Step One: Clean out junk from floor… The wood is ceiling of back porch.
Step Two: R13 Dow Safetouch Insulation, made without glass fibers or formaldehyde. Vapor barrier up- towards the warm side.
Step Three: Reflectix Insulation, which is supposed to reflect 96% of radiant heat- so the radiant bathroom floor doesn’t become a radiant porch/kitchen ceiling.
Next step is putting in the pex tubing. Since the rest of the bathroom floor is above the kitchen, we need to put in the pex before we put the insulation in. The hot water goes thru the pex and heats up the whole bathroom from the floor. Ideally.
This should be especially fun pex to run since the lights, ducts and wires are already in the kitchen ceiling!
madison square home 
As long as there’s an air gap between the radiant source and the reflective foil, you should get some benefit from the reflective foil. But without an air gap you’ll none. I wrote a short article on the subject: http://www.thermaray.com/blog/index.php/archives/218
Oops! I meant “without an air gap you’ll get none”.
I guess I shouldn’t post late at night. 😀