Cropthorne Autonomous House - Notes on the Design
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....
Return to the HomepageGo
to the Project Diary
from the Plans for The Cropthorne Autonomous House
drawings shown on this site are © Neill Lewis, chartered architect.
(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
It should not be lavish, but 'sufficient'.
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
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
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
is to generate more power than
is used, so to produce net zero carbon-dioxide emissions.
The Site Plan
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
How Do You Build a Zero-Heat House?
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
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
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.
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
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
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
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
The Rainwater-Harvesting System
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
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
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.
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.
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,
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.
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.
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.
It Is Now
related publication 'The New Autonomous House'
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.
We Hope To Do It
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.
Vales' Southwell 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.
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.
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.
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.
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.
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
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
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.
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.
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.
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.
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.
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
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.
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.
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
the UKs first grid-connected solar photovoltaic system which exports
power to the national grid when excess is generated.
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.