I will be expanding my bathroom and moving the bathtub, meaning I need to prepare a new bathtub alcove.

I know how to frame and plumb - but preparing for tile is new to me.

I know that there are different products out there to use to prepare for tile, such as concrete backer-board which would apply direct to the studs and get sealed with thinset, vs. waterproof membranes which would be applied directly to greenboard without the backerboard.

What I don't know is - what are the advantages/disadvantages to each approach? Is one better than the other?

The plan is to use ceramic and glass tiles - does that make a difference?

  • fyi, thinset is an adhesive, not a sealer.
    – DA01
    Oct 11, 2012 at 17:19
  • @DA01 Am I getting my products wrong? I thought the backerboard was installed with an expansion gap which was then filled with thinset... Oct 11, 2012 at 17:21
  • I've used thinset for that, but it doesn't seal anything. It's a mortar. You'd still need some sort of waterproof sealant in the 'sandwich'.
    – DA01
    Oct 11, 2012 at 17:23

1 Answer 1


First of all, greenboard is no longer accepted as a suitable product for wet areas. It has been replaced with "purpleboard" or glass-backed products without paper for mold growth.

Consider doing to tub alcove in a cementicious board like Hardi500 or a fiber-glass backed gypsum like DensGuard.

Do not apply a vapor barrier behind these if you intend to seal them from the room side. This will promote the dreaded "moisture sandwich." where any moisture which does get through the first barrier cannot escape the second.

From recently doing a similar project I am a supporter of the Kerdi membrane. You set the wall boards and seam them as usual (any board will do, but I like mold-resistant and water-resistant board)... then you "shingle" on the kerdi membrane with thinset.

EDIT: I changed where I said "blueboard" to "purpleboard" as some googling showed I was using the wrong slang....

  • Kerdi is good. I also like Redgard (which is a paint-on membrane)
    – DA01
    Oct 11, 2012 at 17:20
  • Some mold resistant drywall products are green, some are blue. But don't worry, I won't use any for the tub alcove. Oct 12, 2012 at 18:02

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