The relationship between wood and moisture is dynamic, not static. Even after green wood has been dried, it still absorbs or releases moisture depending on the ambient conditions surrounding it. These moisture content (MC) fluctuations in the wood can be the cause of a variety of wood flooring ills, such as swelling, shrinkage, warping, cracking, splitting, decay, etc.

The keys to preventing the vagaries that can afflict wood products and the structures they constitute are to understand: (1) how wood holds and releases moisture and (2) how to measure the moisture content of wood accurately. By understanding the science behind these two matters, one can take the necessary steps to mitigate moisture-related deformities in wood.

Where water resides

Trees need water to grow. Nature's design imbued wood with cells that soak up the moisture it needs as it matures. Unfortunately, those cells don't lose their capacity to hold and release moisture once that tree is converted into usable lumber.

 Wood cells hold moisture in two ways:

  • The cell cavity can hold moisture in both its liquid and vapor states, as can any void within the cell. This type of moisture in wood is called "free water."
  • The walls of the cell, made up of cellulose fibers called "microfibrils," hold water molecules that have chemically bonded with the cellulose molecules. This type of moisture is called "bound water" because it's literally bound with the wood cell itself.

 The moisture content percentage (MC%) represents the combined total of both free and bound water. However, free and bound water does not impact wood in the same way. Wood can lose its free water fairly easily since this moisture moves through the cells and veins freely. Bound water needs to be pushed out. Also, it's the movement of bound water that has the most significant impact on whether the wood is warping, shrinking, swelling, or cracking.

Here's why the movement of bound water affects wood: a cell holding free water doesn't change its shape as that moisture leaves the cell's cavities and open spaces. The cell's walls, where the bound water resides, do shrink or expand based on how much bound water they hold. Whether the cell walls shrink or expand depends on whether the cell has reached its fiber saturation point (FSP).

 FSP is reached when all the cell’s free water is gone, and the cell walls are holding as much moisture as they can. As the cell walls absorb moisture, the fibers in the walls swell. When the cell's walls are fully saturated, they can't swell any larger. Thus, wood shrinking or swelling only occurs when most of its cells are below their FSP. At FSP, any absorbed water now can only be held as free water in the cell, which doesn't change the cell's shape. While excessive free water won't impact the wood's shape, it does create juicy conditions for decay. Because FSP exists at the cellular level, wood doesn't uniformly warp, check, or split.

Pictured is swollen, wet flooring

Why and how water moves through wood

The ambient conditions (temperature and relative humidity) around the wood directly impact whether wood cells take on or release moisture. Air temperature affects how much moisture the air can hold. Warmer air can hold more moisture than colder air. Most importantly, changes in air temperature change how much moisture the air can hold. Thus, changes in temperature impact the relative humidity (RH) of the air. RH is the amount of moisture the air is holding as a percentage of what it could hold.

Wood absorbs or releases moisture in reaction to RH. A low RH pulls moisture from the wood. That same dry wood sucks up moisture under high RH conditions. Wood attains its point of equilibrium moisture content (EMC) when it's no longer adding or losing moisture. Attaining EMC requires the ambient conditions around the wood to remain constant so the moisture in the wood can level out in relation to its environment.

The primary guideline for working with wood is to ensure that it is dried to the proper MC. The wood’s MC should be equal to the EMC where it will be used. Inside heated buildings, one should typically aim for use of wood with an MC of about 6-8 percent, but can vary due to different environmental conditions and locale.

Slabs about to enter a kiln

Methods to measure moisture content

There are two methods to measure the moisture content of a piece of wood. The most accurate – and most time consuming and expensive – is the oven-dry method. The oven-dry method starts by weighing cut sections of wood from the stack to be used as test pieces or "moisture sections." This initial weight is the wood’s "wet weight." As the wood dries in the oven or kiln, the moisture sections are removed periodically and re-weighed. When the drying process is finished (when the change from the previous weight measurement is less than 0.1% MC), the final weight is known as the wood’s “oven-dry weight.” The wood's moisture content can then be quickly calculated by subtracting the oven-dry weight from the wet weight, then dividing by the oven-dry weight, and multiplying that by one hundred.

The simpler and faster way to measure wood's moisture content is to use a handheld moisture meter. A moisture meter is a small tool that's placed on the wood to read its moisture content. There are two types: (1) resistance moisture meters and (2) dielectric moisture meters.

The resistance moisture meter inserts electrodes (or "pins") into the wood. The meter measures the electrical resistance between the two pins. Since moisture conducts electricity, increasing electrical resistance between the pins indicates the wood is dryer than with a lower resistance. Pin meters typically convert the resistance measured into a moisture content percentage. The downside to pin meters is that they can only measure moisture content within a limited range, usually 7 to 30 percent. Also, using them requires inserting pins into the wood; sometimes as deep as two and a half inches.

In contrast, a dielectric moisture meter doesn't require physical intrusion into the wood. These pinless moisture meters use flat plate electrodes that are placed on the surface of the wood to detect conductivity within the wood using electromagnetic waves. A pinless meter makes moisture measurement faster, is 100% non-intrusive, and can be designed for measurement at specific depths. A pinless moisture meter can also provide a broader range of moisture content measurements, from 4.5 to 32 percent.

Technology advances

While handheld moisture meters are convenient and helpful, they have certain drawbacks. As noted already, pin meters leave unsightly holes in the wood sample. In addition, the readings of some pinless meters may be overly affected by surface moisture or ambient temperature.

Newer technology used in more advanced pinless meters has overcome many of the drawbacks, and can facilitate measurement at specified depths within the wood, so that readings are virtually unaffected by surface moisture.  Another recent advance in pinless meter technology is the ability to do true in-field calibration of meters.

Regardless of the specific method one chooses to measure moisture in wood, knowing how wood dries is just as essential as knowing the actual moisture content. With this understanding of the dynamics between wood and moisture, one is better prepared to make good decisions about wood’s readiness for use.

Source: Jason Spangler is the flooring division manager for Wagner Meters and has more than 25 years’ experience across a broad spectrum of industries. For information call 800-505-1406 or visit WagnerMeters.com.

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