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I picked up some DiamondBack CBU to tile a shower enclosure, as it seems like it would be a lot easier to work with than Hardibacker (I can't cut the stuff without making a huge dusty mess, so if you have hints, I'll take them) and also matches up to the 1/2" green board that abuts it. When I look at the installation instructions, they don't have a waterproof plastic sheet (say, 6 mil plastic) nor a waterproof coating (such as RedGard) on the surface.

Incredulous, I called their customer service line to verify. While friendly, their customer service rep seemed to have as much knowledge of the product's ability as I did. He said that the outer layer doesn't require it and that I need to butt the panels tightly against one another, seal changes of plane (corners, etc) with a flexible sealant and use an ANSI (i forget the number) rated thinset.

Has anyone used this product? Has anyone used this product without adding a waterproof layer? I am a bit nervous and, with the slicker coated surface on the DiamondBack am afraid that, if I defy the instructions and waterproof that I'd have issues with it and, ultimately, the tile's adhesion to the board.

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According to the manufactures install specs, a separate vapor barrier is not required if you seal between panels with a waterproof sealant/caulk. I found the complete install specs for you here: http://www.certainteed.com/resources/CTG-2509_Diamondback_Install_Guide_Eng.pdf

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