Radiant heat

Here is an idea that many will criticize as being unfounded or to far off the wall.
Lets assume a 5" slab, every 16 or 24 inches cast a 2X4 on the flat directly into the slab flush with it's surface, you can put screws into the edges not driven all the way in, 'round here they call it porcupining the wood, to bind the wood firmly into the cement. Now you have nailers for the flooring. Wood cast into wet cement will not rot as the cement is absorbed into the wood surface and protects it from rot. Over the years I have done this several times, to give me embedded plates for fastening walls etc, years later the wood is still sound. Makes screeding & finishing the cement a breeze.

I did a few jobs where two layers of 3/4 play were glued and nailed to the slab, hardwood nailed over that. It give you better thermal conductivity compared to 2X4 sleepers nailed over slab.

I have not found a real eloquent way to put nail down hardwood over a slab. An engineered hardwood would be ideal for wood over concrete.

Ideally you want to use pressure treated wood when it is in contact with concrete. My experience is that any dimensional PT wood shrinks and twists as it drys, so that needs to be managed somehow.
If you glue and screw them to concrete you can keep them fairly straight, they will still shrink a bit.

I can't imagine concrete installers/ finishers would working around suspended 2x4's so easily. Possibly for a very small DIYer project?

I too would be concerned about wood sleepers. My suggestion (assuming it's a slab on grade) is to embed the tubing and instead of wood flooring suggest porcelain tile that looks like wood. Very stable, looks fantastic with the wide plank style.

If client insists on hardwood, I'd suggest something like Warmboard over joists.

Not sure your building a slab, or over a crawl space or basement.

nibs said:

@Zman embedding 2x4 on the flat is easy you just fix it to the rebar and adjust the el to suit. Makes the finishing & placement a breeze.
The OP says owner wants a slab.

@nibs
If I remember correctly, you did your floor by hand mixing in small batches. That works fine for a HO who has time to spare.

No professional concrete crew, anywhere would do it that way.

Sometimes it is the job of the pro to educate the homeowner and help them make a good decision. "The customer is always right, unless they are wrong in which case it is your job to tell them so...."

Edit: Lost connection to brain.