I too used aspen for years and it was fine. Did have to lay folded up wet paper towels on top of hides in the winter. I switched to the cocochips aka ReptiChips, Exo terra Coco Husk and other of the same stuff and it is awesome. Well, I look at it this way, I check where said snake comes from and what environment they live in and try to replicate the humidity by that. Since BPs live in Africa in mostly termite mounds and abandoned rodent dens, the humidity for those 2 places would be fairly high. The temps in the termite mound would be pretty warm too I imagine while the rodent den would be cooler. I do let the stuff dry out more on my BP though. I generally don't respray it until it pretty much completely dries out as I have her in a ProLine cage which holds humidity pretty good(not as good as AP cages though).
Now my boas and retic, they are kept 80% humidity as they come from a much more humid area. If you check the temps and humidity for places like Peru and Colombia, the humidity gets up to 90%+. The surprising part though is if you look at Colombia average temps now, you will see it drops to the 40s at night and 60s during the day. That's why I also have a 75-79F cool side for my boas and their 90F hot spot. A lot of people told me to offer them a mid 70s cool side. I really didn't believe it until I actually looked up the weather in those areas and saw by god they were right.