View Full Version : polystyrene as backer for shower mortar

01-29-2008, 05:05 PM
I have figured that Polystyrene would provide a good backing for floated mortar bed shower walls. It won't promote mold growth and is very water resistant (extruded (XPS) more than expanded (EPS)). Either XPS or EPS will allow vapor transmission though.I looked into the Wedi product, which is extruded polystyrene with a cementitious layer on the face for tile, but it is very expensive and I don't need the cement layer for tiling upon.

I need to add 1/2" layer to the studs to bring out flush with the 1/2" ply that backs the shower pan but I want to stay away from gypsum based products....too many mushy rotten tearouts that I've seen.

My plan was to line the walls and ceiling with EPS (could not find XPS in 1/2" sheets) and then add a vapor barrier , durock the ceiling, float the walls, float the floor and tile with stone.

Does anyone see any problems with my plan?
Any suggestions for another non-gypsum based 1/2" sheet material that would work?


01-29-2008, 05:20 PM
Why not use CBU? That's exactly what it's designed for. Lots of brands out there. Put a layer of plastic or roofing felt on the walls first.

Are you going to do a conventional liner and sloped mortarbed for the floor? Check out www.johnbridge.com (http://www.johnbridge.com).

01-30-2008, 12:54 AM
I plan to float the walls because :

1. My layout for my 12x12 tiles requires different backing thicknesses from the studs.
2. I'm not keen on the new method of attaching the cbu above the sloped mortar bed and caulking the bottom for wicking prevention. For support, I would add some thinset behind the unattached lower portion of the CBU and water would wick up that route anyway, right? Just seems like a weak connection.
3. I want flat blumb wall.
4. Fat mud is cheap and I have the time.
5. I want the experience.

Yea, I already have the chloraloy pan in and I will float the floor too.

On that note, is the practice of aplying a waterproof membrane on the floated walls and deck still considered a good idea??


01-30-2008, 05:15 AM
A floated wall is a great way to biuld a wall in a shower, but you don't want it on something a flexible as the polystyrene.

You'll get much better help at www.johnbridge.com (http://www.johnbridge.com). Code requires the waterproof shower pan to be on a sloped bed. If you have the liner down already, you've started on the wrong foot right off the bat. With the liner on the floor, it acts like a basin to anything that does make it beyond the first layer (tile and grout is NOT waterproof in the long term).

If you are worried about wicking and stuff under the tile getting wet, consider Kerdi from www.schluter.com (http://www.schluter.com). It would work fine over floated walls. It requires its own drain. Watch the video on their site...really slick system.

01-30-2008, 02:04 PM
Thanks for the tips Jim,

The flexbility of the EPS would not be an issue, considering a floated wall (with a scratch coat) spanning studs has no backing....or are there other issues with the EPS backing? One of my concerns is with the permeability of the EPS in a moist environment, even though I would have a vapor barrier between the EPS and the fat mud.

This is a permitted project, so yes there is a preslope under the pan.

I'm seeking help at JohnBridge tile forums also.