With a surface applied membrane, you do have a thermal break, in a good enough kind of way.

The slope is the most important thing. Membraned of course.

Heat: a subjective thing, in many ways. But, any heat you add to the tiled bathroom will be a plus, a good thing, not a bad thing.

Without any heat in the bathroom, some people will want to spray hot water around the walls to warm up the tiles, some won't. It's subjective, it's personal, and I cannot say whether or not the shower area will feel cool or cold or too cold, and to whom. I wouldn't worry at all, since it's not on a cold exterior wall.

If you do heat it in any way at all, that heat will spread out and end up adding warmth and comfort to the (no doubt cool) basement. So, instead of heating adjacent rooms' air with convection, you can heat the bathroom tiles somehow. Example: a heat lamp, or any incandescent bulb. Example: anything that is warm.

I once had a heated fishtank in the coldest corner of a room. It equalized the heat across the 3-d space. Being permanently on, it spread (a small quantity of) heat permanently into the rest of the space. Tiled areas feel cold because tiles are a heat sink. So, it is nice when they are slightly heated. That's my final answer.

First..do a moisture test on your existing slab. You want to see if moisture vapor is coming up into the basement from under the slab. Tape a 2' square sheet of poly on the floor and see if there is any moisture under it after 48 hours.

If the moisture is significant, it might play into what type of membrane you use on the floor. Memebranes like Hydroban and Kerdi have similar properties when it comes to liquid moisture, yet differing properties when it comes to moisture vapor transmission.

Significant moisture drive could actually lift and delaminate a topical membrane from the slab.

If you have under-slab drainage or radon ventilation pipes, and foundation perimeter footing drains, you should be in fine shape.

I'd make it a wet room, pretty much follow Bill's advice;

-slope the entire floor to the drain.

-topical membrane on the entire floor and up about 6" on all the walls, or just fully membrane all the walls. Fully membrane the shower walls.

-ventilation. Don't skimp. This room will be cool and the basement itself is typically humid, so latent drying action will be less than in an upstairs bathroom. Do what you can to eliminate moisture vapor as it is generated.

-I'd not insulate or heat the floor.

You might have to balance relative humidity in the basement as a whole with a heat source in the bathroom. If you do have vapor drive into your basement, it might help to have something as simple as an IR bulb on a timer to help with the initial drying of the room.

For a humid, slow-to-dry shower environment;

A non-porous porcelain tile might be better than a porous natural stone tile.

An epoxy type of grout might be better than a portland-based grout.

Great advice from the pros! Thanks so much. Will check for moisture coming up but I remember doing that when we first moved in and didn't get anything.

I will see what the electrician says about adding a heat lamp overhead (cheaper than coils but there is a bunch of HVAC in the ceiling). The room does have forced air and is on a whole house fan system. Maybe we should put a fancy humidity-triggered switch in there to make sure it really dries out after showers?

I have asthma, so we run the furnace fan all the time and have dehumidifiers and UV lights attached to both furnaces.

Any votes on preferred membrane? Is cork not an option because of running it up 6" or more on the sides?

cork is great but not a shower membrane.

Hydroban. That way, even if you DO have moisture coming up through the slab, it won't lift, being that Hydroban is not a vapor barrier, while Kerdi is.

I wa t to remove the tub and I stall a curbless snower in the 6x8 bathroom. The house is built on a slan. Can this be done? Is there enough room to slant the floor toward the existing drain in what was the bathtub?

Reviving my old thread to ask the pros for further advice. Serious illness sidelined the designer/GC so she is only now finishing that bath.

What is recommended for board on the walls and ceiling? Every trade has a different idea. We have leftover board from the plasterer and a little leftover cement board. I was sure the whole wet room should be cement board, but the last guy (stone fabricator, we are using remnant from kitchen) said that cement board will wick moisture up from below (even with membrane?) and recommends a board sold by The Tile Shop for this purpose. Help!

Our two upstairs showers had mud jobs with curb all around, then cement board above. Seemed to be the local standard practice.

Our tile installer used greenboard in the bathroom around the sinks/tub and cement board in the shower. He used Hydroban or a similar product to coat all of the cement board and the entire floor (slab) before any other material (tile) was installed. What is the exact board your guy is recommending?