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After much research, I have a lot of conflicting advise about the need to add a vapor barrier before installing the backer (hardy / cement) board for my bathroom shower.

The "expert" at Home Depot, and another guy on youtube seem to think this is a very good idea, and even suggest sealing it with roofing tape. (The guy on Youtube tapes it directly to the sill of tub.)

However, when I asked about the tape and how far down I should go at Lowes, they told me they'd never hear of anyone doing that, and that it didn't seem like a good idea. Likewise, Ask This Old House only recommends putting down caulking on the sill before applying the backer board, and then again after before adding tile, and then caulking a 3rd time under the tiles before grouting.

Should I add a vapor barrier between the frame and the cement board? Why or why not?

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3 Answers 3

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You should NOT place a vapor barrier BEHIND the backer board.

Where, pray tell, would any such collected vapor/water go?

Answer:There's nowhere proper for such moisture to egress.

The current best practice is to place waterproofing OVER the backetboard and just UNDER the tile. Thin sheet membranes like Kerdi or NobleFlex are examples . Paint on membranes can also be used, such as RedGuard or AquaDefense.

This a is a recent job I completed illustrating Kerdi over Hardibacker.

Kerdi membrane over Hardibacker

enter image description here

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While I agree the waterproofing should be on the inside as much as possible (I've used Redgard and like it) the use of plastic sheeting behind the cement board has been common practice (at least in the US) for a long time and may even be code in places (if you're not using the alternatives you've listed) –  DA01 Mar 4 at 3:25
    
@DA01 is it also standard practice to leave gaps in the caulking to let the water drain from? –  virtualxtc Mar 4 at 8:03
    
@virtualxtc it depends on the system you are installing, but most modern tubs have a lip that the plastic and cement board would overlap. Whether or not to caulk that spot is debatable. Some say that you want any water behind to eventually drain. Other's say that water should never get there to begin with and the plastic is just a last line of defense. (FWIW, I push Redgard as a better alternative, as the waterproofing is now closer to the water) –  DA01 Mar 4 at 15:28
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Do I put a plastic sheet up so local inspector doesn't throw a fit? Yes. Is it totally useless with HB? Yes. HB is basically waterproof. I have let these boards sit in water for months with no affect so why do I need a sheet behind it? (Little secret of mine... I might accidentally puncture said plastic in several places while installing HB on top) - The whole point is where is the water coming from that the plastic would help? I am more worried about the wall on the other side of plastic sheet - if water is trapped in plastic sheet for a long time, mold could grow on nearby walls. –  DMoore Mar 4 at 16:56
    
@DMoore not all backer boards are the same and they're not all rated to be moisture barriers. Whether one should have a separate moisture barrier is highly debatable, of course, and is why I tend to vote for the 'inside' method ala Redgard to get around the whole debate. –  DA01 Mar 4 at 22:37

You should, so that any moisture that gets behind the cement board runs into the tub or shower instead of into your wall cavity (causing rot or mold/mildew).

If you overlap the moisture barrier with the raised lip around the tub, any moisture has only one path to take due to gravity -- down the barrier and into the tub. I usually use thick-mil plastic sheeting, overlapped in shiplap fashion (overlapping, laid down from bottom to top -- like shingles).

Do caulk between the tub and tile, but leave small gaps for any water runoff down the sheeting to escape. I always take a level to the tub base to see where the water would run to if it had the chance, and place my holes appropriately.

You'll staple the sheeting to the framing -- don't worry about these minor penetrations of the barrier.

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I'm confused about the shiplap, can one can buy rabbited thick-mil plastic sheeting? and I assume you mean horizontal not vertical shiplap such that the water will run down, correct? If I caulk the tub, wouldn't that just make it puddle behind the cement board? What tape should I use? water-tight aluminum? tyvek tape? –  virtualxtc Mar 3 at 13:59
    
The shiplap reference simply refers to the upper layer covering the edge of the lower layer, like roof shingles. Gravity does the water proofing. Although I am not an advocate of solid impermeable membranes, the small area behind the tile, should not create any mildew problems with the blockage of moisture that will need to pass through the walls. I just renovated my 1989 hall bath shower, tile over Wonderboard. When I pulled it down to the studs. there was no evidence at all of any water passing through the 4X4 tile. except in one corner where I never nailed the corner tight. I fixed that. –  Jack Mar 3 at 15:27
    
When you caulk around the tub, you need to leave small 1cm gaps every so often in the caulk for water to escape. –  Ethereal Mar 3 at 21:06
    
Ask TOH is usually right on, but how old was the segment? A vapor barrier behind the hardi-backer or concrete board is SOP today. These walls are also insulated for sound proofing so even more important to water seal the outer layer from the inner cavity. If the plastic (6 mil min) is not large enough to cover the entire envelope in one piece, it should be installed at the bottom first, overlapped so water runs over the lap downhill, and taped at all joints. –  shirlock homes Mar 3 at 23:47
    
@Ethereal: Your comment about leaving 1cm gaps is relevant to me. I built a shower last year and wondered how any water coming down the vapor barrier would get into the shower base because I caulked the gap where the vapor barrier lipped over the shower base's outside edge. Having read your comment I'm now wondering whether I should drill some small weep holes through the caulk. Would you? –  getterdun Mar 4 at 2:16

I called USG,the maker of Durock cement board Next Gen, and they advised using no vapor barrier so that the wall cavity can breath and allow any moisture to evaporate. Their online instructions/diagrams show no use of a vapor barrier.

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