What Is Equilibrium Moisture Content and What Does It Means for Wood

Woodshop Environment

What Is Equilibrium Moisture Content?

Equilibrium moisture content (EMC) is the moisture level where the wood neither gains nor loses moisture since it is at equilibrium with the relative humidity and temperature of the surrounding environment.

Let’s define some key terms needed to talk about the effect of relative humidity on the moisture content of a piece of wood and reaching equilibrium with its environment.

These terms are the variables in the complex mathematical equations used to determine when wood is at its EMC. In the next few sections, I’ll detail these variables, how they’re calculated, and how they’re used in other calculations.

What Is the Moisture Content of a Piece of Wood?

The moisture content of a piece of wood is the total amount of moisture, including both water and vapor, contained in that piece of wood.

Mathematically, moisture content (M or MC) is the mass of wood with moisture (m) less the mass of wood without moisture (mod or the oven-dry mass), divided by the mass of wood without moisture. It can be expressed as:

MC = (m – mod)/mod

To obtain %MC, multiply MC by 100%. The typical moisture content varies among regions and species. Green moisture content is that which is found when the wood is still in its natural state, before being kiln-dried or equilibrated to its ambient conditions. For example, the average moisture content of green wood Douglas fir (Rocky Mountain type) is 43%, while it’s 108% for a California red fir. Similarly, the green heartwood of eastern white pine has an average moisture content of 50% and the green heartwood of ponderosa pine averages 40% MC.

The average EMC ranges vary too. The Midwest has an EMC range of six to nine percent, which is the middle ground of U.S. EMC ranges. The average EMC in coastal areas is above nine percent and the dry southwestern United States average EMC is below six percent. These EMC differences are caused by differing ambient relative humidity and temperatures.

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 free water. The water held by the intermolecular attraction within cell walls is bound water. That is, it’s literally bound up in the cellular structure of the wood.

The fiber saturation point (MCfs) is the point at which all free water has evaporated and the cell walls are completely saturated. When wood reaches its fiber saturation point, it can hold no more bound water. Below fiber saturation, the wood will start to shrink or swell as its moisture content changes in concert with the ambient conditions around it.

For most wood species, the fiber saturation value of MCfs is about 30 percent. However, the fiber saturation point can vary from species to species and piece to piece. Some tropical species from Africa have a fiber saturation point as low as 17 percent. Furthermore, the surface dries first, allowing for it reach its fiber saturation point before the wood’s interior.

What Is Relative Humidity?

Relative humidity is the ratio of partial pressure of water vapor (H2O) to the saturated vapor pressure of water at a particular temperature in an air-water-vapor mixture. That’s the hard science-y way to put it. More simply, think of relative humidity as the amount of moisture in the air as a percentage of how much moisture the air could hold.

Temperature affects the relative humidity of the air because it affects how much water vapor the air can hold. The following equation is generally used to calculate the relative humidity of air:

ϕ = (ew/e*w) ×100%

where
ϕ = relative humidity expressed as a decimal
ew = the partial pressure of water vapors
e*w = the saturated vapor pressure of water at a particular temperature

More About Equilibrium Moisture Content

As long as the fiber saturation point is not reached, the relative humidity and temperature of the atmosphere significantly affects the moisture content of wood.

The moisture content at which wood neither gains nor loses moisture is the EMC. The equilibrium is dynamic in nature because the ambient conditions, such as relative humidity and temperature, are constantly changing.

When a piece of wood is placed in a controlled environment, it tries to achieve equilibrium with the environment over time. The moisture content adjusts to the relative humidity and temperature of its surroundings. After a certain period of time, the wood’s moisture content stops changing. This moisture level is termed as the EMC.

As long as the relative humidity and temperature of its surroundings don’t change, the wood’s moisture level remains at its EMC once reached. That’s why acclimating wood in its final surroundings is vital before using it. Once it acclimates to its environment and reaches the EMC, the risk of physical changes occurring in the wood is minimal… unless the environment changes.

How EMC, Relative Humidity, and Temperature All Relate to Each Other

The relationship between moisture content, EMC, and relative humidity can be studied and approximated for a given temperature.

For each value of relative humidity for a given temperature, there is a corresponding EMC percentage. Therefore, EMC can be plotted as a function of relative humidity for a known temperature. For most of North America, 30% to 50% relative humidity corresponds to 6% to 9% EMC. It’s worth noting that 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 (a mathematical equation that calculates the time it takes to reach a specified point)
ϕ = relative humidity expressed as a decimal
T = temperature in Fahrenheit

Coastal AreaThe Hailwood-Horrobin equation can also be used to estimate the complex relationship of EMC, relative humidity, and temperature mathematically. This is a more complicated formula but provides more accurate results.

Meq = ( 1800 / W ) * ( (kh) / (1 – kh) + ( k1kh + 2k1k2k2h2 ) / ( 1 + k1kh + k1k2k2h2 ) )

where
Meq = the %EMC
T = temperature in Fahrenheit
h = the fractional relative humidity

W = 330 + 0.452T + 0.00415T2
k = 0.791 + 4.63*10-4T – 8.44*10-7T2
k1 = 6.34 + 7.75*10-4T – 9.35*10-5T2
k2 = 1.09 + 2.84*10-2T – 9.04*10-5T2

Generally, temperature doesn’t affect the moisture content as significantly as relative humidity does. For this reason, coastal areas like Miami, Seattle, and Japan, which have higher values of relative humidity, also have higher EMC values 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 content in wood.

The real question isn’t whether the wood is wet or dry. The real question is if the wood is at the correct moisture content for its end use and in-service location. What is the piece of wood’s ideal moisture content for how and where it will be used?

As discussed above, the specific geographic location of the wood has a significant impact on what it’s EMC will be. This is especially true for wood with an external in-service location. Simply put, wood planks that will be used to build a house deck need to be treated differently than planks to be made into internal wood flooring.

Since the average EMC for the Midwest ranges between six to nine percent, that would be the ideal moisture content range for wood to be used as flooring indoors in Chicago. However, if the woodworker wants to use the wood to build an outdoor deck in Chicago, where the EMC ranges between 12 to 15%, then this six to nine percent range is too dry. This is why, for example, it’s critical for wood flooring to acclimate in the exact environment where it will be used before it is installed.

Why Is Equilibrium Moisture Content Important?

Consequently, the risks of not knowing the moisture content of the wood or EMC of its in-service location are high. It’s also critical to understand how the in-service location’s EMC fluctuates throughout the year.

When the temperature and relative humidity change, the EMC changes along with it. When that happens, the wood starts to release or absorb moisture to re-acclimate to the new EMC. This period of re-acclimation is when warping, cracking, and splitting can occur in the wood.

These moisture-related failures are more likely to occur based on how often the EMC changes and how large the EMC swings are. New Orleans is one of the most humid cities in the United States, yet it’s also one of the most temperate. That is, there aren’t great swings in temperature or relative humidity throughout the year. Thus, a flooring installer doesn’t need to account for as much swelling and shrinking when installing boards as does a flooring installer in Oklahoma City, which has more seasonal climate variety.

The worst-case scenario is installing wood with a moisture content that’s too high. This wood is still in the process of equalizing with its surroundings, which means significant shrinkage is inevitable as it reaches for equilibrium. Under such circumstances, total moisture-related disasters are likely.

The Importance of Correctly Measuring Moisture Content

Wagner moisture meters are designed to quickly and accurately measure moisture content in a piece of wood so that very costly mistakes are avoided. With a Wagner moisture meter, wood can be measured accurately and as often as one might need before making any important decisions.

Try Our EMC Calculator

Last updated on October 13th, 2020

31 Comments

  1. Larry Loffer 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 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. Larry Loffer 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 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. Larry Loffer 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 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. Larry Loffer 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 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. Larry Loffer 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 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.

  6. Larry Loffer krati pandey says:

    Mr. Loffer,
    I went through your article. I wanted to calculate the moisture content at equilibrium for Pre-compressed board which is again a cellulose material with a lesser density as compared to wood. Can the same formula be used for calculation or not?

    • Larry Loffer Larry Loffer says:

      Hello Krati,

      The equation “MC=(M-Mod)/Mod” will work with any solid substance as long as you do not lose any of the material during the drying process.

      Thank you,

      Larry

  7. Larry Loffer Misha Brown says:

    Hello Larry. I have a question that I am hoping you can answer with all the knowledge you have about wood. My kitchen cabinets are producing some form of liquid and there’s no piping so i am wondering if it could be that the wood is having a problem because of fiber saturation. Could that be possible?

    • Larry Loffer Larry Loffer says:

      Hi Misha,

      It isn’t likely your kitchen cabinets are that wet. Kitchen cabinet manufacturers use dried wood to prevent other problems like splitting, warping of the wood.

      If you are sure that a food item is not leaking, then perhaps there is some sap or resin that is coming from the wood. If that is the case, it should be small and eventually stop.

  8. Larry Loffer Misha Brown says:

    Thank you very much for your help. I will research it and pray thats what it is!

  9. Larry Loffer Knut Samal says:

    Hi Larry,

    I have oak parquet planks that have become curved. They have been lying in a room with 73 degrees F and 33% relative humidity for 6 weeks. This should be more than enough time for the planks to reach EMC. However, I measure the MC to 0% with a pin moisture meter. According to WoodH2O the EMC should be 6,6. Can wood really reach 0% MC, or must there be something wrong with my moisture meter?
    Besr regards
    Knut

  10. Larry Loffer Mariella Karen Aldana says:

    Mr. Larry, thank you so mucho for your amazing article. I have a question about the moisture content, if MC is MC = (m – mod)/mod, the mod ( mass of wood without moisture) is considered with bound water or only free water?
    I mean, the cells are hydrated and only the free water is consumed (vaporated) when the mass is without moisture?

    Best regards

    • Larry Loffer Larry Loffer says:

      Hi Mariella,

      The equation MC = (m-mod)/mod as you listed is good for bound water (water within cell walls).

      Free water is defined as the water outside of the cell structure. This is also called fiber saturation which occurs around 30% moisture content. It is very difficult to accurately measure moisture content in fiber saturated wood above 30%.

  11. Larry Loffer Komal Thakare says:

    Hello Mr.Loffer,
    Can you please direct me towards source for finding EMC Vs Relative humidity data for Sodium Dodecyl sulfate.

    • Larry Loffer Larry Loffer says:

      Hello Komal,

      Wagner Meters has a free APP called “WoodH2O” that can be downloaded for iPhone or Android. This APP will give you the EMC value expressed in percent moisture content by weight. You need to know the ambient temperature and humidity.

  12. Larry Loffer Elaina Hughbanks says:

    Hi Larry,
    You mention in the article that the Hailwood-Horrobin equation can be used to estimate EMC, but that wood composites generally have lower EMC values. Do you know of any work done on modelling of EMC with humidity and temperature changes for chipboard or paperboard products? I am working with chipboard pieces at very low relative humidity (approx. 10%) and somewhat elevated temperatures (approx. 120-140F). Any help or guidance toward references would be greatly appreciated.

    • Larry Loffer Larry Loffer says:

      Hi Elaina,

      Yes, the EMC’s of most wood composites are lower since they are not as absorbent as wood.
      I’m not aware of any tests being performed with chipboard.
      You can start by searching for “chipboard tests”.

  13. Larry Loffer Farish says:

    Hi Larry,

    I am a furniture manufacturer from Indonesia and have been exporting to india lately. We majorly use species like Teak, Mahogany and Bangkirai
    ( Tropical Hardwood). We attain a MC of about 8% for all our furnitures at the time of dispatch from our factory. The shipment takes about 40 days to reach its destination. It’s highly humid in southern indian peninsula. The furnitures,once placed at the desired spot starts to shrink at the joints leaving it with hairline gaps(that looks dirty, trust me).

    I believe this Is due to the wood trying to attain it’s EMC at its new surroundings. Would you be able to help me with a ‘simple’ measure to overcome or minimise the issue. Where in I would be able to keep in check the variation in wood Moisture.

    Kind regards

    • Larry Loffer Larry Loffer says:

      Farish,

      I’m a little confused because wood will increase (Swell) in size if placed in a more humid environment. You claim that it is shrinking once it reaches its final destination. That tells me the wood was higher in moisture coming from your factory. Perhaps it is gaining moisture on the journey? It is best to know the conditions of the environment it will live in, to match the conditions at your factory. Perhaps wrapping the wood in an airtight plastic will help.

  14. Larry Loffer John S. says:

    Thank you for the detailed explanation of EMC. I have a charge of Oak 8/4 in my kiln right now. I have insulated pins at center (1 inch deep) and a set near the surface. How do I know when my wood has reached EMC? The center is always higher than the surface which makes sense. Do I take the average? If I’m shooting for a 8% moisture content for a finished product, again – is the average or do I need to get the center reading to 8%? Thank you – I’ve been struggling with this concept for awhile.

    • Larry Loffer Larry Loffer says:

      John,

      Ideally, you will want to get 8% throughout the thickness. This can be achieved by “Conditioning” your kiln charge.
      In other words, put less drying stress on the wood for an extended period of time. This should help even out the moisture content.
      A good resource to help with drying is http://www.kilndrying.org.

  15. Larry Loffer Peter Enyonam Kornyoh says:

    Thanks for the question and answer opportunity we have here. After reading through the comment, does it mean that if I have obtained the equilibrium moisture content (EMC) of a concrete slab, there is no way to be able to estimate the moisture content (MC) of the concrete using the EMC values?
    Thank you

    • Larry Loffer Larry Loffer says:

      Peter,

      The article you are referring to relates to wood and wood-based materials.
      The moisture content in wood is measured as percent moisture content, whereas, in concrete, it is measured as relative humidity deep inside the slab.
      Equilibrium Moisture Content (EMC) has no relevance in concrete.

  16. Larry Loffer Jost Lunstroth says:

    Mr. Loffler – I build climate controlled wine rooms in Texas and am always researching rH, temperature and moisture related issues. The goal in the wine room is temperatures in the 55 – 60 range and an rH of 50 – 70. Anything over 70rH can be problematic for any number of reasons.

    The climate around the wine rooms walls is the ambient of the house – usually in the high 60’s to low 70’s temperature and 48 – 52 rH.

    Usually the walls inside and outside the wine room are sheet rocked.

    My questions are:

    * If the room is heavily insulated and sealed with closed cell foam on all walls, ceiling and floor, do you think it is possible that humidity could enter the room through the studs and or a wood door? The two narrow sides of the studs are not sealed or in contact with the closed cell foam.

    * If we stay with the above scenario and I “seal” exposed side of the stud with some sort of sealer, it follows that humidity will travel thru the wood from outside the wine room and then ‘stop” at the sealed side which is in the colder, damper wine room. If this is correct, do I need to worry about the stud holding the moisture and decaying?

    • Larry Loffer Larry Loffer says:

      Hi Jost,

      Make sure you purchase kiln-dried studs when building your wine room walls.
      It is recommended you check each stud for proper moisture content (less than 20% MC) before purchasing. The small differences in RH will not be a problem as long as you buy kiln-dried lumber, and, of course, the studs do not get wet.
      Here is a link to one of our meters I would recommend for measuring studs if you do not currently have a moisture meter.
      https://www.wagnermeters.com/shop/orion-910-deep/

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