As NWFA’s VP of Education and Certification, I attend a multitude of events around the country and even outside of the U.S. on behalf of the NWFA. This gives me the opportunity to meet a wide variety of wood flooring professionals, all of whom face different challenges in their work. One thing that’s for sure is that each region has its own unique requirements for successful wood floor performance. That said, there are key areas that need to be considered regardless of which region you are performing an installation.
Temperature/Relative Humidity – Environment
In order to understand why temperature and humidity affect wood, we first need to understand the relationship between temperature and humidity. Temperature is simply a measurement that indicates how hot or cold something is. In the United States, we use the Fahrenheit (°F) scale; the majority of the rest of the world uses Celsius (°C). Humidity is the amount of water vapor
in the air.
Relative humidity (RH) is the ratio of the amount of vapor moisture in the air shown as the total amount of moisture the air can hold at a given temperature. These values are very much temperature dependent.
- Heating the air will increase its ability to hold moisture; therefore, the percentage decreases.
- Cooling the air will decrease its ability to hold moisture; therefore, the percentage increases.
Humidity is important because wood products exchange water molecules from the surrounding air based upon the amount of moisture in the air. One issue that comes up often is the wide disparity in humidity levels that are present in different parts of the country. As our Wood Flooring Installation Guidelines and Methods state, normal temperature should be somewhere between 60 and 80 degrees Fahrenheit, and relative humidity levels should range between 30 percent – 50 percent to ensure a successful installation. As those in our industry also know, this range is often hard to achieve in certain areas of the country.
There is no doubt that humidity issues vary depending on where you live and perform the majority of your installations. Recently, I talked at length with an installer who told me that no matter what our Guidelines recommend, he will never be able to achieve what NWFA defines as “normal” in his area of the country because of the extremely humid conditions he sees year-round. When I ran my own installation company in Colorado, I had exactly the opposite problem: normal humidity that was much lower than the recommended range throughout much of the year.
So is this an “issue,” and if so, how do we address it to ensure successful installations?
Equilibrium Moisture Content (EMC)
Equilibrium Moisture Content is defined as the moisture content of wood below the fiber saturation point, which is a function of both temperature and humidity of the surrounding air. EMC is the point at which the moisture content of the wood is neither gaining nor losing moisture, and is in equilibrium with its environment.
The average equilibrium moisture content for flooring installation can vary from extremes of 4-12 percent, depending on variables including geographic location and time of year. Additionally, a wide range of relative humidity can be experienced between individual job sites in the same locale, such as an oceanfront or lakeside home versus one that’s a few miles inland.
Wood flooring will perform best when the interior environment is controlled to stay within a stable environment and the wood is installed at a moisture content corresponding to those interior conditions, thus providing a condition for wood to be at EMC. Most wood flooring manufacturers dry their flooring to 6-9 percent MC, which directly coincides with a relative humidity range of 30-50 percent and a temperature range of 60 to 80 degrees Fahrenheit. Fortunately, that’s about the same comfort range most humans enjoy. It’s important to understand that the range suggested by the NWFA is just that: a range. But this range is not arbitrary. Wood is a hygroscopic material, which means it takes on and gives off moisture in direct relationship to its surrounding environment.
The chart at below indicates the moisture content wood will likely have at any given temperature and humidity combination.
Note that the recommended temperature (60-80°F)/humidity (30-50 percent) ranges coincide with the 6-9 percent range within which most hardwood flooring is manufactured. Using this graph is a simple way to tie EMC and relative humidity together; for example, a relative humidity of 25 percent gives an EMC of 5 percent, and a relative humidity of 75 percent gives an EMC of 14 percent. A 50 percent swing in relative humidity produces an EMC change of about 9 percent. How that affects wood flooring depends on several variables including which species is being used and how it is cut from the tree. Wood flooring is constantly exposed to both long-term (seasonal) and short-term (daily) fluctuations in relative humidity and temperature of the surrounding air. Thus, it is always undergoing at least slight changes in moisture content. These changes are usually gradual, and short-term fluctuations tend only to influence the surface of the wood flooring. The rate in which a wood flooring product reacts to these changes varies by species and construction. Moisture content changes can be slowed, but not entirely prevented by protective coatings.
How do you determine what EMC should be for where the floor will be installed?
As the installer and influencer of which wood flooring products should go where, it is your responsibility to have a good understanding of the area in which you are installing floors. Here are a few suggestions on how to make this determination:
- Outside the facility: Check out historical weather trends by using websites such as weather.org, weather.gov, or wunderground.com. These sites allow you to pull historical weather data from a specified zip code or city. Use this information to determine highs, lows, and averages. These exterior conditions usually will affect what the HVAC system is doing inside the structure.
- Inside the facility: Check the home’s/facility’s HVAC system. You do not need to be an expert on HVAC systems, but you should be able to identify the type of heating/cooling source, thermostat settings, humidification/dehumidification system in place, hydrostat settings, and finally if any of these systems are operating properly. Use a thermohygrometer to check and record the ambient conditions in the space to see if they coincide with the HVAC system settings.
- Help the customer make an educated decision: Finally, help your customer choose a flooring product that will coincide with what their facility can maintain.
You should understand what will/won’t work in your area from a product selection and installation practice perspective. While every effort has been made to produce accurate and accepted guidelines for our industry, the principles and practices in our guidelines are not universal requirements, and may not necessarily reflect the most-accepted industry practices in your geographic area.
As previously mentioned, it is the installer’s responsibility to specify/use a product that coincides with the environment in which it will be installed. NWFA’s Guidelines stipulate that wood flooring should be acclimated (or conditioned) to the normal (expected) living conditions in which it will be used. So, if these conditions are higher than what our Guidelines state is “normal,” give your customer the option to choose a floor that you know will perform within those parameters.
Conversely, in the dry, arid climates, like those in Colorado, as long as the materials are properly specified, acclimated/conditioned, and maintained at the expected living conditions, the flooring will perform as it was intended. If this were not true, there would be no wood floors in desert areas or coastal regions. Obviously, this is not the case.
Many engineered and factory finished flooring manufacturers explicitly state that they do not require acclimation before installation. When this is the case, you will also find that they clearly specify the conditions in which their flooring must be maintained. They also are ensuring that their flooring was manufactured and pre-conditioned to those specified conditions, and, assuming the environment is at those conditions, their flooring will be at EMC. If the manufacturer recommends that the wood flooring is acclimated before installation, see the article on page 70 for tips on acclimation success.
With all this said, the most important thing you can do is do your research on the area and the facility where the flooring will be installed before even selecting a product. There are many options available on the market that allow for an array of alternatives an end-user can select. For example, a 4” white oak floor can come in many different cuts (plain sawn, quarter sawn, rift sawn, live sawn), solid or engineered, unfinished or factory finished. Educate your customer about these options as they make their wood floor choice. You can install any type of floor, but to minimize potential shrinkage and swelling, you are doing your customers a huge favor by explaining to them in writing and in person the risks and benefits of their choices. Don’t be afraid to say, “No.” This is your responsibility to the customer and the industry.
Work with your distributors to understand the products you are installing, and work with your customers to help them understand their wood floors and their role in the successful performance of their floor. Build these relationships through communication, and you will have many successful installations and many happy customers.
Addressing challenges like this is one reason NWFA’s training is offered regionally. It gives us an opportunity to adapt our curriculum to meet the training needs of contractors based on their specific geographic area. Check the NWFA’s training schedule at nwfa.org/tech-ed-schedule.aspx to see when we’ll be in your area.
Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at firstname.lastname@example.org.