Tub Surround - Water Proofing

[Melanie0303]

In the process of re-doing a tub surround (alcove style). We put up cement board (Wonderboard light) but did not put any poly or roofing felt behind. Only one of the three walls has poly behind it because it is an exterior wall protecting insulation.

After doing extensive reading here, still confused what to do at this point?

Pull down the cement board and put poly behind the other two walls?

Paint on a membrane? Redguard or Hydro Barrier?

Or put up Kerdi?

Hubby has not done the sealing of the seams yet with fibreglass tape and mortar.

Also, how do you not create a "sandwich" barrier on the one exterior wall that has poly behind it?

Any recommendations from the pros on this forum would be greatly appreciated!

Thanks!
Melanie

[Tool Guy - Kg]

Welcome to the forum, Melanie.

First, let me shed some light on your last question:

Originally Posted by Melanie0303

Also, how do you not create a "sandwich" barrier on the one exterior wall that has poly behind it?

On the outside wall that has poly, either you leave the poly behind the cement board and avoiding using a surface waterproofing on the front of it, or...you omit the poly (or defeat the poly by cutting several long slashes in it). It's a one or the other deal, not both.



Okay, the first thing to decide is if you'd rather use poly behind the cement board with no surface waterproofing on its face or if you would rather have surface waterproofing. Surface waterproofing is considered by most pros to be superior because it decreases the mass behind the tile that can be infiltrated with moisture, thereby allowing the tiled walls to physically dry out faster between showers...which greatly suppresses mold's ability to grow in that environment. Both roll-on and sheet membranes will work if you pay attention to the directions and take your time. But make no mistake, the "traditional" method of using roofing felt or poly behind the walls (and overlapped onto the front side of the tub's tiling flange) has been done successfully for many, many years.

Because you don't have the cement board taped and mudded as of yet, please don't let the fact that it's already fastened to the wall affect your decision much. Regardless of how long it took you guys to originally cut and fasten them in place, it doesn't take long to remove them and reinstall them (assuming you used screws).

[Kman]

Originally Posted by Tool Guy

(assuming you used screws).

That's the catch right there. If nails were used, you could wreck the cement board pulling it down. If that's the case, I'd try to use a surface waterproofing.

Just to clarify, this is just a tub with no shower head, correct?

[Melanie0303]

Hi Tool Guy...thanks for your reply and the welcome!

We live in Canada and have to have a poly over the insulation on exterior walls so I don't think we can slash it. By the way, the poly was run from ceiling to floor on the studs on the one wall before tub was installed.

Can we do surface membrane on the two remaining cement board walls and not on the exterior wall with poly under it....? Leaving poly intact.

Would it make sense to have a combination of waterproofing? That is, one wall with poly under it and other two with painted membrane? Or would it be better to put poly under remaining two by taking down cement board?

Thanks again!

PS We did use Rock On screws. This is a tub with a shower head.

[jadnashua]

The moisture sandwich issue relates to the fact that there's a difference between waterproofing and vaporproofing something...waterproof surface membranes, for the most part, are not vapor-proof, but are waterproof. That means that some moisture vapor could get through, be stopped by the poly (which itself isn't vapor proof, but maybe more so than the shower waterproofing), condense, and there'd be little chance for that moisture to escape, causing it to drip down, into the structure where it might then have a chance. Yes, it can be an issue. Not, it doesn't always become a problem. Lots of things combine to determine the scope of the issue. Best to only have one layer in there.

[Tool Guy - Kg]

Assuming you were to apply surface waterproofing to the cement board, you absolutely could slash the poly on that exterior wall with insulation on it. The surface waterproofing on the front of the cement board would provide the same benefit the poly is currently performing. Like Jim said, the poly isn't vapor proof, but it is more so than the surface waterproofing. That means that vapor that gets deeper than the surface waterproofing would be stopped at the surface of the poly. That's the cold point upon which vapor would condense. If there was more condensation than evaporation going on, it would collect and drip down the plastic wall.

There is nothing wrong with either system. But the poly on the outside wall, if you want it to stay intact, should be flashed onto the front side of the tub's tiling flange. You'd need to remove the cement board to facilitate this. It would be quite simple to remove the board, make a 30" horizontal slit 3" up from the tiling flange, then insert a 6" tall x 30" wide piece of poly behind the slit and onto the front side of the tub's tiling flange.

Realize that when you come onto this forum, we're going to do our best to tell you the truth with proven methods to back it up. We aren't going to tell you what you want to hear because we really do care about helping folks. We volunteer our time to help, not just hear ourselves talk.

And like Jim said, it isn't always a problem. You might be able to get away with applying a surface waterproofing on 2 1/2 of the walls, overlapping onto your exterior wall. But it's best to practice "best practices". And that's what the John Bridge Forum is about. Educating folks into understanding what the best practices are.

[Melanie0303]

Thanks Tool Guy...we definitely want to follow best practices. Looks like we will pull down the cement board and run some poly....

One other question...any water that drips between the cement board and the poly...how does it drain? ....is the space between the tub and tile not sealed?

[Kman]

Originally Posted by Melanie

.is the space between the tub and tile not sealed?

Now you're going to start another heated discussion.

[Tool Guy - Kg]

Since you are leaving the cement board face un-waterproofed, it's not caught between two barriers. It has something of an open path back out the same way it came back in. It will evaporate back out or fluctuate to a varying equilibrium moisture content, depending on how often the shower is used.

Without going on and on, it's like this: With moisture management, you want to make it difficult for the moisture to get into places you don't want it to get to, yet as easy as possible to get out when it does get in. With the system of poly behind your cement board, you have a system that has proven to work. It will allow more moisture into the system than one with surface waterproofing, but it indeed works.

[Melanie0303]

One other thing we're not sure of is of where our cement backer board meets with an outside dry wall corner. The cement board is about 1/2 " shy of meeting up with the drywall. Is this fixable or do we have to re-do the cement board to meet up flush at the 90 degree angle?

Here are two pics I took.

[cx]

Is that a wet or dry area?

[Melanie0303]

The outside corner is a dry area....the cement board is the wet area (tub/shower surround). The cement board wall in question is the wall with the shower head and valves.

Would a j bead be alright to fill in that gap?

[cx]

Can you back up a little and show us some perspective on the location?