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Cropthorne Autonomous House

I went on a most interesting visit yesterday, to the Cropthorne Autonomous House, claimed by the owners to be the most energy-efficient house in the country.

It’s a long story, which started a good few years ago, when Christopher and I were getting our extension built. We planned to replace our existing detached garage, which was made of pre-fabricated concrete blocks bolted together. It was frankly an eyesore. In fact the local planning officer told us that we were doing the neighbourhood a favour by knocking it down! However, it was sturdy, lockable and watertight, so it would have been a real waste to consign it to land-fill. Our architect said that he had another client who was about to embark on an ambitious “Grand Designs”-style self-build, and needed a temporary lock-up for the duration.

So one cold and wet January day, Mike turned up to disconnect the electricity and take away the garage. He and his partner Lizzie planned to use it as a builders’ brew room, a tool-store, and a refuge from the weather until their house was water-tight. Once the build was finished, they would pass it on, via Freecycle, and I understand that it’s now being used as a log store somewhere outside Worcester.

Mike and Lizzie had some really ambitious ideas for their plot of land. They wanted to build a house that had a minimal environmental footprint – with no mains water or drainage, no central heating, no boiler, and producing as much electricity as it consumes. I was an avid reader of their blog during the long and challenging construction process. Last weekend, the “Passivhaus” movement for highly energy-efficient houses had an open-day, with houses all over the country opening their doors to interested visitors. Cropthorne was on the list, so I contacted Lizzie and asked if I could join one of the tours.

I think that the other seven people in the tour group were all interested in building a Passivhaus of their own – they all had notebooks and pens, some had clipboards, and all were taking copious notes. One couple even seemed to have brought their structural engineer and their builder with them to pick up some tips! I was there out of sheer nosiness – I wanted to see what my garage had helped facilitate!

Cropthorne Autonomous House

Cropthorne Autonomous House

At first sight the house looks modern, but not particularly unusual. The first thing I noticed is that it’s not aligned with the other houses on the road, but is rotated to face due south. On the south-facing roof elevation you can see some solar hot water heaters, and there are photo-voltaic panels to generate electricity just visible in the garden to the far left.  There are lots of windows on the south-facing side, to maximise the solar collection, and much smaller windows on the north and side faces.

Mike took us on a guided tour of the house, including the “engine room” in the basement, and it was absolutely fascinating. The basic idea is to have a thermally-massive and maximally-insulated house so that it acts like a giant storage heater, storing the sun’s warmth during the summer, and giving it up slowly in the winter. So structurally, it’s made of a huge mass of concrete, with triply-glazed windows and vast amounts of insulation to protect the concrete core. It’s virtually airtight, so they have to use mechanical ventilation to draw fresh air into the bedrooms and living room, and extract it from the kitchen and bathrooms. They have composting toilets, which apparently aren’t nearly as yucky as they sound, and save on huge amounts of water, as they don’t need to be flushed. All the rainwater is harvested and is stored in the basement in huge vats which were originally used to import concentrated orange juice from Israel. The rainwater is slowly filtered through gravel and sand and comes out drinkable – though it needs to be pH corrected as it is surprisingly acidic.

Outside of the give-away facilities in the basement, the living area in the house didn’t appear to compromise at all on comfort for green credentials. In fact, it was extremely comfortable. There was a utility room with washing machine (best used when the sun was shining, to use their own PV-generated electricity, rather than importing it from the grid), a modern kitchen and four bedrooms. Perhaps the only give-away that something unusual was going on was that the house was “upside-down” – with the bedrooms on the ground floor,and a large open-plan living area upstairs. Since heat rises, the downstairs is slightly cooler than upstairs, which is better for sleeping during the summer. A side benefit is that the panoramic windows in the living area have stunning views out over the Worcestershire countryside. Despite the fact that it was a pretty cold day in early winter, the house was very warm inside. About 23°C according to Mike’s data-logger (everything is logged, so that he can keep an eye on the system performance) – which is significantly warmer than my house! All the visitors were taking off their coats and jumpers. Apparently, even in the depths of January the house stays at a bearable temperature, not dropping below 17.5°C even on an overcast day in the middle of winter.

Overall, it was really interesting to see such an innovative design for a house, and to understand how it functions. It clearly was a massive labour of love by Mike and Lizzie, and they are both passionate about the environmental benefits of a minimally-invasive house. It was great to finally see for myself what our garage had, in its own tiny way, helped to bring into existence!