> As great as modern insulation is at keeping hot and cool separate, a modern insulated wall doesn't cool it's surroundings like a high thermal mass wall would.
Why does modern insulation hold less thermal mass? Is it just that trapped air has less mass than stone?
That's exactly the reason. Technically it's actually the amount of energy needed to heat a volume of material, not the physical mass, that is important. But for many materials the two go hand in hand.
Touch a cold blanket and cold stone countertop and tell me which feels cooler, then do the same thing for a hot blanket and a hot countertop.
Sure, the stone is more conducive meaning you feel the temperature sooner. But it also has a lot of thermal mass, meaning it can give off or absorb more heat.
Why does modern insulation hold less thermal mass? Is it just that trapped air has less mass than stone?