The Cropthorne Autonomous House - Notes on the Design
Updated by: Mike on 28-12-2009 (Added thermal mass & insulation. Still to come: Passive solar heating)
This page may never be complete, but will evolve along with the house....

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Coloured Elevations
Coloured Elevations from the Plans for The Cropthorne Autonomous House
Any architectural drawings shown on this site are © Neill Lewis, chartered architect.

A Brief Summary
(This paragraph explains all the basics. The further you read down this page, the more detailed it becomes...)

The design aims for the house are that it should have minimal impact on the environment and should, as far as possible, obtain everything it needs from the land around it. It should also be attractive, and provide a pleasant, comfortable living environment, comparable to (but not necessarily the same as) a conventionally serviced house.

It should not be lavish, but 'sufficient'.
With an internal floor area of roughly 150mē, this is a compact four-bedroom house. Sufficient for a notional family of four, but not unnecessarily large, which would be a waste of materials.

For the servicing needs:
Very high levels of insulation, an airtight structure to reduce heat loss, and a mechanical ventilation system to reclaim heat from the extracted air.
Thermally massive - high-density concrete construction inside the insulation envelope to help to store heat in the winter, and keep the house cool in summer, without needing air conditioning.
Highly efficient triple-glazed windows with minimum heat loss and maximum solar heat gain. These, combined with the insulation and mass, should mean we need no heating system at all. After a couple of seasons in the house, we'll know if the 'zero-heat' approach is working. If not, there are various options for back-up heating in exceptionally cold weather.
Positioning of the windows to make the best possible use of natural daylight, reducing the need for artificial lighting.
A dry composting toilet system to turn all of the human waste into high-quality garden compost. It also saves water – a conventional house uses up to 40% of total consumption in toilet flushing. With adequate storage and sensible management, rainwater can meet the entire needs of the household, so there's no need for a mains water connection.
The waste (grey) water which the house produces will contain only soaps and carefully chosen detergents. The Environment Agency rates this as less toxic than the discharge from a septic tank, so it can go directly into a soakaway, meaning the house needs no main drainage connection. In effect, we will 'borrow' the rain which falls on the building, use it, and return it to the land afterwards.
Solar panels on the roof will provide most of the domestic hot water - in the summer months probably all of it.
Photovoltaic panels mounted on a pergola in the garden to produce electricity, which will supplement that drawn from the national grid - the only external service connection. Excess power generated will be sold back to the electricity company. Eventually, we may add some kind of wind power, to produce more electricity. The overall aim is to generate more power than is used, so to produce net zero carbon-dioxide emissions.

The Site Plan

The house is 'skewed' relative to the road to achieve maximum solar heat gain into the rear glazed areas, which is essential in the design of a 'zero-heat' house. Fortunately, the local authority were sympathetic to our aims and allowed this slightly unusual orientation.

Building materials have been chosen largely for minimal environmental damage. Lime mortar and plasters are being used where possible, and PVC has been kept to a minimum. The outer skin bricks are reclaimed, and the roof tiles, although clay (for planning reasons), have been sourced from a local manufacturer.

The garage is deliberately only large enough for one car, but with plenty of space for bicycles and a repair workshop. Being in a rural location, we felt doing away with a car altogether was unrealistic, although hopefully a very low-emissions model will come along in due course.

How Do You Build a Zero-Heat House?

The thick walls of traditional stone cottages have considerable thermal mass - but externally they're in direct contact with the outside air, so the heat they can store eventually leaks away.

Consider ancient cottages and farmhouses: often very picturesque and charming, they're traditionally built using local stone, with walls that can be 18in thick or more.

All of this stone provides 'thermal mass' – a huge heat store, which helps to stabilise temperatures inside the house, delaying any change in temperature on the outside from reaching the inside. As a result, these buildings are often praised for their comfort - a sudden drop in temperature as winter arrives takes a few days to cool the walls down, delaying the need for internal heating.

Similarly, these old buildings are often prized for their ability to remain cool in summer. A very warm spell of a few days duration doesn't warm the mass of the walls fully, so the interior remains cool. It's like having free air conditioning.

Eventually, of course, these buildings do cool down in winter, because although they're thermally heavy, the outside of the walls are in contact with the external air, so heat flows out of them freely, because there's nothing to stop it.

Now, imagine taking one of these old buildings and wrapping it in a great big thick duvet – not just around the walls, but over the roof, and under the floor too. You've insulated the thermal mass – the heavy stone walls – from the outside environment, and it will take much, much longer for any temperature change to make its way through to the inside, because the mass of the building is no longer in direct contact with the outside air.

In essence that's how the Cropthorne Autonomous house works. 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.

All of this mass is then insulated from the external environment by 375mm (15 inches) of high-performance insulation, meaning the temperature inside the building will take a very long time to follow the temperature outside.

Finally, the outer skin of the house (reclaimed red bricks and lime-rendered board) contributes little to the thermal performance, but is there mainly to protect the structure from the elements, and to provide an attractive external appearance.

The Composting Toilet System

In a conventional house a considerable amount of energy is consumed in providing a water supply and disposing of waste water. Supply, capture and purification, plus pumping through the mains to your premises involve a lot of infrastructure and energy. Most of the water which enters via the mains leaves again via the sewage system, with various things mixed in with it. In particular, we mix two very useful substances: human waste, and clean drinking water, and produce from these a problem substance – sewage – which we then expend energy pumping through miles of pipes to a plant which uses a lot more energy separating it all out again and discharging some of it into nearby rivers. Typically, 30 to 40 percent of the clean drinking water supplied to the average house is flushed straight down the toilet.

The Clivus continuous composting toilet system. The composting chamber will be in the basement, with two 'straight through pans' in the house above, connected by 15-inch stainless steel chutes.

There must be a better way - and in all but the more crowded city and town centres, where central servicing is a more sensible option, the strategy adopted in the Cropthorne house can be used successfully.

We'll be installing a UK-manufactured version of the Clivus Multrum composting toilet. The system has been proven over many years around the globe, and the Kingsley Clivus continuous-composting system produces clean, safe and odour-free garden compost and liquid fertiliser from human waste without using any water or additional energy. It also needs little maintenance.

It consists of a large composting chamber with a sloping floor (see illustration - right) and can accommodate two toilets, which have to be sited directly above the chamber. These toilets have no U-bends, but discharge vertically into the compost chamber via 15-inch diameter stainless-steel chutes. In laying out the house to accommodate this system, we basically had to start with the toilets, and then design the rest of the house around them. This is because of the need to site the toilets directly above the chamber. It also meant the inclusion of a basement - a relatively expensive addition - but we can install a lot of other services there as well, including the tanks for the rainwater-harvesting system.

In use, the toilet is much the same as a conventional one, and despite slight initial misgivings, people get used to them quickly, and don't have any problems at all. There's no flush to pull, and occasionally it's necessary to throw a trowel-full of wood shavings down the pan to provide a bulking material which helps to keep the compost pile healthy. A small fan draws air down the toilets and out through a vent in the chamber. This aids the aerobic digestion process, but also means a gentle down-draught draws any 'bathroom odours' straight down the toilet, making this system virtually odour free. If you install an extractor fan in a conventional toilet, it has to suck smells out of the pan, into the room and past your nose before they're drawn outside.

In the Cropthorne house, we won't be using the dedicated fan shown in the illustration, but instead will be connecting the composter to the mechanical ventilation system, meaning no additional energy use at all, and the recovery of a certain amount of heat from the compost pile which will contribute to keeping the house warm.

Having 'handed over' all human waste to the composting toilet system, the remaining waste produced by the occupants of the house will be nothing more than soapy water, which the Environment Agency has already advised can be discharged straight into a soakaway.

So the Cropthorne house will dispose of all of its own waste on site, requiring no complex infrastructure and with virtually zero overall energy consumption.

The Rainwater-Harvesting System

Demand for water in certain parts of the UK is beginning to outstrip supply. With further housing developments planned, areas like the south-east will be particularly hard pressed, with excessive water abstraction potentially causing serious environmental problems. Also, as noted above, considerable amounts of energy are used in purifying and delivering mains water.

The Cropthorne house is designed to run entirely from
rainwater harvested from the roof, and with careful design there's no reason why many other dwellings couldn't operate in the same way, meaning the amount of water pumped from rivers or underground aquifers could be cut drastically.

The composting-toilet system is absolutely key to the water-saving strategy, as eliminating toilet
flushing immediately leads to water savings of up to 40% compared to a conventionally serviced house. But that alone isn't enough: further steps need to be taken to reduce water usage, and the house needs to have sufficient water storage to cope with extended dry spells.

A 1520L Rotoplas tank. 10 of these will store rainwater, another a slow sand filter, and a twelfth pre-filtered clean water, giving over 16,000L of storage.

Water savings begin with the occupants, who need to be responsible, and not waste water. That doesn't mean terrible inconvenience, it just means having to think a little. Once you're clean, turn the shower off and get out.... Futher savings can be made by careful choice of plumbing fittings, and water frugal appliances such as A++ rated washing machines. With all these measures it's possible to achieve a water usage per head of less than 50L per day, so under 200L per day for our design family of four. Based on the roof area available to collect rainwater, and the average annual rainfall figures for the area, I've specified over 15,000L of water storage, which will supply the water needs of our design family for around 100 days without any rain at all. So there's plenty of capacity.

The rainwater will be stored in the basement, in an array of ten 1520L 'Rotoplas' tanks. They look like giant plastic milk churns, 1.8m high (see image - left) and they arrive in the UK from Israel, full of concentrated orange juice. Once emptied, they're not re-used, so essentially become a 'waste' product, often sold off as giant garden water butts.

Ten of these will be used to store rainwater, piped into the basement from the house and garage roofs. In case of very high rainfall an overflow will allow water to run off straight into the soakaway, to avoid flooding the basement. Water will be pumped from the storage tanks to another Rotoplas container with the top cut off, and housing a slow sand filtration system - a miniature version of that used by many water companies.

The filter system is being designed by Dr. Tim Pettitt at The Eden Project. He's one of the UK's leading authorities on sand filtration, and is confident this filter can provide water of the required quality. He'll be monitoring the system once it's installed and water from it won't be used for drinking until its purity has been verified. No chlorine will be added - it's only necessary in mains water to prevent build-up of bacteria in the miles of pipework between the water works and your home. So no chlorine, and beautifully soft rainwater to wash in.

After the sand filter a twelfth Rotoplas tank will store 1500L of pre-filtered water, after which it will be pumped to a header tank in the loft, and from there to a relatively conventional gravity-fed domestic plumbing system, operating at low pressure. All pipes will be copper, as this is ultimately more reliable, and more recyclable, than plastic.

So again, the Cropthorne house water supply fits in with the aim of supplying all the needs of the house from its immediate surroundings. In dispensing with a mains water supply it avoids the energy use associated with centralised servicing, but also makes the occupants of the house aware of the value of water as the scarce resource it is. In a period of drought, for example, the occupants can regulate their use of water to prevent the tanks running dry, although the volume of stored water makes this extremely unlikely. However, if they were to ignore the warnings and actually run out of water, they alone would be responsible, and they alone would suffer the inconvenience.

With centralised servicing people can side step these responsibilities, such as those who continue to water their gardens with a hosepipe despite a drought and a hosepipe ban being in place. When eventually the situation becomes critical, everyone suffers the inconvenience of stand-pipes in the streets, including the responsible majority who took steps to save water. The government's initiative to massively increase the uptake of water meters should encourage people to think a little more about their water consumption, but the Cropthorne house is already well in advance of that.

A recent satellite image of the Vales' house at Southwell, Nottinghamshire. The photovoltaic panels are clearly visible, mounted on a south-facing pergola in the garden.

How It All Started

In the mid 1990s, I was working as a cameraman and editor in the ITN science unit alongside ITV News science correspondent Lawrence McGinty. One morning I was asked to shoot a story on 'some new kind of house'. One of my cameraman colleagues who was keen on environmental matters had been planning to do this particular assignment, but was otherwise engaged on the day, so I was asked to go instead.

The location I was heading for was Southwell in Nottinghamshire, and on arriving I learned the 'some kind of house' was a then fairly revolutionary design by leading eco-architects Robert and Brenda Vale, which was designed to do as little damage to the environment as possible, and to meet all of its servicing needs from its immediate surroundings.

Robert Vale showed us round the house, starting with the basement where the rainwater storage tanks were located. As the tour continued I became more and more
impressed by the house and all the thought that had gone into it, although I'm not sure I made it obvious at the time.

By the time we left I was well and truly intrigued - what I'd assumed to be a fairly routine assignment had turned out to be something quite fascinating. I didn't realise it at the time but an idea was forming in my mind, which only came to the fore some 12 or more years later, when a series of changes in my circumstances suddenly brought the startling realisation that I could actually consider trying to buy some land and build an 'autonomous house' with my partner Lizzie, for the 21st century.

The related publication 'The New Autonomous House'
How It Is Now

Increasing awareness of
environmental matters in building and other areas could elicit the suggestion that the Vales' house no longer represents the cutting edge. That may be true in some areas, but on looking in greater detail many of the design decisions they made still make a great deal of sense today, and their publication 'The New Autonomous House', is still regarded as an essential text book (and text book it undoubtedly is) by many eco builders.

So the design aims for the Cropthorne House are based on those of the Southwell house, namely:

1. The house should make minimal demands on the environment during construction, its working lifetime, and ultimate demolition/disposal.
2. In making minimal demands in use the house should obtain all its servicing from its immediate surroundings, i.e. it should obtain water, heating, lighting and dispose of its waste all within the land on which it is built.
In fact the proposed Cropthorne house will stand in quite a large area of land, so the aim is to deal with all of the servicing requirements in the area immediately surrounding the house, to demonstrate that a similar system could easily be extended to a house in an urban area with a relatively modest garden. If done well, local servicing of houses with sufficient land can eliminate much of the energy used in water treatment and delivery, plus sewage pumping and cleaning. This model wouldn't work so well in densely populated areas like city centres, where efficient central servicing is a better choice.

How We Hope To Do It

The Southwell house incorporated a number of innovative design features for its time. The main ones are listed below, together with the approach we're taking for the Cropthorne house. Although some 16 years on, many of the basic principles are the same.

The Vales' Southwell house The Cropthorne house
'Super-Insulation' - with no real UK standards to work to at the time, Robert & Brenda Vale used the highest levels of insulation that were reasonably practical. They used triple glazed argon-filled windows from Scandinavia. At the time these were practically unheard of in the UK. Similar aims, but working to the AECB Gold standard, comparable to the German Passivhaus standard, believed by many to be more useful than the current UK Code for Sustainable Homes (CSH). We have a greater choice of windows now, but are aiming for higher performance......
Heavy 'high thermal mass' construction to even out temperature variations in winter and summer - particularly to avoid overheating in hot weather and the consequent need for air conditioning. Much the same philosophy, using more-or-less the same types of materials, but with more insulation and even higher mass. Also detail differences due to external style.
Space Heating  
Broadly designed to be 'zero-heat' i.e. to be so well insulated the heat generated by solar gain through south facing windows, plus the heat output from the occupants and their activities would keep the house at a comfortable temperature even in the depths of winter.
As a backup, a small wood-burning stove was installed, in case winter temperatures became unacceptably cold. In practice this has been used a lot.
Also designed to be 'zero-heat', but unlike the Vales' house ours is optimally oriented, thanks in part to an understanding planning authority. Large areas of south-facing glazing will maximise winter solar heat gain. Experience of living in the house will determine whether or not we need additional winter backup heating.
Water Heating  
Water was heated entirely by an immersion heater in a heavily insulated conventional hot water cylinder. A grid-connected photovoltaic array in the garden contributed some of the power used to heat the water. Water to be heated by solar panels on the south-facing roof which will be used to charge a super-insulated 500L hot water tank. Supplementary backup in winter from a small intelligently managed immersion heater. We're looking into installing a device to reclaim heat from outgoing shower waste water, rather than using it to warm the soakaway.
It's essential not to lose any of your precious winter heat, so airtightness is vital. Think of it as extreme draughtproofing. The Vales aimed for a high level of airtightness in construction. A number of years later when the house was evaluated using modern testing equipment, it was found not to be particularly airtight. We will have to work to the highest possible standards on site, to ensure the Cropthorne house is as airtight as possible. Building regulations now require testing, and if any leaks are found at this stage we'll have to correct them and try again. Tiny leaks lead to massive heat loss so we have to get this right.
Two small ventilation/heat reclamation units in the kitchen and bathroom areas, turned on manually as required, plus trickle vents in the windows which could be opened to increase ventilation but at the expense of some uncontrolled heat loss. A ventilation system servicing every room of the house, and running continuously, with battery power backup. The incoming air travels through a 'ground tube' which warms it in winter and cools it in summer before it enters the house. Additionally, in winter, heat is extracted from the outgoing air and used to warm the incoming air. Our windows won't have trickle vents, but they will be openable.
Waste Treatment  
The Vales installed a Clivus Multrum dry composting toilet system. Some people might baulk at the idea but it's proven to be reliable, hygienic and odour free since 1993. No water is required for flushing - up to 40% of the water used in a conventional house is flushed down toilets. Eliminating this waste means enough rainwater can be harvested from the roof to meet the entire water needs of the house. The waste water from sinks and showers contains no sewage so can be fed into a simple soakaway. We're installing a UK manufactured version of the Clivus Multrum. Kingsley Clivus of Winkleigh, Devon have built a special composting chamber with enhanced filtration so it can be connected to the mechanical ventilation system.
As the composting toilet will deal with all human waste, we can use a simple soakaway system for grey water, meaning the house won't need to be connected to mains water or sewage.
Water Supply  
All rainwater from the roof passed into storage tanks in the basement, filtered and used to supply the house via a 12-volt pumping system. Storage provided using 20 recycled orange-juice transport containers, which are normally only used once then disposed of. Again, rainwater will be stored in recycled orange juice bulk transport containers in the basement, but based on calculations for our house & the Vales' practical experience, we will only be using 12 of them. A similar pumping system is envisaged, with a slow sand filtration system, designed by Dr. Tim Pettitt at The Eden Project.
Electricity Supply  
Used the UKs first grid-connected solar photovoltaic system which exports power to the national grid when excess is generated. Again, a grid connected solar photovoltaic system which will be our only connection to an external service. We aim to generate on average more power than we use - this may involve the later installation of a wind turbine, but at the moment this is a very low priority.

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