From Gene Wengert, forum technical advisor:
From time to time, there are postings that involve the movement of wood. Here is some information that may be helpful. Wood does not shrink or swell in use except if its MC changes. Its MC changes when the RH changes. (Of course, if the wood is at the wrong MC when first put into use, it will adjust to achieve equilibrium with its environment and therefore may shrink or swell initially quite a bit.) Temperature alone does not cause any significant size change in wood. Heating does causes moisture changes to occur faster.
The basic relationship between MC and RH is given below, with a third column for the equilibrium moisture content (EMC) which is a property of the air.
0% RH = 0% MC = 0% EMC
30% RH = 6% MC = 6% EMC
50% RH = 9% MC = 9% EMC
65% RH = 12% MC = 12% EMC
80% RH = 16% MC = 16% EMC
99% RH = 28% MC (approx.) = 28% EMC (approx.)
Note that most heated homes and offices will run 6% EMC in the wintertime and even a bit lower. In the summertime, 9% EMC is common. Outside in most of North America, the outside is 12% EMC, summer and winter. In coastal locations, 16% EMC outside is common.
When air is heated, its RH drops unless moisture is added (that is, the air is humidified). For example, if it is 30 degrees F outside and 100% RH = 28% EMC; snowing or foggy perhaps) and that outside air is brought into a home or office and heated, the following will be seen:
Heated to 40 F will result in 68% RH and 13% EMC.
Note in the above that a home with plants, cooking, bathroom showers and so on will add moisture, so these values might be a bit low, but not much. Of course, using a humidifier will increase these values; offices are not humidified too often, although such higher humidity would help keep static off of paper used in high speed copy machines, etc.
Because purchased wood is often around 9% MC (actually too high, but that is reality for some folks; I suggest 7.5% maximum MC) and a shop can run around 5% EMC, some folks are tempted to increase the shop air to about 50% RH. Indeed this will eliminate problems in manufacturing, but it actually only postpones the shrinkage until the customer gets the wood in his/her dry home or office. Do not over-humidify a shop.
Checking and recording on paper the MC of wood you are using and the RH of your shop using fairly inexpensive instruments is a prudent thing to do. It provides documentation to prove that you did the right thing regarding MC so that subsequent MC problems are not your fault - assuming you would not use wood that had incorrect MC readings and that you would not keep a shop at the wrong RH or EMC. (Sometimes this approach is called CYA.)
Once a piece of wood or a wood product is put into a fairly tight container, including wrapped in plastic or into a closed truck or trailer van (that is, liquid water cannot get in and there is little RH exchange with the outside), the MC will not change in transit or storage, regardless of the temperature. For 100 pounds of wood to change by 2% MC will require the addition or loss of one quart of water...that is a lot of water to be brought into a container by merely exchanging a small amount of air with the outside. It would involve bringing in more than 1000 cubic feet of humid air and this is just for one 100 pound piece of cabinetry, furniture or flooring. I hope this helps. Does anyone have any questions?
Forum Responses
(Architectural Woodworking Forum)
From contributor J:
Wood does expand and contract with temperature. Doors I built and installed in south Florida move quite a bit with temperatures at over 100 degrees in the summer to sometimes in the 30's in the winter. This is with keeping a MC (depending on species) of 6% year-round.
I learned the hard way (as we all do) to increase my astragal/active reveal to 3/16" on outswing doors. This may raise some eyebrows and it has with a couple of builders. But even if I rely on a perfect install, doors seem to do funny things in the heat regardless of MC or RH.
In a chart I found called "Rates of Thermal Movement for Common Materials" (coefficient of expansion) published by AAMA states: "Range of movement based on a100 degree change using an 8 foot section of material". Range of movement for wood: 1/64" to 1/32". In most situations those numbers are negligible, but I believe it's not to be overlooked. For what it's worth, spend the extra money for a good meter.
However, we do not deal with dry wood. The problem is that when wood is heated its response to RH changes; that is, the EMC vs. RH curves change; wood that is at 28% MC at 100% RH at room temperature will be only around 22% MC at 200 F. So with heating at constant RH, the MC will change and this MC change causes wood to change size almost immediately. Of course, as I stated, the heating of air will lower the RH and this effect will be larger than the EMC effect, but these moisture changes take time to occur.
So, it is virtually impossible to heat wood, such as a door, without changing its MC. For wood with moisture in it, here is what the Wood Handbook says: "Wood that contains moisture reacts differently to varying temperature than does dry wood. When moist wood is heated, it tends to expand because of normal thermal expansion and to shrink because of loss in moisture content. Unless the wood is very dry initially (perhaps 3% or 4% moisture content or less), shrinkage caused by moisture loss on heating will be greater than thermal expansion, so the net dimensional change on heating will be negative. Wood at intermediate moisture levels (about 8% to 20%) will expand when first heated, then gradually shrink to a volume smaller than the initial volume as the wood gradually loses water while in the heated condition."
I can get around the low accuracy of pinned meters in one way, just by controlling the humidity. Mine is at 35%. I visited the southern CA machine shop last month and was stunned to see a RH reading of below 5% on an instrument in an adjacent building. If that is true, could the MC in my product be 1% after a few weeks of equilibration?
This leads to a question on finishing at these low MC’s. My customer does their finishing work in southern CA as well, without humidity control. Might I anticipate problems here as well? They are used to working with laminates, but not thick solid hardwoods so maybe some issues will come up. The most damaging issue I can think of here is that if they finish the products at 5%RH, then ship around the world where any RH could be encountered, later swelling might pop joints or hardware.
The pinless meter does read to lower MC’s well. Specifically to your case, as long as the wood is in a bag, the MC is constant. Your 4.8% MC reading is not at all accurate. The MC could have been 6.1% MC, 4.8% MC or somewhere in between or even drier. We just do not know or have any idea. One idea for you is to weigh the wood piece when you put it into a bag. This will establish the weight at 7.0% MC. Divide the weight by 1.070 and you will have the oven-dry weight estimate. Your customer can then check the MC by having the piece weighed again before they begin processing (grocery store, meat market etc.).
It is nearly impossible to estimate the time involved for the piece to change MC when exposed to a drier EMC. It would be very unusual to see an environment of 5% RH, although it does happen (I have seen it in Denver, CO, but it is transient.) If they are really that dry and are shipping to the rest of the world, they need to take a small area and make a plastic enclosed room and then put a humidifier in that room set for 35% RH. They would then store all the work that they have in progress (that is, after they take it out of the shipping bag from you) in this room, except for the hours they are working on it. Once finished, it can stay out for a longer time, but even then, it would be best to put it back into this room if it is not wrapped in a plastic bag. They could also, whenever long storage is anticipated and they do not have room in this RH controlled room, wrap the piece in a plastic bag, which will prevent any moisture change from that point onward until the piece is unwrapped. This procedure is easy, inexpensive, effective, but does require a bit of dedication to these guidelines. Does all this make sense to you?
Hard maple is known to have a lot of chip out as the grain is never perfectly straight. So, in small spots we can be planing or machining against the grain and that gives us tear-out. This defect is more common with dull steel or with carbide tools as well. I suspect that you will have chip-out as well, unless you have very sharp tools, good feed rates, etc.
These parts are curved. Is there a pinless meter that works on curved parts? For hard maple, are you recommending steel over carbide? There certainly is non-uniform grain in a lot of the chip out areas. I find that there may also be denser or harder areas that are not obvious defects, but may cause chip out. They may also be areas of grain slope change that just isn't obvious from observing the surface.
Secondly, can you suggest a method or rule of thumb for estimating how quickly a piece of lumber might respond to changes in RH? I'm especially curious about how the thickness of a piece of lumber might relate to the speed of dimensional changes. I understand that there are a lot of variables here, but it seems obvious that a piece of veneer will respond very quickly while a butcherblock countertop might shrink or expand much more slowly. I guess I'm wondering whether large pieces of lumber might change dimensions slowly enough that they never come to equilibrium during a given season, i.e. that the fluctuations in RH between dry winters and damp summers might happen so quickly that the wood never moves as much as simple calculations might suggest.
Since in a shop we do not cool off during the early morning and reach 100% RH, the average in a shop with the windows open will be a bit drier, maybe 9 to 11% EMC. If we need it to be drier (in the old days without central heating, being drier was not an issue) then we can add a dehumidifier or add heat. Adding heat in the summertime is not a good idea, so then we can dehumidify. Our first attempt may be to run the DH at 45 to 50% RH when the dust system and doors are closed, including all day on the weekends. (Of course, the DH unit must be large enough to actually achieve the desired conditions within a few hours.) We also could air condition (money). Finally, we could build a small plastic lined room or area where we keep all the wood when it is not being worked on. Once a piece is finished, then we also need to put it in the room or wrap it well in plastic to prevent MC changes.
Let's come back to the heating idea. I do have one customer in Houston that bought an old 18-wheeler trailer van. He painted it a dark color and then stores wood in the van. A small fan assures that the hot (and therefore dry) air is circulated throughout the trailer. The fan is run only when the sun is shining.
A/C and dehumidifiers are the same thing only an A/C unit puts the heat outside so you might as well get the added production of cool employees. The shop has 20 foot ceilings so we hung the unit at one end and used a 3" diameter sock running the length of the shop to distribute the cool dry air. Two big pleated filters on the intake let us use the unit as an air cleaner too. We hardly notice the difference in the electric bill from summer to winter.
