If you go without the cement board, you can count on that floor failing within 3 months. Not because you're going directly over plywood, but because you're going over a single layer subfloor. That's instant failure to a tile floor. The floor below (johnmari's floor) was basically the same thing as what you're proposing-- Suntouch over CBU and plywood, but instead of octagon and dots, it was 1" hex. Also, instead of SLC, I used thinset, but either way is just as acceptable, and I also did it over 1/4" CBU instead of 1/2". Unless you're looking for the added height to match up to existing floor, there's no need for using the 1/2" CBU. It has practically no structural strength whatsoever. It's there only as a bonding surface, so if you'd rather, your floor will be just as strong with the 1/4".

We've done radiant floors (from warming systems, so it's just the wire, no mats) and we did one with 1/4" Hardibacker and one with Ditra. Your steps are correct, though with the Hardibacker we just did a single coat that covered the wire and provided a bed for the (large) tile. The instructions that came with the wire offered the 2-layer process as an alternative.

We just did our kitchen with Ditra, and laid the wire down underneath it with non-modified thinset under the Ditra (if you don't have wire, you use modified thinset underneath), and non-modified thinset on top under the tile.

As to cost, Ditra is about 2x the cost of Hardibacker, but WAY less labor because you cut it with a utility knife (no cement sawdust) and you don't have to screw it in every 6 inches. I don't know what the self-leveling mortar you mention costs.

I'm a new Ditra convert, and a big fan.

And how would the whole thing work if I wanted to use ditra? Would it be cbu, suntouch mats, ditra and then tile?

So it's basically just a difference between using some kind of self leveling mortar bed or ditra?

weedyacres, I think it was your post where you mentioned being a "convert" to ditra because you didn't have to mess with CBU that made me start thinking about using ditra instead of cbu.

I have installed the radiant heat mats before and first things first - good choice. We loved it, they were easy to install, relatively inexpensive and a pleasant treat in the morning.

As for install, it is really quite simple. Don't over think it. DO NOT SKIP the subfloor - install whatever is necessary first - we had the subfloor and then mortar under the cement board with lots of screws as usual. Fill the joints and then lay down the mats in the layout you need. So far the only difference is the addition of the mats. It may seem like a big change but in my experience it just was not a big deal. I did not do a layer of self leveling or anything of the sort. Just trowel on the mortar bed, be careful not to damage the wires and lay right on top. It worked for us.

Again, do not skimp on the prep though or you will have to tear it out and redo as a result of cracks, etc.

Good luck!

If you use Ditra, you don't need cement board. The layers go:
radiant heat mats
unmodified thinset
Ditra
unmodified thinset
tile

If you want to see photos of it going together, see the thread on our kitchen remodel below. There are some photos of the half-done floor that show how it came together.

I've never used SLC, so I'll have to defer to our resident tile gods on that, but my opinion would be that if you use Ditra there's a lot of "stuff" between the mat and the tile, so you wouldn't have a need to do any more leveling (other than, say, the "2x4 with a piece of carpet on it" deal) to avoid a "princess and the pea" thing with your wires potentially telegraphing through. When I put down the Ditra, it was a tad "bumpy" (perhaps said tile gods can give me some pointers on that too :p), but after another layer of thinset with the big tiles on top I had flatness.

Here is a link that might be useful: Kitchen remodel

I haven't been hanging around here lately but someone mentioned to me that this thread was here and I just wanted to clarify something. Bill, you didn't use CBU on my bath floor - you used the double layer exterior-grade plywood instead. I remember ALL too well that dang GC pitching such a snit-fit about not only your using a plywood substrate instead of CBU but about having to install the second layer of ply in the "picky" way you specified and taking off with the CBU in the shower undone! My documentation on that project is still buried in a box somewhere but I think it was 1/2" exterior-grade plywood over the original 3/4" plywood subfloor? Can't remember why you chose that route though, I think I've blocked out a good bit of the nitty gritty details of that project! LOL

BTW, Bill, we finally closed on 12/6 and the buyers absolutely LOVED that bathroom, particularly the tiled shower. :-)

You're right-- I completely forgot about that. However, I would NOT recommend to anyone to tile directly over plywood unless they're very familiar with the process and know exactly what they're doing.

For whatever it's worth I was not looking to tile directly over plywood but rather the choices were CBU vs. ditra and which method would do a better job of making absolutely sure that no texture from the wires would show through the smaller mosaic type floor tile. Ditra seemed easier but I wasn't sure it would do the job. Since they don't recommend you go smaller than 2x2 anyway I guess that answers the question.

BTW, Bill, we finally closed on 12/6 and the buyers absolutely LOVED that bathroom, particularly the tiled shower.

I'm glad to hear you were able to sell it somewhat quick. I'm only sorry you weren't able to keep it long enough to enjoy it!! That's okay-- the next one, we'll REALLY do it up!! :-)