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).
How Do You Remove Moisture From Wood?
Kiln Drying
Freshly cut wood has a lot of moisture in it. Eventually, this internal moisture will evaporate by itself. However, kiln drying is used to speed up the process. Some of the unfinished wood you see on the market has been kiln-dried to reduce its water moisture content to around 8% so that it won’t suffer from moisture-related defects like warping and buckling. However, many building materials may have been dried down to about 15% moisture content.
But, that’s not the end of the story…
Wood moisture content is always varying. It’s never constant. Wood – freshly cut or kiln-dried – is always interacting with environmental moisture. Therefore, just because the wood is kiln-dried doesn’t mean it has lost the ability to absorb moisture. It will continue to absorb and release moisture until it comes into balance with the surrounding air.
What are Acceptable Moisture Levels in Wood?
The acceptable moisture levels of wood and lumber are in the range of 6% to 8% for interior and 9% to 14% for exterior wood or for building envelope components within constructed assemblies. The acceptable moisture content in wood depends on two factors:
The wood’s final use.
The average RH of the environment where the wood will be used.
These two factors make it difficult to say anything specific about acceptable wood moisture content. It’s more important to understand that the wood is kiln-dried down to a certain bell-shaped range of MCs. There will be statistical outliers on both the low and the high end and you’ll want to catch these by using a quality moisture meter.
