I am constructing a mortar/cement board shower and i need to waterproof it. Is their anything wrong with using a plastic vapor barrier enter image description here / pvc shower pan liner enter image description here for the bottom layer
and then also liquid membrane for the top layerenter image description here are would this be a double vapor barrier and create problems?

2 Answers 2


It looks by what you're saying that it would creeate a double vapor barrier (or vapor pocket), however with the materials shown, it may not create "problems" per se.

That said, I don't know why you would do it using those materials. You could easily grout a tile a shower fairly cheaply and get a more rugged finished product. Personally that's what I would do because it is the standard, easy and correct thing for a shower...

Having said that, you could also just use your liquid sealant layer and heavily coat your motor board and get an equally water-proof later.

  • This is all underlay for tile. It must be waterproof. Their are two main methods of waterproofing. I am trying to over engineer my shower by doing both. I am worried that if I do this I will run into problems of a double vapor barrier.
    – joe
    Apr 6, 2017 at 20:34
  • Ah, got it. I though you were trying to use those materials as your top layer. I'm updating my answer a little with that information in hand. Apr 7, 2017 at 21:54

**it will make a problem ** as mentioned on floorelf.com


If you are using a topical waterproofing membrane such as a liquid like redgard or hydroban, or a sheet like kerdi, do not use a moisture or vapor barrier behind your substrate. If you have a vapor or moisture barrier behind your substrate do not use a topical membrane on the front of it. This combination creates two waterproof barriers with your substrate sandwiched between them. any vapor or moisture trapped between them has absolutely no way to dissipate. This is lovingly referred to as a ‘mold sandwich’. It is not tasty. Use either a moisture or vapor barrier behind your substrate or a topical membrane on the face of it. One or the other – never both.

With that said, if you want to use a topical liquid such as redgard on the seams of your backerboard, after you tape and mud them, you can do so without problems. If your moisture barrier and backerboards are properly installed there is no real reason to do so – but if it will help you sleep at night go ahead and do it.

If you are using a topical membrane and you have an exterior wall with either plastic facing or kraft paper facing you need to cut slits into that facing before installing your substrate. If you do not it will create the aforementioned mold sandwich. Give moisture or vapor somewhere to dissipate.]1

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