Tricky to explain so bear with me.
We're extending the basement of an existing listed building, the image below shows a simplified ground floor (left) and basement plan (right). Blue shows existing footprint and green shows new areas.
Due to listed status our options are limited so it's IWI to all walls. New build basement walls will be retaining so generally we're maintaining the same approach here with solid masonry walls and IWI, any EWI would likely be crushed. Yellow walls are lined out to achieve the same U-value (0.28 W/m²K) as above ground.
What I'm wondering is how much we need to insulate the new basement walls shown in red. They are not 'perimeter' walls in the sense that they will not be exposed to outside air, as they are completely beneath the ground floor footprint above. Following FosterTom's theory that the soil directly beneath a building will reach an equilibrium point if continuously heated, I'm wondering what our options are for these walls.
It would be ideal if we could reduce the IWI depth here as space is very restricted. I concede that the perimeter 'enclosure' for the soil here is just the existing building's foundations, so won't be insulated, but is there still a case for reduction?
Finding it difficult to find ways to model this scenario, although I'm gradually getting to grips with THERM if anyone can give me some pointers?