I am trying to reconcile the following 3 points but cannot:
- 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.
- 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.
- 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:
- Not that important, or
- Entirely possible, or
- 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:
- 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.
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.