Is there a radiant system out there that will allow a retrofit over an existing concrete slab?  I have visited the Radiantec website and don't believe this is an option.  Thanks.

Roth Panel is a good product for that.

Thanks, Rob.

We installed a hydronic radiant floor over our existing concrete slab when we did a seismic retrofit in 2001 of our old warehouse space.  (Still have the original single glazed metal sash windows which are almost 40% of the wall area.)
Systems Description:
6x6 WWM shot through R-Foil radiant bubble pack
   R-Foil was turned up Sills and extra blocking about 6"
Pex Tubing wired to WWM
2.5-3.5" of highly plasticized concrete over all(slab was uneven)
Taco pumps and controlers with a custom soldered manifold (no pre-made ones available at the time)
50 gallon Bradford White DWH.

Results have been superior.