I have an issue with a floor installed back in April.

Brentwood CA (near Stockton ). Low humidity, high heat in summer. Winters mild, generally heated to 68-70. Bellawood 3 1/4 x 3/4 solid brazilian koa, second floor, 2 inch staples, 15 lb roof felt. The wood spent a week in the installed room to acclimate.

When it starts to heat up this time of year and throughout the summer, the temp upstairs can fluctuate between 70 and 80 or so. At night we use a whole house fan that generally gets the temp nearer 70 in the AM. Since April, we've been happy with the floor, noise is low... But, since it's started to heat up the last few weeks... After a warm day ( which we expect to be every day for the next 4 months ), in the middle of the night ( when the house starts to cool ), we hear the floor - occasional "crackle/popping" sounds. And when we get out of bed, it's full on crackle on every step. Once we walk on the floor for a bit, it's back to quiet.

We have "tiger wood" solid glue down downstairs (3/8"?) ( which we absolutely love ) - going on 5 years now, so my plan had been for the whole house. The temp fluctuates much less downstairs. But, with the temp changes upstairs and the crackling, I'm wondering if a solid 3/4 is a bad idea?

So, my questions:
With a daily 10 degree change in temp, is 3/4" bellawood brazilian koa just a bad idea? Would engineered be more appropriate?
Will the morning crackling go away or just get worse with the higher heat/temperature fluctuation? What else causes the crackling?

Thanks for reading...

Didn't hear what the subfloor or underlayment was underneath ie; whats it fastened to, plywood, OSB or Particle board?

Even if it's plywood, exotics move alot. So either get the upstairs under control or live with it. I suggest another zoned AC unit for up stairs and a humidifier or 2 for the house.

All the heat rises to the upstairs. I'll bet the RH is loooow up there.

The subfloor is OSB - I can't find any markings on it though on any that is exposed right now.

Given the fact that it's OSB and I expect temps to fluctuate a bit (more in the summer, although we will cap now at 80 via AC, winter upstairs stays between 68-70) - would a 1/2" engineered (still bra koa) and say a 3/8" ply underlay be a better idea? I have more rooms I'd like to do, and now's the time to change paths.

What do you think?

Split tongue's, maybe bad OSB. Screwing ne ply over it would help immensely. And try a 18ga cleat nailer instead of staples.