What is the joist spacing and how thick and what kind of concrete? Perhaps you are thinking of gypsum concrete. You're putting a lot of material under the tile for someone who wants to keep it low.

Joists are 7.5" x 1.5" , 16" OC. It's a self leveling compound.. Levelquick or something like that. I used it in my other bath. I only pour enough to cover the heating wires, like 1/4" or so.
I ordered 1/2" Advantech, to go over the 3/4" planking.

Level Quick is not concrete; it's a polymer reinforced cementitious trowel-on underlayment that is NOT self-leveling. Be sure to properly seal any wood substrate.

Be sure the electric heating pad manufacturer recommends the installation method you describe. Are you sure a grout bed wouldn't be better?

You mention plywood but say you bought OSB. Make sure that is OK with the Level Quick and heat mat instructions. Plywood and OSB are very different in surface characteristics and some adhesives will not work on OSB.

Electric mats should not be considered part of a radiant heating system; they are really just floor warmers.

I never use such large tiles in a bathroom; they never look clean; they are more prone to cracking on a wood subfloor; and they often have to be cut awkwardly.

Good luck

Thanks for your input.

I've used the level quick before and it's fine for this purpose. I do prime first with their primer. Good catch on the Advantech, too-- I'd just realized that and have switched my order to regular fir underlayment (1/2") for that top layer.

I've also installed the electric heating wires (not mats) before in another bath. It's purely to warm the tiles, and is quite nice! I follow the manufacturer's instructions exactly; no grout or mortar bed is advised. I don't really NEED the SLC, but I feel better protecting the wires that way before I start tiling with sharp trowels...

I'm curious about your comment above. LevelQuik "rapid set self-leveling underlayment"... right on the bag.

Why do you say it's not self-leveling? It self-leveled last time I used it :)

Well, I didn't see the bag, only your comments.

Level-Quick by Umasco Products and Quick Level by Environmental Coating Systems and are not self-leveling.

LevelQuik by Custom Building Products is self-leveling.

Exterior grade plywood is listed as a substrate for LevelQuik but OSB (Advantech)is not. The Extended Setting time version is best for DIY.

1/2" fir ply will be fine.

You've used SLC before so you know about sealing any leak points.

Depending on what RFH mat you're using, I'll sometimes use a garden sprayer to apply the SLC primer after the mat is down, especially if over a large area. Yours isn't too large so no worries there. The sprayer gets the primer where it needs to be, in and around the mesh matting. If using just spooled cable and clips (no mesh) I don't use the sprayer.

Thanks, mongo.

Last time I primed before laying out the wire (it's wire, not mat) and I can't recall if that was from the manufacturer's recommendation or what. Any reason not to do that this time?

Also: what's you favorite way to seal all the holes, edges of the subfloor, etc. I didn't do it well enough last time and I did lose a bunch of SLC down into never-never land!!!

I did lose a bunch of SLC down into never-never land!!!

Uh, is that Star Lord Commander(s) or Swingin' Love Corpses?

macv, even though I'm just an observer on this thread, I just want to say Thank You for posting the distinction about the various "Level Quicks". That's just the sort of thing that would have tripped me up!

Stacy, I'll caulk anything that needs to be caulked, any larger gaps will be foamed with canned foam.

Around the perimeter of the room I'll usually use foam backer rod.

It really depends on how much will be going where, and what you need to do to contain it. Caulk, foam, backer rod, tape...your pinky toe...plug anywhere you don't want it to escape.

Thanks, mongo!

I'm not sure about compatibility with the radiant heating system, but from what I understand cement backer boards (Hardi-board?) are intended to be used to stiffen subfloors before installing ceramic tile. I installed a 50 square foot ceramic section for a wood stove a few years back (12" square tiles) with backer board beneath, and found the process to be pretty straightforward. As with everything else in the ceramic/masonry world though, the boards are heavy. IIRC, they're typically sold in 3' x 5' sheets; I think a 4' x 8' sheet would be entirely too large/heavy to handle.

Joe in PA

"but from what I understand cement backer boards (Hardi-board?) are intended to be used to stiffen subfloors before installing ceramic tile."

Cement backer board adds no additional strength to the floor.

It does create a stable substrate and acts as an isolation membrane between the tile and the wood sub-floor.

For 12x18 tiles you are going to need at least an inch of solid wood perpendicular across the joists (not at 45 degrees).

Adding cement board is not going to reduce the flex from an inadequate wood sub-floor.