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The Relationship Between Wood and Moisture Is Complicated

It all starts with the fact that wood is hygroscopic.

We won’t bore you with the etymology of this Greek word. Just know that it means wood is something like a sponge in that it has the ability to both absorb and release water vapor. We’ll go into more detail about this in just a bit. However, the short of it is that anyone who works with wood, from weekend hobbyists to experienced woodworking professionals, needs to take wood’s hygroscopic property into consideration.

Ignore it, and your projects could be ruined.

We’re not being dramatic. The wood and moisture relationship really is that important. If you work with wood, you need to know the information presented in this article.

The Relationship Between Relative Humidity & the Equilibrium Moisture Content (EMC) of Wood

Understanding the EMC

Whether you’re laying down a wood floor, building a cabinet, or constructing some other object made from wood, understanding the relationship between the moisture in wood and the relative humidity of the surrounding environment means the difference between failure and success.

This is because, as we just mentioned, wood is hygroscopic. It absorbs and releases moisture in response to changes in the ambient relative humidity. Here’s how this works…

When wood’s moisture content is less than the surrounding relative humidity, it will absorb moisture from the environment. When its moisture content is greater than the surrounding relative humidity, it releases moisture back into the environment. This process continues until the wood is in balance with the relative humidity of its ambient environment. That is until it’s no longer absorbing or releasing moisture. This point of balance is called wood’s EMC.

So, why is this important for anyone who works with wood?

It’s important because as wood is absorbing or releasing moisture, it’s physical size is changing. That is, when wood absorbs moisture, it expands, and when it releases moisture, it shrinks. Once it reaches its EMC this ceases.

If the wood you’re using in a project hasn’t reached its EMC, you’re going to have problems because it’s still shrinking or expanding as it slowly comes into balance with its ambient environment. Quality wood projects depend on precise measurements that don’t change. This isn’t possible when you’re working with wood that has yet to reach its EMC.

Wood Moisture Meters and Why You Need One

Wood moisture meters are hand-held devices small enough to fit in your palm. They’re used for measuring the moisture content of various materials, including wood (most common), concrete, soil, paper, and drywall. Both hobbyists and pros use them because too much or too little moisture can ruin their projects.

As we pointed out above, prior to using it in any type of project (big or small), wood needs to have already reached its EMC. If this doesn’t happen, you’ll have problems, guaranteed. This is why anyone who works with wood needs to own and use a quality moisture meter.

Wood moisture meters come in two types: pin (sometimes referred to as pin-style or pin-type) and pinless. We’ll go over the basics of each…

Pin Moisture Meters

Pin wood moisture meters have two small pins (or probes) that you need to push into the wood in order to take a moisture reading. After the meter is turned on, an electrical current passes between the two probes and measures the resistance encountered.

The technology behind pin-style wood moisture meters is based on the fact that water (along with salts and impurities) is a great conductor of electricity, but wood isn’t. So, the drier the wood, the more resistance the current will encounter.

Pin meters are very sensitive to the temperature of wood as well as its chemical makeup, which varies from species to species. That’s why pin meters should come with temperature correction charts and should give you a way to enter the species of wood before you measure the wood moisture content.

The length of a pin meter’s pins determines the depth of the moisture reading. For the deepest readings, you can use something called a hammer probe. This is an external device that attaches to the pin meter.

Although quality pin moisture meters can be very accurate, there are a couple of downsides to them…

  • Pin meters create holes in wood. Each reading with a pin meter creates two unsightly holes in the wood, and unless your piece of wood is very small, you’ll need more than one reading in order to get an accurate picture of its moisture content. This is because each reading with a pin meter only measures the moisture content between the two pins. Remember, each reading with a pin meter creates two holes. If you need to take five readings, you’ll create ten holes. This fact means that pin meters usually are not the meter of choice for measuring the moisture content of fine wood furniture and hardwood floors. Pin meters are often used to measure the moisture content of lumber and firewood.
  • Pin meters aren’t the quickest way to measure moisture content. If you have anything other than a very small piece of wood, you’ll need to take multiple readings in order to get an accurate overview of the wood’s moisture content. This is because — as we pointed out above — pin meters only measure the moisture content between the two pins and nowhere else.
  • The pins tend to bend or break. Some types of wood are extremely dense, and the pins have a difficult time penetrating the surface. When this happens, the pins can either bend or break. Broken or bent pins need to be replaced, and if you use your meter a lot, the cost could really add up.
  • Pin meters are sensitive to wood temperature. This means that you’ll need to use a temperature correction chart if you want an accurate reading. All quality pin meters will come with such charts or ways to account for temperature differences.

Pinless Moisture Meters

In contrast to pin meters, pinless wood moisture meters don’t need to physically penetrate the wood in order to take a moisture reading. This makes them perfect for measuring the moisture content of things like fine furniture and hardwood floors.

Pinless meters are also able to quickly glide over the wood’s surface and measure the moisture content of large volumes of wood (truckloads, batches of wood used in hardwood floors, etc.) in just seconds. A pin meter cannot do this because of the fact that it only measures the moisture content between the two pins.

Unlike pin meters, pinless moisture meters aren’t sensitive to wood temperature. However, they are sensitive to wood density, also known as “specific gravity.” Therefore, you’ll need to enter the specific gravity setting for the wood species you’re measuring before you take a reading.

How to Use a Moisture Meter to Measure the Moisture Content of Wood

Pin Moisture Meters

  1. Push the meter’s pins into the wood.
  2. Turn on the meter and take a reading.
  3. If necessary, repeat steps 1 and 2 until you have an accurate picture of the wood’s moisture content.

Pinless Moisture Meters

  1. Turn on the meter.
  2. Press the scanning plate firmly to the wood and take a reading.
  3. Move the scanning plate on the wood and take another reading.
  4. Continue until you have an accurate picture of the moisture content.

Read the article on how to properly use a moisture meter.

Almost All Wood Defects Are Related to Moisture

According to R. Bruce Hoadley, Professor Emeritus of Building and Construction Materials, somewhere between 80-90% of wood defects stem from too much moisture. In other words, almost all wood defects are related to moisture.

Woodworking professionals especially need to produce quality work. They need every piece to be perfect. They can’t afford to have clients paying money — sometimes thousands of dollars — for custom wood pieces with problem joints, cracked finishes, or uneven tops. All it takes is one unhappy customer to ruin their reputation and endanger their livelihood. Therefore, they need to make sure the wood they use has already reached its EMC before they start working with it, and then they need to make sure it stays at its EMC during building and usage.

The only way to do this is to use a quality moisture meter to monitor wood moisture levels.

We can’t stress it enough…

If you work with wood, you absolutely need to use a quality moisture meter to ensure the success of your projects.

If you’re in the market for a quality wood moisture meter, check out our reviews of what we consider to be the top moisture meters available today.