In Italy, building professionals often tell you that thick stone walls will keep you warm in winter. Our first geometra said so. And recently my friend’s architect told him it wasn’t worth adding insulation to his walls since they were porous tufa stone, which the architect claimed was a good insulator. But it’s not true.

A good insulator has a high thermal resistance - it prevents heat from flowing from the warm side to the cool side. Polystyrene, rockwool, and sheep’s wool are all examples of good insulators. In many cases you can compensate for lower thermal resistance by increasing the thickness of material: if your insulation isn’t good, just use more of it. But with stone, the thermal resistance is so low that in order to offer a reasonable level of insulation, the walls need to be unrealistically thick. 

A typical 500mm thick stone wall, for example, has the equivalent insulating value of only 15mm of rockwool. In comparison, to pass building regs a new house in the UK needs the equivalent of 150-200mm of rockwool. The typical stone wall lets out about fourteen times as much heat as a wall in a new house. To meet UK building regs you’d have to make it seven meters thick.

A 500mm wall of porous tufa stone is a bit better with a thermal resistance equivalent of 40mm of rockwool. Medium weight masonry is about the same. But you’d still need a wall two and a half meters thick to achieve a good level of insulation.

Why do people buy into the myth that stone walls are a good insulator? If you turn off the heating, it will take longer for a house with 500mm thick stone walls to cool down than a lightweight house with a pitiful 15mm of rockwool.  This time lag gives the impression that the house with stone walls is the warmer of the two. But that’s only because it took more time and energy to heat up the high mass house in the first place: more mass means it’s slow to warm and slow to cool.

Now turn the heating back on. The lightweight house will warm up quickly and the heavyweight house will take much longer as the mass in the walls soaks up the heat. But once the inside of each house is up to temperature (say 20°C), the rates of heat loss are identical; i.e. each house has the same amount of heat energy flowing from inside to outside. They’ll have the same heating bills and require the same size boiler.

So if you have the choice during renovation, insulate the walls of your high mass house. But make sure you put the insulation on the outside and not the inside; otherwise you lose the effect of mass that will keep you cool in the summer. More on that later.