We are starting to design an above-sub-floor sandwich radiant heating system for a 2500 sq ft home in Maine that is currently under construction. At a design temperature of -7 degrees the heat load will be about 67k. A mod-con gas boiler w/ indirect domestic is planned. The flexibility and kink resistance of Onix is very attractive, as is the idea of 100% contact between subfloor and finished floor and the elimination of aluminum plates. The floor between level one and two is 12" I-joists. It would be helpful if we did not have to insulate in those floor joist bays since most batt insulation is not cut to fit well between I-joists and filling that cavity with blown in insulation would be overkill. What are the pros and cons of not insulating between the two heated floors?