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Moisture Issues with a White Oak Lay-Up
Question
Forum Responses
From Gene Wengert, forum technical advisor: As you are probably aware, oak changes size quite a bit with small changes in humidity (and therefore moisture). So, you may have trouble because you are using oak and not because of your construction techniques. Also, the construction technique of the core can be a source of problems. I suggest that the core grain run parallel to the face grain. When re-sawing the 4/4 into 1/8" pieces (I hope you do not mean that you will actually be splitting the wood, but you will be sawing it instead), if the lumber was not properly stress relieved (that is "conditioned" to remove casehardening), you will not have flat 1/8" pieces. It would not be a good idea to try and flatten the pieces while pressing to the core. Most people would use a non-oak core in order to save money and weight, and to have a flatter door. One potential problem with an oak core is that if you have some pieces that have sloped grain, when their MC changes, they will twist and warp, causing flattness problems with the door.
From the original questioner: Thanks for your response. I was thinking of using an oak core so the core and the skin would move the same, if we used a plywood core and the moisture went up what would happen? I would rather use a plywood core 11 ply Baltic birch, that would be my preference. Back when I first started in the 70s I glued some 16" wide, oak panels to a slab door. One year latter I replaced the oak panels .They shrunk and split and I dont know what the moisture content of the oak or the doors were (the shop was in Houston TX so I am sure it was high). I was just worried about that happening again. Would using a plywood core keep the oak from expanding? We are being careful with moisture this time, all will be 6% to 8%. We have the heat on and the air has been very dry. The houses were all the panels will be going will have heat and AC during the instillation. Our shop on the other hand does not have AC and when the weather turns wet and warm I am worried about problems.
From Gene Wengert, forum technical advisor: As you appreciate, the only reason that wood shrinks is that it is losing moisture. With oak, a 3% MC loss is 1% shrinkage (and the reverse for moisture increase.) The skins shrink first, as they are exposed to the drier air first. If you use oak that is too wet for its final environment with an oak core, the entire door will shrink, the skins first and core second. With a plywood core, the wet skins will shrink as well. So, in either case, using wet oak, you will get a failure. With an oak core, the entire door will shrink and leave you with a door that is too small. In short, you must control the MC and make sure that the components are at the correct MC when assembled, using a MC meter. (In most cases, the maximum MC for the wood should be 6.5% MC to 7.0% MC, but it depends on the final environment's conditions). You should also use a finish that will restrict moisture (vapor) movement, so that any changes in MC are slow and more even, face-to-core. Finally, the customer must recognize that wood is a dynamic product and some changes will occur even with a perfectly built door (including a top notch finish) if the RH changes quite a bit. We cannot change the properties and behavior of wood in a changing environment.
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