Effect of relative humidity on edge-glued panels

Q. I would like your advice on the effects of relative humidity on edge-glued panels. I would like to know if the RH of the environment in which are kept finished panels (glued and sanded) has an influence on the glue joint's strength and if it can result in a glue joint failure? Do you know of any standard or written recommendation on what relative humidity level to store edge-glued panels?

A. The relative humidity greatly affects the strength of the joint. However, within 24 hours, the strength is completely established, so the storage process has no effect on strength. However, humid storage conditions can cause the wood to gain moisture and swell; dry conditions can cause moisture loss and shrinkage.

The stress caused by swelling or shrinkage can stress the joint and cause failure if the joint is not very strong. The problem is not the storage (or even the in-use conditions if the failure occurs later), but that a poor joint was made initially. Many poor joints result because the wood is not glued immediately after machining. The choice of adhesive has little effect, as all adhesives can make a joint that is stronger than the wood itself.MARK To minimize storage related shrinking or swelling, the ideal storage conditions in most cases would be 38 percent RH, as this is close to the average RH of most customers.


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.