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.
I’m intrigued by this. I’ve been planning a green house here in Canada. One of the risks in the location is fire, namely forest fires and grass fires, partly due to climate change drought. So I wanted a poured concrete exterior (perhaps with stone facing to look better), with the insulation inside. That makes it more or less fire resistant.
But summer heat is a definite problem. The location in Southern British Columbia can get up to 45 or 50 degrees Celsius as clear skies let the sun blast away. Will my insulation (R45 on the walls, R60 on the roof) not work well to keep me cool?
“More on that later” – I’ll be looking forward to that.
Alex
(By the way, I run a free green radio download site, plus a 24 hour all environment Net radio station, at http://www.ecoshock.org)
Hi Alex, thanks for the comment.
The insulation will definitely help to keep the heat out in hot weather, especially the roof insulation. But a well-insulated house without exposed thermal mass inside will heat up pretty fast from heat gains: e.g. warm outside air brought in for ventilation, solar radiation coming through your windows, occupant gains, eqiupment gains, and so on. You can keep some of the gains out using shading, low-energy lighting and appliances, and a good ventilation strategy, but in most cases you’ll need mass as well. I’m hoping to post an entry on mass soon.
I’ve taken a look at your site and it looks great. I’m just downloading the James Hansen interview now.
Alex,
The simple solution to your question is to place the insulation on the external side of the structural concrete walls. That way you get the insulation value, but also retain the mass too! The building will respond slowly to heat input during winter, but the same is true for the summer. Casey’s comments regarding shading etc are also important.
This does cause complications regarding chasing in cables/pipework etc to the poured concrete. I know there are methods for including small service ducts within the poured concrete, but this tends to be on commercial or high spec. buildings. Well, at least in the UK. You could try a mixture where you manage to rationalise the location of cables adn pipework to certain segments of wall which could be thinner concrete with a dry-lined finish? (to keep the internal wall line the same).
to explain a bit more, if you managed to located all cable and piped services to run within internal walls adn floor voids, you wouldnt need to worry about chasing in to the concrete.
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