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Author Topic: Kiln dried lumber vs air dried  (Read 3905 times)

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Offline Wisconsintimber

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Kiln dried lumber vs air dried
« on: November 25, 2014, 02:48:34 PM »
Ok, so just throwing some questions or scenarios out there...

Starting with kiln dried boards...if they are stored in a non climate controlled shed after being kiln dried won't  they take on moisture and acclimate to whatever the cilmate is?  If so, was there any advantage or other reason to kiln dry?  The reason is  I have seen lumber advertised as kiln dried and then stored in a pole shed, non climate controlled, so just wondering... I understand the target is 6-8% Mc for interior use, but does it stay close to that % after being dried no matter what?

Also a comparison if using kiln dried lumber say for building a piece of furniture vs lumber that has been air dried then acclimated for a month in your house before building with it.  Will the kiln dried still be more stable than the acclimated?

One other related question... say if you need a 10" wide board, will a glue up have less shrink/swell with the season changes than a single 10" board?

Offline beenthere

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Re: Kiln dried lumber vs air dried
« Reply #1 on: November 25, 2014, 03:11:01 PM »
Quote
.if they are stored in a non climate controlled shed after being kiln dried won't  they take on moisture and acclimate to whatever the cilmate is?

Yes, pretty much.
south central Wisconsin
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Offline red oaks lumber

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Re: Kiln dried lumber vs air dried
« Reply #2 on: November 25, 2014, 06:59:07 PM »
with kiln drying you are removing the core water, when it acclumates the core water is still gone. the surface will re adjust to the conditions its exposed to.
 a few years ago i wanted to get rid of a stack of kiln dried hard maple i had in my storage shed for several yrs. the hardwood mill took a couple of random boards for testing. the center of the board still tested alittle over 7% mc. just my thoughts :)

i never glue up peices wider than 4"
the experts think i do things wrong
 over 18 million b.f. processed and 7341 happy customers i disagree

Offline Ianab

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Re: Kiln dried lumber vs air dried
« Reply #3 on: November 25, 2014, 07:22:40 PM »
The whole board will eventually acclimate back to whatever it's environment is. Question is how long it takes.  large dead stacked pile of oak, it will probably take years.  A smaller piece that's exposed to the air? It will change in weeks,

As an experiment a few years back I microwave dried a sample board, to work out it's exact moisture content. Zapped it at low power in the microwave until it stopped loosing weight (on a digital scale). So I was able to calculate it had started at ~14%, which is about EMC for our climate. Then I left the sample on my desk, and weighed it again every day. After 3 weeks it was back up to 12%, again, in line with the EMC of the room.

Now this was a small piece of softwood, that dries (and therefore gains moisture) easily. Other species, or not having it exposed to so much air, it would take longer.

On a related subject, if furniture builders etc here in NZ import a container of kiln dried wood from the US it's still around 6-8% when it arrives. Dead stacked in a sealed box, it doesn't change. Before they use it they have to pull it all out and sticker it for a couple of weeks to come UP to the local moisture levels before it's stable enough to use.

Stability of glue ups? Only thing you can really affect is the way it may cup. If you have a wide flat sawn board it will have a tendency to cup as the moisture changes. If you glue up a table top with the growth rings alternating up and down in each board, then the whole board wont cup in the same direction, and you don't notice the effect on each 4" piece. If the wood is Q-sawn, it's not prone to cupping, so a glue up vs single piece has little effect.
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

Offline hacknchop

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Re: Kiln dried lumber vs air dried
« Reply #4 on: November 25, 2014, 10:50:36 PM »
You asked whether or not kd made a difference, framing is kd more so as to equalize the wood as well as have the grading be more reliable as some defects such as warp  twist efects of compresion wood usually manifest themselves during the drying process.Most softwood species SPF are considered seasoned at 19% but I have seen spruce 2x4&2x6 twist and ruin a finished wall whereas kd material even though it picks up moisture and the loses it again remains stable.
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Offline beenthere

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Re: Kiln dried lumber vs air dried
« Reply #5 on: November 25, 2014, 11:11:21 PM »
hacknchop
Do you see any framing material (dimension lumber) SPF that is not kiln dried? I didn't think any processor would air dry dimension lumber due to the time element.
Or are you referring to S-dry 19% as different from kd ?
south central Wisconsin
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Offline GeneWengert-WoodDoc

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Re: Kiln dried lumber vs air dried
« Reply #6 on: November 26, 2014, 07:50:00 AM »
If you air dry a piece of lumber to the same average MC that you kiln dry to, then the two pieces will essentially be identical, except
1) the KD piece, if heated above 133 F in drying, will be free of insects, eggs, and living fungi.
2) if it is a softwood species, then the sap (or resin) will not ooze out if the kiln was at 160 F or hotter...that is, the pitch has been "set."
3) if you ran the kiln over 160 F, the wood will likely appear to be more brittle when machining, especially if the humidity in the kiln was very low.  A kiln can be operated to avoid this effect however.
4) If you kiln dry from the green MC, you will develop drying stresses, also called Casehardening but the case is actually not harder, that must be removed at the end of drying by using a high humidity environment called conditioning.  In air drying, the high humidity every morning relieves these stresses during the drying cycle.  So, AD followed by KD does not have drying stresses.
5). KD is much faster than AD, and oftentimes time is money
6) KD is more expensive than AD...perhaps $25 per MBF.
7) in almost all locations, you cannot AD under 12% MC, as the outisde humidity is not under 65% RH.  (Some people will take AD lumber and then put it in an attic or heated shop to achieve a lower MC.  They do not use a kiln, but they are also not using air drying totally either.  Almost everyone defines air drying as meaning that the wood is exposed outside.)
8) A kiln will often have a shell to core MC difference unless the kiln uses equalization to avoid this...most hardwood lumber kilns do equalize.  Slow air drying has a fairly uniform MC, shell to core.
9). Warp in air drying is quite low if stacking iprocedures are good.  Warp in kiln drying to the same MC as in air drying has the potential to have even less warp if stacking is excellent (the wood is bent flat), but only with perfect stacking.  In practice, KD wood is lower in MC than AD, and the drier the wood, the more warp, but at the same final MC, KD will be flatter.
10) due to the potential for fast drying in a kiln, the color will be brighter and whiter.  On the other hand, the very hot temperatures in a kiln can also darken the color, so proper kiln operation is required for best color.  The risk of surface checks and honeycomb is higher if the kiln is not run correctly.  In air drying,  the color and checking are controlled by the weather, so do you want to bet your profit based on the weather?

The assumption for these ten items above is that the wood is at the same MC, AD vs. KD.  But, in practice, KD lumber is almost always drier. 

For softwoods, air dried lumber is sold as S-GRN or S-dry.  The MC is above 20% often.  However, KD is sold at 12, 15 or 19% MC.  It is possible to achieve 12% MC in air drying, but the time is very long and potential for insect or fungal damage is present.  Most softwood framing will eventually be around 10% MC in a house wall, or roof or floor joists, so by achieving this low MC in manufacturing by KD, the wood will not warp or change size in use. 

For hardwoods, which are generally dried to 6-8% MC, it is not possible to achieve this low MC in air drying (outside, in a garage or in a shed).  Hardwoods shrink about 1% in width or thickness (maximum, but oak is more) when the MC drops by 4%.  So 12% MC lumber used in furniture will move by over 1% as it dried to the 7% MC that most houses and offices will have.

If you air drying and then use a heated room to achieve the low final MCs, compared to properly run Kiln drying, the two pieces will be identical when it comes to machining, gluing, stability, and so on.  But due to color, insects, sap oozing, and other items mentioned, the KD piece is  often "better."  But note that I did state "a properly run kiln."  The truth is that a few kilns...many more than I would like to see...are not run properly, so their wood is not always the highest quality.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline GeneWengert-WoodDoc

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Re: Kiln dried lumber vs air dried
« Reply #7 on: November 26, 2014, 08:14:18 AM »
Why is SPF kiln dried and often to 12% MC?  The answer is that it is lighter weight (lower shipping costs and easier to handle by carpenters), it will not warp in use, it is free of sap, nails easily, looks clean as it is bright and white, etc.  Compare a piece of SPF to a SYP piece.  The SYP-19 will warp in place, or even before installation, is much heavier, is very difficult to nail without bending the nail, etc.  But the SYP is stiffer and so can span longer openings.  SYP is also easy to treat with preservatives.  So, maybe this is why over 80% of the framing lumber sold in GA (the most forested state in the South) is Canadian SPF, but almost all trusses are SYP and all decking lumber is treated SYP.

If SYP was kiln dried to 12% MC, grading losses would be very high due to warp and nailing would be very difficult.  Maybe this is why SYP is seldom dried after treatment.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline serg

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Re: Kiln dried lumber vs air dried
« Reply #8 on: November 26, 2014, 09:34:16 AM »
Hello friends!
 I began to make the first wooden house bar 85 x 125 mm. Drying high temperature of 320 F. When did the installation of the walls of a wooden house, constantly measures the humidity. After 90 days the walls assembled quickly and easily. No swelling, stable geometry. Humidity 4.1% - 4.6%. Drying heat really gives changes in physical and mechanical properties of pine and other woods. No mold, fungus, beetles.











Offline Jemclimber

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Re: Kiln dried lumber vs air dried
« Reply #9 on: November 26, 2014, 11:00:15 AM »
@GeneWengert-WoodDoc  I think I've heard you say this term before. What is the name that describes high moisture wood (fresh cut) and low moisture wood (KD) placed together in a certain Rh and the EMC of kd will still be lower than the EMC of the fresh cut?  Not sure I am explaining this correctly..
lt15

Offline Wisconsintimber

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Re: Kiln dried lumber vs air dried
« Reply #10 on: November 26, 2014, 12:12:51 PM »
Thanks for all the replies!  Good information, exactly what I'm looking for!

Offline GeneWengert-WoodDoc

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Re: Kiln dried lumber vs air dried
« Reply #11 on: November 26, 2014, 05:05:21 PM »
Let's say that we have 50% RH.  Green wood exposed to this condition will dry to 9% MC.  Wood that was previously dried to 6% MC and is now exposed to 50% RH will increase to about 8-1/4% MC or maybe 8-1/2% MC, but not 9% MC.  This difference is called the hysteresis effect.  Is this what you are looking for?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Jemclimber

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Re: Kiln dried lumber vs air dried
« Reply #12 on: December 01, 2014, 12:34:56 PM »
Yes, thank you very much.
lt15


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