You might also look into Supra, that offers tin-coated copper wires in a variety of gauges and that come in a "heat and aging resistant PVC jacket."mitch2 beat me to it. Supra is constructed in a similar way to what you want and is made for walls, under floors, etc. That, and I see you already use it and it does meet all the requirements you need, not to mention, all the codes.
All the best,
Nonoise