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 the 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?
In 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 to 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 to 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%
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 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 to 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
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
The 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 ) )
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.
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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 equilibrium. Under such circumstances, total moisture-related disasters are likely.
The Importance of Correctly Measuring Moisture Content
Wagner wood 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.
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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.
Last updated on December 7th, 2021