Heating from Basement – GreenBuildingAdvisor

I have a build I am planning and am going back and forth on how to heat it. For the purpose of this discussion let’s set aside any cooling needs. 

I want to heat with radiant, not because I want warm floors I understand in a super insulated air tight home I will not have warm floors, but I like them for other reasons, they are quiet, and I like not seeing any evidence of heat registers and I like no blowing of air, etc. 

I plan to use ICF basement walls so I will have R-22 minimum below grade, I could be swayed to add another inch of foam internally if makes a huge difference. Likely R-10 under slab. Above grade walls will be atleast r-40 and roof r-60. 

In my current design there is an open stairwell to the basement with only a railing style half wall. Main floor is an open floor plan with vaulted ceilings and a half loft open to below.

I have read articles both here and elsewhere that in an airtight super insulated home the entire envelope can often be heated from the lowest level with a small temp differential maybe 2 degrees or so being seen with each level. 

I am trying to decide if I heat just the basement slab or if I should also put radiant in the main floor, or even if I should not heat the basement slab at all and only heat the main floor. But the basement is getting poured either way and adding pex is easy enough. I have read a couple posts of people claiming they did both levels and the thermostat on the main floor never calls for heat if the basement is on. 

For context this build is for an efficient home for my aging mother. All the main living space is on the main floor, the basement will likely have a play room for grandkids and other storage but there will be no bedrooms or any reason for her to need to go down there. The open loft will be mostly a library for her large collection of books, there might be a guest bed up there but it will be very infrequently used. I’m am basically unconcerned with the temp in the loft (and only minimally concerned with the basement temp), but if I could heat the basement to say 69-70 to keep the main floor  67 without adding the expense of a second zone that seems like maybe the way to go?