Discalimer: I'm clearly no expert . . .

It seems to be builders' preference. My tiler was used to doing Durock for commercial applications--stronger, more vandal-proof. I dislike Durock's "crumbly" edges (warranted or not), so I chose Hardibacker instead. It's easy to cut. Unlike Durock, however, Hardi wicks water easily (even though it's "waterproof," it's not vapor-proof, like a Kerdi, Tyvek, or other vapor/water-proof membrane. Hardi just doesn't warp or get damaged by water). That in mind, installed correctly, I'm sure all of the available commercial cementboard products do their job satisfactorily.

I did my guest bath in Hardibacker with a Tyvek vapor barrier between the Hardi and the studs. However, in the master bath shower, I really want to do a Kerdi shower. The Kerdi system seems like the best system for no-worry, never-leak membrane systems. The peace of mind will be well worth the $400-$500 in materials cost (which I think is trivial in light of the cost of all the other materials, fixtures, and labor you're about to invest in).

Just my novice opinion . . . I'll let the real experts chime in now.

I did not even think about Tyvek....is this the same stuff used on the house exterior. Our last bathroom had the clear poly plastic sheet...which I guess replaced the tar paper method of days past?

Half-inch Durock over poly is a fine system for a tub surround. You'll have no worries.

Do NOT use Tyvek behind the durock though, Tyvek is not a vapor barrier. Tyvek is vapor permeable, it has a perm rating of 58. To qualify as a vapor barrier the perm rating as to be less than 1.

As for Durock, Wonderboard, or Hardiebacker, they all perform well. I personally prefer Durock and Wonderboard over Hardie, but again, that's a personal preference.

When I start getting into things other than basic tub surrounds, that's where a material like Kerdi can shine. Kerdi is by no means a necessity though. Especially for a tub surround.

Feel confident to use Durock over poly.

Mongoct:

Didn't know that about Tyvek! Mistake number 4,381! What are you supposed to use behind the cement board (e.g., specific brand name)? We threw up the Tyvek, slapped on the Hardi, then slopped some smelly waterproof black gooey stuff (didn't know about RedGard). Shouldn't really be a problem should it? I saw the 50-year-old tar paper behind the old tile and thought, well, Tyvek should be much better!

Well you're right! Tyvek (which is a "breathable, air-intrusion membrane), has a permeability of 56, while poly-based membranes have a permeability of approximately 0.1. I read on the johnbridge.com forum that one guy recommended 6-mil. poly sheeting. I guess it's that Hefty-brand sheeting sold in rolls at Home Depot. The irony is that I already have a huge roll of 6-mil. poly sheeting just sitting at home. It's as if both my tile guy and my plumber never built a single bathroom before--I'm learning all this after the fact.

If the "black gooey stuff" was a topical waterproofing membrane (with characteristics similar to Redgard), then no worries, that'll be a sufficient barrier.

The two most common things put behind cement backer board are 6-mil polyethylene sheeting or tar paper. 6-mil poly is a true vapor barrier, tar paper is more of a vapor retarder than a true barrier. Tar paper's perm rating varies based upon ambient humidity.

Tyvek is designed to prevent liquid moisture from passing through, but it allows the passage of moisture vapor.

Now if the "black gooey stuff" was a true vapor barrier too, then you did a good thing by using Tyvek, as you don't want two vapor barriers in a wall assembly. See, there's always a bright side!

Now not to nitpick, but to clarify one other thing that was mentioned for future readers...

Cement boards are not waterproof. Water can saturate and pass through them. But they are not damaged by repeated wetting and drying cycles, which is what I think you were trying to infer.

Semantically yours,
Mongo

Thanks, Mongo! I feel a bit more relieved now, after reading your post.

Yeah, they slopped on this real smelly black goo on the seams and on the fastener heads. Anyway, yes, it was some sort of topical waterproofing goop (which they said was real expensive stuff), and that they use it in all their commercial bathroom installations (McDonald's, Wendy's, Toyota dealers, etc.). They slopped it on fairly liberally but didn't coat the entire lower half, like I've seen in pictures recently of other tub surround installs. Anyway, we're tearing out the other side of the surround's wall to do the master bath--is it even worth attaching poly just between the studs, from the other side?

Also, I've been wondering about that double-membrane issue, trapping the moisture. I've seen photos of people coating quite a bit of their tub surround's cement board with RedGard (like the whole lower half), and I assume they had also installed a poly barrier behind. What is the proper amount of RedGard to apply that you recommend in a surround, assuming you have a sheet of poly behind as well?

Thanks again for your comments!

What is the proper amount of RedGard to apply that you recommend in a surround, assuming you have a sheet of poly behind as well?

None. Remember, it's an either/or situation. You want a barrier either in front of the cement board (RedGard or similar) or behind the cement board (6-mil poly or similar) but not both.

If you are going to apply RedGard, there are certain mil-thickness requirements for the material to properly function. It might be two 30-mil coats, for example. But it depends on your specific situation. The easiest way to gauge thickness without measuring is by applying the membrane at a certain coverage rate, so many sq ft of coverage per gallon of material.

Just use the Kerdi over drywall. What, you're concerned over a couple of hundred bucks when we're looking at a totally waterproof and mold-free enclosure? You eleminate the pre-slope and the vinyl liner...which saves you money. Frankly, I don't even offer my clients an option. "This is the way I do it.....and I guarantee it for life." Don't like this? Fine, hire some hack and take your chances...

Oh no, now I'm getting ready to panic. For our tub/shower enclosure, our contractor put in durock - with no vapor barrier - and with no tape or anything between the seams or at the corners. How much of a problem will this be?

The contractor only tiled about 3' above the tub before we "parted company." Is there a fix we can put in above for the rest of the walls before we put in the tiles? Oh how I hope you don't tell me that we don't have to sledgehammer out all of the (really too expensive) tiles that are already up....