I am in the early stages of planning a radiant floor retrofit in an existing (1979) wood-frame house in Oregon.  I expect to start talking to installers soon, but first I'm busy trying to educate myself as much as possible.  In doing so, I guess I have some ideas of what will work in this case, and I'd be very appreciative of any fedback anyone is willing to give.

Some basics:  House is 1550 sq. ft. gross; heated floor area is somewhat less.  Split level design suggests 3 zones.  First zone is LR/DR/Kitchen, which total about 550 sq. ft. of heatable floor (not counting under cabinets, fireplace area, etc.)  in about 650 sq ft total.   In fact, I'll just limit this to that first zone, as that will be the first area installed (the rest will be a year later.)

Possibly relevant detail:  The floor structure is unusual: Post and beam, beams on 4 ft. centers, with 2x6 T&G on top of that.  We plan to put radiant above the T&G, with hardwood floors.

Worst case heat load in this zone is 10,300 Btu/hr with 40F temp difference to outdoors.  (That includes conduction and infiltration.) 

Actually, I'm not sure you care about all that.  To get to my questions:  We are considering using aluminum heat plates supported by 3/4 plywood above the T&G, with the wood floor directly on top of that.  The tubing will probably be 1/2" pex, at 8" spacing; maybe 10".

  - Does anyone have any pointers to specific details on the heat transfer coefficient from the pex tubing to the plate?  (I've got numbers for just about everything else.)

  - Or, any rules of thumb or experiences with this approach?

  - I worry about floor squeaks or other noises with this idea.  Any comments?

  - Or, should I simply use something like ThermalBoard, a pre-fabbed wood/alum board?  (My concern is it won't take 1/2 tubing.

Well, that's much more than I can ask anyone to readand comment on, so if yo udo, I am in your debt!

Thanks.
Paul