Equilibrium Moisture Content Is Determined by Relative Humidity

Woodshop EnvironmentEquilibrium moisture content is the moisture level where the wood neither gains nor loses moisture since it is at equilibrium with the relative humidity of the surrounding environment.

In order to properly understand the effect of relative humidity on the moisture content of a piece of wood and the equilibrium with its environment, a basic understanding of some commonly-used terminologies is extremely important. Before going into any details, one should simply keep in mind that the relative humidity (along with temperature and other factors) does affect the moisture content of wood as well as the equilibrium value of moisture content, as long as the fiber saturation point, discussed later, is not reached. The mutual relationship of relative humidity “h” or “ϕ”, moisture content “MC”, temperature “T”, and equilibrium moisture content “EMC” is governed by complex mathematical equations.

What Is the Moisture Content of a Piece of Wood?

The moisture content of a piece of wood is the total amount of water (or moisture) contained in that piece of wood. Mathematically, moisture content “M” or “MC” is the difference between the mass of wood with moisture “m” and without moisture “mod” (oven-dry mass), divided by the mass of wood without moisture “mod”. It can be expressed as:

MC = (m – mod)/mod

What Is Fiber Saturation Point?

Several types of WoodIn almost all kinds of wood, moisture can exist as either free water or bound water. The water vapors or liquid water in the cavities and cell lumina is termed as free water, whereas the water that is held by the intermolecular attraction within cell walls is termed as bound water. Fiber saturation point “MCfs” is the point at which no water exists in the cell lumina but the cell walls are completely saturated.

Another way to think of fiber saturation point is to think of it as the moisture content level beyond which the properties of wood do not change as a function of moisture content. Typically, the value of MCfs is about 30% for wood but can change from species to species and piece to piece.

What Is Equilibrium Moisture Content?

As long as the fiber saturation point is not reached, the relative humidity “h” and temperature “T” of the atmosphere affects the moisture content of wood considerably. The moisture content at which wood neither gains nor loses moisture is known as the equilibrium moisture content, or “EMC”. The equilibrium is dynamic in nature because of the changing relative humidity and temperature.

When a piece of wood is placed in a certain environment, over a period of time it tries to achieve equilibrium with the environment. The moisture content changes to adjust to the relative humidity and temperature of the surroundings. After a certain period of time, the moisture content stops changing. This moisture level is termed as the equilibrium moisture content “EMC”. It stays as it is as long as the relative humidity and temperature of the surroundings are not changed.

What Is Relative Humidity?

Generally expressed as a percentage, relative humidity “h” or “ϕ” is the ratio of partial pressure of water vapors (H2O) to the saturated vapor pressure of water at a particular temperature in an air-water mixture. Temperature and air pressure affect the relative humidity of the air. The following equation is generally used to calculate the relative humidity of air:

ϕ = (ew/e*w) ×100%
where ew is the partial pressure of water vapors and e*w is the saturated vapor pressure of water at a particular temperature.

EMC, Relative Humidity, and Temperature

The relationship between moisture content (as well as the equilibrium moisture content “EMC”) of wood and relative humidity can be studied and approximated for a given temperature. It is clear that for each value of relative humidity “h”, there is a corresponding value of EMC. Therefore, EMC can be plotted as a function of relative humidity. True for most of North America, 30% to 50% relative humidity corresponds to 6% to 9% EMC. The EMC values of solid wood are generally greater than wood composites.

For a reasonable estimation of the true target EMC at any value of relative humidity and temperature, the following equation may be used:

EMC = [ -ln (1 – ϕ) / 4.5 x 10-5 ( T + 460 ) ] 0.638
where

ln = natural logarithm

ϕ = relative humidity expressed as a decimal

T = temperature in oF

Coastal AreaThe Hailwood-Horrobin equation can also be used to mathematically estimate the complex relationship of EMC, relative humidity, and temperature.

A general trend, as you may also have noted from the equations discussed, is that temperature doesn’t affect the moisture content as appreciatively as relative humidity does. Coastal areas like Miami, Seattle, and Japan have higher values of relative humidity and subsequently higher values of EMCs than non-coastal areas.

When Is Wood Wet and When Is Wood Dry?

The moisture content of a piece of wood at a particular time determines how wet or dry that piece of wood is. The lower the moisture content, the more dried the wood and vice versa. A Wagner moisture meter can be used to determine the moisture level in wood. Relative humidity and temperature affect the equilibrium moisture content, as is evident from the equations discussed before.

The Importance of Correctly Measuring Moisture Content

The moisture content of a piece of wood and the equilibrium moisture content can be determined by the relative humidity of the air surrounding the piece of wood. The stakes of unknown moisture content are way too high. At the worst, excess moisture content in wood can cause total flooring disasters. Wagner moisture meters are designed to measure any excess moisture content in a timely manner so that million-dollar mistakes are avoided. Moisture content in wood can be measured accurately and as often as one might need before making any important decisions. It is a nice way to ensure that moisture content in wood is in accordance with industry standards.

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Larry Loffer

Larry Loffer is a senior technician at Wagner Meters, where he has over 30 years of experience in wood moisture measurement. With a degree in Computer Systems, Larry is involved in both hardware and software development of wood moisture measurement solutions.

10 Comments

  1. mark votapek says:

    Mr. Loffer, i happened upon your article when searching information on care for a couple of 18th century wood musical instruments i own. I’ve always been hyper-aware of keeping them at a relative humidity between 40-60% (ideally nearly 50%), but i still detect big differences in how they respond at different combinations of temperature, altitude, and maybe even barometric pressure, even when the relative humidity is kept absolutely constant through humidity controls.

    It was suggested to me that partial pressure water vapor might be a better way of measuring the moisture content (your EMC, i think). Can you direct me to more written about this that’s readable by layperson? Or do you have more to add?

    best,
    mark

    • Larry Loffer says:

      Hi Mark,

      As you know, wood is similar to a dense sponge. It can acquire moisture as well as give it up depending on the surrounding conditions. The Equilibrium Moisture Content (EMC) you referred to is the percent moisture content that wood will eventually reach given the ambient conditions remain constant.

      If you have a smartphone, Wagner has a free APP that will determine the EMC based on constant humidity and temperature. It is called “Wood H2O”.

      We also have a plethora of free wood articles on our website at https://www.wagnermeters.com/wood-moisture-meter/.

      Thank you for your question.

  2. TAREK says:

    Hello Mr. Larry Loffer,

    I have been reading your interesting article. Me I have to do an experiment on a wall, I placed sensors (for temperature and relative humidity) at different depths of the wall, I would like to determine the Moisture Content at those points through determined values of relative humidity and temperature, so my question is : how is that possible? I insist on Moisture content MC and not equilibrium moisture content EMC, because I want to study the transient performance of the wall, so I am not going to wait for equilibrium at every value of relative humidity… I hope my question be clear for you.

    Best regards

    Tarek

    • Larry Loffer says:

      Hello Tarek,

      Humidity readings (ambient moisture in the air) are only useful for determining equilibrium moisture content over a long period of time. The humidity readings will not help you if you need to know the moisture content now.

      I don’t know what material you wish to measure, but I assume it is wood or some other building material such as sheetrock or drywall? A moisture meter designed to measure these materials will give you percent moisture content readings as soon as they are placed on the material to be measured.

      I recommend the below model for measuring many different kinds of building material:
      https://www.wagnermeters.com/shop/bi2200-building-inspection-moisture-meter/

      Thank you

      Larry

  3. Anju says:

    Hello Mr. Larry Loffer,
    I have been reading your interesting articles. Could you kindly explain me the relation between Fiber Saturation Point and Equilibrium moisture content in wood?

    • Larry Loffer says:

      Hi Anju,

      These are two different things.

      There are small cells in the fibers of wood. Fiber Saturation is when these cells can’t hold any more water. Water accumulates outside these cells and is sometimes referred to as “Free Water”.

      Equilibrium Moisture Content is the moisture content that wood will eventually reach given exposure to the same conditions around the wood. Room temperature and humidity are the factors.
      As an example, if a piece of wood is in a controlled environment that is a constant 70 degree Fahrenheit and 50% humidity, the wood will “Equalize” at a moisture content of 9.2%.

      I used Wagner’s free App called “WoodH2O”.

      Thank you,

      Larry Loffer

  4. Kenny says:

    Mr. Loffer
    Thank you for your explanation of the equilibrium moisture content.

    I am planning to purchase a large amount of expensive wood paneling that will probably be milled in a damp climate for installation in my home in the Mojave Desert (Las Vegas). I’ll have to sticker it up here until it reaches equilibrium before I install it. I’m wondering about the usefulness of a pinless vs pin moisture meter for use on wood that is already coated with a six sided finish. Will a pinless meter be able to observe the moisture beyond the finish barrier? Should I just saw off a piece to be able to get an accurate measure or should I used a pin meter?

    • Larry Loffer says:

      Hi Kenny,

      In my opinion, the only advantage of a pin moisture meter is that you can pound the pins into different depths of the wood, provided you get insulated pins. This would allow you to get past whatever finish is on the outside.

      Wagner pinless meters reach down into the wood about ¾ inch and is the way to go provided the finish is only a couple millimeters and is non-metallic.

  5. Ed says:

    Mr. Loffer,
    Thank you for your excellent article. Are there any rules of thumb for estimating how long it will take to reach EMC? For example, if I have a piece of basswood of some thickness at a known MC, say, 7%, and introduce it into a chamber with, say, 60% relative humidity at 70 degrees F, is there a way to estimate how long it will take the basswood to reach its new equilibrium (EMC=10% if I used the formula above, correctly). Thank you.

    • Larry Loffer says:

      Hi Ed,

      First of all, I get an EMC of 11% using your humidity and temperature values when I use the free App “WoodH2O”. There is no formula for determining the time needed to reach equilibrium because there are too many factors involved such as wood species and dimension. The heavier dense woods will take longer. It is best to monitor the moisture periodically using a moisture meter.

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