Kiln drying and storing

[DPatton]

I have been kicking around the idea of building a kiln for a couple years now. The problem I have is I don't have a climate controlled building to store dried wood in once it comes out of a kiln. I have a place to store it where I can keep it out of the rain under a roof, but I don't have any control over the temperature or humidity. I do know that I loose many buyers because my products are not kiln dried. I've always been 100% honest about this with potential buyer's and very few are comfortable with purchasing air dried lumber and slabs. 
So I have a question to you experienced members who dry and store wood. How important is it to have a climate controlled storage facility for wood coming out of the kiln. What happens to kiln dried wood if it is not stored in a climate controlled enclosure? I know it will pick up RH but will the wood give up that RH and stabilize once it is taken back into a climate controlled atmosphere like the buyers shop or home? 
Your experience and expert input will be greatly appreciated! 

[WDH]

Water evaporates from wood into the air if the forces of equilibrium point that way.  Water vapor can also leave the air and move into the wood if the forces of equilibrium point that way.  If the wood comes out of the kiln at 7.7% moisture content, it will remain at 7.7% if the relative humidity is 40%.  If the relative humidity is 65%, then moisture will move into the wood from the air and the wood will increase in moisture content to 12%.  As wood regains moisture, it also changes shape in that the wood will swell and become thicker and wider just a bit as it equalizes to the higher relative humidity.

In most places outside, the relative humidity at night will increase, many times to over 90% every night.  Your wood stored in a shed will be exposed to this higher humidity every night even if the daytime humidity stays around 60% or so, so the driving force is for moisture to move out of the air into the wood.  This happens every day in most cases so it is only a matter of time before your kiln dried wood becomes air dried wood again.

If after kiln drying and then placed in non climate controlled environment, and if after the wood regains moisture in equilibrium with the diurnal relative humidity in your area and becomes "air dried" again, and if after being used to make a project and then placed in a climate controlled environment, the wood will lose moisture to the lower relative humidity inside and will shrink in place.  Simply put, it will move as it changes moisture content and this movement can cause issues with whatever the wood was made into.  Would this movement make a significant difference?  Sometimes it does and sometimes it does not, especially if the expected movement was taken into account from a design standpoint. 

Many customers simply do not understand these relationships.  They buy my kiln dried wood and leave it outside or lay the wood flat on a concrete garage floor for weeks or months.  Then stuff happens and the wood regains moisture and moves.  Recently a customer bought some 5/4 18" wide pecan boards from me.  Was going on a trip so unloaded the SUV and put the wood laying on the porch for 5 days and nights while on his trip.  It was cloudy and rainy continuously for several days over this period.  The 18" wide pecan boards cupped.  One side was exposed to high humidity from the air for 5 days.  The other side was flat on the porch floor.  One side gained more moisture than the other side.  One side moved and swelled more than the other.  Seeking equilibrium, the boards cupped.  These same boards were in my lumber room for 6 months and were perfectly flat.  However, they stayed in a stable environment kept at 40% relative humidity.  If he had put them inside his house and not left on the porch in the rainy weather he would have been fine. 

[doc henderson]

how much volume are you talking.  i keep dry wood in a storage container.  and run a home dehumidifier occasionally.  if I do not open it up, I rarely run the humidifier.  like once a month or so, or as it cools down in the fall.  i set the DH at 40%, but if no water is coming out, I turn it off.

[kelLOGg]

If kiln-dried wood is left at ambient conditions will it uptake bound water? My understanding is that it will not. If it does not what difference does that make in joinery in projects? 

[WDH]

My understanding is that all water in wood is bound until you exceed fiber saturation point. 

[GeneWengert-WoodDoc]

The term "bound water" refers to water that is held within the cell wall by hydrogen binding.  This bounding is not permanent and therefore is reversible.   That is, water can leave if the wood or the surrounding humidity is lowered, or can gain if the surrounding relative humidity increases.  The water that leaves whenever the wood dry is  because the RH drops is bound water.  Water that goes into the wood because the RH increases is bound water.  In fact, water is not fixed, but even without a change in humidity, some molecules leave and others come in to replace those that left.

If you have wood dried to 12% MC at 65% RH, and then the wood is exposed to 50% RH, the wood will dry to 9% MC.  Now, if you go back to 66% RH, the wood will only go to about 11% MC.  The. If you go back to 50% RH, the wood will go back to close to 9% MC.  Now, if you dry to 30% RH, the wood goes to 6% MC.  Then go back to 50% RH, the wood will gain back to about 8% MC, not 9% MC.  The seemly different values when losing versus when gaining MC is called the hysteresis effect.  The EMC values for air in the tables of humidity and MC are for drying or moisture loss. They also are for spruce.  Actually, the spruce values are, from a practical point, ok for all major North American species, except southern pine.  They are off for many tropical hardwood by as much as 8% EMC for high humidity values.  Everything agrees at 0% RH, although the oven test for sappy softwoods can be in error because some softwoods will lose both water and resin and the oven-dry test counts the resin lose as water loss.

I know this is highly technical and maybe confusing when reading this for the first time.  On the other hand, if you were ever asked to be an expert witness or even just an expert for a lawyer, you need to know this.  

This hysteresis effect also explains much of the reason that when drying, we shoot for lumber a little on the drier side, as this 1% lag when exposed to higher RH helps when dealing with shrinkage and movement after drying.  That is, if we dry to 7% MC, if the wood is exposed to 30% RH, it will dry 1% MC more.  If dried to 7% Mc and is exposed to 50% RH, it will gain 1% MC and not 2% MC.  Humidity in a typical house or office in North America humidity will range from 30% average RH in wintertime to 50% average RH in summertime.

The original question was about wood storage.  If you have a fairly tight building, with plastic on the walls, with no moisture from the floor, and if you paint the roof black, and maybe have a very small fan to stir the air, you will get an average of 7% to 8% EMC.  You might be off in the winter due to limited hours of sunlight and low sun angles, but water movement in cold wood is really slow when cold.  Even at 90F,  ovement is not that fast.

[doc henderson]

you can think of the wood as at least two components (keeping it simple).  the wood structure for one, and water as number two.  If you remove water, the weight decreases and the thickness and width will change predictably.  If you made a tight glued joint, like a dove tail box, it may destroy itself over time, and you have an unhappy wood consumer.  I made a cedar chest from lumber yard wood.  the design was rail and stile with raised panels.  made in about 1992.  the boards were 12 inches wide, and I got glue on the edges at assembly, as I really did not believe it was an issue.  several of the panels are split down the center, as everything shrank.  there is not much point in kiln drying, if moisture is just going back up, except you may sterilize and set pitch.  after it is dry, you can wrap bundles tightly in plastic, but hard to show customers.  you could line a room with vapor barrier, and make a tight door.  tell customers it is your wood vault.     you could use plastic with a zippered door as is used on commercial construction projects.  It will not last forever.

[doc henderson]

here is a pic of my old project.  In a hall way so not great pics, but you can see the panel with a split, but mostly intact.  the wood was from a lumber store and kept inside, but not dried to low MC.



 

 

 

 

[boonesyard]

So, what happens to kiln dried wood, lets say to 7%, when stored in a non air-tight shed when the temps are below freezing? For instance, today the humidity is 83% with a temp of 22 above? 

[Larry]

Don't know the answer to that one......just thought I would post a link to a handy dandy EMC chart.  The chart doesn't go to that low of temperature.

I printed it out years ago and it seems to be an accurate predictor of MC.

https://www.finewoodworking.com/FWNPDFfree/equilibrium-moisture-content-chart.pdf

[Andries]

@boonesyard; Larry's chart is one that I have in my shop as well.
It's accurate when woodworking, but it's good to have a rule of thumb when the chart isn't close by.
Divide RH by five to get EMC. (A Rough but Ready estimate)
So, look at the weather report, get your relative humidity.
Divide that by five, that the moisture that your wood is eventually moving to.
You said 83% relative humidity, that becomes 16.6 %
However, at 22 degrees, with five degrees of frost in your 7% kd. wood, it might never get there as long as it stays frozen.
As @WDH sez: us glaciated types do things different.
.
That's just my blather, I'd be interested in @GeneWengert-WoodDoc response, the man's a walking WikiPedia !

[doc henderson]

the temp helps regulate the speed (energy) of water movement.  when the water is a solid, it evaporates very slow, but can go from a solid to a gas in a dry environment,  freeze dried.  even below freezing there is water in the air, i.e. 83% relative humidity.  the relative means how much water is in the air relative to how much can be held (100%RH)  at that temp.  so at night as the temps go down, the RH goes up, even though the grams of water per unit volume may not change.  since the water gain or loss from a board is slow, we speak in terms of the average daily RH, and the resulting EMC that will be achieved.  so if the 22° and 83% RH is at night, the average RH is lower as the temps rise during the day.  temperature is heat density, so even ice has heat in it.  and the actual amount (in grams) of water in the air at 22° and 83% RH is much less than at 100° and 83% RH.  so I "think" it will still gain moisture in the wood, but slower.  @GeneWengert-WoodDoc .

[Andries]

. . .  temperature is heat density, so even ice has heat in it. . . . 

Doc; now you're starting to talk like a Canadian!
:D :D :D

[firefighter ontheside]

I was in the same situation as you.  I was selling air dried lumber and slabs.  I could not charge what I wanted to, so I built a 10x16 shed while my county was under a stay at home order this spring.  It is insulated only to R13.  It efficiently dried air dried wood down to 7% using a residential DH.  In the summer it would get to over 100deg in there easily.  Then when I wanted to sanitize the lumber I could not get over 130deg until I added 2" foam to the walls and ceiling, but only in a chamber that I created withing the shed.  This chamber is about 5x12 and can be disassembled when not in use.  I still do not have a place to store the kiln dried wood, so I am selling out of the kiln/shed.  Once all or most is sold I reload for another kiln run.  Of course the next goal is storage so I can dry and sell at same time.  I don't think that will happen til i build a bigger shop.

[Stephen1]

Don't know the answer to that one......just thought I would post a link to a handy dandy EMC chart.  The chart doesn't go to that low of temperature.

I printed it out years ago and it seems to be an accurate predictor of MC.

https://www.finewoodworking.com/FWNPDFfree/equilibrium-moisture-content-chart.pdf

I printed off a few copies for my shop.Thanks for the post

[farmfromkansas]

I had plans to buy a 53' storage container last spring, but things got put off.  My plan is to store dry lumber in the storage container so I can fill my dryer up with some air dried lumber. Sure is nice to open the doors and load up a pickup load of lumber dry enough to go straight to the shop and cut up for projects.

[YellowHammer]

Kiln drying does many things from a sales standpoint, all are beneficial for the customer, all are beneficial for the seller, and after going through the effort and cost of kiln drying lumber, its makes sense to have a place to safely store it.

It's a classic case of the kiln itself is just the beginning of the costs.

When I first started, I had open air lumber racks under the wing of my barn where I would sell my kiln dried wood.  I monitored my moisture content, and even though I dried my lumber to 7%, I never overloaded my racks, and I stored my extra lumber in my wife's garage.  When the racks would get low, I would restock them.  Basically, I timed my sales with the amount of wood I had out for sale.  I could turn over my wood fast enough where it would get sold before it gained back much moisture.

The situation finally reached a critical point when I had all of our 3 car garages full of lumber, and we built a large building, but buy that time we were selling enough kiln dried lumber to mare than make up the cost.

While kiln dried lumber demands a higher price, its real benefit to us is that we will sell 100 times more of it than green lumber.    

 

[boonesyard]

the temp helps regulate the speed (energy) of water movement.  when the water is a solid, it evaporates very slow, but can go from a solid to a gas in a dry environment,  freeze dried.  even below freezing there is water in the air, i.e. 83% relative humidity.  the relative means how much water is in the air relative to how much can be held (100%RH)  at that temp.  so at night as the temps go down, the RH goes up, even though the grams of water per unit volume may not change.  since the water gain or loss from a board is slow, we speak in terms of the average daily RH, and the resulting EMC that will be achieved.  so if the 22° and 83% RH is at night, the average RH is lower as the temps rise during the day.  temperature is heat density, so even ice has heat in it.  and the actual amount (in grams) of water in the air at 22° and 83% RH is much less than at 100° and 83% RH.  so I "think" it will still gain moisture in the wood, but slower.  @GeneWengert-WoodDoc .

My head hurts  say_what

[GeneWengert-WoodDoc]

This style of door is called a raised panel door.  The center panel is fastened tightly only at the midpoint, top and bottom.  Then small, soft rubber balls or similar, called space balls, are placed in the grooves for the edges before the door is assembled in order to keep the panel edges from rattling.  The panel is free to expand and contract?  Oftentimes the panel is prefinished so when it shrinks it does not expose unfinished wood of a different color.  Such construction prevents the panel from splitting when it shrinks and allows expansion without breaking the frame joints.

[GeneWengert-WoodDoc]

The EMC chart is incorrectly labeled.  EMC is a characteristic of the air.  It is not correct to call the chart the "EMC of wood."

Note that for a given RH, the EMC changes very little as the temperature changes.  Also note that except for the very low and very high RH values, the EMC is very close to RH divided by 5.

Now EMC is based on the MC of thin strips of spruce when drying.  There would be a small difference if they had used a different species and if the strips were thicker.  In North America, only southern pine has a big difference in MC from the predictions of EMC.  Tropical wood species are often quite far off.  Further, after drying and then the moisture regained, the actual MC is about 1% lower than the EMC.  This is the hysteresis effect.  Also, note that if the wood is dried at over 160 F, give or take, the actual numbers change substantially...more change as the wood is dried hotter.  So, wood veneer and chips that are often dried at temperatures over 200 F, will have MC values much lower than the EMC values.  

[GeneWengert-WoodDoc]

Here is a question...will wood shrink if put into boiling water and kept submerged?  Answer:  yes, as the FSP of wood drops to around 22% MC.  Interesting as the amount of liquid water in the hollow pores of the wood changes very little, but the water in the walls, bound water, changes and results in shrinkage.

Here is another..Does the water in the walls when the wood is under FSP, let's say 10% MC, freeze when the wood is at 0 F?  Answer: No.  we think of water as being solid, liquid or gas, but when absorbed by the wood cells and connected by hydrogen bonding, the water is not solid, liquid or gas, but is held in what we might call a molecular form and so is not solid, liquid or gas.

Confused?  A nice egg nog or similar will allow you to forget about this and not worry.

[doc henderson]

thanks! and Happy Holidays Doc!  @GeneWengert-WoodDoc 
:new_year:

[Walnut Beast]

So it seems that with all the technical information that if your kiln drying wood and storing it in a non climate controlled environment and it sits there for any length of time that you are wasting your time. And if your selling kiln dried wood from that example it really isn't 

[doc henderson]

you gain a bit if it is sterilized and the pitch is set, and sounds like with hysteresis, it might stay 1% under EMC.  but you can dead stack it dry and wrap tight with plastic and preserve the MC at 7% say, but hard to show the wood.  would work ok if you sell a whole bundle, and not too hard to repackage.  

[YellowHammer]

Wetting wood after drying sometimes can cause marginal movement, sometimes its very bad.

The worst case is when we have people buy our kiln dried wood on a rainy day and want to load it in their open bed pick truck, and drive back home, in the rain.  We won't "let" them and give them plastic sheets to wrap their wood in.

Some folks are surprised that water will damage or even affect kiln dried wood.  So I throw a Yellowhammerism at them "If you take a sheet of paper and get it wet, it will swell and wrinkle.  When it dries out, it will stay wrinkled.  What's paper made from?  Wood.  What do you think your wood will do if you get it wet?  The same thing.  It'll dry, but it won't ever be flat again."