If installing floor above grade over cement board is a vapor barrier still required or can it be installed directly on top of cement board?

Hello William - thank you for your inquiry. No vapor barrier is needed with this type of subfloor application. Please keep in mind - Gypsum substrates must be and remain dry, fully bonded and structurally sound, clean, smooth (ridge-free) within a flatness tolerance of ≤ 1/8-inch over 6-foot and 3/32-inch over 1-foot in all directions. If required, smooth using a suitable gypsum based leveling or patching underlayment, written instructions from the manufacturer. Hope this helps answer your question(s) Thanks -