Are they different to saw, chisel and plane? Does it matter?
Note: This is the second article in our series where we do deep research into thorny woodworking topics. The first article was on mahogany, a beautiful material that carries environmental and human rights baggage. This month, we look at wood-drying processes. So buckle up.
For the first seven years I was in the woodworking business, I used kiln-dried lumber exclusively. Not by choice. That’s just what was available at the commercial yards here in Cincinnati.
But my fellow employees talked a lot about air-dried wood. How its color was better. And working with it? It was much more pleasant to saw and plane. And the kiln-dried wood we were building with every day? They told me this:
Kiln-dried wood is brittle.
You can’t bend it with steam.
Its cell walls have been collapsed, so it can’t take on much moisture.
It moves less with the seasons.
It’s harder to work.
It’s filled with stress and honeycombing.
It’s much more likely to warp.
I always wondered if their ideas about air-dried wood were a “grass is always greener” thing. After all, wood is incredibly variable. Not only from species to species, but from tree to tree – even within the same tree. How can you make blanket statements like “it’s harder to work?”
In 2003, when I started building chairs, I got my first taste of working with air-dried ash and red elm. My conclusion?
“Yep. It’s wood alright.”
Since then I’ve worked with wood that has been dried by every means available. And I still struggle to make generalizations about the differences between air-dried and kiln-dried stuff. So we asked our managing editor, Kara Gebhart Uhl, to spend weeks researching the published literature and talking to people who live and breathe the drying process.
As it turns out, these are some differences. They surprised me. Here is her report.
— Christopher Schwarz
Folks have strong opinions about air-dried vs. kiln-dried wood. But lately we’ve wondered if there really is much of a difference when it is being worked on the bench? It turns out that little research has been done to compare the two. So, we talked to some experts to find out.
First, a Good Explanation of Moisture Content (MC)
“The total amount of water in a given piece of wood is called its moisture content (MC),” writes James E. Reep, wood products and utilization specialist at the University of Kentucky’s Cooperative Extension Service, in “Drying Wood”.1 Although we are accustomed to the fact that 100 percent signifies the total amount of something, the MC percent of wood can be greater than 100 percent. This occurs because the water can weigh more than the wood, and the MC of wood is usually based on the ratio of the weight of the water to the weight of the wood after it has been dried.”
What Happens to Wood When It Dries?
In simple terms, wood shrinks when it loses water and swells when it gains water, says Stavros Avramidis, professor head of the Department of Wood Science, Forest Sciences Center at The University of British Columbia and current president of the International Academy of Wood Science.
“A dry piece of wood will exchange water molecules with the surrounding air according to the level of atmospheric relative humidity,” Reep writes in “Drying Wood.” “Loss or gain of moisture in wood products may cause such troublesome results as shrinking or swelling, interference with paint adhesion, and increased susceptibility to decay and stain.”
Avramidis agrees.
“There are all kinds of defects that may happen when you take water away from wood,” he says. “That’s one of the major problems that we have with this material. Water is part of the life of the tree, so it’s part of wood.”
Avramidis likes to think of a piece of wood as a 2-year-old at bedtime. If you abruptly tell the child to stop playing and go to bed, “you’re going to have a reaction, and sometimes, it’s going to be really bad,” Avramidis says. “But if you do it slowly and use the right words, you might not have a negative reaction.”
A 2-year-old having a tantrum may cry, kick, throw, hit, scream and go limp.
Wood can also react strongly. It can bow, crack, crook, check, cup, shake, shrink, stain, twist and warp. Removing water too fast can also cause internal stress.
As such, for centuries, we’ve sought out ways to avoid tantrums when removing moisture from wood.
The History of Drying Wood
In 2019, near a river basin above Kalambo Falls in Zambia, archeologists discovered “two interlocking logs joined transversely by an intentionally cut notch,” according to a 2023 article in Nature. Using luminescence, the archeologists estimated this rare find was 476,000 years old.
Woodworking, it seems, pre-dates Homo sapiens.
We don’t know when humans realized the benefits of drying wood, exactly, but we do know people have been air drying wood for thousands of years – and trying to speed up the process. Avramidis says that 5,000 to 6,000 years ago, Egyptians were putting wood over fire to accelerate the air-drying process.
For years, air-dried wood worked well in houses that lacked efficient heating and cooling methods and insulation.
“You didn’t get really low-equilibrium moisture contents in houses in the winter, so there wasn’t as much need for kiln drying,” says Mike Milota, professor emeritus of Wood Science & Engineering, College of Forestry, Oregon State University. “And then as housing changed and we got more heating in houses, it became more of a necessity.”
According to a 1923 article in The California Lumber Merchant, contributed by the Moore Dry Kiln Co., “The dry kiln came into commercial use, in a limited way, about 1870. … The builders kept pace with the changing and enlarging mill conditions and developed the dry kiln to meet demands so that from 1890 to 1900 it became a valuable part of the manufacturing equipment in some sections. At this time, it was so decidedly apparent to many mills that their dry kiln boards indicated a difference between profit and loss. In these early days when the lumber industry was young the dry kiln through sheer merit proved itself a new source of increased profit.”
William Smith, professor and director of the Wood Utilization Service in the Department of Sustainable Resources Management at SUNY College of Environmental Science and Forestry, says the basic kiln-drying process – controlling temperature and relative humidity – has not changed much.
“Over the years, there have been continual incremental improvements in more precisely controlling these conditions,” Smith says.
For example, we’ve learned how to dry softwoods relatively fast, using bigger fans for better airflow.
Air-dried Lumber
“This is an old and very traditional form of drying,” writes Lost Art Press author Richard Jones in “Cut & Dried.” “First, converted planks are swept clear of sawdust produced by the milling to deter mould growth during subsequent drying. The boards are piled up with stickers between each layer for air to circulate. The ground should be firm; a concrete base is ideal.”
Air-drying is done by home woodworkers and large companies alike. Avramidis recalls visiting huge operations in the southeast U.S. where millions of board feet of wood were air-dried on thousands of acres.
Different types of air drying include forced air drying, accelerated air drying, low-temperature warehouse pre-drying and climate chambers.
“Air drying outdoors is the least-controllable method to dry wood,” Jones writes. “The speed at which the material dries and the final moisture content of the end product is rather dependent on the weather and the climate.”
For example, if it rains nonstop for days, there will not be much air drying, Avramidis says.
“Of course, the wood needs to be protected, not exposed to rain or sunlight,” Avramidis says. “You cover the wood, but you don’t have air circulation. It can be windy one day, but when there is no wind, the drying slows down significantly. You can’t control air temperature, humidity and velocity. This is why humans decided to build kilns.”
Kiln-dried Lumber
Kiln drying relies on three things: heat, humidity and air circulation.
“The higher the temperature, the lower the relative humidity, the faster the water comes out of the material – it’s as simple as that,” Avramidis says. “And a dry kiln is nothing but a box where you can control the humidity, the temperature and the air circulation inside the box.”
The fundamentals of kiln drying have remained the same for more than a century.
“What’s changed is the technology,” Avramidis says. “Some remote places still use old-technology kilns from the ’50s, ’60s and ’70s. They have analog systems. Then, the CPU appeared in the ’80s, and now everything is digital. Some kilns now use AI for better control of the conditions – the temperature, the humidity and the air velocity inside the kiln. Some systems are actually very sophisticated where the velocity on one side of the kiln is different than the air velocity from the other side of the kiln because of different moisture contents.”
Today, Smith says most hardwood companies utilize kiln samples, which are boards about 30" long, to monitor moisture content levels and adjust temperature and humidity schedules appropriately.
“A substantial proportion of mills use steam, mostly generated by burning wood waste such as sawdust and chips, to heat kilns, with vents to exhaust excess humidity,” Smith says. “Boilers are also heated by some with natural gas, oil or propane when economic conditions make sense and attractive alternative markets exist for mill residues. Dehumidification drying is also utilized, where excess humidity is removed via heat pump condensation.”
According to “An Overview of Drying Hardwood Lumber” published by The Ohio State University Extension, the final moisture content for kiln-dried hardwood is typically 6 to 8 percent in North America, with residual stress relieved (more on that below). In Europe, the target moisture content for furniture-grade wood is 12 percent, plus or minus 3 percent. (You can read more about this in section 8.10.1 of “Cut & Dried.”)
“Hardwoods tend to be pretty small sawmills whereas softwoods tend to be large,” Milota says. “I was in a softwood mill last week and they dry 800,000 board feet a day. That’s more than most hardwoods do in a month. That mill was running 14 kilns, and they were changing them out all the time. They were running a 30- to 45-hour drying process.”
In addition to technology, there have been some other kiln-drying discoveries along the way.
“In the Appalachian Mountains, there were and still are some really small family operations that dry hardwoods,” Avramidis says. “These are small sawmills, and they have small kilns, and at 6 p.m., they go home. Nobody stays back so they have to shut off the kiln. They cannot let the kiln run overnight because what happens if something goes crazy? Nobody’s going to be there to fix it or change things that need to be changed. So they turn it off and then they come in the morning and they turn it on again. Over time, they discovered that this produces a very relaxed and very high-quality wood. Why? Because during the night, the wood relaxes.”
This is called oscillating drying.
In the ’70s, Avramidis says, Sweden developed continuous or progressive kiln drying.
With batch drying, also called compartment or traditional, drying, “You have a box, you open the door, you push the wood inside, you close the door, you do your drying, for one, two, three, four, five days,” Avramidis says. “When the drying is done, you open the door, take the wood out, put fresh wood in, close the door.”
Progressive drying is basically a long tunnel.
“The wood moves slowly through the tunnel, goes in wet, comes out from the other side dry,” Avramidis says. “The tunnel might be the length of a football field. And it might take three or four or five days for the wood to move from one side to the other side. It’s an old idea, but the Scandinavians commercialized it back in the early ’80s.”
If you’re only drying one kind of species, progressive drying is great.
“But here we have softwoods, we have hardwoods, there’s lots of variability in North America,” Avramidis says. “In Europe, when you talk about commodity lumber, 2x4s and whatnot, for construction purposes, it’s Norway spruce.”
For this reason, North America shied away from progressive drying for a while. But some kiln manufacturers couldn’t stop thinking about the ease of the process.
“Why bother opening doors, closing doors, doing this, doing that, and then not knowing how much you produce every day?” Avramidis says. “Progressive drying is like sausage production – in and out, you know exactly how much kiln-dry wood you have every day.”
Eventually, a North American went to Europe and bought a progressive kiln.
“And you know how it is,” Avramidis says. “You have to go to the dealership and kick the tires before you can buy something. So once this company installed a progressive kiln, others came looking. ‘Oh, look at this! Oh, you don’t have to open and close doors! You don’t have to have people typing drying schedules! And oh boy, we know exactly how many thousands of board feet of KD lumber we produce every day. I want one of those.’ Then it was, ‘I want one of those,’ and ‘I want one of those.’ Now we have, I don’t know how many, I’ve lost count.”
For some of the largest lumber companies, such as Interfor and Western, progressive kilns have been a boon as they’ve shifted from 2x4s to cross-laminated timber (CLT).
“But if you want softwoods to manufacture windows, doors, furniture and things like that, the continuous process is not the one to go with,” Avramidis says. “You have to go back to the batch process. Why? Because you cannot have stress relief in the continuous process. And, of course, hardwoods should be dried in boxes only.”
Other specialized types of kiln drying exist, such as vacuum drying, electrical seasoning and superheated steam vacuum (SSV) drying, but not many companies use them.
“We develop the Ferraris and the Lamborghinis, but very people use those cars,” Avramidis says. “Most of the population, they use the Hondas, the Chevys.”
Stress
Another important development in kiln drying has been the introduction of stress relief, also called conditioning or case-hardening relief.
Read the original article
Comments
Wood expands and contracts with moisture content. More moisture makes the fibers "fatten up".
The interesting thing is that this is anisotropic: the expansion/contraction occurs across the grain, NOT along the grain. The rate of expansion also depends on the local characteristics of the grain itself (hence the effects of warping due to uneven expansion) ... Also there's a big difference between the direction "across the growth rings" (i.e. radially when it was still a tree) and tangentially to the growth rings. And these surfaces are curved, of course. But one thing we can always say is: the wood doesn't significantly change size along the grain.
Design and construction methods can make wooden artifacts more or less susceptible to cracking and distortion from this. For example dovetail joints can be pretty good as all the wood expands/contacts together the same way. Especially if the pieces are joined together from the same piece of wood. Stuff like that. Or at the other extreme, metal fixings like nails don't move with moisture at all, which can cause problems with relative movement and stress can accumulate.
Edit: and the repeated cycling of moisture content induced stress can eventually lead to cracking, in a similar way to metal fatigue. Old wood just cracks sometimes, this is probably why.
A good mental model for wood is that trees are a bunch of stacked cones (growth rings) on top of each other.
In the spring it fills with water and the diameter grows but the tree does not get longer because it needs to support a large mass on top and the lengthwise fibers are not able to grow and shrink (they need to be stiff to carry the weight).
Because of this, the circumference of the outermost growth rings need to grow more than the inner ones.
Now cut a board out of it and look at the end grain. Think what happens when the rings closer to the outside need to shrink more than the inner ones for the same humidity change. For a flat sawn board, you will always see it cup so that the concave side is on the outside.
This doesn't explain why boards twist or bow but cupping is the most prevalent wood movement in typical flat sawn boards.
By jermaustin1 2025-06-1118:21 Both are actually explained the same way because in bowing, it dries slower in the middle of the board creating the bow, and twisting is just different type of uneven drying typically due to some open grain drying faster.
By cardamomo 2025-06-1115:27 One nit: there are times when you do want to use a metal nail or screw in a joint, particularly if there is some sort of cross-grain joinery going on and you want to allow for wood movement. Chris Schwarz (publisher of this article) makes the point himself: https://blog.lostartpress.com/2015/07/11/the-bare-bones-basi...
I’ve been taught that in the length it can expand/contract at most 1%, but in the width at most 10%.
This is also why properly designed tabletops are attached to the frame with a “floating” construction that can handle those changes.
This is correct but the numbers are off by an order of magnitude. The annual movement of wood is maybe 2% width wise and almost negligible lengthwise.
This is for wood that is dried and stabilized, the shrinking is a bit more from green wood to seasoned lumber (but not an order of magnitude more).
You can use online calculators such as this one for estimates based on the species of wood and your location: https://kmtools.com/pages/wood-movement-calculator
The numbers here match my experience, a 600mm wide spruce table top shrunk and expanded by about 12mm during a year of being outdoors but under a roof at temperatures from -25C to +30C. The structure had sliding dovetails to allow growth but keep it flat.
See my other comment - they are closer than you think in some sense, but there are too many missing variables to say anyone is right or wrong.
The annual movement of wood depends (basically) on the local RH swing, thickness, absorption/diffusion rates, and swelling coefficients.
So giving any percents here without more data is just incomplete.
This is assuming bare wood too, with no coatings/etc.
A lot of the bare percents you see are making assumptions of various sorts. Usually they ignore the diffusion rates/etc and shoot for EMC at some parameters (the calculator you linked does) because doing it for real require more complex math. The calculator you linked is better than most for sure, but it is still a simplification of reality where it may be off by orders of magnitude depending on thickness.
It will be much closer to reality for thinner pieces than thicker ones.
The figures I gave are annual movement. Initial shrinkage is larger when drying from green wood. The numbers (from the calculator) match my empirical observations very closely.
By the way your relative humidity figures assume constant temperature. Wood cares about absolute humidity (mass of vapor per volume of air), and temperature is the dominant factor in absolute humidity. Rainy day at +1C (100% RH) is less absolute humidity than a sunny day at +30C.
This matters to me a lot because half of my woodworking projects are outdoors or not temperature controlled indoors.
"Initial shrinkage is larger when drying from green wood"
?
It's not - it's exactly the same as anything else. The wood doesn't know it's green.
The calculator you gave is shortcutting it, and has an entire article in how they shortcut it the same way as anyone else, based on the swelling coefficients/etc, but assuming thickness is small enough to not matter.
If your projects are outdoors, you will be affected by more than just humidity - UV will also have a significant effect on the properties of your projects :)
The moisture transport is also not as simple as you are making it out to be, and has a not insignificant effect.
See:
https://gupea.ub.gu.se/handle/2077/54179
https://www.mdpi.com/2076-3263/8/10/378
https://www.sciencedirect.com/science/article/abs/pii/S12962...
By exDM69 2025-06-1116:37 Yeah, the coefficients are the same but the initial moisture content in green wood is much higher than the wood will ever get to after seasoning, it won't suck that much moisture from the air (unless you're in a swamp or something). So the annual absolute change in millimeters is lower than from green to seasoned.
I have my woodworking projects in temperatures ranging from -25C to +100C (sauna) and extreme humidity changes from near zero to 100% RH. It is a form of art to make wooden things survive that, and I don't always succeed.
Ummm, most wood starts at a much higher moisture content (50%-200% noting that this is as a percentage of fully dried so it can easily be over 1) than it will ever have after drying (typically 5-15%).
Frankly, the idea that a piece of wood after initial drying was moving even an in/ft (eg "only" 8%) would be pretty shocking. Even good joinery won't deal with much more than a quarter of that ~2%.
You missed my point, which is that initial drying is absolutely 100% not different than any other time. Wood, especially outdoor projects like the person is making, will definitely see states where the moisture content is as high, if not higher, than when the wood was originally dried.
Also, most green hardwood starts at about 30%.
No reputable lumber supplier is kiln drying hardwood that is 200% moisture content. That's crazy town.
Even if they dried it super slowly, it would end up as mostly checked/warped garbage that they couldn't sell.
Beyond that, wood is moving a lot, sorry you don't believe it, but its still gonna do it.
Rather than say it's "pretty shocking", and dismiss it, care to present any studies that back up your assertion?
I sent plenty, both in here and other comments. I'm not aware of any sourced, actual scientific research that says anything other than what i did, since I was careful to use cited figures from actual research studies, and not random pages on the internet about "wood movement"
I think you are also assuming a lot about how it moves and what 8% radial swelling/shrinkage really means that isn't necessarily true.
Also your point about joinery doesn't seem to make a lot of sense. While it's true that most joinery can't handle lots of flex, if everything expanded or contracted uniformly, it wouldn't be a problem.
You seem to be assuming the opening will not expand the same as the thing going into the opening. It will. That is why you try not to mix conflicting grain directions in joints, and why you see so many joints that go out of their way to do that (IE 90 degree mated dovetail boards are not made by two conflicting grain directions, the pins and tails are made of the same grain direction that happens to mate at an angle)
https://cad.onshape.com/documents/3e489410fcf65e1f0f82663d/w...
I made two tabs for you, one with a 25% transform and one without.
Notice the opening gets larger when scaled. So would the mate. They would still fit fine. The same is true if you made a dovetailed box. It would just become a bigger/smaller box.
I didn't bother to scale it differently for tangential vs radial but it wouldn't matter as long as the same scaling factors apply equally to the mate, and the mate is made the same way. As is true of most woodworking joints, on purpose.
So the only issue is if (assuming 2% was the limit) the non-uniformness lead to >2% difference somewhere that mattered.
All of this is also why wood glue has such expansion/contraction characteristics. if wood was only changing 0.1%, it wouldn't matter.
So far i've seen a lot of doubt but nobody else actually seems to be bringing any real scientific rigor to that doubt, or saying some silly things.
Please feel more than free, i'd love to see papers with real measurements that suggest something else.
Sorry, it's not true that "initial drying is absolutely 100% not different than any other time".
https://extension.oregonstate.edu/catalog/pub/em-8600-wood-m...
Fiber saturation is a thing and it rarely exceeds 30% for usable lumber.
I'm mostly going on what I've read though I went outside to measure a 30" fir just now and it's 100% (REED Pinless). The 100 yr old apple trees were over 80%, but that's not peer reviewed (nor is it pay walled). Maybe people can cut down trees and toss them straight into an Alaskan for exactly this reason? Firewood sitting covered outside is <10%.
As for pretty shocking. Yes it would be, if the 36" door (flat sawn book matched) to my house didn't open, because it was an inch too big (it accommodates <1/4"/ft). It's about a hundred years old, and I'm pretty sure that's never happened.
Again - Nothing in any of this says anything about the equations being different for green wood.
I don't even know what you are arguing and why - it seems to change with every post.
So I give up - you still haven't actually shown me a study that says it's wrong, and now your argument is "my door would be too big".
This is a silly discussion.
Since you still haven't given me a single scientific study suggesting the movement doesn't actually occur, I guess i'll offer you this and then walk away:
Is your door surrounded by brick or something rigid? Or is it surrounded by wood and blocking, like most doors? What species is it? What are the radial/tangential shrinkage rates? Is it painted or otherwise sealed in a way that would affect rate of absorption, like most doors?
As an aside, did you know that basically no door company will warranty unpainted doors because of exactly the issue you say doesn't happen? Just about every single one will say something on the order of "this door must be painted or stained within x days or the warranty is void", where x is usually <7, and will unequivocally state that unpainted and unstained doors will warp. Because they do! Like potato chips, a lot of the time.
There are some made to be bare unpainted wood, but it's not common and it requires different construction techniques. Most of them are not solid wood either, they are 1/4" or 1/2" veneer pretending to be solid wood. Otherwise, doors left exposed to the elements often totally fall apart in years. All the time. I can show you one that fell apart due to movement in <5 years.
Beyond that -
Doors surrounded by brick or rigid things frequently become too large to open/close at various times.
My home was built in 1929, and the doors are painted, but the jamb is surrounded by limestone or brick on all sides. Not a facade. The jam is up against well-set brick or limestone. This is actually a super-bad construction technique, since in most cases, the brick/limestone is a facade to avoid this issue. I can send you videos if you want to see what happens.
In the winter, it is about 1/2-3/4 inch smaller than it is now overall. I've measured it. It does in fact, become unopenable in the summer. It actually is right now. I plane it until it can be opened again. It will show a very large gap in the winter.
This is on a painted door, so not even one that is totally exposed to the elements.
This is uncommon, again, because most doors are not surrounded by highly rigid materials. If they are, it's a facade instead of structural. Those doors that are structurally unable to move, will in fact, break apart. This is one of many reasons totally solid wood doors are uncommon (besides weight and cost)
Since you seem big on anecdote, and your door is your baseline, there's a door for you.
Most people with historic homes would laugh at what you are saying. Since you say your door is >100 years old, i'm sort of shocked at your view.
For example, my wife's interior office door, is wildly out of square and plumb. By about 2 inches. The concrete foundation and tile is exactly in the same place, and perfectly level and square. No tiles have broken or cracked, and they are original to the home. Only the things made of wood are no longer where they should be. The two exterior doors in her office on opposite sides were built identically ~100 years ago. They don't even close to line up any more, and are easily 1" off. Again, foundation is exactly where it should be. only the wood has moved.
But still, i'm out since we aren't actually having a useful discussion that involves more than vibes about doors.
By kurthr 2025-06-1123:37 I don't know what you're saying either, because not only is it completely at odds with the measured data presented in a lumber university textbook, but you seem to be unclear on how moisture is even measured.
Please reread the section about fsp and measured MC, because it explains clearly why wood does not expand beyond it's fsp ~30%. Then look at the simplified MC% vs RH% table and read data for shrinkage vs MC for various wood types. No pay walled university papers required, it's not that complicated.
If your wife's door is 2" out of square you probably have a house framing issue not door expansion due to indoor humidity.
I've sawn enough timber and built enough decks out of them that I know wood moves.
By ComputerGuru 2025-06-1119:54 Yeah, the Midwest is cursed for this reason. Humid summers with incredibly high RH for being inland while temperatures push 100° F followed by bone-chillingly dry winters with temperatures falling to -20° F (and relatively little relative/absolute moisture). But all our homes are built of wood and the consequences are pretty drastic.
By bee_rider 2025-06-1120:57 Are there designs that exploit this effect? I want a house with walls that intentionally become more permeable in the summer, less in the winter, haha.
By roberthahn 2025-06-1112:351 reply I think you’re off by an order of magnitude. With those numbers, a 12” board would expand and contract 1.2”, and an 8’ long board would vary by almost an inch.
Much more reasonable would be 1% across the grain and 0.1% along it. You can confirm this in some of the wood movement calculators found online.
To those learning about wood movement, these ratios are decent but approximate; if you end up caring about these things you’ll want to check the species of the lumber you plan to work with.
Serious woodworker here:
They aren't off by that much. You are further off if you assume some standard parameter ranges :)
But in the end, it depends on factors i didn't see listed.
Overall, the percents are usually calculated by swelling coefficient. Swelling coefficient is percent change in radial/tangential for each 1 percent of moisture change. There are well-known sources for these that calculated them in sane ways. The US forest service is one of them, and they publish their methodologies/etc for how they determine them. See, e.g., https://wfs.swst.org/index.php/wfs/article/download/1004/100...
Take standard flat sawn red oak. The swelling coefficient is 0.001-0.002 for radial (0.1% per 1%), and 0.004-0.005 for tangential (0.4% per 1%).
So in initial drying, which is usually 30%->15%, it will move 1.5-3% radial and 6-7% tangential.
Without humidity control, houses swing from 30%<->60%. Sometimes per day, sometimes per month, sometimes per season. So even more than initial drying. But because the swing varies, depending on thickness/etc, how much moisture change you get in the wood, and how fast, will vary a lot.
If you assume it causes a 10% change in moisture content over the year, throughout the wood, we get 1-2% radial movement, and 4-5% tangential movement for red oak. But that is both swelling and shrinking, not solely one or the other.
So the GP would be off by a factor of 2 in one, but not off in the other.
It's obviously trickier in practice to calculate the actual rates because the moisture is going to diffuse through the wood at some rate, and as long as the RH is changing faster than the diffusion rate, the wood will not really have a consistent moisture content all the way through. To be accurate, you'd have to slice it into enough pieces to capture the different moisture levels in the wood, apply the coefficients to each slice, and, etc. Worse, because boards are rarely square, and instead often much wider than they are thick (IE 12"x1") , you'd have to slice and calculate it one way to deal with this for radial, and slice and calculate it the other way to deal with tangential.
I'm too lazy to calculate how coarse/fine of a slice you'd need to get within say 5% of the "real" number.
I'm also assuming you are trying to do it by hand, since this is obviously an integral of some sort that you could also just directly solve. I'm sure it's in a paper somewhere.
This is all for bare wood too, with no topcoats. The topcoat would seriously affect absorption rates, etc, even assuming you applied it to all sides.
Nobody does any of this calculation in practice, we just accept large error bars and build floating tables :)
30-60% RH range in a house surely must not be this strongly related to moisture content of wood? ("10% change in moisture content over the year")
https://www.wagnermeters.com/moisture-meters/wood-info/how-r...
This table shows up to a 4% moisture content seasonal difference in a climate controlled house (20-50% RH).
By DannyBee 2025-06-1116:39 I can't tell where their data comes from, and they don't cite it.
The 10% number was not meant to be real, i just was giving an example :)
Real is much harder.
4% is not a horrible guess from as best i can calculate (but see below because this page has some crazy claims). Studies suggest that wood RH tracks RH pretty closely, slowing down with depth. Transport also appears to depends on temperature, independent of humidity itself. But if you assume it's going to track RH closely and throw out the rest, you can just assume the wood will always fall within the EMC range for the RH range.
If you look at
https://www.fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapte...
You can see that between 30-60% RH, you really don't get more than like a 7% span (i'm eyeballing it) of EMC that the wood could vary around at any temperatures likely to exist in your house.
So 4% is probably not a horrible guess.
However,the site you link to says some very wrong things, interestingly:
"Temperature Has No Significant Effect on Wood MC"
This is 100% wrong, in more ways than one.
First actually even wrong if you ignore humidity entirely, because studies suggest wood moisture transport changes at high/low temperatures, even ignoring humidity. The exact mechanisms are not pinpointed (AFAICT from skimming), but that's what real data says.
Second, the temperature affects the EMC (and relative humidity).
It's very weird for them to go on and on about how humidity affects would but then say temperature doesn't matter at at all.
You can't actually separate these things, and say humidity level matters but temperature doesn't, because they are linked.
If you want real data/simulations to try to figure out more, here's some references - i didn't read all of them, busy morning, but i did at least look at most of them.
https://www.sciencedirect.com/science/article/abs/pii/S12962...
https://gupea.ub.gu.se/handle/2077/54179
Given limited absorption rates, does that mean varnish etc helps keep the internal moisture content more steady over time (and therefore less variation across the wood internally as well)?
By DannyBee 2025-06-1120:15 It depends on how vapor permeable they are. Some of them are good at resisting liquid water but not vapor, and some are good at resisting both.
But all things being equal, yes, they generally can only help keep moisture content more steady over time.
By rags2riches 2025-06-1114:56 A panel door is basically designed to minimize warping as the wood expands and contracts. There is leeway for the panels to move inside the edge pieces (sorry, not sure about the terminology here) and the edge pieces have the grain along the sides of the door. Stuck doors or doors that will not close are no fun.
By arturocamembert 2025-06-118:552 reply Small addendum: some traditional wooden joinery is deliberately prepared to account for the varying rates and effects of drying across the timber.
This is particularly relevant in timberframing, where you want to work with the wood when it is as green as possible. Green pine, though heavier to lug around, is significantly more receptive to a chisel than drier lumber. In a classic mortise and tenon joint [0], it's common to leave the outer edge of the shoulder slightly raised from the inner edge to account for the natural warping as the exterior of the beam dries more aggressively.
Although it's more outside my area of experience, I believe fine carpentry also has a few techniques that see a higher frequency of use in areas that enjoy seasonal swings in humidity. The split-tenon is the only one that comes to mind, but, now that I think of it, I realize my mental model isn't great. More surface area to account for seasonal swelling / shrinkage? Maybe someone else can chime with a better explanation of this one.
[0] https://www.barnyard.com/sites/default/files/styles/full_pag...
Timber framing uses dry wood as well, slightly different techniques but in the softwoods and some of the hardwoods its is not all that harder to work dry than green and in some ways easier. It depends on the tradition and location as to the exact process and technique, some preferred dry timbers, some green, some something between.
In US farm country it was common to fell the trees in late fall/early winter after the harvest was all taken care of and then leave the trees where they dropped until the ground froze. After the ground froze you haul them to the build site, much easier to drag logs on hard frozen ground than on soft wet ground. Then you would forget about them until after the spring planting is taken care of and build in the summer. Those big timbers would be far from dry but they will have lost a fair amount of weight and will be more stable which makes everything easier.
By arturocamembert 2025-06-1117:001 reply I can only speak to my own experience of doing this professionally in northern climes without power tools for ~5 years, but both of your suggestions are foreign to me. I take this as a nice reminder that there is lots of regional variation to this craft around the world, which isn't surprising.
Even then, building a barn with dried pine or hemlock is much more tedious and incurs many more trips to the sharpening wheel. It is in no way easier.
By ofalkaed 2025-06-1120:07 The joints used in dried are dictated by the operations which are easier to do in dry wood and are not influenced by what the wood will do as it dries. Dried you get to use a saw with considerably less kerf and a thinner plate, augers can be more aggressive and take better advantage of lead screw and spurs. Chisel work will be a bit slower when chopping across the grain but not harder and if it incurs many more trips to the sharpening stone you are most likely trying to chop that mortise as you would in green wood.
By HeyLaughingBoy 2025-06-1115:131 reply I read a biography of the earthmoving equipment maker R.G. Le Tourneau, and it was really eye-opening how much this was a thing before mechanized equipment was readily available. A lot of moving was put off until winter because it was so much easier to drag logs, boulders, buildings, etc. over ice than over thawed ground.
By potato3732842 2025-06-1119:591 reply Or waiting for things to freeze real good so you can dig the kind of hole or trench that would make HN clutch its pearls or simplify de-watering problems.
I’ve never tried to dig in frozen ground - isn’t that going to require blasting equipment or techniques closer to mining? (Heavy pneumatic jackhammers)
By HeyLaughingBoy 2025-06-1216:261 reply Well, in the era we're discussing, there wouldn't be jackhammers. Mainly a bunch of guys with pickaxes.
By ofalkaed 2025-06-1220:14 Generally you just build a fire on the ground you want to dig up, possibly throw in some good sized stones to hold the heat longer. If the frost is deep might turn it all over once the flames have died down and bury those coals and stones so their heat is more contained and not just going up into the air, or have a second fire after you have dug out the thawed soil.
Green woodworking is an entire field of its own. Not very common in industrial scale but it was a common method a few centuries ago.
Examples of things where green woodworking is common: spoon carving, bowl turning, chair making, etc.
The idea is that wood is worked while green to make 80% finished blanks, which are dried slowly for some months or years before finishing the rest of it. This gives less distortion to the shape as it dries. And the drying times are faster because it's all small pieces at that point. The time from tree to product is shorter.
It is an almost extinct craft but it is a lot of fun for woodworkers not under schedule pressure.
I just finished a green wood post-and-rung chairmaking class last week. The posts are split out and steam-bent, while the rungs are dried in a makeshift kiln (a box with a heat lamp). The posts are then above ambient humidity, while the rungs are dried below it. As the entire chair equals out, the posts will dry out and compress onto the tenons of the rungs, which will swell up a bit and lock in place. We did use glue but you don't really need to. Neat stuff.
By exDM69 2025-06-1120:24 Cool. I've also built a bar stool with green wood but it's a fairly crude shop stool rather than a fine chair.
A green wood specialty in my neck of the woods is sauna ladles (used for throwing water). You can buy wooden ones but they are made from seasoned lumber with CNC machines and don't survive more than a year before they crack. The one I made from green wood is still going strong after 7 years in extreme humidity and temperature environment.
By sevenseventen 2025-06-1215:02 It's absolutely routine for hobby and artisan turners and carvers, though. In between the first turn and the second turn, you can air dry, kiln dry, and other techniques. With air drying, you actually want to slow the drying so that it happens more evenly. Otherwise, the outside of a vessel dries faster than the inside, which splits the wood. In general, packing a vessel inside and out with wood shavings helps even the process.
I've also had great results using silica gel on smaller items, although it can be hard to scale it to larger vessels. Much faster drying than air alone, with greatly reduced distortion and cracking.
For some reason this reminds me of :
https://www.thefenlandblackoakproject.co.uk/our-story
In particular - the section on drying - air drying would have been too rapid/harmful to the wood - so they put it into a purpose-built dehumidifying kiln for 9 months.
(It was briefly discussed here a few years ago: https://news.ycombinator.com/item?id=36912861 )
I saw a thing once where a guy would make 3 large cuts at the bottom of a tree, in a particular pattern. This would kill the tree, and it would essentially air-dry over the course of a year or two. I wonder how that compares.
I should note he was a homesteader doing this to provide dry wood with easy access during cold months.
By superb_dev 2025-06-120:222 reply That’s pretty clever! I’ll have to keep this in mind if ever get my dream homestead.
I’m no arborist, but I’d guess the cuts sever all of the tree’s microtubules without felling it? I think 3 would be the minimum amount of cuts you’d need.
By jaderobbins1 2025-06-1219:40 Yeah, girdling: https://en.wikipedia.org/wiki/Girdling
I hadn't heard it used to get dry wood for harvesting but I recall it being useful because it actually kills the tree, so if you cut the tree down (after it's fully dead) the stump will decompose instead of trying to continue to grow.
Seems like it might be a good way to waste wood. Dead and dry wood tends to shatter and break apart when felled. Green wood has a LOT more flex. Depends on what you’re after I guess.
By esseph 2025-06-2112:43 You don't want "green" wood for firewood, you want dried seasoned logs.
By superb_dev 2025-06-1518:22 If you’re using it for firewood, that sounds like a bonus!
By jimnotgym 2025-06-1116:20 I used to be a carpenter and joiner. I once had a batch that was badly kiln dried. We called it 'case hardening', I guess it was done too fast. If you planed a flat face on it and it instantly warped again!
If you sawed it, it would either pinch or spring apart. I made the sales rep come and see it.
