Doesn a layer of insulation through the middle of each wall cut the thermal mass in half?
No. The thermal mass inboard the insulation moderates or averages out the temperature fluctuations inside the house and stores the free passive solar energy. The thermal mass outboard of the insulation can also store free passive solar energy and raises the average temperature of the exterior wall. Because heat loss across the insulation increases with the temperature difference across the insulation, both walls act to reduce that temperature difference and hence reduces heat loss.
