There has been an ongoing discussion on this topic at another building related discussion forum. Might be of interest.
Here is a link that might be useful: Discussion of radiant heat
Thanks for pointing that thread out. I will show it to my husband.
I have a collection of links on radiant floor heating that you may find useful. I was looked into retrofitting my slab-based home and decided it wasn't in the cards, but there is a lot of information available, and many interesting and innovative approaches are being tried. Try these - they ought to put you to sleep a few nights. I am not a customer or employee of any of these places.
Radiant Design Institute and Radiant Direct
A.I.M. Radiant Heating
US Department of Energy Brief on Radiant Floor Heating
Hope these help.