Hey,

New to here and appreciative of all the learning I’ve done pouring over all the info.

I have a couple questions related to designing the foundation for my home.

I want to have a perimeter concrete foundation (either frost-protected or to below frost line, haven’t decided yet) and ideally a finished earthen floor on grade for passive solar use (and less hard on the feet than concrete). I’m wondering about a couple of the details:

1. I’ve read that roxul’s comfortboard IS is possible to use sub slab – http://www2.buildinggreen.com/article/move-over-foam-sub-slab-mineral-wool-here – anyone have any experience with it’s use under slab? I’m not thinking under the footers, but under the earthen floor that will be 2-4″ thick (haven’t found specs on thickness of earthen floor for passive solar yet). The house is a 1 story octagon.

2. I’m trying to figure out how to insulate such a foundation – would it make the most sense to just insulate up the inside foundation wall from the sub earthen floor insulation? Or is exterior insulation a better choice? It seems that either way there is a weak point where heat can flow out at the corner where foundation meets wall assembly. With interior insulation, there is a corner point (between wall insulation and foundation insulation) that would have low R values. With exterior insulation, would the heat flow out the foundation through the footer?

Thanks for the help

Moses