A local radiant installer I've worked with has gotten fed up with gypcrete installers and their ridiculous prices and has started installing radiant floors with what he calls the dry sandwich technique.  He starts with a 3/4" plywood subfloor insulated underneath with blue board.  On top of that he nails down 8" wide strips of 5/8" OSB as sleepers with 5/8" spacing between them to accept 1/2" pex.  He then grouts the voids between the sleepers and tubing with thinset and applies a skimcoat over the top of the sleepers to put down 1/2" hardibacker over which he lays tile.  When he installs hardwood he just nails it straight onto the OSB sleepers - no aluminum plates.  He claims it works very well and is much faster to install.  Anyone else done anything like this?  Has it worked well?  Are there any reasons any of you WOULDN'T install this method?
-Spencer

I'd use plates, to dissapate heat more effectively. I'd be concerned for heat striping otherwise, and/or higher than necessary water temps. I find it hard to believe he's getting a 4.5" even lateral spread from the pipe with that, but hey, maybe I'm wrong.

But it's a viable method, sure. I'd probably spec a tighter than normal on center to make sure floor temp was even, like 6" o.c. or so. But if his clients can say there isn't heat striping, that's fine.

The question I would have is why not just do a thick mud base and use 3/8" pipe, and skip the plywood altogether, or maybe use strapping at a wider on center and more wet stuff? That would work better, I would think.

we pour 1 1/2 concrete overlays with staple down tube.
peastone mix with fibre. Cracks a bit but much cheaper than gypcrete.