This distinctive house is completely ‘water neutral’ and carbon negative…
- The ‘Cropthorne Autonomous House’ sits on a semi-rural site on the edge of a village in Worcestershire, with south-facing views to Bredon Hill and the Malverns beyond. The internal floor area of the new house is around 144 m², consisting of four bedrooms on the ground floor, and a large open plan living area above, with adjoining kitchen, plus an office and toilet. There is also a large basement under most of the house. The inverted layout was chosen to maximise warmth in the living areas during winter, and avoid overheated bedrooms in summer.
- The aim was to replicate a house built by Brenda and Robert Vale seen in Southwell, Nottinghamshire in the mid-nineties. At the same time the self builders wanted to improve on it by taking advantage of the greater choice of components and materials now available. They also wanted a home that was less utilitarian and more suited to its village location.
- A single home had stood on the site previously, but had been demolished following a serious fire. A property developer had then attempted to build several houses there, but was thwarted by planning and sold it on as a self-build plot for £295,000.
- This self build project is unusual in that it not only meets, but exceeds Passivhaus standards (although has not been officially certified as such), is completely ‘water neutral’ as it makes use of harvested rainwater and composting toilets (and therefore does not require mains water or drainage) and makes intelligent use of renewable energy making it carbon negative.
- The first challenge was that, as part of the specification, all of the materials were to be assessed for environmental impact, and sourced locally wherever possible. This included everything from basic building materials with a high recycled content, through to paints and finishes.
- Plans for the site were drawn up by Neill Lewis, a one-man practice based in Malvern. As a Sustainable Building Association board member, Neill had considerable experience in designing similar low-impact projects. The project also needed additional specialist input in the form of energy consultant David Olivier.
- The core construction used a lot of bricks and concrete, to provide high thermal mass. The floors are concrete, around 50 tonnes each, plus the staircase to the first floor living area. The internal walls are built from high-density concrete blocks ; a standard builders merchant item, except for a few internal walls which were built from reclaimed storage heater bricks.
- This was surrounded by a large amount of insulation, working to the AECB Gold Standard (comparable to the Passivhaus standard), with close attention paid to making the entire building envelope very airtight. The windows are triple-glazed, with timber frames and aluminium cladding.
- The house is also designed to be ‘zero heat’ – i.e. it is so well insulated that it captures solar heat through the south-facing windows. This heat is absorbed by the high thermal mass in the floors and, combined with the output of the occupants themselves, keeps the house at comfortable temperature all year round, without the need for a normal heating system. This is also aided by the ‘upside down’ layout of the house – whereby the main living area and kitchen is upstairs, thereby maximising the use of the available rising heat.
- As the house is so air tight, with little margin for heat loss, ventilation is provided by a mechanical ventilation with heat recovery (MVHR) system which works by warming the incoming fresh air with heat reclaimed from the outgoing stale air. The system is almost inaudible, except in boost mode and brings cool air into the bedrooms at night, while extracting kitchen smells quickly and efficiently.
- The house has many thoughtful interior design touches such as stained glass windows made from recycled glass bottles by a local artist in the curved wall adjacent to the kitchen. The kitchen worktops themselves are made from recycled vending machine plastic cups and the balcony decking is made from recycled plastic, which, while more expensive than timber, is maintenance-free.
- One of the many striking elements of this self build is the large basement, housing the chamber for the UK-manufactured version of a Clivus Multrum composting toilet system. The system creates clean garden fertiliser from human waste, without using any water for flushing. The use of compost toilets did create some design challenges as you need the ‘store’ directly under the location of the loos, as everything falls into the store by gravity! Beneath the loos is a large composting chamber with a sloping floor that accommodates two toilets (one on the first floor and one on the ground floor entrance level). The toilets discharge vertically into the compost chamber via 15-inch diameter stainless-steel chutes. Incredibly, there is no unpleasant odour at all from the toilets or the sealed system chamber itself, and the occupants report that they actually now prefer this method of sanitation to standard flushing toilets.
- The basement also houses ten 1500 litre plastic water tanks, originally used by Britvic to transport orange juice concentrate. These tanks store water form the rainwater harvesting system. The system uses slow sand filtration to ‘clean’ the water so it can be used for all the occupants’ domestic water needs. Together with the composting toilet system and a grey-water soak away, this means the house operates largely autonomously, with just a mains electricity connection, assisted by a grid-connected photovoltaic array.
- The running costs of the Autonomous House are significantly lower than a typical house with a modern condensing boiler, standard double glazing and loft insulation – where utility costs would typically be around £2,000 per year. In this house, the only bill is electricity, which is expected to be £200-£300 per year. However, the owners have installed a photovoltaic array that brings in £800 – £1,000 each year and they earn a further £200-300 from their solar hot water panels. Therefore, they are effectively £3,000 per year better off compared to a similar-sized property. The £3,000 annual saving will, of course, increase in line with fuel prices, so a 10-year saving of £40,000 to £50,000 is realistic.
- The total construction cost was £360,000, with a site cost of £295,000, funded by the owners’ sale of their house in London. However, this isn’t a truly final figure as there are some outstanding items such as site fencing to be completed.
- The current market value was estimated (in 2012) by a local surveyor to be £780,000 for a buyer who is environment and running-cost aware. Therefore, if sold, the owners could expect a profit of £125,000, although this was never the main motivation for the build in the first place.
- Dec 2006 – Plot purchased
- Sept 2007 – Architect appointed, plans drawn up
- January 2008 – Clearing of plot begins, planning submitted
- July 2008 – Planning approved
- June 2009 – Building begins
- October 2011 – Owners move in
- To make this house work the owners had to effectively build a third, basement level to accommodate the ten large water storage tanks and the toilet composting chamber. This extra floor has inevitably added to the overall cost in the region of £45,000. So if you want to go off-grid in a similar way; be sure you understand the additional costs in terms of the build and the associated technologies that are involved. If they did it again, they would possibly spend much less time researching and designing specialists systems such as the composting toilets and rainwater harvesting systems; as these days there are more ‘off the shelf’ products widely available.
- The house is an extreme example of how to create an environmentally-conscious building, so it would be possible simplify the design, making it cheaper and more mainstream, while still retaining a similar energy performance. But at the very outset of the project, their mantra was “If we’re going to do this environmental building thing at all, then I really want to go for it!”