Hardibacker and Ditra are substitutes, so you just need one or the other. Just delete the item above with the one you won't use (I vote stick with the Ditra) and you're good to go.

Mats go on top of cement board but under Ditra.

the cement board is already cut and screwed down, so i may as well leave it hadnt i, and i was gonna use ditra on top

Here's your appetizer: You might want to pull the cement board and start off by adding another half inch of plywood on top of that 5/8" of osb.

We'll get to the entree next.

i was afraid you were gonna tell me that, need at least 1 inch of subfloor?

1-1/8" is recommended, though there are ways to get around it. But not with your 5/8" existing and the desire to use rfh. The rfh sandwich sort of begs for a sturdier subfloor.

Also what kind of tile, ceramic or stone?

If you go with the ditra it's latex modified under the ditra and unmodified to install tiles ontop, not unmodified for both top and bottom.

stan, that's true without radiant heat. With radiant heat it's modified under the mat, unmodified between the mat and the Ditra, and then unmodified between the Ditra and the tile.

Have we all made barredrock's head spin yet?

no im im getting all this. add 1/2 inch plywood or should i use more tongue and groove???? skip the hardibacker. im gonna use a slc over the heated mat. then modified under the ditra and unmodified over the ditra then ceramic tile. and preferences on the grout??? should i use the metal expansion deals in all the change of plane areas

For the added 1/2" use plywood not OSB.

With the added information:

1) Add your 1/2" plywood over the existing osb.
2) Dryfit your heating mat. Take a reading off the ohm meter and record that reading. Then pull up the mat and set aside.
3) Caulk all joints/holes in the floor to contain the slc. Add a dam at any doorway to contain the slc.
4) Add 1/2" thick foam around the perimeter of the floor to give a cushion between the slc and the wall's sole plate.
5) Prime the floor with slc primer
6) install the heating mat, test the leads with an ohm meter and record the reading. At a minimum use a squawk box.
7) Put slc primer in a garden spray can and spray another coating of slc primer on the floor and on the heating mat.
8) Mix and place the slc, have a squeegee to coax it into place. While it flows well, it's truly self leveling only if you help level it yourself.
9) record another reading off the ohm meter.

10. Tile over the slc after it's fully cured.

Plan on pouring about 3/8" of SLC, have enough on hand for a 1/2" pour just in case. You can always return unused bags, but if you have to run out mid-pour to get more product, you;re installation is toast.

That 3/8" depth is for a perfectly level floor. If you floor is sloped, plan on having the thinnest part be 3/8", that thickness will fully cover the wire mats.

Now that you're planing on using slc, you don't need cement board, you don't need ditra.

Good to know there are plenty of folks here to help out when I get busy! :-)

what kind of foam for the perimeter, i thought i seen somewhere to use the 1/2 inch sticky foam you put around windows?????

also i already have the hardibacker bought and cut, but not screwed down, hadnt i just use it too???????

Window gasket foam is fine.

If you want to use the cement board you can. But in general, it's not a great idea to put cement board on top of SLC. SLC is fairly thin and when you drive screws through it, it can crack and you can break its bond with the plywood underneath. It can give a bit of a crunchy sound when walked on. That said, some folks use cement board over SLC and have no problems, but for DIY it might be best to not take that chance. I've never encountered that problem but I've heard from several people who have.

If you really want to use the cement board, then add Step 1.5 to the previous list:

1.5) Thinset the cement board to the subfloor and screw it in place. Use unmodified thinset (to save a few $$, but you could use modified if you wanted to) and a 1/4" square notched trowel to spread the thinset.

If you don't use SPECTRALOCK for the grout, then I would suggest TEC XT. It is mold and mildew resistant, crack resistant and does not need to be sealed. Really a fantastic grout.

i was gonna use the hardibacker on the subfloor, and then put the slc over the hardibacker with the heated mat, i bought the home depot brand slc, along with the same brand primer, and window gasket foam, and a large squeegee on a stick

in your other thread, you said "there is a slight lean on the floor" -- does this mean you really need SLC??? I think not. The more extra work you can avoid, the better, and this is really good advice when you are new and canmake mistakes. SLC can be a big problem. A flat floor that is not level is OK if you can see it's flat but not unlevel.

Point 0 in the other thread was about the joists under the wood. Are they big and strong? Can you look and see and then write it out here?

Screw the new wood into the first layer while avoiding screwing it into the joists.

HTH

Here is a link that might be useful: other thread, about radiant heat floor mat ..

floor leans slighty but is level all the way across, it is 3/4 inch tongue adn groove, when i originally thought it was 1/2 inch. i bought the slc but have not used it yet. and i was gonna go ahead and screw down the 1/4 inch hardibacker to the 3/4 tongue and groove. lay the radiant mat and cover with slc, let dry then tile. the joists were 2x6 on 12 inch centers

The joists are close together; that is good. They are only 2x6"; that is small. How much distance do they span between supports? Is one of the supports under one of the bathroom walls?

I would use thinset to hold the cables to the cement board. Simpler than SLC. Do it the night before tiling.

Your floor is flat and it will stay flat after tiling. Can you live with a flat floor? It's not level but only slightly. Not using the SLC means you get your money back and you avoid the problems it can create.

Actually, in the tiling operation you can make the floor a bit more level. With thinset or medium set. If you are comfortable with thinset, you'll probably be comfortable with medium set too.