The Repti-Bark is fine. The added humidity shouldn't harm your lizard unless mold starts to grow. Sand is bad news for most reptiles, however most snakes, (like your Rosy Boa), probably enjoy it a great deal. If you don't want to use sand, (ad if you do be sure it is sanitized play sand), you may also try a product called 'Lizard Litter'. It's a little on the expensive side, but it resists mold, is absorbant, digestable, and isn't harmful like some pine-based substrates. Good luck, I hope this helped!