How do you insulate a basement slab to keep the benefits of thermal mass in a passive solar home?
Will I get the benefit of thermal mass with a foundation where the inside of the foundation stem walls are insulated and the rest of the horizontal part of the foundation is foam insulation mon on the bottom followed by 4" of AB topped with 5" of concrete?
From what you describe above, no, your stem walls will not offer any thermal mass heat regulation, but your slab will. Read more here -
How thermal mass in a home can help regulate temperatures for comfort and efficiency
Thermal mass that is separated from the heated space by a layer of insulation, such as ICF construction, it will offer virtually no heat regulating assistance. Concrete inside the thermal envelope will absorb and release heat, concrete outside of it will not.
Are the stem walls also insulated on the outside? I'm wondering if they are ICF, as I can think of no reason to insulate the interior only of a below grade stem wall.
It sounds a bit like you are in a cold climate and building a slab with a stem wall, is that the case? Perhaps you just mean foundation wall and are not referring to a frost wall. If you are in fact building a slab, stem walls are a very costly and unnecessary component, here is our slab on grade best practices DIY building guide page.
The only reason to build stem walls / frost walls is to get below the frost line, and you can accomplish the same thing a lot easier and cheaper by just doing a slab on grade with a perimeter skirt of insulation. Laying a footing and stem wall to get below the frost line is a building technique that dates back to a time before the advent of insulation, where dirt was the insulator. Now insulation can be the insulator.