I just noticed no one has responded to this yet
Crested geckos require elevated humidity. In fact, it is good for them to have a few hours a day with the humidity spiked up to 80% and even up to 100%. That being said, you can use moss at the bottom. This can help you keep the humidity up. Do you plan on fogging or drip system at all in your set up? Your crested will spend much of its day elevated but will sleep close to the bottom. I would recommend moss, or even doing a moss/eco earth mixture.
- Justin