How is it possible to let wood reach equilibrium moisture content?

Question

I am trying to reconcile the following 3 points but cannot:

1. It is extremely important to let wood sit for a while in the environment it is intended to be used in before using it for a project so that it reaches equilibrium moisture content (EMC) with the environment. Evidence: This advice comes up time and time again here, in forums, in articles, everywhere.
2. It is impossible to do so where I live, where the EMC has varied from 2.8% to 29.1% on an almost daily basis over the last 2 years. Evidence: See below.
3. Building furniture from wood apparently works anyways. Evidence: I live in NYC. There are 15 million people here with furniture that isn't constantly tearing itself apart.

How can an extremely important requirement for well-built furniture be apparently impossible, yet well-built furniture exists anyways? One of the three points above must be false. It is either:

1. Not that important, or
2. Entirely possible, or
3. Nobody's furniture in NYC lasts longer than a year or two before warping itself to pieces.

Which is it? What am I missing here? In such a wildly changing environment, how is it actually possible to let wood reach its EMC, or is the advice misguided, and does it not actually matter?

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Sources for point 2 above:

• Hourly weather records for Jan 2014 thru Jan 2016 for NYC, source Weather Underground.
• The Hailwood-Horrobin equation for calculating EMC from temperature and relative humidity, via CSGNetwork.com's EMC calculator.

Here is the calculated EMC according to the above data for a typical 10-day period in my area, showing a highly varying range from about 6% to 23% even over the course of single days:

Here is the calculated EMC (hourly) over the past 2 years, showing very little correlation between EMC and season, a high noise distribution between 2.8 and 29.1:

Here is temperature (C) and relative humidity (%) just for completeness over the same 2-year period. There are clear seasonal variations of course but within a day / week it varies greatly:

Here is my weather and calculated EMC data (uncompressed).

How can I make all this make sense? Evidence suggests that the advice to "let wood sit for a few weeks or months to reach equilibrium with the environment" is not actually correct (because it appears impossible - what would the target MC be?), but I have to assume I am missing a key point because of my inexperience.

As the question and answers are evolving I am becoming clearer on what I am confused about: My assumption is not that the wood's MC changes quickly, it's that in the presence of a highly varying environment I question if it changes too slowly to ever really reach EMC, at least in the conventional "weeks" or "months" suggested resting times. Also I question conventional accepted wisdom regarding season and location (I am gathering more data on these lines as I type this, however). It is given, to me, that live wood has a high MC and needs to be dried; what is really not clear to me after looking at answers and data is if it actually matters that wood rests near its final location, and how much a couple of weeks or months really makes a difference. By all traditional experience, it does, but by the very limited observations I have made so far, it does not seem to.

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The question ends here. The following section contains misc. data related to comments.

Seasonal temperature / humidity / EMC statistics for past 2 years (21,457 samples) (full results). I've included median and IQR, if you swing that way:

          min   max   mean   std  median  iqr    REL HUMIDITY (%)
Winter     11   100   59.9  20.2   56.0  36.0
Spring     12   100   62.6  22.7   63.0  43.0
Summer     24   100   63.2  16.9   62.0  26.0
Fall       17    97   64.2  17.8   63.0  30.0

          min   max   mean   std  median  iqr    TEMPERATURE (C)
Winter  -16.7  21.7    1.3   6.2    1.1   8.6
Spring   -6.1  31.7   14.7   7.1   15.0  10.0
Summer   11.7  35.6   23.8   3.6   23.3   4.4
Fall     -5.6  28.3   11.9   6.3   12.2   9.5

          min   max   mean   std  median  iqr    EMC (%)
Winter    2.8  29.1   12.4   5.1    10.5  7.8
Spring    2.9  29.1   13.1   5.8    11.6 10.3
Summer    5.1  28.8   12.4   4.3    11.3  5.4
Fall      4.0  25.7   13.0   4.5    11.9  6.9

          min   max   mean   std  median  iqr    EMC (%) BY MONTH
Jan       4.2  25.2   11.7   4.5    10.0  5.7  
Feb       5.1  22.9   12.0   4.5    10.6  6.5
Mar       2.8  26.0   11.7   5.4     9.7  9.0
Apr       2.9  29.1   12.0   5.9    10.2  8.9
May       3.9  25.9   13.1   5.6    11.6  9.5
Jun       4.6  29.0   14.3   5.2    13.2  9.4
Jul       5.4  28.8   12.9   4.6    11.7  6.0
Aug       5.5  23.2   11.6   3.9    10.8  4.7
Sep       5.1  23.3   12.5   3.9    11.7  4.9
Oct       5.7  25.7   12.9   4.3    11.9  6.7
Nov       4.0  23.4   12.6   4.8    11.3  7.9
Dec       6.0  29.1   14.8   5.1    13.6  8.8

The major variable that affects the above is Winter/December 2015 had record-breaking high temperatures (72F/22C was not a white Christmas). From this I learn two things:

• The tighter range of seasonal EMC averages (12.4-13.1%) and medians (10.5-11.9%) suggest a reasonable target EMC. This is good news.
• There is no statistically significant difference in my area's relative humidity and EMC per season (or even monthly, really). This leads to other questions outside the scope of the original version of this post.

This raises some additional questions for reflection: Most notably 1) is a "few weeks" or a "few months" really enough time to let wood sit, and 2) that season doesn't matter contradicts some accepted experience, although this may be unique to my location.

Answer 1

In such a wildly changing environment, how is it actually possible to let wood reach its EMC, or is the advice misguided, and does it not actually matter?

It does matter, and the moisture content bit is a bit easier than you're making it out to be. Let me explain.

Humidity fluctuates throughout the day, as we all know. However, on average, an area's humidity levels can be given to a fair amount of precision. For example, we'd all expect a rainforest to have a higher average humidity level than a desert, even though both can fluctuate quite a bit.

How can I make all this make sense? Evidence suggests that the advice to "let wood sit for a few weeks or months to reach equilibrium with the environment" is not actually correct (because it appears impossible - what would the target MC be?), but I have to assume I am missing a key point.

The important thing to remember is that wood starts out containing more water by proportion than the air it's in. Nature always seeks to achieve equilibrium. So, if you have a piece of wood with more moisture than the air, the air is going to try to remove moisture from the wood to balance the system.

At some points, there will be days where the air is more moist than the wood and no loss of water will occur in the wood. However, if given enough time, the wood will lose enough moisture that, on average, it is as moist as the air.

After this point, daily fluctuations will move the moisture content in the wood up and down, but this is usually localized to the outside fibers of the wood. Think of it as "moisture inertia." The thicker the piece, the more moisture inertia it has, and the longer it takes to equilibrate with the air. Conversely, the more moisture inertia it has, the more resistant it is to moisture fluctuations once it reaches EMC.

Building furniture from wood apparently works anyways. Evidence: I live in NYC. There are 15 million people here with furniture that isn't constantly tearing itself apart.

Well-built furniture takes these movements into effect. Poorly-built furniture will have cracks and opened joints (among other problems) where they have not been properly accounted for.

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Further Reading

1. Rethink Moisture Content for Workbenches (Lost Art Press)
2. But is it dry enough? (Lost Art Press)
3. Water vs. Wood: grudge match (Lost Art Press)

Answer 2

There are many techniques for designing a piece of furniture that will not tear itself apart with environmental fluctuations, but the key point you're missing is that you ideally want to work the wood while it is at the EMC of the environment in which it will ultimately reside.

All the climate data you have crunched is presumably for a piece that sits outdoors without any climate control. If it is going to sit in an air-conditioned house or apartment, however, you need to work with the EMC of that environment.

Another point worth noting is that the wood's moisture content will fluctuate more gradually than the environment's. If you let your materials come to the EMC of a bedroom then take them into your shop, they won't instantly come to equilibrium with the shop.

Answer 3

To warp or to twist, that is the question

It is extremely important to let wood sit for a while in the environment it is intended to be used in before using it for a project so that it reaches equilibrium moisture content (EMC) with the environment. Evidence: This advice comes up time and time again here, in forums, in articles, everywhere.

This is very true. Wood is designed by nature to be absorbent of moisture. That's how the leaves get the water they need to create sap. The problem here is that moisture changes the shape of the wood. When it's moisture content changes that results in a change in the shape of the wood, and these changes are what most people don't want to happen.

I've purchased expensive wood and stacked it in the garage for just 1 night. The next day it was warped beyond repair. I did not put spacers between the boards to let it flex with the changes in moisture. What I've learned about wood is that it will bend or twist but never return to it's original shape.

The same problem can occur if you take new wood and cut it. In a few days it will all be a different shapes.

So letting the wood sit is important, but how you stack it is also important. Give it lots of support, don't put extra weight on it and allow space for the wood to breath.

But it's going to change shape. That's a fact. Your job is to minimize this as much as possible.

It is impossible to do so where I live, where the EMC has varied from 2.8% to 29.1% on an almost daily basis over the last 2 years. Evidence: See below.

Wood breaths so it will expand and contract as the humidity changes. The key is to allow it to do that naturally. With fluctuating levels it just means that you are more likely to warp wood if you do not store it properly. It does not mean that you can not equalize it to your environment.

Ikea Is Not Carpentry

Building furniture from wood apparently works anyways. Evidence: I live in NYC. There are 15 million people here with furniture that isn't constantly tearing itself apart.

The majority of people buy manufactured furniture. This stuff uses man made materials, has been painted and is of generally poor quality. It was just made to look nice. If everyone had furniture made from solid wood they would have to learn how to take care of it.

A good carpenter knows how to build stuff that won't change shape. That is a whole other topic of wood working and I'm sure there are books on the subject, but if you follow those guidelines you won't have any problems.

It's All Relative

In such a wildly changing environment, how is it actually possible to let wood reach its EMC, or is the advice misguided, and does it not actually matter?

It's all relative to where the wood was previously. Wood that is already in your shop and has been there for a while has already reached EMC. Yes, your environment is constantly changing but now that wood is changing in an expected way because it's now relative to your environment.

Wood that comes from the mill came from a different environment and it now must change. The closer the previous environment is to yours the less change required.

You get what you buy

It's one thing to let wood rest when you receive it. It's another to expect bad cuts, imported woods or very different moisture levels to yield you a nice wood product later.

I can't count how many times I got so distracted by trying to find wood without knots, cracks or length that I needed and forgot to check the end grains. Later after letting it rest the wood warped in a way that could have been predicted.

It's more often the buyers mistake than it is an environment problem.

Answer 4

I believe that the main problem with your question is assuming that the wood moisture content can change in an instant. This is not the case. Wood fibers do reach a moisture equilibrium with the surrounding air's relative humidity, but this is a very slow process, especially for thicker pieces.

The calculator you used to determine EMC is well and good, but it doesn't tell you the rate at which the EMC changes. The daily fluctuations in relative humidity and temperature don't have an immediate impact on the EMC. The gradual changes over the year do, and this is slow enough to smooth out any fluctuations (even if they can be pretty extreme when viewed up close, on an hourly basis).

Consider that kiln-drying felled lumber takes weeks, and air drying felled lumber takes roughly a year per inch of thickness, and you can see that wood's MC changes slowly. The same rates apply to furniture in your house.

Lastly, as pointed out by some of the other answers, all of the data you presented is related to wood that is outside. For indoor furniture the temperature and humidity variations are probably less severe than outside, again minimizing any extreme fluctuations. However, it is worth mentioning that the area of the house does have an impact - there is a big difference in EMC for a piece of furniture in the basement or in the bedroom.

Answer 5

Just a simple note on your climate data: it is for outdoor. Since most of wooden furniture will sit indoor, you have to calculate for indoor climate. Which - thanks to heating and A/C - will pretty much differ from and much more stable than outdoor climate.

To have an impression, here is a diagram of calculated equilibrium moisture content in my workshop, based on RH-T logs I started to record recently:

Answer 6

The theoretical predictions for equilibrium MC are approximations from empirical research done in the 40s. All well and good and they predict the MC of wood when left long enough at the stated temperature and humidity.

What is long enough? As a few have alluded to it takes significant time in a kiln or in the open air for drying to occur. One will never achieve 8% MC in an air-dried situation unless you are in the Arizona desert in winter with a heating source. In practice low humidity and high temperature are difficult and costly in energy to achieve so kiln drying time is somewhat dictated by economics.

Nonetheless the times to kiln dry are dominated by the safe rates that wood can be expected to give up their waters, which is dependent on species and thickness. Using the kiln-drying paradigm to gain a time estimate to achieve equilibrium is not advised. The rate at which air drying occurs is perhaps a little closer to providing a model of time to equilibrium.

If one assumes that the free water has been removed and the wood is somewhere above desiccated (0% MC) and below the fiber saturation point (~30% MC), then MC is dictated by thermodynamics with the main factors being relative humidity and temperature. These are generally not independent but if it is assumed that they can be set independently then an increase in temperature with a fixed RH will cause the process to move faster than at a lower temperature. In addition, the same can be said for a greater change in RH for a fixed temperature. If it can be further assumed that the rate due to temperature differential and the rate due to RH differential have a proportional effect on the drying rate, then it can be shown that the response to a step change in either or both will result in a first order rate change in equilibrium MC.

Thus a single time constant representing the time it takes for the equilibrium MC to reach 63% of its final value is enough to characterize the wood's rate of change in MC. To my knowledge there is not a study that deals with the time constants of wood species or thicknesses to MC changes but if there is I would not be surprised to see time constants on the order of 5-10 days. Given it takes about 5 time constants to reach 99% of the final MC value, this would indicate 25 to 50 days. In this time the temperature and RH will have changed significantly in both directions so the EMC will never be reached.

Thus the MC of wood will always be chasing the environment.

The lesson here is small(large) changes in environmental conditions mean small(large) changes in EMC, which mean small(large) dimensional changes in tangential and radial directions. Air drying takes so long because the drying process is significantly delayed by temperature, RH, and air flow limitations. As to the wisdom of letting timbers equilibrate to the shop …. It is true that lumber dimensionalized can (and will) show dimensional changes the day after milling but probably not to EMC but rather to the internal wood stresses equalizing after removing material the day before. That is why milling to thicker and wider than needed is prudent so that after the wood has settled (1 day) what remains can be dimensioned. Also, construction techniques that allow for the dimensional changes due to the EMC changes are always recommended and these changes are seasonal (on the order of 90-180 days).

Averaging the outdoor temperatures and RH for each month in your local area and computing the theoretical EMC will reveal that there is an average probably greater than the 8% that was produced at kiln drying with a small deviation of maybe +/- 1.5%. For a Kansan that's 12.3% +/- 1.3% from Sept 2002 to Sept 2021.

The same will be true for indoor conditions as well, but with a different mean and deviation. EMC fluctuations indoors are greater than outdoors because forced-draft heating will dry the air and EMC is much more sensitive to RH than to temperature. For instance in the Summer with AC indoor temperature/RH of 70/50 will bring an EMC of 9% but in the Winter a 70/30 will bring an EMC of 6% even though the same wood outside never sees below 11%.