The humidity in my house is around 35%. Despite that, I have no problem maintaining humidity in the enclosures (all of them are currently at least 70% humidity), just because the plastic tubs really hold it in. In fact, even with paper towel substrate and only a single water bowl in each enclosure, I actually have to make sure the hides and decor have time to dry after washing, otherwise they bump the humidity up past 80%. I do not have to mist the enclosures at all.
As for downsides to paper towels, these are the only ones that come to mind for me:
- They're not very aesthetically appealing, haha. I might give other substrates a try in a few years, once the snakes are fully grown and have moved into nicer, adult-sized cages as their "forever homes."
- If they get soiled with pee/poop, you have to toss them out, wipe things down, and replace them with fresh paper towels each time, rather than "spot-cleaning" which you can get away with for a while when using thicker and more odor-absorbing substrates. With that said, my boas only pee every 1-2 weeks, and poop even less frequently than that, so it's pretty manageable.