Insulating Above-Grade Slab -- Need Creative Ideas
I hope someone can help with me with creative ideas. While I have freat respect for building codes I am getting really frustrated because now I am trying to engineer a solution to meet code and will end up spending probably hundreds of times more money on the solution than the annual energy savings that R-30 provides vs R-10 (which I could fit easily).
I am having a great difficulty finding an acceptable way to insulate an existing above grade slab without compromising the addition I am considering. I am told that in climate zone 5 I need R-30 for floors. This is 6 inches of XPS foam. The top slab which is the ceiling of the garage is only 2 inches below the top of the floor joists in the house, that means that with the insulation and assuming the same subfloor and floor thickness as the rest of the house I have a step of 4 inches from existing house to addition. Given that I am making an addition to the kitchen a 4 inch step in the middle of it is unacceptable as is any sloping floor. Rising the floor in the existing house is also not desirable as I have only 7'8" of height, so I cannot afford to lose any height without making the place look like catacombs.
While I can put the insulation below the slab inside the garage I see no reasonable way to insulate the slab's edges given that the slab is preexisting and on the sides it rests on a masonry blockwall plus some supporting I-beams above the doors. The garage masonry wall has brick veneer on the front side and is completely below grade on one side and the back. The other side is towards the house (no insulation on the shared basement masonry wall -- 1950 construction).
Are there any other creative options that I am not thinking of? Something with an R value better than XPS? Some other way to insulate an existing concrete slab that is hanging in mid air and is supported by thick masonry walls? Something else?