There are a few ways to adjust the humidity. I use a room humidifier in the snake room.
If your rack is vented to much, you can cover some of the holes to help hold in more moisture.
Misting the enclosure or adding damp moss will raise the humidity.
Cypress works well to hold moisture and resists mold much better than aspen.
As I have been told...."read your animals, they will tell you if it's to dry"..."We don't need no stinking hygrometer" and the most helpful tip of all..."This ain't rocket science".
Having said that, I keep my room around 50%, sheds are in one piece, snakes are breeding.
Dave