PKM, film curl is pretty much dictated by gelatin <> base ratio. Humidity affects this by altering the contraction/expansion of the gelatin against the base.
People have tried all kinds of tricks short of parking their car on top of cut negatives, but in the end the gelatin will still almost always pull the base inwards because that's what it wants to do. Jacking up the humidity helps counter-act this, but if you're in a low humidity environment then fully expect super-curl to happen.
Some films are better than others in the curl department because, IMO, the manufacturer has less aggressive gelatin to base ratio. PE would have some thoughts on this for sure.