Insulating a raised slab
Alright, so here’s one that I can’t find in the great treasure troves of this site: how to handle insulating my raised slab on our new house. Quick details on the project: climate zone 6, 3500sqft, two story, zip-r external insulation/sheeting (R6), fiberglass batts in 2×6 walls, combination vented roof w/ R60 cellulose and unvented roof with R50 foam, low U 4th surface fiberglass windows, and a combination of slab on grade and vented crawl w/ R48 foam. The concrete will be polished as finish floor in slab areas. Heating is entirely mini-split, no in floor radiant.
Project is built over a very steep hill, with the steepest portion the vented crawl, everywhere else we built up concrete walls and back filled for slab sloping back to grade. As such the slab on grade portion ranges from 6” above grade to 10’ above grade… currently we have R10 under slab with edges turned up and cut at 45° around perimeter. The engineer required integrated rebar ties to walls around entire slab, 3’ rebar stingers from wall are bent at 90° to tie into slab. No vertical insulation was installed on inside of concrete walls below bottom of slab.
My concern is thermal transfer from external wall system to slab… is the only option insulation on the exterior? If so, how far down the walls would need to be insulated to keep the slab warm? I can find little information on thermal transfer properties of concrete walls, and am curious how much geo warmth we’ll get from ground temp up through the back filled areas and if that will help combat cold transfer from external walls.
Appreciate the help!