A few questions before I get started on my Kerdi Shower

[sdmarkus]

Man, I love these sites...what a wealth of information I have received from all of you already-thank you!!!

Cut out our old cast iron tub and have pretty much everything down to the studs in the bathroom. Putting in a Kerdi shower with tile surround and floor.

Used 3/4" treated T&G ply for the subfloor. In regards to the floor, should I throw down some roofing felt before moving forward with the Hardibacker or is the treated ply sufficient? Would I run that underneath the pan area as well?

Does one typically used Hardibacker underneath the pan or only outside of the pan area where the floor tiles will be laid?

I'd like to insulate the walls while I have them open, one being an exterior. What's the best way to avoid the dreaded moisture sandwich?

Thanks in advance,

Mark

[jadnashua]

Treated wood INSIDE a house is not a good idea. It often comes from the factory almost dripping wet. Plus, when it dries out, it tends to twist and warp. If you've glued and screwed it down well, you might be okay if it has been down awhile and dried out first. Also, ensure that the ply has at least 'C' or better sides (i.e., no 'D' faces). Have you verified that the joists are strong enough for tile? A good subfloor only protects things BETWEEN the joists...the joists must be strong enough ALONG them. And, keep in mind it is not the size of the room that counts, it is the length of the unsupported span of the joists underneath, whether that extends beyond the room or not. You didn't mention if this was going to be ceramic or stone. If stone, then you need a second layer of ply, installed properly.

You need to read the instructions on installation of HardieBacker, or any cbu...no roofing felt. You must install the cbu in a bed of fresh thinset, then screw (or nail) it down with the prescribed fastener schedule then, either as you tile or before, you need to use the special mesh tape on the seams. It's easier to do while you are tiling, then you won't end up with 'speed bumps', which can occur if you do it beforehand, then the thinset cures.

You don't want felt or cbu under the shower pan. Over a wooden subfloor, you do want to put some metal lath in the mix if you aren't going to use the Kerdi pre-formed pan. The mudbed needs to be around an inch or more at the drain over wood, and taper up at 1/4" per foot to the longest corner. That will set the height of the mudbed all around the whole shower. You can either stack 2x4's for your curb, or use a preformed one. On the 2x4's, you'd need to install some drywall before you install the Kerdi.

On the exterior wall, just use unfaced insulation (I assume you are planning fiberglass). The Kerdi is an effective waterproofer, and drywall on the walls is the preferred surface to apply it to.

Check out www.johnbridge.com for help on tiling things.

[johnfrwhipple]

Applying Kerdi over drywall is not accepted here in Vancouver. This is some common miss information given every day online.

Check with your city hall before going this route. Much better to install cement board and then Kerdi.

Here is a link to the best Kerdi "How To" I have found online;

The link won't work but search for "Mongo Kerdi Thread Part Deux" and you will find it..


It's free.

Consider using 1/4" cement board or Green EBoard for the bottom 6" of the shower to reduce build up of the folded layers of Kerdi. This trick is easy to do and make s the tile install so much easier.

Good Luck.

John Whipple

[sdmarkus]

Thanks for the info Jim, i'll run the numbers again but pretty sure I'm good for ceramic tile.

The subfloor is already back down and I think it's going to work out pretty well.

I've got the Kerdi pan, finally starting to take shape

[geniescience]

"best way to avoid the dreaded moisture sandwich?" = only one membrane, any membrane. A liquid paintable membrane for example.

[sdmarkus]

Thanks for the replies gentlemen.

I'll run the numbers again to make sure I'm good for ceramic tile.

The unfaced insulation should work well and I will research the kerdi/drywall combo a bit more.

Time to get 'er done!!!

[johnfrwhipple]

The moisture sandwich problem...

This is a tricky beast and one that requires much thought on how to approach. You simple can't say use Kerdi or Hydro Ban and your good because that might be wrong.

For example if the long side of your tub is against an exterior wall - how is the area below the tub protected from moist vapour? In construction the vapour and insulation inspection happens before the drywall and backer board is installed. If poly is not installed around the tub this becomes an issue. Some one has to wait and explain this science to the inspector - then debate if necessary...

I have had great success with a simple band of Kerdi applied to the tile flange with Kerdi Fix and to the poly with Tuck tape. Then CBU or Green EBoard. Some slits with the razor above this flange ensures trapped moisture can escape before you board.

This moisture sandwich is not so critical when Liquid Waterproofing is installed verses the likes of Nobel TS or Schulter's Kerdi (assuming the poly is left intact). Hydro Ban will not trap moisture behind it and the liquid topical waterproofing allows trapped moisture to escape. Kerdi does not. Nobel does not.

If you are building a steam shower Hydro Ban needs poly. Kerdi does not. Nobel does not.

The moisture sandwich is real issue - but an easy one to approach and build right.

It is not rocket science - but it is science - building science...

JW