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Thread: Can I tile and/or install a mortar bed directly over polystyrene (solid) insulation?

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  1. #1
    DIY Member benze's Avatar
    Join Date
    Aug 2010
    Montreal, Canada

    Default Can I tile and/or install a mortar bed directly over polystyrene (solid) insulation?

    I'm looking at redoing my basement bathroom, and one of the things I want to do is put in insulation in my subfloor. Currently the subfloor is the slab (house was built in '67). My original thought was to put in 1" extruded polystyrene insulation and 3/4" ply on top, and then tile ontop of the ply. However, looking at the spacing in my basement, I would really rather save the extra 3/4" for headroom. Esp in the shower, where I want to have an overhead showerhead.

    Can I lay down Dietra right over the insulation and tile directly on that, or will the insulation be too soft to use as a subfloor?

    What about in the shower? If I am building a mortar bed to slope towards the drain anyhow, can I put the mortar bed directly on the insulation, or is that not recommended?

    If not, what is the min thickness of ply I can use? 1/2"? 3/8" Does it make a diff whether I use OSB or ply?



  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    It probably would depend on the density of the foam. For example, Schluter has a foam shower tray. You don't tile to it directly, but you do after installing their waterproof membrane, Kerdi. I used one of their pans when I did a remodel at my Mother's house and it works fine. You may want to consider maybe Ditra directly on the foam, but I'd call Schluter first. Also, at least with their foam tray and curbing, it is designed to be thinsetted to the subflooring. If you did the whole floor except the shower area with sheets, then used their pan for the shower, you might be able to build a curbless shower if you used Ditra outside of the area, and used the Kerdi-band on the seams to waterproof the whole thing.

    Call Schluter and run the ideas by the people at www.johnbridge.com. Should be interesting.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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