Hardwood Floors October/November 2025

AT THE SITE What is Hysteresis? (Continued)

NWFA Wood Flooring Installation Guidelines define hysteresis as “The phenomenon in which the value of a physical property lags behind changes in the effect causing it. In wood, the EMC that wood attains at any given relative humidity and temperature combination depends upon many variables, including the direction from which the EMC is approached." EQUILIBRIUM MOISTURE CONTENT

WHAT IS HYSTERESIS? NWFA Wood Flooring Installation Guidelines define hysteresis as “The phenomenon in which the value of a physical property lags behind changes in the effect causing it. In wood, the Equilibrium Moisture Content (EMC) that wood attains at any given relative humidity and temperature combination depends upon many variables, including the direction from which the EMC is approached. During desorption (moisture loss), the EMC will be higher (sometimes by several percent moisture content) than during adsorption (moisture gain).” In plain speaking, this means if you acclimate a floor to a temperature of 70 º F with an RH value of 40 percent, the wood’s MC should be around 7.7 percent (based on the EMC chart). When that wood then goes through the dry season, it will lose moisture. During the following humid season, a much higher RH than 40 percent will be required to get the floor back to the same width the floor planks were at during installation. In the chart, the same moisture content would only be achieved during adsorption at a relative humidity of 58 percent. This is significant!

24

20

16

12

DESORPTION

8

MOISTURE CONTENT %

ADSORPTION

4

0

COURTESY OF WOODWORKINGNETWORK.COM

0

10

70

30

20

50

60

90

40

80

RELATIVE HUMIDITY %

THE BOTTOM LINE There are many things to factor into the chosen relative humidity at which you will acclimate your floor. Things such as expected living conditions, species, and width of plank being used, just to name a few. Undoubtedly, hysteresis is something that should be considered when explaining the importance of acclimation to your customers. If you have more questions on acclimation, check out NWFA’s Wood Flooring Installation Guidelines’ acclimation and conditioning section. A big thanks goes to the late, great Dr. Eugene Wenegert who first exposed me to this topic at the NWFA Expo Symposium in Milwaukee and also took my phone calls after that time to help further explain this topic. Ryan Jazdzewski is the owner of Jazz Custom Wood Flooring in Burlington, Wisconsin, and an NWFA Certified Installation Professional, Certified Sand and Finish Professional, and Certified Wood Flooring Inspector. He can be reached at jazzflooringco@gmail.com.

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