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)
I have been living in a two story ICF (insulated concrete form)house since 2005. The south facing wall was designed using 10% of the square footage of the house for solar glazing. The walls are R55 and the vaulted ceiling is R76. Our temps here in the high desert (6,300 ft) of Arizona range from an average of 90 degrees to winter lows that will dip below zero but average 25 degrees.
The quietness of the insulated concrete walls ( with a concrete core of 8″ with 3″ of foam on each side is sided with tongue and groove cedar) and the even thermal comfort combined with the effective solar gain and low utility bills is out of this world!!
During the summer the sun is directly overhead and the insulation and reflective barrier in the ceiling work beautifully to keep the heat out. During the winter, the sunlight comes in the windows perfectly and heats to the back of the house. It stays cool enough / warm enough to turn off the A/C or heat for long periods of time (oftentimes weeks during the early or late parts of the summer or winter seasons and not at all in between). Admittedly we like it at 63 degrees to sleep in the winter and its windows open any time the weather above 50 degrees allows – which happens a lot.
We have faced wildfire, drought, freezing and heavy (70 mph) winds without issue. The basement has a closet that is more like a vault – a good place to keep valuables during fire evacuation.We are being forced to move for health reasons and the thought of not being able to build another just like it is stifling. I know we will never find a new home so satisfying.
I would suggest facing your house for solar gain and using a proper design on that south facing wall in addition to utilizing landscaping that allows light in the winter and shade in the summer will also greatly improve your energy efficiency.
(I can’t seem to leave a general comment to the main post so I’m using yours!)
The one cosmic issue Casey has not mentioned is ***moisture**. If your 500mm thick masonry (stone/cement/brick/whatever) wall is damp then its thermal performance will be poor. If it is dry then it will be relatively good. Ergo, take sensible measure to deal with wet (good gutters and decent overhangs where possible) and dry stone walls are not only effective structurally and for sound-proofing but are also reasonably good at insulating, too. That does not mean they would not need help, especially if you are looking for something even approaching passive status.
So many old stone-built properties (such as the ones here on the Isle of Colonsay, Argyll ie southern Hebrides, built around 1850 and the one I am sitting in typing this) have been tragically unsympathetically rendered externally with cementitious render – the original stone mortar being lime, of course. The lime mortar readily permits the movement of moisture through the wall allowing winter wet to evaporate away externally as well as permitting interior sources of damp (cooking, bathing, breathing) to do the same. Clarting the outside of the walls with cement effectively stops that, dead. Obviously, the justification for rendering in the first place is to prevent rain from penetrating walls but a/ if the pointing was properly maintained this would not be an issue in the first place and b/ when the render cracks (as it inevitably does) the cracks permit water ingress *directly* and then it is trapped causing damp interiors, more cracking of the render when the trapped water freezes and rotting joists and other timber protrusions into the stonework.
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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Seems a mite simplistic to this engineer. Let’s see…slow to heat, slow to cool…if you’re in Italy and it’s your house stone is absorbing warmth all day…it can release it all throughout the night, warming the house. The next day all that cool stone is cooling the house and resisting heating up. Kinda works that way in the Southwest with Adobe too, doesn’t it? The real question is what temperature swings you see, in what time period. Meanwhile, I agree you can do it with high thermal inertia, low mass… I’m in Arizona with a Straw Bale house…I can heat with a candle…cool with an open window at night. check out “Sedona Straw Bale” on Amazon.
Interesting post, I am considering rebuilding a stone house in the mountains of Greece and one of the questions was: do you keep the +700mm stone walls or a build a hybrid. After a heated debate about costs with my wife last night I think we came to the conclusion that we would go for 350-400mm of stone, 100mm of cavity wall insulation and 215mm of medium density concrete block work. Could have gone stone, insulation, stone, but you pay for each stone face you dress.
With regards Alex I would suggest that they shouldn’t think “which side” for insulation when they can equally have mass both inside and out. I would suggest that a cavity wall design would be stronger and have a good thermal mass inside as well as out. Although if one is worried about being green then concrete is something of a hypocrisy and I would suggest stone & limecrete. Concrete isn’t fireproof, only fire resistant.
I am inclined to agree, a general rule of conduction in physics being that the denser the material the better the conductor of heat, that would apply to stone walls too, being denser than insulation it would absorb the heat, so you are effectively surrounded by a heat sink. IMHO it is better to insulate the interior particularly in a colder climate.
Sorry, but this is patent nonsense as 2 minutes research on the thermal performance of stone would reveal (https://www.naturalstoneinstitute.org/designprofessionals/technical-bulletins/rvalue/).
The problem is *moisture*. The classic UK scenario is a 500-600mm thick stone wall originally built in the mid to late 1800’s with *lime mortar* that was rendered with cementitious render at some point in the early 20th century ‘to keep the rain out’. This was done as it was cheaper than getting the wall properly re-pointed. All it does is effectively seal the outer surface (bar the inevitable shrinkage cracks that always happen with cement-based renders) preventing the wall from ‘breathing’; letting moisture generated within by us breathing, washing and cooking to escape.
It is the moisture content (and connected air gaps) that lowers the thermal conductivity of a stone wall, not the thermal characteristics of the stone!
Cement-based render also prevents what rain that penetrates the shrinkage cracks from getting back out, too. Traditionally, these buildings would have had poorly fitting doors and windows by today’s standards which provided adequate ventilation to convey moisture to the outside. Now, this escape route is closed and the walls just get damper and damper, encouraging the decay of timbers embedded in them, causing fungal growth on the inside and making it next to impossible to ever get the house warm.
Get rid of the render and ‘just’ properly repoint the stone with lime mortar and these beautifully built, hugely attractive thick stone walls would easily meet the current UK building regs part L thermal performance.
(Ooops – just noticed I had this little rant last year – so moderator; please feel free to delete this latest one!)