Understanding Wood Moisture Content: The Key to a Long-Lasting Floor
Wanting to ensure your hardwood floor remains beautiful and damage-free for years to come?
Whether you’re a homeowner or flooring professional, understanding wood moisture content is crucial to doing so.
In this comprehensive guide, we will delve into the science behind wood moisture content, discuss its relevance in flooring, and offer insights on how to measure, control, and maintain it to ensure a durable, long-lasting floor.
You’ll find the following topics:
- The basics of wood moisture
- Wood moisture measurement techniques
- Optimal moisture content for flooring
- Wood drying methods
- Subfloor preparation
- Preventing moisture-related problems
- Troubleshooting and repair
The Basics of Wood Moisture Content
Wood moisture content (MC) refers to the amount of water contained within a piece of wood, typically expressed as a percentage.
Wood is a hygroscopic material, which means it naturally absorbs and releases moisture from the surrounding environment.
Water exists in wood in two primary forms: free water and bound water. Free water is found within the wood’s cell cavities, while bound water is present within the cell walls.
Once all the free water has dried from the wood, it’ll begin losing the bound water. And it’s this moisture loss that causes its dimensions to change. It’ll expand or contract with changes in its environment.
This dimensional change can lead to warping, cupping, or other forms of distortion if not properly managed. Additionally, wood’s strength and stability are significantly influenced by its moisture content. Wood with high MC is more susceptible to decay, mold growth, and insect infestation, while wood with low MC may become excessively dry and prone to cracking or splitting.
Several factors affect wood moisture content, including:
- Humidity: Relative humidity (RH) is the amount of moisture present in the air relative to the maximum amount the air can hold at a specific temperature. When the RH increases, wood absorbs more moisture, and when the RH decreases, wood releases moisture. Consequently, controlling the humidity levels in the surrounding environment can help maintain a stable wood moisture content.
- Temperature: Temperature plays a crucial role in determining the rate at which wood absorbs or loses moisture. Generally, higher temperatures accelerate the process of moisture exchange between wood and its environment. Wood exposed to high temperatures in low relative humidity will lose moisture more rapidly, while wood in cooler environments with high relative humidity will absorb moisture more slowly.
- Wood Species: Different wood species exhibit varying moisture content levels and absorption rates. Dense hardwoods, such as oak and maple, typically have lower moisture absorption rates than softwoods like pine or cedar.The wood’s grain pattern and porosity also influence its moisture content, with tighter-grained species being less prone to moisture-related issues. Understanding the specific moisture content characteristics of the wood species you are working with is essential for optimal performance and longevity.
Since wood moisture content is so important, how can you determine the moisture content of your wood? On to that next.
Wood Moisture Measurement Techniques
Measuring wood moisture content accurately is essential to the durability and performance of your flooring.
There are several methods for doing this:
- Moisture meters
- Oven-drying method
Each method has its own set of advantages and disadvantages, so it’s crucial to select the most appropriate one for your specific needs.
In most cases, though, wood moisture meters will be the most convenient. You’ll find out why.
Moisture meters are portable, non-destructive instruments used to measure wood MC.
They come in two primary types:
- Pin-type meters: Pin-type moisture meters have two metal pins that are inserted into the wood surface. These pins pass an electrical current between them, and the instrument measures the wood’s resistance to this current. Since moisture increases the wood’s conductivity, a higher reading indicates a higher MC.Pin-type meters provide localized measurements and can detect moisture gradients within the wood, but only if using insulated pins on a hammer probe. These meters are also highly sensitive to the chemical composition of the wood, which is affected by both the species of the wood and the specific soil that tree was grown in.
- Pinless meters: Pinless moisture meters use electromagnetic signals to measure wood MC—and without puncturing it with holes. These meters can cover a larger surface area and provide a more comprehensive assessment of the wood’s overall MC.
Pinless meters are especially useful when the wood’s surface must remain undamaged, or you need to take many readings fast (as you should when measuring flooring before installation).
The oven-drying method is for the scientific among us. It’s a traditional, laboratory-based technique that involves weighing a wood sample, drying it in an oven at a specific temperature (usually around 103° C or 217° F) until all moisture has evaporated, and then weighing the sample again. The difference in weight represents the MC, which is then calculated as a percentage.
Although this method is highly accurate, it is destructive (you’re not going to be using the sample afterwards) and time-consuming. Not ideal for measuring flooring on-site!
Though the oven-drying method may provide highly accurate results, you’ll probably opt for the more convenient moisture meters. And if you choose a good quality one, the accuracy will be close enough to fit your purposes as a flooring installer.
Once you’ve measured your wood’s MC, how do you know what to look for? It all comes down to figuring out the optimal moisture content for your project.
Optimal Moisture Content for Flooring
When you prepare your flooring materials for installation, you’ll want them to be at the MC that coincides with the final location. We call this the equilibrium moisture content, or EMC.
EMC refers to the moisture content at which wood neither gains nor loses moisture. It’s in balance with the surrounding environment.
Achieving this number is crucial for the stability and performance of wood flooring. If the MC is too high or too low, it can lead to excessive expansion or contraction, resulting in various issues, including cupping, crowning, or buckling.
To determine the EMC for wood flooring, you’ll want to consider the climate where it will be installed. This involves measuring the relative humidity and temperature of the installation site and calculating the wood’s EMC using specialized software or charts.
Here’s why it matters.
A location with a high humidity level requires wood with a higher moisture content so that the wood will remain stable rather than swelling excessively. On the other hand, a dry location requires wood with a lower moisture content to avoid cracking or splitting.
Though you’ll still want to figure out your own EMC, let’s look at some appropriate ranges for various flooring materials.
Solid Hardwood Flooring
The ideal moisture content for solid hardwood flooring is typically between 6 and 9%. To maintain the wood’s stability, acclimate the flooring to the installation site’s humidity level and MC before installation. Otherwise, the floor could end up at risk for warping, cupping, or buckling.
As you determine the specific Target MC for your wood, be sure to consult the manufacturer’s recommendations or a wood flooring professional.
Engineered Hardwood Flooring
Engineered wood is typically more resistant to moisture than solid wood, but it still requires careful moisture management. The ideal moisture content for engineered hardwood flooring is typically between 6 and 8%, depending on the location you’re installing into.
Like with hardwood flooring, acclimate it before installation, measure the wood itself, and make sure you’re installing over a subfloor with good moisture conditions as well.
The ideal moisture content for bamboo flooring is typically between 6 and 10%, depending on the manufacturer’s recommendations and your local EMC. Bamboo flooring, too, should be acclimated before installation, and the subfloor should be properly prepared to prevent moisture infiltration.
Moisture content matters for laminate flooring, too. Typically, it should reach between 6 and 8% and will require some subfloor preparation, depending on the manufacturer’s recommendations.
So, what if you measure your wood’s MC and find that it’s much higher than the EMC? Then what? If they’re a percent or two off, with enough time, you may be able to acclimate them to what you need. Any more than that, though, and you’re probably sending it back to the manufacturer or supplier as a wet claim.
Before installation, wood flooring materials should undergo an acclimation process to adjust to the surrounding environment. This means storing and stacking the materials in the installation area for a specific duration.
Use a moisture meter to check the progress of the wood and make sure it reaches within an acceptable tolerance near the EMC before installation.
You’ll also want to consider the subfloor to prevent moisture issues.
Preparing the subfloor is essential if you don’t want to deal with moisture issues later.
For one, the subfloor should be clean, dry, and level, with no protruding nails or screws.
If it’s a wood subfloor, use a wood moisture meter to check that the subfloor is within 2–4% of the floor’s MC.
If it’s a concrete subfloor, use appropriate concrete relative humidity testing to ensure the slab is dry enough for installation.
Usually, the manufacturer will provide the specifications. If you can’t get an answer from the manufacturer, anything under 75% will mean you’re safe according to ASTM F2170. You can also check out RHSpec for guidelines with various materials.
Second, if you’ve got a concrete subfloor problem, either wait until it’s dry or install a moisture mitigation system to prevent moisture from infiltrating the wood from below. Follow the floor manufacturer’s recommendations when choosing a moisture barrier.
Preventing Moisture-Related Problems
Once the floor is installed, moisture management is not over! There are things that you (if you’re the homeowner) or your client can do to prevent common moisture issues and help your floor stand the test of time.
Here, we’ll identify the common issues, potential moisture sources, and the things you can do to avoid them.
Common Moisture-Related Issues
Cupping: Cupping occurs when the edges of wood planks curl up, forming a concave shape. This is caused by excessive moisture on the underside of the wood, causing it to expand.
- Crowning: Crowning occurs when the center of the wood plank is higher than the edges, forming a convex shape. The moisture is probably coming from the top side of the wood and causing it to expand. The other possibility is that the floor was previously cupping, was sanded flat, and then the moisture conditions went back to their previous state.
- Buckling: Buckling occurs when the wood planks warp and lift off the subfloor. It’s usually due to excessive moisture infiltration from below the subfloor.
Common sources of moisture problems include high humidity levels, water leaks, and insufficient ventilation. Inspecting the subfloor for moisture infiltration and checking the moisture content of the wood flooring can also help identify potential issues.
Take the following steps to protect the hardwood floor in your own home, or educate your client on them:
- Humidity control: Maintain a stable humidity level in your home. Depending on your climate, you may need the help of a dehumidifier or humidifier.
- Regular inspections: Regularly inspecting the wood flooring for signs of moisture-related issues can help detect problems early on and prevent further damage. Check for signs of cupping, crowning, buckling, or excessive gaps between the boards.
- Proper maintenance and cleaning: Wipe up spills immediately to prevent water from infiltrating the wood. Also use appropriate cleaning products and methods to maintain the wood’s natural moisture balance.
- Floor Sentry: Floor Sentry is a device you can install on the underside of a wood flooring plank, allowing you to monitor the temperature and relative humidity conditions of the floor and the subfloor. If conditions go out of acceptable limits, it’ll alert you via your smartphone. This way, you can deal with moisture problems before they happen!
If you—or your clients—didn’t take these precautions, a problem might have resulted. Or sometimes, a moisture problem is outside of your control—such as a leaky pipe or flooding in your home. We’ll take a look at what to do in those cases.
Troubleshooting and Repair
The first step in dealing with a moisture issue is assessing the damage to figure out the source of the moisture and the extent of the damage. Only then will you be able to decide whether to sand and refinish the floor, replace a board, or completely replace the floor. Sometimes, you may have to call a professional to evaluate the situation and recommend the best course of action.
Some more detail on those steps…
Determine the Source of the Moisture
Figure out where the moisture is coming from so that you can stop it.
A leaky pipe may need to be repaired. Or you may need to install air conditioning or a dehumidifier to prevent fluctuations in temperature or humidity.
Once the moisture has been stopped at the source, you can repair the damage and not have to worry about the problem happening again.
Assess the Extent of Damage
Look for signs of moisture-related damage, such as cupping, crowning, or buckling. Check the moisture content of the wood as well as the subfloor to determine how far the moisture has penetrated.
Depending on the severity of the damage, choose one of the following options once the source of the moisture issue has been solved and the wood is at the proper moisture content:
- Sanding and refinishing: If the damage is minimal and limited to the surface of the wood, you can simply sand down any damaged area and refinish the floor.
- Board replacement: If the damage is more severe but limited to a few boards, you may have to replace those boards—but not the whole floor.
- Complete floor replacement: If the damage is extensive and affects a large area of the flooring, complete floor replacement may be the best option.
If you’re unsure of how extensive the damage is or what steps to take, your best option is to call a professional. A flooring expert can provide you with advice and ensure that the repair is done correctly to prevent further damage.
And prevention is always the goal.
Invest in Preventing Wood Moisture Issues
Wood moisture content is the key to saving time and money in costly flooring repairs. Knowing the tendency of wood to move with its environment can help you to work with that tendency—rather than against it.
And with that knowledge, you can:
- Get your materials to the needed EMC to function well in a given environment.
- Acclimate your flooring materials and prepare the subfloor properly—including relative humidity testing for concrete.
- Use a wood moisture meter, such as a high-quality Orion pinless meter, to check whether your wood floor is ready for installation.
- Safeguard your hard work with Floor Sentry.
Ultimately, investing time and effort into proper moisture content management will result in a beautiful, long-lasting floor that enhances the value and comfort of your home.
Wagner Meters is a family-owned American business that aims to provide solutions in moisture measurement technology that will enhance the quality and value of each customer’s project. With an almost 60-year legacy of innovation, Wagner continues to be a resource for both individual craftsmen and high-performance commercial endeavors.
Last updated on December 7th, 2023