That is a really good question! It really varies from state to state, even from city to city. In Minneapolis double-wall is required with any glycol, but not outside the city, for example. I’ve heard that Montana recently dropped the double-walled requirement in their code. Then it can sometimes come down to the actual inspector and how he interprets the code; go figure.
I know we all hate taking the efficiency hit with double-walled HX and the indirect tank manufacturers dislike producing them.
All that being said, the Caleffi SolarCon storage tanks, available with single or dual HX (not double-wall), have been used successfully all over North America. It is possible that our SolarCon tanks, when used with the Caleffi SolarHD non-toxic (food grade, NSF approved) propylene glycol might be accepted where a double-wall HX was specified.