Had a grow where the humidity was at a constant 30-35. Didn't have a any problems and the bud was actually much more stick icky...I don't like it to drop that low but I had a new dehumidifier in there and it took a while to get it dialed in, but in the end, I didn't run into any problems. Lower than 20, even 25 might be a little on the too dry side. Also, if you happen to have spider mite problems, that % is gonna give you a hell of a problem with the mites.