Question on wood floors and water vapor

Hello,
I'm confused about the way solid wood floors are traditionally installed, given my understanding of the wood and water vapor.
First, I understand that wood floors are typically nailed to a plywood substrate. It seems that fixing the wood in place would not allow any room for the planks to expand or contract across their width, presenting the possibility of cracks in dry weather or buckling in damp weather. Why isn't this an issue?
Also, laminate floors are typically installed as a floating floor, with an expansion gap around the periphery. Is this difference in technique because laminate floors are less stable with respect to humidity and temperature than solid wood floors?
Second, I believe that wood floors are only finished on their top surface. It seems that this would create a big difference in the permeability of the top and bottom surfaces, and that the uneven water vapor transfer could cause the planks to cup. Why isn't this an issue? It seems like it would be easy to finish the underside before installation, but I've never heard of this being done.
Thanks for your comments.
Cheers, Wayne
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Great post Wayne but as you can tell, from the lack of responses, this is a question probably best posed to manufacturers such as Bruce, etc.
My two cents would be that humidity issues are probably on the low side (ie not enough) when it comes to hardwood inside. As to nailing, because the nail goes into the tongue any cracking would occur there allowing movement of the board. If I recall correctly, the wood swells widthwise, not lengthwise.

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Nailed to either plywood or 2x stringers. There is a major difference in the flooring summer to winter. A floor layed in a school gymnasium in the heat of the summer even when running the air handlers will have up to 1/8" gaps between the floor boards during heating season. DAMHIKT

Solid floors also must have expansion around the perimeter. Laminate floors and prefinished hardwood floors are probably more stable than site installed solid wood flooring.

I have never seen one back primed, the technical wording for what you are referencing.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing. . . . DanG
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