Insulating cob and earthbag houses. And do you need to?
“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?
The short answer is no. The long answer is hybrid. But first, it pays to properly grasp 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 this 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.
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. In fact there are an awful lot of extra factors to consider.
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 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 cloudy 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 cope a lot better with 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 shortish 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 desperately 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 may well get away with it.
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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