You're definitely going to need a vapor barrier so your floor doesn't cup. Is it unfinished or prefinished?
You could put a coat of finish on the back side of your boards to keep them from gaining/losing moisture.
I'd use #15 roofing tar paper as underlayment. Overlap the seams an inch or so and staple it sparingly - just enough to keep it flat. That should be all you need.
+1 on Brians comment. I laid in hardwood flooring when I lived in CA and used a vapor barrier it was a must. My advice when it comes to laying your own flooring...don't skimp on anything. I don't have any knowledge with using tar paper inside a home though
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