I also use cypress mulch. Unlike bark, it has lots of 'shreds' of cypress mixed in, with makes the substrate better able to hold moisture and also hold tunnels and burrows better. A guy on the crested gecko forum made a good point one day about relative humidity, which is what I assume you are referring to. In some places in the US, the humidity gets up to 90-100%, but that does not mean those people/animals in that place NEED it to be that high to thrive. I don't quote this or know that it is scientifically true, but it makes since to me and all my animals are definately thriving. For my tegu, I just give his burrowing side of the mulch a nice heavy misting about every 2-3 days, I stick my finger deep in the mulch to see if it feels moist. If not, I mist the hell out of it. It usually seems to stay pretty moist for anywhere from 2-4 days. He sheds great every other week and is growing like a weed (approx. 1.5 inches per weeek). So I don't have a number for you. But mabye like me, you will make more sense out of this than an actual number. Besides, with being inside and requiring the high temps, you will have a near impossible time keeping relative humidity at any steady number, especially in winter, so don't worry. Kinda long but I hope this helps.

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-Bill
1.0.0 Albino California Kingsnake
0.1.0 Colombian Red Tail Boa
0.1.0 Leucistic Patternless Leopard Gecko
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