You would probably have better luck getting this answered in the cages forum - but as someone else mentioned if you are planning on turning your closet into a vivarium type setup for your ball - it's not a good idea balls don't require that much space - I could understand tho if you are going to be putting racks or cages in it. As far as mold - I know when we redid our bathroom there is some kind of sheet rock (? I dont remember excactly what it's called) that you put behind the walls to keep the moisture from penetrating into the wood maybe you can use that?
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Tosha 
"One of these days i am going to wake up..look around...and realize my place looks more like "Ace Venturas"s than my own." Coldthumb
7.33.0 Ball Python (Harry and Fluffy and gang)
1.0.0 Angolan Python (Skywalker)
0.0.1 Green Tree Python
0.2.0 Feline (Pippen and Pandora)
0.0.1 Dessert Tortoise (Pope John Paul "JP"
2.2.1 Fish (1,2,3,4)
0.0.2 frogs rescued from pool skimmer
0.0.1 lizard rescued from vicious feline