Dimensional Instability of Kiln Dried Lumber

Hardwood lumber is typically kiln dried and used for interior applications such as furniture, paneling, cabinets, flooring, etc.  The final moisture content of the lumber coming out of the kiln will vary but will typically be around 6 to 10 %.   This is ideal because most houses are climate controlled and the combination of relative humidity (RH) and temperature in the house will create an equilibrium moisture content (EMC) for the wood.  Wood between 0 and 30% moisture content will shrink or swell as it gains or loses moisture content.  So, to keep the wood movement minimal, we want to install interior wood products as close as possible to the EMC.  This is the reason why wood flooring is placed in a house with the air conditioner running for several weeks so the new flooring can reach the EMC of the house and not shrink, swell, crack, finish failure, warp, etc. after it is installed.  In general, higher density species such as oak will shrink and swell more than lower density species. 

For the wood worker or hobbyist, it is ideal to store wood at a RH that will main the moisture content at the correct level.  For example, at 30% RH, the air is 6% EMC.  If you store kiln dried lumber, you should have an electronic RH sensor to monitor humidity and a moisture meter to monitor moisture content.  Some other key numbers are 50% RH = 5% EMC, and 80% RH = 16% EMC. 

Years ago I served as an expert witness in a case in which hardwood flooring had buckled within two years of installation.  The wood had been kiln-dried to the proper MC, but it had then been stored at ambient conditions in a non-ventilated building for several years.  The moisture content was not checked at the of installation, but it certainly was at an elevated MC when installed. 

More information? 

For more information, please read Seasoning to Prevent Defects in Green Wood.

Todd Shupe is the President of Wood Science Consulting, LLC. He is a a well-recognized expert on wood preservation, insects and fungi, wood decay and degradation, and wood species identification. He has a broad background in new product development, quality management, and marketing and sales in both the public and private sectors. For more information please visit DrToddShupe.com.