I was probably still in plumbing school in 08, and I remember discussing systems with and without insulation, which makes me think it was still optional back then.
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"I like to quote my own quotes" ~ Dewey Cox
Code clause- no I do'nt think so. And likely no one checking on it even if it was in the code.
It is a best practice to have more R value under the floor than over it, but rigid foam is only one option. There are multiple products available for this application
Most of your heat loss in a floor is perimeter rather than directly down, especially if you have heavy clay. I built my house and garage in 2000 and did not insulate the underside of either with in floor heating. I did not want frost underneath the garage pad and my floor has never cracked. No issues with higher heating costs, my bills have always been very reasonable....of course I don't know what they would have been but it's not a huge trade off. I do industrial refrig for a living and install refrigerated and heating floor systems.
There is a requirement for insulation under a heated slab although the variety is not specified, it's just that styrofoam is the best option available.
I'd have to look up when that became code, I certainly put it in my own 2009 build.
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"The trouble with people idiot-proofing things, is the resulting evolution of the idiot." Me
Your build would fall under the 2006 alberta building code which did not require insulation under a heated floor.
There was a standata issued in Dec 2019 for the 2014 code which requires R5 minimum if the floor is below the frost line or R7.5 if the floor is above the frost line.
Question for construction/concrete guys.
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