I'm not a pro at this, but I am a DIY Engineer.

For tile, you need to compute the deflection of the floor. Anotherwords, the depth of the floor joists, their span, and the space between joists determine how think the floor must be, without flexing. The John Bridge forum is a good place to get that info.

Do you want the PEX to be the primary heat source for the bathroom? If the room is already heated, I'd use electric mats to warm the floor.

Hi Geoff thanks for responding
Yes I'd like the pex to be the primary heat source for the room and I'm prepaired to lower the sub floor to between the joists to get a 1-1/4" mud bed. I'm just concerned that the 1/2" pex tubing (5/8od) will make the actual thickness of the mud bed, where the tubes are, will be only 5/8" or less.
My floor joists are 2x10-12 spf' 16oc I'm going to go to the John Bridge site this evening.
Thanks Jeff

Jeff, since this is DIY, what is the source for the hot water that will be circulated through the bathroom PEX? And what is the temp of that water?

Will it be pulled off a boiler that is supplying baseboard heat to the rest of the house, or will it be pulled off a water heater dedicated to this bathroom loop?

With all that written, have you looked at any of the low mass options? ie, no concrete?

I understand that the floor is striped to the joists. Do you have access to the underside, ie, from a basement, for a staple-up?

As far as the spacing of the tubing, while there are myriad options, typically two runs of PEX per joist bay is sufficient.

Thanks for responding mongoct

The source is a Renai tankles water heater with a heat exchanger for the hydronic

The system is set at 125 degrees right now

I don't want to go hotter because one of the zones has wood floors while the other 2 are tile

I already have 3 zones working and I have this one (4th zone) ready to be tied in

I'm unable to get at the under side of the sub floor as this bath is on a second floor with a finished living area below

My main concern is the thickness of the Dry Pack mud over the pex

The dry pack will be about 1-1/4 " except where the pex is
There it will only be about 5/8"

I was thinking of about 8" spacing of the pex

does anyone know of a site that would address hydronic heat in a floor as it would relate to the thickness of the mud floor

Your previous info is all good, and it's good to see you have a plan. Not everyone does, thus my questions.

Yes, you can go with a 1-1/4" thickness of mud with the PEX embedded within that depth. That's about the minimum I'd ever want.

Thanks again Mongo

I'm going to come clean with you.

I'm a semi retired remodeling contractor, who was either too cheap or too picky so I generally did all my own work when ever possible

Including residential plumbing and electric drywall tile carpentry and trim. Roofing siding and concrete when I was younger.

So after 35 yrs of doing this stuff I've made lots of mistakes
(usually only once) and learned by watching the pros and asking questions.

The work I've been inquiring about is my own house and I'm too old to make those mistakes this time.

I've done an awful lot of tile including mud floors and shower pans.

I've yet to use the Schluder/Kerdi systems, maybe on this one?

This would be my first curbless/doorless shower and first hydronic pex heat system

All my heated floors have been electric

This would also be my first trench drain

I have all the tools or access to them and the ability to do what is necessary

So if you tell me that you'd prefer to see the joists cut down 1/4" to 1/2" to get a bit more mud over the pex I can do that (I don't necessarily want to)

I think I might need to cut the joists down in the shower so I can get enough pitch to a trench drain.

The shower is 4.5 x 6.75 with a bench along one of the short walls so the floor will be 4.5 x 5.25 and I envision the trench drain in front of the bench

I was thinking of creating a small speed bump at the door to help with the floor pitch if necessary

I've always used diamond mesh with the bath floor mud but not in the shower over the "Cereloy" liner

I'm concerned about abrasion to the pex and Cereloy

I've used a plastic mesh when I run wires for electric heat

Is that a good alternative for this application

Well that's a lot for you to digest right now

I'm sure I'll have lots more questions as time goes ON

Thanks for your time
Jeff

Ps
I read your FAQ about shower valves, preasure balanced multiple head and jets etc.
Found it very interesting and informative

Jeff,

1-1/4" of mud should be fine. Just give it plenty of time to cure before you fire up the heating loop. You might want to even go to a richer mix than typical mud for added resistance to any thermal stresses the mud might see.

If you're open to all things Schluter, I recommend you consider Ditra over the hydronic mud bed outside the shower, as a buffer between the mud and the tile. It'll ease any thermal stresses the RFH might want to put on the tile. Plus if you use a 4" wide strip of Kerdi on the Ditra seams, you'll have a waterproof installation. You'd use Kerdi (not Ditra) inside the shower.

A watertight floor can help in a curbless.

Nobel makes good membranes too. So depending on which linear drain you use, that might direct you towards a waterproofing "family"; Nobel, Schluter, Laticrete, etc.

If you lean towards Kerdi and aren't terribly familiar, several years ago I posted a series of photos from a Kerdi install.

There are a lot more online resources about these membranes today than there were 10 or 12 years ago when that photo shoot was done, so the info there might not help you at all.