Everyone who works with wood needs to understand how wood interacts with moisture in the environment. Whether you’re a woodworker making cabinets, a wood flooring professional installing hardwood floors, or if you use wood in construction, wood moisture content (MC) should be always on your mind.
Wood is hygroscopic. It gains or loses water moisture as the relative humidity (RH) of the surrounding air changes.
These varying humidity levels of the surrounding air cause wood to not only gain or lose water moisture but to expand or shrink as well. As the humidity increases, the MC increases, causing the wood to expand. As the humidity decreases, the MC decreases, causing the wood to shrink. When the wood neither gains nor loses moisture, we say that the wood has reached its equilibrium moisture content (EMC).
According to Dr. Eugene Wengert, professor and specialist in wood processing at the University of Wisconsin-Madison’s Department of Forestry, wood should be dried to a water MC that’s within two percentage points of the EMC where the wood is going to be used.
Before we explain what this means, let’s make sure we have our definitions down.
- MC = the wood’s moisture content
- The EMC (equilibrium moisture content) of the location where the wood is at the moment or the location of where the wood is going to be used = the MC that the wood will eventually attain if it’s placed in that location.
Humidity of the in-use
EMC of the in-use location Corresponding MC the wood will attain at this location 19-25% 5% 5% 26-32% 6% 6% 33-39% 7% 7% 40-46% 8% 8% 47-52% 9% 9%
So, using this chart, we know that in an area of the country where the RH inside a home or office is anywhere from 26-32%, both the EMC of the in-use location and the wood moisture content kept in that location will be 6%.
This means that wood intended for interior use in this location should not only be dried to around 6% but should be kept at this moisture content both before and during the manufacturing process.
So, the wood must always be allowed to acclimate or come into balance with the RH of the end-use location. Failure to do this will result in warping, cracking, and other problems after the construction of the wood product.