“Alas if the space heating limit is 15 kWh per m2 per year you have only a few options to tweak. One of those is sub slab insulation.
Once you have maximized the cost benefit of wall insulation (which is the most expensive because it increases roof area and fastening costs) and the attic is full to 120 how else do you meet the heating? Well even though sub slab is not that effective it is an available option to meet that unyielding target.”
John Straube, BuildingScience.com