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.

You wrote: "the ones going across the old ceiling (vs. yanking them out), " What you wrote stood my hair on end! It is very likely you can not remove the horizontal timbers without your house's structure being compromised. Those beams are most likely doing a whole lot more than just supporting the ceiling and the attic floor. They are connecting the sides of your house. Before you even think of this, please hire an architect or structural engineer to assess it. Given the age of your house (1840) and location it seems highly likely your house is a timberframe structure. Those horizontal beams are ESSENTIAL in timberframe houses! The lower ceilings in older houses are one the characteristics of the style. You can expose the beams, but not remove them. After they are exposed you could cover them, but it would be a lot of work. In buildings the age of yours they might be hand hewn, and attractive, but rough. Many people find the old timber look very appealing as they shown the hand-made nature of the structure. The other issue is that once you have a room with such a tall ceiling (going up to the roof rafters) you will kake it very hard to keep warm at "ground level" as all your heated air will go up, far above your head. Even with good roof insulation (can be done, but does present some other technical issues) keeping warm downstairs will be a challenge, unless you have something like radiant heat in the floor. When the house was built, and every bit of heat was from logs cut, hauled, split, stacked and loaded, by the occupants they knew that lower ceilings were warmer than soaring spaces. MOlly

True. There will be heat lost to the ground. Nonetheless, do not conclude there is no point at all in having a layer of insulation: any break is better than no break at all. Think of wearing one layer of light clothing as protection against the cold: better than bare skin. Any small R (e.g. R2, R3) still directs more heat upwards than not. To place radiant heat directly on the slab means more and more heat is lost as floor temperature rises, not just 50% but a far greater fraction. "thin" , "not very" , "useful" -- terms undefined. A separation layer is better than none. Not to overthink this, just to drum it in. "Something" is more than nothing, and this is the significant point, not whether the R factor appears low or high. Once your floor temperature rises, you want to prevent sending most of it into the ground, which is what happens when there is no insulation / isolation / separation at all. Note that the definition of R rating testing does not allow one to distinguish between heat transfer occurring by conduction or by radiation. In many circumstances this is not critical. If you want to get into this level of detail, it will take a few dense paragraphs. Suffice to say that R ratings will be misleading and inaccurate for this application so do not rely on the apparent differences between R numbers. They are all small and that is OK. They are almost meaningless in this situation of a slab-floor separator with a thickness of from 2 to 16 mm. It is obvious that with electric cables you have more room for a thicker insulation layer. HTH -david

Glad we now agree that 85 is an established threshold that radiant floor designers use as a recommended maximum for floor surface temperature. Foot contact with the floor makes the second way high temperatures lead to discomfort. 85 is evidently too hot for those expecting warm feet and will definitely lead to uncomfortably hot room temperatures, in efficient homes. There still seems to be confusion regarding water temperature and floor surface temperature. The temperature of the floor surface will be lower than the water temperature in the tubing. The water temperature entering the tubing loop from heating appliance, will be warmer than the temperature exiting loop, on its way back to the appliance to be reheated. The only mention I made of water temperature was that 125 seemed too high for an efficient home. Perhaps not for a long tubing run with short spacing but I still think it seems high and engineers generally don't like long tubing runs. I've personally installed tubing on a spec of mine and for other builder friends. We installed short runs with wide spacing for better zoning and they were for efficient envelopes. Few ended up being hooked up to an actual appliance. Why spend that money when the required AC can handle the heating too. The "at" which I corrected to "near" room temperature comments were in regards to floor surface temperature, which is a widely accepted fact for those involved in efficient construction. From All About Radiant Floors: "Why isn’t my floor warm? Homeowners who look forward to walking barefoot over warm floors are often disappointed by homes with in-floor hydronic heating. That’s because these floors are rarely as warm as most homeowners expect." From that article's case study (keep in mind difference of water temperature and floor surface): “On the design heating day [the coldest day of the year], the water flowing through the tubes in the floor will only be at 85 degrees,” said Robb Aldrich, an engineer at Steven Winter Associates" From energy expert Alex Wilson's article describing a level of home performance between passive house and international code minimum: "in a highly insulated house (and I’m talking about a really tight house with at least R-40 walls, an R-50 ceiling, and triple-glazed low-e windows), such a tiny amount of supplemental heat is needed that a radiant floor needs to be kept no more than a few degrees above the air temperature—or else overheating will occur. If a concrete-slab or tile floor surface is maintained at 72 or 75 degrees, it will likely feel cool underfoot—since it’s at a lower temperature than your feet. So you may not get that delightful benefit of a warm floor surface.