I was thinking about how much I hate to do overhead work with my creaky shoulders and came up with this idea as an alternative to staple-up for new construction.  The essential idea is to, before putting the deck on, attach a small ledger piece to each side of every floor joist about 2 5/8ths down from the top of each joist.  You then lay a precut piece of 2 inch thick foil-faced foamboard like Thermax between the joists resting on the ledgers.  You then install the tubing on top of the Thermax.
Attach the deck and you're done.  You don't have heat dispersion like you would with plates, but you have a radiant barrier.  If you were using BCIs you could leave a rim joist off and shove the foam pieces in from that end and then attach the rim joist.
Feedback?  Was I clear enough in explaning the concept?

Myself, I try to minimize the time I spend working on top of open joists. Anything you do there will be slow and attended by the risk of falling. In addition, if your foam is not cut perfectly it will prevent you from straightening the framing as you lay the subfloor. Working on top of I-joists is impossible anyway. They are too wobbly until the subfloor is fastened. Lumber joists are merely uncomfortably wobbly.
rgb

I've seen the "ledger" method promoted elsewhere.

I think it's a horribly difficult installation and I don't know why anyone would want to do it. if it's new construction, do an over-floor sandwich and be done with it. You're not going to dramatically improve the economics of that install with a ledger method for sure.

Right as Rob.

Taking advantage of the innocent for personal gain. Despicable.

Whoa--who's being taken advantage of?  It was just a simple DIY idea I had for my own house and I wondered if anyone had tried it.  I'm not a commercial installer.  Thanks to those who provided useful feedback. 

There are those who promote this idea to the unsuspecting novice in order to sell parts. As Rob correctly points out, no one with experience would do it.

We have a small addition where this was done. Not enough communication between the plumber and the contractor building the addition, and no room was left above the joists to do a sandwich. A 2' crawl space below prevented the plumber from doing the work from underneath, so we put ledgers in and did it pretty much the way you described. It was only a small area 120 s.f. but.. 20/20 hindsight... I would do it differently were i able to do it over.

I'm planning on putting a layer of reflective insulation (insulation4less.com) on top of the subfloor, then installing the sleepers (with screws), tube, plates, etc. I think it's a great idea because it will prevent thermal conduction into the subfloor and joists. Think that will be enough insulation? Did I not think of anything? Thanks.

reflective insulation is useless in the application you are discussing. it requires an airspace to be effective and it's horrible at stopping conduction.

You will need actual R value, most likely below the subfloor.

What NRT.Rob said- if you can't get at least 1/2" of air space the reflective layer is buying you nothing that a sheet of plastic wouldn't. Even with a proper air-gap both above and below the reflective layer, the effective R-value is only mid-single-digits.

If you're cutting foil-faced iso-board to be installed on ledge below the subfloor, give it an inch of gap below the tubing- the reflective layer will indeed improve the performance over the raw R-value of the iso. But it won't approach the performance of 2" of XPS over the sub-flooring with the tubing over that. The joists for thermal short-circuits of only ~R2 in the 2" thick isoboard scenario so the net R-value won't be nearly that labeled on the sheet (probably R12-ish on the label.) But an unbroken layer of R10 XPS over the sub-floor will give you the full R10.

What do you put on top of the XPS?

For hardwood floor it would be subfloor--->XPS---->sleeper -----> hardwood?

For stone or tile I guess you could embed the tubes in a mud bed on top of the XPS.

Thanks for the info Dana and Rob. Speaking of XPS, could XPS be used as sleepers (al a Roth panels)? I know the Roth panels are 90 psi, but the best solution I could find at the hardware stores is 30 psi 1/2" or 1". I contacted one of the XPS manufacturers to ask about prices of the high-load (60 or 100 psi) XPS, but it only came in 2" minimum thickness and the cost was way high - might as well get the roth panels. Thanks.

I personally wouldn't do it, but I'm not a structural engineer ;) Roth is a decent solution here especially if you can do a floating floor over the top of your assembly.

I have seen compromise assemblies where one sleeper was a nailer, wood, and the rest of the assembly was XPS. you get some thermal bridging at the sleeper, sure, but you can get pretty good insulation coverage that way.