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

benze

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?

Thanks!

Eric

rusty baker

Why not put radiant heat under the tile? Wood on concrete, even with insulation beneath will fail eventually and won't be solid enough for tile.

__________________
"I'm twisted, not sick. Sick implies, I'll get better"
Semi-Retired Installer
Installing since 1973

Bud Cline

There are so many things wrong with all of those ideas that I don't know where to begin.

I suggest you bring in some contractors and get some estimates. You will learn a lot from them simply in the conversations you will be having. THEN, decide the best way to do things after you have some legitimate workable opinions.

benze

If we forget about the bathroom for the moment then, and just consider the laundry room, is it still such a horrific idea? I realize the idea of tiling over a wood subfloor in a bathroom is probably a no-no, but I thought that if covered with Kerdi and/or Ditra (as appropriate), it would compensate for the subfloor.

Would you mind elaborating a little on why it would be such a terrible idea? You'd be amazed at what I've heard from some "pros" that I've have look at the job....

Thanks,

Eric

Bud Cline

Acquire for yourself a Tile Council of North America Handbook for Ceramic Tile Installation and show us where in that publication it says your plan is acceptable. http://www.tileusa.com


"Horrific" is your word. Ridiculous would be mine.

Why is it a "no-no"? Where did you get that? You said the subfloor was concrete. Where is the wood coming from? Are you talking about the suggested plywood when you say wood?

KERDI and DITRA will in no way compensate for a proper subfloor. KERDI and DITRA are two entirely different products with two entirely different uses.

No I wouldn't - not at all.

You can install tile directly to concrete easy enough. Covering the concrete as you suggest with foam WILL NOT insulate the concrete in any fashion that would prove to be beneficial down the road. You could heat the substrate ahead of the tile if you wish using one of several available floor-heat systems.

As far as the shower is concerned...we can get to that once you decide on a proper method of tiling the other floor(s).

benze

I realize I can install directly to the concrete slab, but I want to create a thermal gap between the tiles and the slab. My slab is poured right on top of gravel, so I am looking at the XPS to do to act as a couple of things. 1) Vapour barrier, 2) thermal gap. Installing tile directly to the slab won't accomplish either of those things.

I've heard of Wedi board which would essentially accomplish that, but I can't seem to find any locally (only in massive quantities), so I called Schulter to see if they had any ideas. They suggested the following:

1) XPS floating on subfloor
2) 1/2" concrete backerboard adhered to XPS using polymer modified thinset (1/4" x 3/8" notched trowel)
3) backerboard screwed into concrete slab every 8" square, heads countersunk
4) Ditra adhered to backerboard using non modified thinset (1/4 x 1/4" notched trowel)
5) Tiles adhered to Ditra using non modified thinset (1/4 x 3/8" notched trowel)

It's a 5x8' room, so screwing the backerboard into the slab promises to be a lot of work, but apart from that, does this option make better sense?

Thanks,

Eric

Bud Cline

OK Listen.......
I strongly doubt that the person at Schluter has ever actually done such an installation. I didn't know Schluter suggested such ridiculous methods. Who exactly (by name) did you talk to at Schluter? I want to talk to them myself.

Here's the issues you face:
Ridiculous idea! Floating substrates of this nature would only offer movement that the tile installation could not withstand.

Equally ridiculous! Even though modified thinset would (for a time) adhere the cement board to the XPS the bond would not be reliable from a lasting standpoint. In addition screwing through the cement board and through the XPS would simply suck the screws through the cement board and over time result in a dubious installation. Cement board lacks quality control and ripples and lippage will result everywhere.

Sounds easy, doesn't it? First the holes in the concrete substrate must be pre drilled to accept the screws. As the holes are drilled where will the spoils go. Drilling holes in concrete typically result in some spalling and chipping of the concrete...where will these spoils end-up? Will the spoils lift the XPS? Yes, is the answer to that question.

THAT...IS NOT Schluter's recommendation. All DITRA is to be installed using modified thinset. I think someone is confused, very confused. That is also the wrong trowel size for installing DITRA.

That part is partially correct but the trowel notch size depends on the type of tile and the construction of the backside. A trowel of that configuration IS NOT a one-size-fits-all trowel.

At what point in your conversation did this idiot tell you to tape the seams using alkali resistant mesh tape? Or did he even mention that part?

This is not an option...this is the advice of someone that has no practical experience what-so-ever. None!

benze

Well, the guy's name was Mark and he is in tech support in Montreal. Apparently, he even checked with his superior or his senior in the office who confirmed what kind of thinsets to use, etc.

I was wondering about that too, but thought that their suggestion of screws every 8" would be enough to solidify and prevent movement.

Another concern of mine too; I didn't know if the modified thinset would be able to bond to the skin of the XPS.

Hadn't thought of that part. Agreed; there will be some concrete spoils (mostly dust from experience) that will accumulate under the XPS like mini-ant hills around every hole. And no real way to predrill the concrete before laying the XPS and actually get holes to align.

What the heck kind of service/support is Schluter providing then?!


He did ask what size tile, and I mentioned 12"x12".

No he didn't. But I had heard about that already elsewhere; to use fiberglass mesh tape to seam together backerboard using thinset (akin to plaster and paper tape for drywall).

Just for excercise, I did price it out, and quite frankly, it comes out as the same cost as Wedi board, except that it is _way_ more work! My problem is that I can't find Wedi board here for 50sq ft. Only for 1000 sq ft! I've talked to the Wedi rep a couple of times, but he hasn't found someone able to sell me 50 sq ft.

And apparently KerdiBoard isn't approved for floors yet, so Schulter does not recommend its use until it gets certified.

Any suggestions what to do at this point? I'm really stuck on this floor problem.... If ply isn't an option (I have been advised against sandwiching ply between two slabs of cement), and can't use backerboard, what do people do to insulate the floor before tiling?!

I can't imagine I'm the first person to hit this dilema???


Ok - so another followup.
I called Schulter back and spoke to another rep. This is what she told me this time.
1) Thinset XPS (XPS has to be 40PSI minimum) to the slab using non modified thinset (1/4 x 3/8" sq trowel)
2) Ditra to XPS using non-modified thinset
3) Tiles to Ditra using non-modified thinset

Apparently, their showerpans are XPS, and this is their recommend installation method for their showerpans. THe only thing she suggested is that I may have to sand the skin of the XPS a little to remove the shine to increase adherence to the thinset.

Does that make any sense instead of the backerboard? I'm still concerned about the XPS....

Edit - another update. I just checked the Schluter site, and they indicate that their shower pans are EPS - not XPS. Does this make a difference to the adherence to the thinset?

Thanks,

Eric

Bud Cline

Eric this is one of the screwy-est situations I have ever seen Schluter come up with. Now their Rep doesn't know what type of plastic their shower pans are made of.

Here's some of the confusion. DITRA IS NEVER intended for showers. Therefore their recommendation is to always use "modified" thinset to install the DITRA. Typically DITRA is installed over plywood or OSB or cement backerboard or fiber-cement backerboard or even WEDI. Over all of these products the modified thinset offers the best adhesion and at the same time can breathe enough for the modified thinset to dry properly. You want the best adhesion available.

They then go on to recommend you use "unmodified" thinset to adhere tile to the plastic DITRA. Unmodified thinset will cure on its own and doesn't need to breathe for it to cure.

NOW...using Schluter KERDI the rules change a little. The use of KERDI (for a waterproofing membrane) over any of the above substrates you would also use modified thinset to install the KERDI Mat. BUT...using KERDI over the Schluter plastic shower pan slope, they want you to use unmodified thinset for the reasons I mentioned above...no air required for thinset cure. THEN...the same goes for the tile over the KERDI...use unmodified thinset.

Based on all of that I can see why they would recommend what they did. They have no field experience and are reading from a book (web page). They don't know any better.

OK, now, XPS and EPS. IF, IF, IF, the density is suitable for a tile cover then it may well offer some slight insulation properties. The problem with that theory is the plastic must be dense enough to support the tile and at the same time contain enough air-expansion cavities to offer insulating qualities. I don't think it does but I'm not a chemist nor a plastics engineer.

Now that they have changed their tune and they now want you to adhere the plastic to the substrate with thinset that's a slightly different story. Someone has now come to their senses in that respect.

I personally think you are asking for trouble, but that's the difference in a DIY doing a job and a professional installer doing a job. Professionals try to install lasting tile floors and stay out of courtrooms.

You will be surprised at how little insulation value you are going to get from this method. I would suggest installing Suntouch or any of the floor tile heating products available today. Home Depot sells Suntouch and it is priced very reasonable. You have a choice of two thermostats. You would glue the heat mats to the substrate, then cover it with Self Levelling Compound (SLC) then apply your tile. It wouldn't matter which thinset you used and your cost wouldn't be any more than the nonsense you are trying to invent. "You Can Do It We Can Help"

Your floor would only be raised 3/8" under the tile and you would be much much much happier with the results. Much happier. Trust Me!