I have wood chips at the bottom to help keep humidity up. they don't dry well.
I don't recommend doing this at all.
There are other ways to keep the RH up.
Just decide on a misting schedule. And mist several times a day, allowing the cage to dry between cycles.
I used to be fussy about having a separate hygrometer for each cage, but discovered it was useless.
The reading fluctuated like crazy.
I know the cage has good RH because I cover the back, and one, or both sides of the cages with clear plastic.
So even if the room is only 30%RH, I know inside the cages are way higher, and don't need a meter to tell me that.
Sometimes in the winter, dry room air, I use humidifiers.
But I don't get as neurotic over measurements as I used to.