Garazgos

I had checked in to comment on the strawbales-as-floor insulation proposal but I see that I and Bruce King had already done so last year at about this time and the thoughts expressed then are just as valid now.

About the only thing that I might about the proposal depicted in the sketch is that the idea might be serviceable IF the floor joists were extended outwards so that they carry the wall bales.

This would enable the straw bales used as floor insulation to be pulled out and replaced as necessary -- and make no mistake, it will be necessary.

The T&G floor is extremely air-leaky and that will surely result in moisture from the interior getting into the floor bales. That is a fact that can be verified by the experience of other SB builders who did the same.

The 'impermeable membrane" placed on the crushed stone drainage layer under the bales will be the plane where the moisture will condense. Microbial activity will be supported followed by Fetid Goo Brew .

WRT to the query re: Why downward sloped perimeter (foam) insulation ?

It creates an umbrella of insulation which retards the outwards/upwards flow of warmth from deep earth, extending the non-frost-susceptible zone underneath the floor, outwards from the "foundation" a bit, which is a Good Thing if frost heave of the foundation elements is considered a Bad Thing. ie Frost expansion is three-dimensional. Lateral pressure can wreak significant havoc too (as well as popping things upwards out of the ground).