We are looking at an existing house and was wondering if radiant tubing could be installed on top of an existing slab and under new tile. My thoughts are to install wonderboard in 12" strips leaving a gap for the tubing. Then install tile over that. I know that electric pads can be installed but I want to use water.

the question is whether the existing slab is insulated.

If not, you want a thermal break at least.

Roth panels are a good solution for this type of situation.

The existing slab is insulated around the perimeter. Most of the sides are bermed in and insulated on the outside of the concrete. We want to use hot water because we have a free hot water source in the form of a lot of solar panels. I'm just wondering if anyone has used wonderboard on concrete and then tile to install radiant tubing after the fact.

Roth panels are a panel for hot water radiant, that are insulation and aluminum together.

if you mount anything to the concrete directly, you are driving additional downward losses. the question is, how big a deal is that? If you have a nice dry pack soil site without any water table or standing water issues, and the slab perimeter is well insulated, and your load isn't too high, it might not be that big of a deal. Getting a thermal break in is a good thing in any case, and if any of those conditions are not met, you won't like the results without one. So unless you are perfectly situated to skip that, I would not recommend you install radiant tubing (or electric mats for that matter) directly in contact with the concrete.

I certainly would not recommend installing plateless tubing in at 12" o.c.; more like 6" o.c.. especially if you are interested in solar.

I was throwing around this idea as well, but over a plywood subfloor placing a continuous coverage of cement board then cutting additional strips of the cement board so the tubing would lay in the channels. Would this still result in downward losses?

cement board is not a thermal break.

If you're over a plywood subfloor, hopefully you can insulate the joist cavity?

I'm not getting what these strips of cement board is supposed to achieve. If it's not continuous, it's not useful as a tile backer, and it wouldn't hold a plate very well either. why not just put the tubing in the thinset and make the thinset bed thicker?

I, too, am trying to retrofit hydronics over an existing (uninsulated) slab. If I were to use a Roth panel, what would be the appropriate overlayment prior to installing tile? I'm assuming plywood is undesireable because it will insulate the tubing- Do I install cement board, then instead?