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Okay-- a couple of things. First, I don't know if you realise it, but the preslope goes in BEFORE the membrane, hence the name PREslope. It's there to slope the membrane so that any moisture that gets thru the mortar bed will follow gravity to the weepholes. As for your question, NEITHER will work. You want a vapor barrier against the studs-- either 6 mil poly or 15 pound tar paper-- It would then tuck into the pan membrane, and THEN Durock directly over that, using 1/2" Durock. You don't want greenboard anywhere in that mix.
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