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Hi Dave
Just to be clear when you say the 'top of the Roxul', the top horizontal edge of the slab insulation has more insulation sitting on top, so there is no exposed 'top' as it were. But I'm thinking by 'top' you may mean the top vertical portion that would be exposed to the weather, so I'll answer that one, let me know if I misunderstood :)
When we installed the Roxul insulation on the exterior of the slab we included a cement board inside the form, and we rigged up our own solution to attach it using plastic tie wraps in order to avoid the thermal bridging that would come by having used long nails as some builders do.
So when we removed the forms the cement board was anchored through the insulation into the concrete, after that it is a simple matter of parging the exterior of the cement board.
What we did worked, though in hindsight given the compressibility of the Roxul (certainly at 8 inches thick) we would have included more fasteners as we found it to be a little spongy for our liking. This still can be remedied by using a flexible parging product, which is sprayed on and leaves a little room for movement without cracking. Here is a link to the video of us installing the cement board fasteners in case you want to take that route, and if it doesn’t make total sense I’d be happy to describe it in greater detail for you, let me know. https://www.ecohome.net/guides/2257/finishing-the-exterior-of-a-slab-on-grade-floor/
As a point of interest, earlier this week we just broke ground on our next demonstration house that will have an air-heated radiant floor where most of the heat is provided by solar air heating panels. It is also a slab-on-grade but the entire problem of exterior finishing is alleviated as the insulation is the form itself - you simply lay the pieces down as per the building directions (then wire mesh and heating system of course), pour concrete inside and then parge the exterior of the foam. There are no forms to build and take apart. Keep an eye out for more on that soon (see a first pic below), and here is a link to a page on the air heated floors as well. https://www.ecohome.net/guides/2217/slab-on-grade-prefab-form-kits/