You need to have ventilation in the bathroom for any type of drywall to last. You should use 5/8" drywall on the ceilings with 24" OC spacing. See if you can find mold free drywall like denshield. Technically 1/2" from some manufactures can be used for this spacing. But 1/2" green or purple drywall is not as still and cannot be used with this spacing. You will need to add furring strips perpendicular to the ceiling joists and set them at 12" OC for these type of drywall panels.