As part of a proposed garage-to-living space conversion I plan to create a built-up floor over the existing concrete slab to allow for insulation and to give a good substrate for flooring. This will be done with pressure-treated 2x's ripped to the slope of the slab. However, it is desirable to hold the floor elevation as low to the slab as possible as ceiling height is a bit of an issue. I know that in Washington State the prescriptive requirement for floor insulation in a crawlspace is R-30. As low as I would like to keep these "sleeper" joists there won't be a deep enough bay for batt insulation. Can I treat this like a slab-on-grade floor since there is no crawlspace and only insulate the exterior walls and lay a 2ft wide strip of R-10 rigid insulation on the slab between sleepers at the perimeter of the room(our energy code requires a similar detail for heated basements with slab-on-grade)? And since at what was the garage door side there will be more space between the slab and subfloor (due to the gradient of the floor)should I try to at least minimally ventilate? I'm not sure that there will be any significant crossover between joist bays but it is just a question.

Thanks in advance