 14% EMC. It will take a long time for wood to reach under 14% MC if the EMC is so high. If wood changed MC that fast, there would be little need for a kiln. (Most of the 48 states, the EMC is 12% year round.)

Once dried to 7% MC, the wood, when exposed to 12% EMC will not come to 12% MC, but more likely will reach 11% MC on the surface. The core will take many months to change from 7% MC to even 9% or 10% MC. I do wonder if the MC readings you give in your example might be taken with a pinless meter and not with an insulated pin meter. If the EMC is 12% then the surface might be 11% MC (due to hysteresis, it will not reach 12% MC) and the core will be 7% or maybe 8% MC in a month. So the average MC for the entire piece will be perhaps 9% MC, but probably even less after a month or so. We did a big study here in Wisconsin a few years ago monitoring the MC of lumber in unheated storage and found that the average MC gain was under 1% for the first month. Of course, the surface and ends did gain more.

As mentioned before, it takes about four gallons of water to change the MC of 1000 BF by 1% MC, so when 7% MC lumber is reported to be 12% MC, that is about 20 gallons of water that had to go into the wood. This is why using plastic will prevent virtually all moisture change, as there is little moisture that can enter through the wrapping. Also, when the EMC is 12% EMC, the average MC of the surface is likely only 11% and the interior of the wood would be even less. So, it would be hard to imagine that lumber dried to 7% MC can reach 12% MC when stored at 12% EMC (the conditions in most of the 48 states).

Incidentally, if the EMC is 12% EMC, it will be hard to dry a piece of lumber to 12% MC. The surface and ends might reach 12% MC, but the core will be wetter. Of course, in air drying where we have rain on the lumber, the lumber would be even wetter.

Generally, we do not want to air dry much under 25% MC as it takes too long; if the lumber gets wet, checking and warp will be worsen; and color loss is a risk. If we air dry under cover, the quality will be maintained, but the length of time makes air drying to a low MC too expensive. At some point (usually 25% MC), it is cheaper to kiln dry than to continue to air dry.

Note that in Hawaii we usually do not have central heating like we do in much of the 48. Central heating makes the interior EMC around 6% or 7% EMC. In Hawaii with open windows, your interior conditions are close to 10% EMC (50% RH) or a bit higher, so that 10% MC is going to work very well. No need to reach 7% MC unless the furniture is going to a dry interior location.



From contributor Y:
Thank you Gene for responding. Most of the severe moisture gains I have experienced is during rain seasons when the RH doesn't come below 85-90% (inside for weeks and months to a time). This spring /summer was a wet one for sure. Material I dried to 8% (because it will most likely end up in a home with AC and low RH, about 40%) gained until it was 14-15% again then back into the kiln. Yes a pinless meter (wagner) but on most any 4/4 reads like my Delmhorst. I did just change the battery and the readings dropped .9%. Could it be stronger depth readings? Is the core dryer than surface?

I guess my comment was smaller operations and individuals that are drying for instruments and artisan furniture works may not share the same philosophies on drying that larger commercial industry organismís do. Some woods retain a richer and more vibrant color when AD longer before kiln-drying the last bit. Some species and specimen are too precious to be regarded and processed as lumber. When AD to 15% or less I can dry to 8% for less than 12 cents a foot. The lumber is very stress free as well. Some of the kindest to re-saw.



From Gene Wengert, forum technical advisor:
Take a dry piece of lumber and wet one side with a wet cloth. Now read the MC with your pinless meter on one side and then the other. With the wetting, which simulates the high RH you might get and the gain on the surface, you can see the effect.



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