Hi - I'm a carpenter doing a conversion/remodel of a 2 car garage into a 2 bedroom apartment w/ 1 car garage bay. I have a friend that's a plumber assisting with radiant design and boiler install.
The first floor is a raised 2x6 subfloor with 4" of rigid foam insulation (R-26 total) below it, foil faced, and the tubing laid on top of the insulation, with a 3/4" subfloor on top. The final plan is hardwood flooring everywhere except the bathroom, which will be tiled. This is how our plumber recommended installing it, though I believe that this may be the problem after doing further research (I found this presentation by John Siegenthaler, which I wish I had found months ago http://www.homeperformance.org/file/1449/download?token=Q83qlYtX
The boiler is a NTI Trinity FTV 150MBH Combi unit.
I imagine that all the tubing is potentially doing is heating the airspace below the subfloor, and is not effectively radiating heat up without any sort of reflective plate, especially since it's likely that the tubing isn't even in contact with the subfloor.
Here's the sketchup design of tubing layout
Here are pics before we finished installing the subfloor