Looking for opinions and criticism of this sandwich design for new construction.

House is 8k sqft, 3-story ICF, triple pane windows.

The Al plates will fully capture the pex, flats will sit just below the finished floor. Insulation should read XPS for weight bearing. Insulation will not extend under interior or exterior walls. Plywood filler strips will be screwed (ballistic) to subfloor and joists.

I'll start. It's very labor intensive. I have a millwork shop with lots of slack time. Including a gang rip saw that can cut all the plywood in 1/2 day. Would like to keep my guys working instead of laying off good men.

I don't think i'd put the plywood strips down on EPS.

I also would be unlikely to recommend or use such huge pipe unless it were really necessary... it'd be difficult to install them at a proper on center (we stick to 9" o.c. or tighter in sandwiches). I'd be inclined to insulate in the joists instead, or cross strap for stability with insulation between the straps.

Wouldn't be easier to put xps sheets on the subfloor than insulating around the wires,pipes, ducts.... in the bays?

I wouldn't personally try to support my floor with rigid insulation alone in this situation. if a plywood strip is pressed on unevenly or something... I dunno. doesn't sound like a good idea to me.

insulating in a joist bay may not always be fun, but it's generally achievable.

First,
Good of you to keep your employees in mind, they can do this and will learn.
My thoughts on the system.
Pex AL 3/4 is a bear to work with, it is a false assumption that you will some how save something, you wont.
Think 1/2 pex B or pex C
You can make 300 foot runs, you will have a 3/4 sandwich system,
you can work with standard fabricated .019 heat transfer plates that are produced in quantity and inexpensive.
The esp insulation concept is interesting. In essence you are creatinga thermal break between the sub floor and living space.
I assume there is outside air below?if not I do no see the reason. in any case I would add fiberglass to the joist cavity.
But to the foam blanket, I do not see why this would not work, your finished wood floor will distribute the load evenly so I doubt there would be
ware issues. You may look at the 12 inch wide heat transfer plates as they would stitch the floor together more than singles, this would run 2 1/2 pipes on a 8 inch pattern. The ply wood would be ripped to a 6 inch pattern. no waste. plates on top, pipe snaps in the heat transfer plates and then your final floor ether nails or is applied as a floating floor(preferred) who needs nails in a hydronic floor.
Dan

Got you on the 1/2 versus 3/4 pipe. But, I thought Al oxygen barrier was a requirement in hydronic heating systems? Especially the condensing boilers with Al or SS heat exchangers.

in a light plate sandwich I do strongly recommend the use of PEX-AL-PEX, not PEX, to reduce expansion issues. at 1/2" it's not hard to work with in this application.

though if you do use PEX, PEX-A is also a fine choice, not just B or C.. it's typically more flexible than B or C, and it reforms with a heat gun if you kink it, without needing a splice.

PEX can have an O2 barrier as well, they use a chemical barrier called an "EVOH" barrier. but not on all PEX, so you have to verify it is there. if you use PEX-AL-PEX, the AL is an oxygen barrier as elwl.

Hold yer horses there Rob,,,,
You will recommend an AL for this sandwich system, Ok I'm fine with that, sell it and have personally laid miles and miles of it, stiff, holds form as you know but then you shift gears and x Diss pex B and C over Pex A because A is more flexible?
Let me present my improvised bendy chart from most flexible to least flexible.

Pex A reforms with a heat gun if you kink it Solidification of cross linking Chemical barrier, toxic waste
Pex B reforms with a heat gun if you kink it Solidification of cross linking Chemical barrier, toxic waste
Pex C reforms with a heat gun if you kink it Radiation cross linking external barrier, no toxic waste (Green)
Pex AL Couple mechanical repair if you kink it Not exactly sure of the chemical process, Pex aluminum pex,

Clearly all pipes work and the Bend-o-manometer placing AL last.
Dan

I didn't "dis" anything. Your comment ignored the existence of PEX-A, and I was correcting your oversight. I am not aware reformation properties on PEX-C, my understanding was that it requires mechanical couplings if kinked. am I missing something? I'm happy to be wrong, if so. PEX-A is still a fine choice though, and I'd thank you not to pretend that B or C are the only choices out there for some reason. a more generic "PEX" notation would be much more accurate than specifying "B or C" PEX.

I recommend PAP because workability is not critical in a sandwich and lack of expansion is a much bigger issue: IF you are going to use PEX though, which is a big if, I do think you might as well go for easy workmanship since that is the ONLY advantage pex has over pap. Why trade down if you aren't going to get some advantage?

I don't know much about the production of the 3 methods in terms of actual environmental impact, but I would certainly hesitate to call a non-recyclable plastic product production process "Green". and there is a lot more than one particular component of the waste stream to consider in that comment.

I have a related Q: I have lots of 1" and 3/4" XPS from an auction and will have left-overs after sheathing my house, etc.
Can the XPS be trimmed to be the width of the joists and glued or ... between the joists from below?
Its lower R than the R-12 fiberglass but 1) I own it and 2) its not itchy :-)
Thanks

if you're over heated space, two 2" and you're all set.