I am about to build a 2-story barn in Connecticut and need some advice. Do any of you folks know whether it is best to install the tubing with sleepers above the sub-floor or drill through the rafters and staple the tubing to the bottom. I understand below is easier and cheaper - i'm just concerned about heat loss going through two layers of wood. I know this has been answered in the past, but i cannot find any of the answers on this forum. Helpful advice would be appreciated. Thank you in advance.

Also, I'm told that engineered wood works best for flooring, true? thanks again

There's no heat lost going through 2 layers of wood, but the wood's insulating value (about R1/inch) limits the peak BTU/hour output (and the response time) the radiant can achieve in absolute terms. With the radiant above the sub-floor you'll be able to run somewhat lower water temps to get the same floor surface temp, which can result higher combustion efficiency out of the boiler, assuming the unit can tolerate the lower temp (don't run a cast-iron gas or propane unit with return water under 130F or it's toast in a single heating season. Condensing boilers are happy down to room-temp returns & lower, if your design temps super-low.)

I'm not sure if there's much of a difference between engineered wood and the natural stuff. It kinda depends on the species and the temperature/humidity swings you intend to run it through. There's plenty of radiant running under birch/maple/oak flooring without ill effect.

I get called in on many sub-floor systems that don't work very well. Single-tube down the joist space (stapled or suspended) with or without insulation and cold rooms the result.

A heat load analysis for each radiant floor heated room properly determines what will work. It may be sub-floor with correctly sized and installed emission plates or a sandwich system. If you want warm floors but can't get enough heat through them (as Dana1 so eloquently explains) you may need supplemental heat such a European style steel wall panel.

We try to support a above platform system when ever possible. You will be able to effectively lower the delivered H20 temperatures and maximize the savings with a condensing boiler system. In our view the above floor application will cost modestly more for material on the install but in the long run will save in fuel. With a staple up system you will need to run the system at considerably higher temp to "push" the heat through the floor system. This is particularly ineffective when the joist is open to a crawl space. There will be more downward heat loss.
Capturing the pipe on the surface of the platform minimizes this. From my view the install of a surface mounted system is easier than a staple up as well. Some high quality engineered floor systems can lay like a blanket with out nails,  advantages are that there is the ability for the wood to expand and contract, and no potential nail damage to your pipes from the flooring installers.
There are many manufactures of these products, our is the RHT floor panel system.
Dan

Dana
I'm in agreement with the above floor installation being suggested. Presuming the design is R2 above and R30 below, how does the R of the wood limit the peak btu output and at what reduced percentage?

It the fundamental physics of the situation:  The more R-value between the heating element (the hot water in the tubing) and the room air, the bigger the delta-T is required between conditioned space<-->heating water to get the same BTUs/hour across the R-barrier.  Water being water, there's an absolute upper limit to the temperature you can use (called "boiling point of water".)   There's no "percentage" to calculate unless you're comparing two independent R-values between heating element <-R-> room air.

The R-value of the insulation below the floor has no effect on how much heat you can get through the wood floor, but a HUGE affect on your heating bill. Since heat flows from warmer to colder, you want to limit heat flows into areas outside of the intended zone with big R-values between the heating element and the adjacent region.

If you're running with electric radiant you have a somewhat higher absolute upper limit on the heating element's temperature-  called "kindling temperature of wood". ;-)  (Most controls for electric radiant will keep it well under that temp.)