My 2 cents worth. I used plywood for framing, particle board for sub roadbed, homasote for roadbed all 3 are not affected by humidity Length-wise. I live in central Oregon, not to be confused with Oregon where it rains a lot. Here, east of the mountains it remains very stable humidity wise, on the dry side. I don't need to worry 'bout no stinkin' swell and shrink. Hah! Lucky me!
I laid my code 70 and 55 without gaps, and thought I'd be fine. The temperatures in Central Oregon are another story, they can and do vary 50 deg from day to night occasionally. My layout is in a finished, sheet rocked insulated outbuilding. I heat it now, but heater is still optional. I never let it freeze.
Welp. guess what? Early in my layout construction the temp swings wrot absolute havoc, tore up my home made switches and kinked what little section of hand laid track I'd gotton so far. Thankfully this was early enough to save what could have been way more work.
I redid everything, repaired the switches and used Micro Engineering flex track, no hand laid ('cept the switches) this time left suitable gaps, especially at switches. Since then it has endured from 35 to 95 degree temp range with no problems
So, Whether you have humidity variations or thermal variations, or both, build accordingly, and be safe, err on the safe side.