I suppose you could....but it's unnecessary.

Redguard isn't compatible with greenboard, so no, just the Kerdi. With the correct drain interface.

Redguard's manufacturer gives specific instructions for installation over Lightweight Cement and Gypsum Surfaces on page 2:

Here is a link that might be useful: Page 2

"Redguard's manufacturer gives specific instructions for installation over Lightweight Cement and Gypsum Surfaces on page 2:"

Trebuchet, that Lightweight Cement and Gypsum Surfaces section you referred to is for gypcrete, not for gypsum wall board.

Gypcrete:

{{gwi:2133872}}

This post was edited by mongoct on Mon, Dec 15, 14 at 0:50

RedGuard can be used over plaster and lath. 11th comment down:

Here is a link that might be useful: P&L