I have had a couple of contractors give differing opinions about how to insulate 3 living spaces. I am half way through building a new house. I have a three car garage. The floor of the garage is hollow core with concrete overpour and infloor radiant. Above the garage is a living space. Below the garage is a basement living space. In the ceiling of the garage is staple up infloor radiant. heating the space below the garage is geothermal heated high velocity forced air. All three spaces have their own zones. Garage doors are insulated R17, walls are R33. Basement is insulated R36. I don't need the garage to be room temperature, just warm enough to make the cars comfortable enough to get in (I live in Winnipeg, Manitoba, temperature swings -35C in winter to +35C in summer). Should I insulate the basement ceiling and or the garage ceiling. And if so what should I use and how much.

both ceilings should be insulated as if the garage were not heated (R30+). Because it may be kept chilly enough to be basically similar to outdoor temp for most of the winter.

Yes they should be insulated, but not to the same degree. It is about temperature differential and loads. You will naturally have a very low heat load and require then a lower design temperature. Lower design temperatures require create lower delta Ts and it follows, lower insulation values.

I design of 10°C garages and generally insulate to R11 between floors, but this number is mainly dictated by the load served by the panel.

All should have been laid out in the original heat load and design.

I think it's important to treat garages as unheated spaces, even if they are currently heated. they are the first spaces turned off when energy prices rise. they are often aggressively reduced in temperature long before that.

New garages cost very little to heat and the people that heat them don't watch the cost of fuel much.

As for treating them as "unheated spaces"; they must be added to the heat load if they will be heated. If you mean adding antifreeze, you know I don't.

I mean all spaces adjacent to the garage must be capable of being economically and comfortably maintained if the garage were not heated.

in the last fuel spike I had several people call about taking the garage offline. I can only surmise more did not call. since it's "optionally heated" space, I think insulating the garage borders as if it were cold is prudent in all cases.

Ok, that is helpful. I will insulate both.

Thanks