> Don't know about r-value though. At that point you would have essentially 8" of solid concrete, so an R value around 1.5 Adding 3" of rigid foam on the outs

> Don't know about r-value though. At that point you would have essentially 8" of solid concrete, so an R value around 1.5 Adding 3" of rigid foam on the outside (then waterproofing and siding on top of it) would bump that R value up to around 15, with all the thermal mass on the inside (which can help if there's large day-night temperature variation)