I am building a garage with granny flat above. The garage is unheated with slab on grade.Code requires air barrier between garage and residential. I am using SIP panels for all exterior walls and for the floor above the garage where possible, it meets my criteria and code. I have a section under the bathroom/kitchen where I plan on using Triforce open joist system because of span and to run all utilities. I need to insulate that floor area. My preference in Rockwool, but it probably will gap at the joists. And I will have ducts and pipes. Any suggestions for insulation?

I think a combination of spray foam (to seal) and Rockwool (to provide most of the R-value) would accomplish your goal.

Is damp spray cellulose readily available in your area? If so, I would look into that.

You're going to be installing a double layer of 5/8" drywall on the bottom of the joist. Go to the Homarama and rent their blower and install some celly. Cover the vertical areas with a double layer of Iso board that you foam in place. You don't have to be as worried about heat loss in a floor so the insulation doesn't need to be tight with the floor. Just make a sealed insulated box below it.

I'd go with spray foam for air sealing would probably be all you'd need, depending on how thick the ICF is.