Overhauling the very misleading thermal mass versus insulation discussion.
“I live high in the mountains, and it snows regularly. Will earthbag be warm enough?” This is the type of question I field on a regular basis. So are cob, mud or earthbag homes good for cold climates? Mostly you will be told the same thing. 'No', the experts will say.
There are, in my opinion, a lot of myths floating around the natural building world, and the thermal mass debate is right up there in the land of misinformed theory. Speak to people who actually live in cob houses (as opposed to those who only know straw bale and theorise about cob), and you get a very different picture to the one we are routinely given. It is far more complex and nuanced than most people think.
But first, let's take a look at the theory behind the difference between thermal mass and insulation.
Thermal Mass (Earthbag, cob, wattle and daub, and adobe all provide thermal mass).
Earthern walls provide thermal mass. This means they absorb the heat and store it (at a rate of about an inch of wall an hour). If you are in a warm, dry climate with plenty of sun this is what happens: The house absorbs the sun’s heat in the day, and then at night when the temperature drops the walls radiate the heat back into the house. By morning the earth has released all the stored heat and has absorbed the cool in its place. So now in the heat of the day the opposite occurs; the walls release cool into your house. It’s a type of natural air-con. I experienced it with my own earthbag house in Turkey and it was quite wonderful.
But...you need sun for passive solar to work. If you live in a cold, wet climate, this system won’t benefit you because your walls are going to store cold air instead, which is not what you want. In that case you're going to light a woodburner, and the walls are going to absorb that heat instead.
Insulation (Straw bale insulates)
Insulation is different. It slows down temperature exchange (heat or cold are prevented from moving through the wall). In the natural building world straw, saw dust, reeds, hemp, wool, and paper are the most common insulating materials.
With this in mind, typically you would be advised to build a straw bale house in a colder climate, and an earthern house in a warm, sunny climate. But not everything in this life is typical. Unless you're in Alaska or Finland and experiencing subzero temperatures for months on end, mud can be very very cosy. Furthermore there are an awful lot of extra factors to consider and when it comes to mud buildings. It's simply nonsense to say only thermal mass is at play, and that all mud buildings are alike. They are not.
I've heard over and over again by those who actually live in cob or earthen houses that earth is so cosy in the cold. I experienced exactly the same thing in my earthbag house when it was snowing outside, but there was so much ecobabble online about how bad mud is in the cold, that I doubted my own experience initially. As more and more people come forward to share their experience of mud in the cold, I become ever more certain that the over-simplified thermal mass debate needs a good overhaul.
Compared to concrete/stone, mud buildings are amazingly energy efficient and warm. Why is that?
First, earth actually has some insulation properties (R value of 0.25 per inch compared to concrete which is almost zero). If you've added straw into you mud mix (cob) then that insulation value is increasing. So with an earthen house you don't only have thermal mass, but also a bit of insulation. This is why the walls feel warmer to touch than stone or concrete.
Why are some mud homes warmer than others?
Katherine Wyvern who lives in a cob home in France, makes a good point when she says, "The working of thermal mass is a good deal more complex than just transmitting heat as opposed to insulation. (First), it's pointless to describe 30 cm thick walls as thermal mass. That's a toy wall. Traditional thermal mass buildings have, well, massive walls. 60, 80 cm wide. It is true that thermal mass is an energy-storing battery, and you cannot keep a house comfortable with an insufficient battery. You wouldn't try to power a car with a couple of AA batteries would you?"
People don't understand how to use thermal mass to their advantage.
My earthbag house had walls somewhere between 60-80 cm thick. The house seemed to retain some measure of the heat (or cold) for about three days. So if you have a mud house with decent walls, it pays to prevent the temperature from dipping too low. You might just light your fire for two hours in the evening (what I did in Turkey), and that heat will power the house for another 22 hours. It's actually more energy-efficient than insulation if you know what you're doing, but no one seems to realise it.
Quirks and exceptions to the rule.
1. Cold climates are often lumped together in the thermal mass discussion. But in my experience the real killer for earth is cold and wet with no sun. Back in my earthbag house in Turkey, I remember the temperature dropping to -7 degrees celsius where I lived, but because the sun was shining, and I had south facing windows which acted like a greenhouse to catch the solar warmth, the walls still absorbed that solar heat and despite the subzero temperature, I didn’t have to burn the fire until the sun began wane.
My personal experience was as follows: My earthbag house was always warmer on cold, sunny days than on cool wet days. I had to light the fire on rainy days even though the temperature was actually a good ten degrees higher than on cold clear days.
2. Earth will be much cosier and energy-efficient in the cold if you live in your house permanently, and therefore heat it regularly. As mentioned, the walls retain the heat. The thicker the walls, the longer they retain it. Over time the house gets warmed to the core which can carry you over a cold spell. The trouble is of course, if you leave your house for days on end it also gets cold to the core, and may take a couple of days to heat up.
Conclusion: If you are building a weekend holiday home, or a community centre that is not continually lived in, and you are in a cold, grey climate, you will need insulation. On the other hand, if you are building a smallish mud house, with excellent passive solar design, you live in it continually, and the temperature is not perpetually subzero but goes up and down with plenty of sunshine, you will feel pretty darn snug I can assure you. Don't forget, in an earthen house you have the added benefit of the reverse happening in summer. It's a kind of natural air-con.
What if you live in a cold, non-sunny climate and still want an earthen house?
There are reasons you may not want to build a straw bale house (I have a straw allergy for example). So what you need to do is create a hybrid, and insulate your mud walls. How far you insulate will depend on how cold your climate is. You could create a straw-heavy earth plaster for your walls, or coat them in some sort of papercrete. Some people have attached reed mats to walls and plastered over them, Or lined the walls with straw bales. Someone suggested creating a lath and stuffing it with wool, or straw. Cork can also be added to the render, or to a lime render (thanks to Cath Coffey for that one;)
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