Dont use paper. It doesnt need to get wet to mold. Humidity is relative, as the temp changes the moisture in the air soaks into anything that will absorb it, in your case it would be the paper.
The Best thing would be some flat white paint, not gloss, Flat white. Flat white will reflect more light then pretty much Anything.
I would suggest some black/white POLY though, check out BGhydro, you wont find any cheaper prices.