|Principles of eco-design
From all the many theories behind
environmental design, this section pulls out just five core ideas. They
are all important, and they provide the tools for approaching most design
challenges. There are also some excellent books that talk in far more
detail about these concepts (see
In this section:
Working with the sun
Six principles for making priorities
OF ECO-DESIGN: 1. WORKING WITH THE SUN
The contribution of the sun to a houses internal heat is called
the solar gain. A fundamental principle of solar design is that it aims
to maximise the solar gain in the winter and minimise it in the summer.
To achieve this solar design combines three strategies- glazing, orientation,
and thermal mass.
Controlled glazing is the vital component
of environmental design. Glass allows 90% or more of the energy in the
suns rays to pass through and then traps the resulting heat. Glass can
let in indirect daylight (so we can avoid using electric lights) whilst
keeping out draughts.
However, there are problems with glass
too. It is a very poor insulator - a sheet of glass has about the same
insulating power as a sheet of hardboard. Double glazing is twice as good
(or half as bad) because the small air gap between the sheets of glass
is a good insulator. Even so, double glazing still only has the insulating
power of a single layer of bricks. As long as the sun is shining through
the glass, it is receiving more energy from the sun than it is losing
through heat loss. When the sun sets the glass find it hard to hold in
that heat and starts undoing all its gains.
So the use of glass is a juggling act
- having enough glass to benefit from the free heat of the sun, enough
glass to let in plenty of daylight, but not so much that the house overheats
during sunny days and freezes at night. There are two keys to this problem-
orientation and thermal mass. Orientation first:
OF ECO-DESIGN: Orientation
Orientation refers to the location of a house and direction to which a
house points. Orientation is crucial for determining the amount of sun
a house receives, because the direction and height of the sun in high
Northern latitudes like Britain changes dramatically throughout the year.
On the 21st December, the shortest day of the year, the sun can barely
be bothered to get up at all. It rises at 8.30 in the South East, drags
itself barely 12 degrees above the horizon in the South at noon, and sinks
exhausted at 3.30 in the South West. On the 21st June, in contrast, it
bursts out of bed at 4.45am. By 6am it is already higher than it was at
noon in December, and it keeps rising higher until by noon it is over
60° in the sky. The day is also correspondingly longer- 17 hours.
From these seasonal changes we can draw three conclusions that effect
all solar design.
1. Only surfaces facing South receive
sun all year round. The dominant direction of the sun is from the South,
especially in winter. For this reason, solar panels and windows that
will capture solar warming in winter, should face as close to South
as possible. Solar receipts start to fall noticeably outside the band
South-South-East to South-South-West. Surfaces facing South-East or
South-West receive 10% less solar energy during the year than surfaces
facing due South.
2. Surfaces facing North are in the
shade all year round. Whilst most sun is received in the arc South East
to South West, the adjacent quadrant, North-East to North-West, receives
very little sun except at the peak of summer. For this reason solar
design concentrates insulation and minimises glazing on this side of
a house. New build solar houses often bury this side of the house in
the ground and put all glazing on the South side of the house.
3. The winter sun is low, the summer
sun is high. Vertical South facing windows work best for maximising
solar heating in the winter as they capture the low winter sun. Some
solar houses even have their windows angled 11 degrees back from the
vertical to perfectly face the winter sun.
For the same reasons, surfaces that are
more horizontal receive most of their sun in the summer. This is a problem
for glazed roofs and skylights which receive little sun in winter but
may well overheat rooms in summer. They usually need special shading or
air vents (more on the design of roofs for porches
and conservatories). The angle which receives the most sun
across the year is, not surprisingly, 45°. For this reason it makes
excellent sense to mount solar panels flush with a pitched roof.
The high summer sun is a blessing when
if comes to designing shading for vertical windows. Only a small overhang
is needed to completely shade a vertical South facing windows in summer.
This is another strong argument for maximising South facing glazing. East
and West facing windows can be a huge nuisance in summer. They receive
no direct sun in winter, but they fully face the low evening and morning
sun in summer. West facing rooms are particularly prone to overheating
and will need curtains or shades (more on curtains
We can sum up these principles to say:
a well functioning eco house will have as much of its glazing as possible
in vertical windows facing between South East to South West, and as few
windows as possible facing North East through to North West (see Orientation).
OF ECO-DESIGN: Solar principles in existing
Of course, very few people have the opportunity to build a new eco-house
and apply these principles fully. We are stuck with the houses weve
already got, which meet solar principles by accident rather than design.
However, there are still ways that we can apply the solar principles to
1. When buying
a house. An unshaded South facing roof is needed for solar panels.
The ideal house is one that faces North in the front and has unshaded
South aspect at the back- this opens up all kind of creative options
for conservatories and solar extensions on the private rear side of
the house. Solar panels can be attached to the roof without affecting
its appearance on the street side. A South facing front is less useful
but it is still possible to build a good sun porch. Be careful to avoid
South facing houses with large areas of glazing on the North side.
2. When building
extensions, porches and conservatories. There is a lot more design
flexibility when building new structures, which can fully apply solar
design principles. Whether a conservatory is an asset or a drain on
the houses energy depends entirely on its design and orientation.
(more on the design of roofs for porches
3. When fitting
new windows. New windows should be positioned according to solar
principles. On the North side of the house, windows should have a lower
glazed area and preference given to skylights to bring in as much daylight
as possible. On the South side, vertical windows should have as large
an area of glazing as possible. Skylights on the South side should be
When fitting new windows there is a
rare opportunity to make changes to overall window size and shape. Reducing
the size or blocking in a window is relatively easy, though careful
attention needs to be made to insulation (see cold
bridges). It may be possible to achieve energy savings as
large by replacing the existing window with high performance windows
(more on windows...).
It is a relatively easy matter to expand a window downwards by knocking
out the wall under the existing window.
OF ECO-DESIGN: 2. THERMAL MASS
The thermal mass of the house is a measure of its capacity to store and
regulate internal heat. Buildings with a high thermal mass take a long
time to heat up but also take a long time to cool down. As a result they
have a very steady internal temperature. This is sometimes called the
thermal flywheel effect because, like a flywheel, the thermal mass can
store and even out fluctuations in temperature. Buildings with a low thermal
mass are very responsive to changes in internal temperature- they heat
up very quickly but they also cool down quickly. They are often subject
to wide variables in internal temperature.
Everything inside the house contributes
to its thermal mass according to its capacity to absorb and store heat,
known as its 'thermal capacity'. The best materials for storing heat are
those that are very dense, heat up slowly, and then give out that heat
gradually. Brick, concrete and stone have a high thermal capacity and
are the main contributors to the thermal mass of a house. Water has a
very high thermal capacity, so it is well suited to central heating systems.
Air has a very low thermal capacity- it warms up fast but cannot stay
warm for long. Only when the walls and floors in a building have warmed
up will the air stay warm.
Eco-buildings are usually designed to
have a high thermal mass for several reasons:
1. To hold over daytime solar gain
for night time heating. A high thermal mass balances out the fluctuations
in temperature that come from solar gain, soaking the extra daytime
heat into the body of the building and releasing it slowly at night.
The flywheel effect is most pronounced when the suns rays hit a wall
or floor with high thermal capacity. For this reason eco-buildings are
often designed with a dark coloured solid masonry wall and solid floor
behind South facing windows.
2. To keep houses cool during the day
in summer. A high thermal mass will reduce fluctuations in internal
temperature during the summer. If a house has good ventilation during
the night, its thermal mass can be cooled and it can then maintain that
cool interior through the heat of the following day. In the extreme
case of desert regions where daily temperatures can vary by up to 40°,
traditional houses are usually designed to have extremely thick walls
to moderate the internal temperature.
3. Increase the efficiency of a central
heating boiler. A high thermal mass favours small boilers working at
maximum efficiency, slowly and steadily raising the temperature of the
building and then turning themselves off for sustained periods. Buildings
with a low thermal mass, by comparison, tend to have much wider fluctations
in temperature, and the boiler is constantly switching on and off to
PRINCIPLES OF ECO-DESIGN:
Thermal mass in existing houses
The positioning of exterior wall insulation can affect the thermal capacity
of a house significantly. Only the building mass on the inside of the
insulated envelope functions as a heat store. When cavity insulation is
installed the thermal mass of a wall is halved, though the impact this
has on overall on performance is slight. The impacts become more significant
for internal insulation which removes any useful thermal capacity from
Thermal mass can be increased when there is new building.
In an extension, for example, thermal capacity can be increased by pouring
a concrete slab over insulation and by finishing the floor with stone
paving or tiles rather than wood or carpet. This makes particular sense
in situations where sunlight is directly shining on the floor. The thermal
capacity of a wall can be increased by painting it a dark colour. This
is standard practice in eco-buildings and can easily be replicated on
walls opposite South facing windows. Internal walls behind radiators can
be encouraged to store heat by painting the back of the radiator and the
wall behind it a dark colour (black is best).
OF ECO-DESIGN: 3. STACK EFFECT
When air warms it expands, becomes less dense than the surrounding air,
and rises. This process is called convection and is the main process by
which heat moves around a room and the house. When rooms are sealed, convection
is a sealed circuit of hot air rising over radiators and then sinking
as it cools to be heated again.
In reality, though, we dont want
rooms or houses to be fully sealed; ventilation with fresh air is vital
in a healthy house, and convection plays a leading role in natural ventilation.
Hot air rises and escapes through small gaps in the building fabric at
the top of the house. As it does so it draws in new cold air through similar
gaps at the bottom of the house. The powerful suction created by escaping
warm air is called the stack effect, or sometimes the chimney effect because
it is the same process that draws smoke up a chimney or smokestack.
Like many other environmental principles,
the stack effect can either be a problem or an opportunity. It is the
main motor generating draughts and the loss of hot air; and often the
largest single cause of heat loss in a home. It requires particular attention
when draughtproofing a house. However, when carefully controlled it can
produce a low and effective level of natural ventilation. If respected
and built into the house design, the stack effect is by far the most effective
way of keeping a house ventilated in summer. Over the past ten years environmental
building has paid increasing attention to generating a stack effect to
create natural ventilation, especially in large buildings. In a typical
design tall chimneys at the top of the building create a powerful draw
and fresh air is pulled into the building through specially placed controllable
vents around the outside wall.
OF ECO-DESIGN: Stack effect in existing
In existing houses particular attention needs to be paid to the
role of the stack effect in creating draughts and heat loss. A thorough
draughtproofing must pay particular attention to places at the top of
the house where hot air can escape (more on draughtproofing..).
The stack effect can be utilised in any
house to support ventilation and summer cooling by providing easy routes
for air (vents vetween rooms, doors open during the day) and providing
adjustable vents high in the house (more on using stack
OF ECO-DESIGN: 4. THERMAL ZONING
We like different temperatures in different rooms- we like bathrooms to
be very warm, living rooms to be a comfortable cosy temperature, and bedrooms
to be cooler. An efficient eco-house recognises these differences and
creates different thermal zones for the different rooms.
||Bathrooms, airing cupboards, rooms for drying
||Living rooms, study, children's bedrooms
||Rooms that are not in use, storage rooms, garage,
These are approximate figures for living temperatures. There
are very great energy savings to be made from cutting just a couple of
degrees off these temperatures. We live at the low end of these ranges
and find that the maximum temperature we need is 19 degrees.
Thermal zoning tries to ensure the best
match possible between the distribution of rooms and the distribution
of the available heat. The ideal thermal zoning is:
The ideal place for hot rooms is in the very centre of the house, with
no external walls so that the heat can radiate out into the rest of
the house. The next best option is with a South facing window (which
may be better for a bathroom where natural lighting and good ventilation
The main living rooms need constant warmth and light and are best placed
on the South side of the house with large windows and good thermal capacity
to hold any thermal gain through the evening. Kitchens can generate
a great deal of heat. The ideal location for a kitchen is therefore
facing into the centre of the house, with the cooker placed on an internal
Adults tend to make little use of bedrooms except for sleeping and do
not need to be especially warm. In a well insulated house a large part
of their heating can be supplied by warmth rising from a well heated
room below. Adult bedrooms can be placed on the cooler side of the house.
However, they need good light and an easterly window or skylight is
Little used rooms are best located along the colder and darker North
side of the house. Rooms that need to be heated occasionally, such as
a spare bedroom, perform better with a low thermal capacity with insulation
on the inside of the room. Storage rooms need to be kept dry but heat,
light, and thermal capacity are of little concern, though the preference
is for constant cool temperatures. Basements and rooms under the stairs,
if dry, are perfect for storage. Durable items can be stored outside
the insulated envelope altogether- such as under the eaves or in unheated
sheds. In both cases the main concern is keeping them dry.
OF ECO-DESIGN: Thermal
zoning in an existing house
Thermal zoning is an important consideration in newly designed environmental
houses. However, in an existing house the configuration of rooms was decided
by the original architect or builder. One has to adapt to the existing
layout, deciding which rooms are used, and which bedrooms are used for
adults, and children.
There is more opportunity for applying
thermal zoning for changes in room use or new building. In the Yellow
House, thermal zoning was a major consideration in the decision to place
the new bathroom, the utility/drying room and the kitchen at the centre
of the house (more on the design
OF ECO-DESIGN: 5. EMBODIED ENERGY
If we want to reduce the total environmental impact of a building, we
must consider the impact of the materials that have gone into its construction.
Clearly no house can claim to be an eco-house if it is constructed from
materials that had a major environmental impact elsewhere.
For these reasons, the concept of embodied
energy is central to good environmental design. The embodied energy of
a building material is the energy that has been required to extract, process,
and manufacture it and then to transport it to the building site. The
embodied energy in the structure of a new house is considerable, exceeding
the total energy required to heat that house for the next 20 years.
In terms of the energy of manufacture,
the highest embodied energy is found in metals (steel requires 57,000kWh
to produce one cubic metre), and highly processed industrial products
(hardboard and MDF require 2,000 kWh to produce 1m3). The middle range
of materials are simpler to make but require a lot energy in their manufacture
(bricks and concrete blocks need 700kWh/m3). The lowest embodied energy
is in materials that require only simple processing (building timber needs
180kWh/m3 ) or those made from salvaged materials or local natural materials,
which require virtually no energy.
The issue of embodied energy divides
new eco-buildings into two distinct families. One kind of eco-building
aims to obtain the lowest possible energy consumption with the most efficient
available technology, such as high performance insulation and solar panels.
Such buildings have a high embodied energy which they hope to justify
with large savings in their energy consumption, or even to generate and
export a surplus of energy. The other kind of eco-building aims to achieve
the lowest possible embodied energy by using salvaged materials or simple
local materials (straw bales, rush matting, mud bricks). Such buildings
will usually perform less well in terms of annual energy consumption,
and are less durable, but often have a lower overall environmental impact
over the course of their lifespan.
Embodied energy is usually a good guide
to wider environmental impacts, especially for toxic waste and atmospheric
pollution. There are two main exceptions to this:
Cement and Concete, which has
a mid range emobodied energy, but a disproportionately high impact on
climate change. When limestone is burnt to make lime it releases an equal
weight in carbon dioxide. Taken as a whole, the cement industry produces
5% of the worlds human carbon dioxide emissions.
Timber has a low embodied energy,
but can have a very high environmental impact if taken from old growth
forests. More on old
OF ECO-DESIGN: Embodied energy in an existing
Renovating an existing house will always use less energy than building
a new house. Even if a new house is extremely energy efficient, it will
be many years before it can pay off the energy embodied in its structure.
An existing house, by contrast, will only have to justify the embodied
energy of the materials used for the renovation. There are plenty of ways
that the energy cost of renovations can be reduced still further.
ECO-DESIGN: SIX PRINCIPLES FOR MAKING PRIORITIES
materials that have the highest embodied energy. Glu-lam laminated
wood beams will perform as well as RSJ steel beams with a tenth of the
embodied energy. At a pinch concrete lintels are better than steel.
Hardboard, MDF and chipboard are bonded with formaldehyde and are best
avoided altogether and replaced with real wood, if grown sustainably.
materials. Salvaged materials effectively have no embodied energy
other than transport and should be used whenever possible. They can
be obtained from local demolition sites or council dumps. Most salvage
yards sell timber, tiles, bricks and slates (more on salvage...).
raw materials in any new building. For new building (such as
extensions), use local materials such as local stone, straw bales, mud
bricks, etc and prepare them on the site. The Centre
for Alternative Technology has many books on these eco-build techniques.
Text books on ecological building are full of illustrations of mud brick
and adobe houses, straw bale walls, insulation from old newspapers, compost
toilets. All well and good, but in the real world we are faced with a set
of external considerations that temper such fantasies: can I afford it;
will it pass building control; will anyone ever want to buy it? So, a sense
of proportion is vital. Here are six principles that can guide all your
THE BEST MATERIALS ARE RE-USED
ENERGY CONSERVATION HAS PRIORITY IN THE ECO-HOUSE
ASPIRE TO SELF SUFFICIENCY
LIVE LIKE GRANNY!
IF YOU DO IT NEW, DO IT WELL
RESPECT THE ECO-HOUSE NO-NOS
OF ECO-DESIGN: PRINCIPLE
ONE: The best materials are re-used
We must be clear that so called "environmental" materials are not actually
good for the environment, they are merely less damaging then non-environmental
materials. So, re-used materials will almost always be the best option,
even when they are materials that would otherwise be seen as a poor environmental
choice (lead, PVC, rainforest plywood).
There are three inarguable reasons for
re-use. Re-use avoids the hidden environmental costs of materials including
transport; it reduces waste (building waste is the main content of landfills
yet recycling schemes concentrate on domestic waste) and it can be substantially
cheaper than buying new materials (more on salvage...).
The only reservation about reuse concerns
products that directly consume energy, especially heavy energy consumers
such as boilers, cookers and fridges. There is a point at which it is
better to replace an old inefficient product with an energy efficient
new one. If they are old, and a more efficient product exists, replace
them (more on appliances...)
OF ECO-DESIGN: PRINCIPLE
TWO: Energy conservation has highest priority in the eco-house
There are many ways we can reduce our environmental impact. However, we
should be absolutely clear- the greatest environmental impact of the average
house is from the fossil fuels it burns for its energy. No amount of eco-certified
bamboo flooring or low phosphate washing powder can ever compensate for
the impact of climate a gas guzzling house. So, when there is a limited
budget and a conflict of interest, prioritise energy conservation. And,
there is the added bonus: saving energy saves money.
OF ECO-DESIGN: PRINCIPLE THREE: The eco-house
aspires to self sufficiency
The more that a house can meet its own needs, the less of a demand it
is making on the wider environment. Examples of self sufficiency technologies
include: solar space and water heating; using waste grey water and rain
water; saving and reusing waste heat; electricity generation from windmills
and solar panels; food production from the garden or local allotment.
A house that reduces its consumption is a highly efficient low-cost house,
but a house nonetheless. It is when the house starts meeting its own needs
that it becomes a true eco-house.
OF ECO-DESIGN: PRINCIPLE FOUR: Live like
Our grandparents were brought up in a world in which electricity was expensive
and not to be taken for granted; cars were a luxury that virtually no
one could afford; all bottles and jars were recycled; all food was organic;
local communities were strong and crime rates were lower than we can imagine.
We could do a lot worse than living like our grandparents- mending our
clothes before we throw them out, turning off lights when we leave the
room; getting a "work-out" on the allotment, riding a bicycle to work,
enjoying simple pleasures.
OF ECO-DESIGN: PRINCIPLE FIVE: If you do
it new, do it well
This is another Granny principle - if its worth doing, its
worth doing well. When you do work on the house, spend the extra to do
it to the highest possible standard of craftsmanship. If you buy a new
appliance or boiler, buy the best you can afford. If you cant afford
to do it well, hold off until you can. Shoddy work and cheap products
are always a false economy and a waste of resources.
OF ECO-DESIGN: PRINCIPLE
SIX: Respect the eco-house no-nos
There are some things that fail so many environmental
criteria that they can be declared out of bounds for any eco-house. They
Rainforest and timber from intact ancient forests
Large single glazed windows on North facing sides
Heated conservatories or garages
Gas patio heaters
Curtains draped over radiators
Electric clothes dryers
Heated out-door swimming pools and hot tubs
Incandescent light bulbs