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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?


Sources for point 2 above:

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:

enter image description here

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:

enter image description here

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:

enter image description here

enter image description here

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.


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.

share|improve this question
    
Plenty of good information in the answers... the only point I'd make is to consider what season you're building in. If it's summer, expect the wood to be a little on the fat side and allow for shrinkage. (A panel in a door could be a little tighter than you'd normally build it, for example.) Conversely, in the winter, expect wood to be a little skinny, so plan accordingly. (And just to beat a dead horse, all this expansion/contraction is across the grain... not lengthwise.) – Aloysius Defenestrate Feb 2 at 3:04
    
@AloysiusDefenestrate The calculated EMC for my area doesn't really vary between winter and summer. What is the variable there that affects the state of the wood in the way that you mention? Do you mean, expect it to be fat/skinny if I haven't let it rest yet and it's still in whatever state it is in from whatever environment it was originally downed in? Because the living state of it was greatly varied between the two seasons? – Jason C Feb 2 at 4:00
    
Hmm, you're now asking me to back up my 'accepted' wisdom. I guess what I'm going by (and I think this would hold true for NY) is that summers are more humid and warmer. Winters the opposite. (We see this in your black graph, right?) Without digging into the math behind your calculated EMC, I can't really challenge it, but I have the feeling that the uptake of wood is slower than actual measured weather. If I get a chance, I'll look more closely at what you've got and perhaps have a better answer. Or maybe I'll have my mind changed. Dunno. – Aloysius Defenestrate Feb 2 at 5:54
    
@AloysiusDefenestrate In my area temperature is very seasonal although humidity and EMC do not seem to vary with any statistical significance by season or month. I did make the pleasant discovery that the mean calculated EMC seems to remain relatively constant throught the year (and therefore, more attainable). See the table I've added above to address this comment. – Jason C Feb 2 at 17:23
    
Holy statistics batman! – Matt Feb 3 at 1:29
up vote 16 down vote accepted

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.


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)
share|improve this answer
    
So in my above environment, what MC range would I be shooting for before judging the wood as ready? Roughly around the mean of that 2 year period, despite high variance? – Jason C Feb 1 at 19:15
    
@JasonC, that's probably a good place to start. Personally, I think anything under 15% is probably good enough to use for your region. – grfrazee Feb 1 at 19:22
    
When wood is in storage, do you know what the relationship between MC change rate and EMC - MC is? I'd like to run an approximate simulation. – Jason C Feb 2 at 4:11
1  
@JasonC, please see the added Further Reading section to my answer. Some interesting stuff there. No. 3 especially if you want to run a simulation. – grfrazee Feb 2 at 13:58
    
Those are great links. It will take me some time to digest. #2 in particular is changing the way I look at this, esp. that getting close to EMC is not really as important as getting it dry enough, e.g. if it starts at 90% even if the environment's mean EMC is 12%, at least letting it get down to say the environment's max of e.g. 30% is the most important part. This may be my key missed point. However, this is actually strengthening the new doubt I've been having about whether the location (and season) matters at all: If the desired EMC is so high does it really matter where you store it? – Jason C Feb 2 at 18:22

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.

share|improve this answer
    
In the summer my bedroom is so different from the rest of my apartment that based on the "resting wood" advice I wouldn't trust wood stored in my kitchen to be acclimated to my bedroom at all; and this is typical. Still a good point about the A/C. FWIW, although I don't have hard numbers, NYC does have I believe a relatively low % of A/C-equipped households and low energy usage per household. In my case I have a window A/C in the bedroom, and baseboard heating with leaky windows. In the winter my interior environment is still much different than outside. In the summer they are about the same. – Jason C Feb 2 at 4:09
    
When wood is in storage, do you know what the relationship between MC change rate and EMC - MC is? I'd like to run an approximate simulation. (PS I did find that only 20% of households in NY state have central A/C, not sure about NYC -- I would presume lower. The window A/Cs lead to an even weirder variance of EMC in interior environments). – Jason C Feb 2 at 4:11

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.

share|improve this answer
    
While Ikea does have some cheap furniture it was designed with that idea in mind. Most of their products are designed from a price first and materials later. Also many engineered woods are more resistant to handle moisture changes. Water contact is another story though... – Matt Feb 2 at 1:03
3  
Most woodworkers do not consider themselves carpenters and vice versa. – Ast Pace Feb 2 at 1:42
    
Cabinetmaking and carpentry used to be separate guilds, and had different traditional ways of handling the wood-movement problem. – keshlam Feb 2 at 2:45
2  
I resent your implication that everybody in New York buys Ikea furniture. Now, if you'll excuse me, I have to get to Ikea before they close. – Jason C Feb 2 at 4:02

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.

share|improve this answer
    
To be clear: The 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. Also I question conventional accepted wisdom regarding season and location (I am gathering more data on these lines as I type this). It is given 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. – Jason C Feb 2 at 18:12
    
Then I'd say that the main problem with your data is that it is for the outdoor conditions and not the indoor conditions. Since most fine furniture are meant to stay indoors, you'd need to get data for the indoors temperature and relative humidity to make any real conclusions. – Eli Iser Feb 2 at 19:40

Don't forget that once the furniture is created, it has structural integrity all of its own. A single sheet of metal will wobble in all manner of directions, but if you bend it and shape it into a car body then you've given it resistance to deformation, whatever that source of deformation happens to be.

Of course that depends on your design having structural integrity. But then that's down to whether you're a competent woodworker or not. :)

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