Maybe another way to ask the question is, when putting in a radiant floor heating system, what is the maximum R-value that you can put on top of it?

I think I read somewhere that you would want less than R-2.5.

I think I also read that 1 inch of wood is has an R-value of R-1. Would 3 inches then typically be R-3 and too much?

I have no idea what cardecking is and it really does not matter. I am not a construction or HVAC pro. What I can say is that there is no way you can set a cut-off point, 2.5 or 3,.... for insulation in this situation. Less is better and more is worse. There probably is not much difference between 2.5 and 3 in your case. An engineer will have to make calculations to get some relative heating costs.

Some people can only deal with discrete, simple figures. 2.5 is a simple target, but your situation is different from typical so the target might have to be shifted. The drawbacks have to be weighed against what you want in your house with the alternatives in mind. More insulation around the radiant heat will increase your costs and increase the inertia of the system slowing response to t-stat changes.

Putting it another way, all the thermal barrier above the heat in your floor increases continuing operating costs, and increases construction cost due to insulation you have to add under it. It also compromises your design on the lower level. If you don't want to live with that, install some more Runtal or other radiators upstairs or use forced air. You won't get a free lunch.

There is probably a way through the horns of the dilemma by installing heat over the existing floor and a new floor over that, but that might not be ideal either.

Thanks for your thoughts. It sounds like great logic to me!

Since posting we had heating contractors walk through and none would execute the architect's idea of running tubes in the ceiling because there is too much mass between the two levels. All proposed the in-floor warm board because we have a floor slope and might as well get down to the sub floor and try to address that, and while at it put in the coils in the warm board.

We have tons of windows in our house -- 2 of the 4 exterior walls are basically all glass. The contractors seem to doubt that we will be able to get enough heat through the in-floor system, but they will run the numbers. We are planning on new windows (replacing the old aluminum with new Marlin aluminum windows, which will be more efficient, but not as efficient as vinyl or wood clad, which just wouldn't look right in our house). We might be looking at having to have radiators too, although frankly, socks and a sweater might work fine most of the time if we can get the house to 65 most of the time.