Was coal ever used as an insulator?
Coal (especially in cities above a certain latitude) was stored under houses, often delivered down a chute to be handled as little a possible as it is messy &amp; dusty.The cost of removing the leftover coal (an unpleasant job) far exceeded its value, so it stayed, often getting covered up later to stop the dust.
  Heating oil &amp; tanks today face a similar $ dynamic in houses changing to gas, geothermal, electric, etc.