How Wood Responds to Changes in Temperature and Humidity
Everyone who works with wood knows that it reacts to its environment, in particular to the effects of temperature and humidity on its moisture content (MC). It’s the changes in MC that make wood move, and the same species will respond differently in different environments.
Different parts of the United States affect wood in different ways. The Southeast is hot and humid, with a narrower temperature range than the Northeast. The Northeast has warm, humid summers and cold winters, and heated indoor air there gets very dry in winter.
It’s vital to understand how different conditions affect wood, whether you install wood flooring, build furniture, or work with wood in some other way.
That doesn’t mean there are regions where wood can’t be used. It means you need to know your region and the conditions where the wood will be used, and plan for how the wood will react to that climate.
Even seasonal and daily changes in conditions affect wood. There’s no avoiding it: temperature and humidity affect your woodwork wherever it is.
The short story on why temperature and humidity affect wood
Gene Wengert, known as the “Wood Doctor,” explains that wood shrinks and swells in response to changes in its MC. That raises the question: what causes a piece of wood’s MC to change?
Wood holds moisture in its cell cavities and in its cell walls. The cell walls swell or shrink as they absorb or release moisture, changing the wood’s dimensions. The air also holds moisture; that’s humidity. Relative humidity (RH) is the amount of moisture in the air compared with the most it can hold at that temperature. Colder air can hold less moisture than warmer air.
That’s why two environments with the same absolute humidity will have different RH if their air temperatures differ. It’s also why outdoor conditions vary so much by location and season. According to the USDA Forest Service’s Wood Handbook, the average outdoor EMC in Salt Lake City ranges from about 15% in January to about 7% in July, while in New Orleans it stays between about 14% and 15% all year.
Wengert also explains why understanding RH is so important for predicting how wood will react in a specific environment. If the wood and the air haven’t reached equilibrium, they’ll exchange moisture.
Wood with a low MC in an environment with high RH will absorb moisture from the air. If the air has low RH and the wood holds excess moisture, that moisture will evaporate. Either way, moisture moving into or out of the wood can cause it to shrink, swell, warp, or crack.
The goal, according to the Wood Doctor, is to let wood reach its equilibrium moisture content (EMC) before you build with it. EMC is the point where the wood is neither gaining nor losing moisture in its surrounding air.
Wengert charts estimated EMC against RH and temperature. So when you know the RH and temperature where a project will be in service, you also have an approximate EMC. Use that EMC to set the target MC for the wood that will live there.
At typical indoor temperatures (around 70°F), an RH between 30% and 50% corresponds to an EMC of about 6% to 9%. But that range is too broad to set a target MC for specific conditions. You need to consider several variables:
- Is the wood intended for interior or exterior use? Interiors with HVAC systems and stable temperatures usually have less variation in conditions.
- How much do temperature and RH change through the year? Even interior wood is affected, since HVAC settings change with the seasons.
- What’s the species? Some species move much more than others as their MC changes. Teak, for example, shrinks and swells relatively little, while American beech and hickory move considerably more.
Another important issue is where you let the wood acclimate. It should always acclimate under service conditions. Otherwise, its EMC will shift once it’s installed, and the wood and air will go through a period of exchanging moisture, which is exactly what you want to avoid. The wood’s MC should be within an acceptable range of the estimated EMC before permanent installation.
Of course, this requires an accurate measurement of the wood’s MC.
Using the right tool to measure wood’s MC and EMC
Using a wood moisture meter that measures MC accurately is the best way to start. But knowing the wood’s MC is only one piece of the equation. You also need to know whether that MC has reached equilibrium with its environment.
The formula for calculating EMC from the air’s RH and temperature is quite complicated. Wagner Meters’ Orion® 950 Smart Pinless Wood Moisture Meter measures ambient relative humidity and temperature and calculates the EMC for you. That makes it faster and easier to decide whether the wood is ready.
The Orion 950 also comes with an On-Demand Calibrator, so you can check the meter and recalibrate it on site. Wagner Meters also offers a NIST-traceable version of the On-Demand Calibrator for companies with ISO quality requirements.
Learn more about the Orion 950. To explore RH and EMC in greater depth, visit Gene Wengert’s page on WOODWEB.
As Sales Manager for Wagner Meters, Ron has more than 35 years of experience with instrumentation and measurement systems in different industries. In previous positions, he has served as Regional Sales Manager, Product and Projects Manager, and Sales Manager for manufacturers involved in measurement instrumentation.
Last updated on October 8th, 2026
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