Z Setting Problem

I've got four 8" Prusa I3V machines and have been printing for almost a year now. I'm constantly having to set the Z before just about every print. Here's how my day usually goes:

1. Before the first print of the day on all four machines, the nozzles are usually 1/16" or a bit less off the glass even though when I quit printing around 9pm the previous night they were set perfectly. Therefore...
2. All four machines have to have their Z setting tightened up significantly before the first print of the day.
3. For most of the rest of the day, until around 6pm, each printer has to have the z loosened up a bit before the next print.

The only thing I can figure is that it could be the humidity here. I live in East Texas and at night we typically have humidity in the high 80's percentile range, and the lowest it gets is usually in the 60's. I visited my dad in New Mexico recently. He has very low humidity there. He let me use two of his 10" machines and one of his 8" machines. I didn't have to adjust the Z setting at all for that whole week. He told me he has to set them once every few months. Does anyone else have any other ideas on this?