In the northern hemisphere we are coming into winter, and many of us are thankful for the furnaces that are keeping our houses warm. Not Mike Coe and Lizzie Stoodley; they are going into their fourth winter in a house with no heating system at all. Instead, they celebrate mass: thermal mass. Their house is built with thick concrete walls and floors that have a great amount of thermal inertia, taking a long time to cool down or heat up. But then they wrap all this in a thick blanket of insulation, so that it takes even longer, weeks and months instead of days. Add some carefully placed south-facing high quality windows for solar gain, pumping heat into those floors and walls, and the The Cropthorne Autonomous House lives up to its name, staying warm all winter (and cool in summer) without any heating system at all. Mike Coe explains:
It has a thermally massive structure, but unlike the old cottages, this mass isn't built from local stone, but from thick (140mm) high-density concrete blocks. Also, considerable mass is added by the poured-concrete floors, which weigh around 50 tons each, plus the internal block walls, and even the staircase, which is constructed from high-density concrete.
Oh wait, there is one heating system: They have a Clivus Multrum composting toilet at the bottom of the chute connected to that special handmade porcelain tube. The anaerobic digestion of poop creates heat and the toilet has an exhaust fan to keep smells from going back into the house; this is run through a heat recovery ventilator, so strictly speaking, this house is heated by sun and poop.
There is also a huge and elaborate rainwater harvesting and storage system that meets all their water needs (far less than a normal house thanks to that toilet). Interestingly, the system is made from recycled orange juice tanks that are shipped full of juice from Israel and then scrapped. It is then pumped through a sand filter. Fortunately it won't be used for drinking until it is tested; I suppose, like the medieval English, they will drink beer instead. Grey water is soaked up in the garden.
Mike Coe claims that "Intelligent use of renewable energy makes the house carbon negative."- there are big thermal and photovoltaic solar arrays that generate more power than he can use. My first thought was that it may be carbon negative in its operation, but there sure is a lot of concrete in that thermal mass. The production of cement generates a huge amount of CO2; normal concrete ends up having a carbon footprint of 180 kilograms per tonne, and this house has a lot of it.
But a small, very efficient house in the UK burns the equivalent of 10,000 kWh of energy from natural gas per year, at .185 Kg of CO2 per kWh, or 1850 Kg of CO2 per year. That's equal to ten tonnes of concrete. I don't know how much concrete is in the whole house, but that 50 tonne main floor takes 5 years to pay its debt compared to a really efficient house, half that compared to a normal British house. Over the not very long term this house saves a lot more carbon that went into building it, and it is built for the very long term.
The embodied energy of construction is a complex and difficult issue to deal with, as is the whole question of what it means to be autonomous; there are those who might argue that Coe doesn't go as far, say, as Alex Wilson, who also tries to grow his own food and bike to town. On the other hand, Coe has built a house that actually runs on sun and poop in a climate where the former is in short supply and the latter is flushed into rivers. It's made with healthy materials like plaster and tile that will last forever. It looks really comfortable. That is pretty amazing.
Watch the terrific video that explains the whole thing a lot better than I can, and see more at the website for The Cropthorne Autonomous House