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Anyone who installs hardwood floors needs to be on top of countless variables to complete a project with a beautifully installed wood flooring that will last for the long haul. One of the most critical variables is the wood’s equilibrium moisture content (EMC). Without knowing whether the wood being installed has reached its point of EMC, you’re rolling the dice with the wood flooring’s longevity. To grasp why not knowing if the wood has reached its point of EMC is a significant risk, you need a clear understanding of the dynamics of EMC and moisture content (MC) of the wood.

The EMC Basics

EMC is the point at which the MC of the wood has stabilized. That is, the wood is no longer absorbing or releasing moisture with its surroundings.

While this definition is accurate, it’s also a bit misleading. Wood is a hygroscopic material. It naturally wants to absorb moisture or release it, based on the MC of the atmosphere around it. Second, the atmospheric details essential to EMC — the air’s humidity, temperature, and relative humidity (RH) — are rarely static.

For example, you’ll typically want wood for use as hardwood floors inside a heated structure to an MC of six to eight percent before installing them. Yet even indoors with an HVAC system, there are fluctuations in ambient conditions, especially seasonal changes.

So, there’s always some moisture give-and-take between the wood and the air. That’s why Dr. Eugene Wengert, a Professor and Extension Specialist in Wood Processing at the University of Wisconsin-Madison, wrote that “It is critical that the wood you work with be dried down to an MC within 2 percentage points of the equilibrium moisture content (EMC) of the in-use location.”

Understanding the Dynamics of EMC

OK, time to dig a bit deeper into EMC. As alluded to above, EMC is expressed as a percentage, just as wood’s MC is.

You need the wood’s MC to reach within two percentage points of the in-use location’s EMC because you don’t want the wood’s physical properties to change after installation. When wood loses or gains moisture, it can swell, shrink, crack, or experience various other ugly and potentially dangerous deformities. The greater the fluctuations in severity, velocity, and frequency, the more likely physical damage will occur.

When the wood has acclimated, its moisture levels have equalized to its service location’s EMC. At this point, there’s very little moisture flow going back and forth between the wood and the air around it. When the hardwood flooring has reached this state of EMC, it’s no longer likely to experience any adverse moisture exchange with its surroundings.

The question then is: Why do fluctuations in air temperature and RH levels impact the MC of wood?

The simple answer is that if the air is already holding high levels of moisture compared to what it could hold, then the wood will absorb some of that moisture. If the wood has a high moisture level and the air has low RH, the air will suck moisture out of the wood.

Before breaking down that answer in more detail, it’s valuable to talk first a bit about the MC of the wood itself and why changes in MC change the wood’s physical condition.

The Ways Moisture Lives and Moves in Wood

Trees need water to live. That’s no secret. Trees have cellular pathways through which water moves from their roots up through their trunk and into branches and leaves. Those cellular pathways don’t disappear once a tree becomes lumber.

Moisture exists in the lumber in two forms: free water and bound water. Free water is liquid or vapor moisture that moves through the cells and their pathways. Bound water is the moisture vapor that chemically bonds with the cell’s walls. The free water is the first moisture to evaporate from the wood as it dries. The bound water moisture moves more slowly, but its movement has a greater impact.

The wood’s veins don’t change when its free water evaporates or if more moisture enters them. The cells, however, are a different story. A cell holds moisture in its walls, which do swell or contract depending on how much moisture they’re holding. A piece of wood’s cell walls can only hold so much moisture. When all the free water has evaporated, and the cell is retaining all the moisture it can, the cell has reached its fiber saturation point (FSP).

The bound water continues to evaporate because the wood is working to reach the point of EMC relative to its surroundings. With this evaporation, the wood’s cells contract, changing the shape of the piece of wood.

An important point to note is that cells aren’t uniform, and each has its own FSP. That’s why MC can vary throughout a piece of wood during the drying process, and moisture-related damage won’t appear uniformly across that piece. The FSP also varies by species, but most common species reach a general FSP when their MC is around 30%. Thus, when the wood’s MC starts to dip below the FSP, you’ll begin to see physical changes because the cells are beginning to lose their bound water.

Back to the Issue of Why RH Impacts EMC

The RH in the air around the wood is the key driver as to whether the wood will release moisture into the air or absorb moisture from it.

As Professor Wengert reminds us, EMC refers to the MC of the air, not the wood. Scientific research has also found that the rate at which EMC increases or decreases is a function of the RH in the air.

The moisture exchange cycle slows/stops when the wood’s MC is in that two percent range of the air’s EMC. There isn’t enough vapor pressure differential between the air and wood to move moisture around. At this stage, the wood can be said to have reached its point of EMC with its surroundings.

Temperature indirectly impacts EMC because of its effect on RH. Warmer air can hold more moisture than colder air. Consequently, if two rooms have similar humidity levels, but one is 20 degrees colder, the colder room will have higher RH. The colder air can’t hold as much moisture as the warmer room.

The formula for calculating a space’s EMC based on its RH, temperature, and humidity is complicated. However, you can measure the air’s RH using a thermo-hygrometer. Some online tables and calculators can help you determine the EMC for a specific area, using the air temperature and relative humidity percentage. Some wood moisture meters can calculate the EMC for you with greater precision since they can take into account more variables, such as wood species.

You Can’t Know If the Wood Has Reached EMC If You Don’t Know Its MC

Given that the point of EMC wood flooring needs to reach is specific to the wood’s in-service location, hardwood floor installers must give the wood time to acclimate to the in-service ambient conditions. In other words, while the flooring pieces had undoubtedly gone through a drying process when they were lumber, there’s likely still more drying needed.

How much drying the flooring needs can only be determined by measuring the wood’s MC when it arrives at the in-service location. Assuming the location’s temperature and RH are at in-service conditions, you should know what the air’s EMC and the hardwood floor’s goal MC should be. Once you know the wood’s MC upon arrival, you can estimate the drying time needed for the wood to acclimate to the in-service conditions. While acclimation may only take a few days, you shouldn’t assume that’s all the time needed for the wood to equilibrate with its surroundings.

When you know what the target MC for the floor should be given the location’s EMC, the best option is to use a moisture meter. After you have the baseline reading when the wood arrives, continue to take readings periodically. When the MC percentage of the wood stabilizes in that two percent range around the EMC, then you know it’s reached its point of EMC with the location.

Installing wood without verifying its MC is at the point of EMC with the installation location is the best way to risk ugly moisture-related flooring failures. With the wide choice of wood moisture meters, some with the sophisticated functionality professional wood flooring installers need and others with a price point suitable for the DIY-er, there’s no reason to take that risk.