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Engineered hardwood planks: 3/4 thick T&G white oak, 8" wide, 5mm thick top wood layer.

Subfloor is out of the equation: we're using a 100% moisture barrier underneath.

So our seasonal changes in condition will be purely from the air in the room.

This is a closed, finished, nicely built indoor space - tight, but not super tightly sealed.

BUT there is no HVAC.

We are in the bay area so we could have a foggy summer evening when someone leaves a window open to 100% humidity OR during the winter somebody runs a dehumidifier for a week straight.

The big issue is we cannot depend on any particular climate control. Someday somebody will leave the windows open.

Right now the planks measure out at around 9.5% which the floorers say is on the "high end of acceptable". They think I should dehumidify it down to around 8%. BUT if we do that we're installing it "dry" in a space where it's probably going to get "wetter". That sounds like a bad plan to me.

I think we should install it "wet" at the 9.5% number - then we are worried about somebody accidentally over-drying the space which is less common than somebody leaves all the windows open to the fog ... we're also going toward gaps instead of cupping. I don't want either but I would choose gaps over cupping ... I think ?

Am I making a good call to install slightly "wetter" into this barely-climate-controlled space ?

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  • Usually the instructions say to leave space for expanding along the walls. If this is done, then you should be okay.
    – crip659
    Oct 13, 2023 at 22:25
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    In your situation wood is a bad idea. There are other options such as tile or LVP that can handle the climatic changes much better.
    – RMDman
    Oct 13, 2023 at 22:26
  • @rmdman OK, point taken. I'm afraid we've already purchased the material and gone this far ... I also think that engineered flooring gives us a pretty big margin of error since the expansion coefficient is a tiny fraction of solid wood ... so I am trying to stack some more cards in my favor here ...
    – user227963
    Oct 13, 2023 at 22:37
  • With the laminations of veneer that make up engineered flooring, I expect cupping will never happen with a 3-5% change in MC. Now, if the floor got wet by a water leak and water got trapped between the subfloor and finish floor, there is no wood floor that would NOT cup, I think. Just a mention about using wood in your situation. Houses have been built for centuries with out no HVAC, except for a wood burning fireplace or something like that.
    – Jack
    Oct 14, 2023 at 2:41

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Since it is an engineered wood product, the movement between 8% and 9.5%, in my thought would be negligible. Take into consideration plywood, it is 6X wider than your floor and only needs an 1/8 gap, if that. Another personal experience I had was to measure the moisture content (MC) of the framing of a storage building in a lumber yard that had framing in an unconditioned covered area. I was expecting to be be around 10-12% MC, much to my surprise it was 8 1/2%. The building has been there in operation for decades. The wood I tested was a 2X4 guardrail of a loft storage area in the second level of the building. Not heated or cooled, same situation you will have.

With all that said, I would go with the dryer side, or at least let it acclimate for 2 weeks bundled or 1 week with the packages opened up. 9.5% may shrink with absolutely no consequence, but if the MC grew a little, I think it would "snug up" real nice. Do always keep expansion gaps around all walls ect., I believe it is required for all wood floors.

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