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Eco-extended 1920 brick-built Semi
The original house
Semi-detached, three bedrooms (one tiny), built with solid brick walls in 1927. Central heating from a very old gas ‘back burner’. The previous owners had fitted uPVC double glazing throughout and added a small extension at rear for an enlarged kitchen.

At the foot of a hill on the edge of Cowley Marsh, the site does not lend itself to wind turbines. The location is within walking distance of all basic local facilities and very good for bus links to the city centre. We use bus and bicycles for virtually all journeys within Oxford, with occasional use of a Commonwheels car. We have enough garden for chickens, which produce fresh eggs every day, and to grow some fresh vegetables.

Energy use before renovating
We have lived here from August 2007, sometimes with lodgers, while gathering a sense of how best to improve it. All our electricity (and gas) bills are based on estimates which often bear little relation to our actual electricity use. Based on bills for first half of 2008 we can only give a very rough approximation of likely our annual energy use before renovating:

Annual use

Electricity    1,400 kWh
Gas    11,000 kWh

According to Energy Watch the average UK household uses 4,000kWh of electricity per year (based on a 2 bedroom house).  One person’s story of reducing electricity usage in a house shared by 2 people  was that he got it down to just over 1,000kWh per year. So our usage was already relatively low. We used electricity for lighting and running fridge, freezer (switched off some of the time), washing machine and computer. We cooked on a basic gas oven that did not use electricity.
Energy Watch estimates the average household uses 15,000kWh of gas per year (2 bedroom house) so again our annual usage was lower than average even with very inefficient gas space heating.
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This may be due to the installation in late January 2008 of a wood burner which provided much of our winter heating needs.

Design purpose:
•    To significantly reduce carbon emissions. We hope the UK achieves at least
      80% reduction in CO2 emissions within our lifetime: to substantially reduce
      the energy needs of people living in this house is part of our contribution to

•    To increase the size (From 80 to 120m2) and re-arrange to allow more
      adults to share comfortably. The biggest change to overall carbon
      emissions will come from sharing the house’s footprint amongst four or five
      rather than two or three. (Additionally, the global population, and that of
      central Oxfordshire, is likely to grow enormously in the next few decades.
      The density of population will need to increase dramatically in existing
      residential areas, or lots of agricultural land will be lost.)

•    To do so within a limited budget, retaining the conventional appearance of
      the front exterior, and mostly using products readily available. We think
      these sorts of measures need to be mainstream, not marginal, and have
      attempted to approach it in this way.

Design features
•    Adding loft conversion, with lots of insulation and designed to maximise
      solar gain – a large SE-facing bedroom window - with additional bathroom.

•    Adding ground floor extensions to side and rear, providing:
•     Porch to create an ‘airlock’ at the main entrance reducing heat loss when
      people enter and leave and making space for hats and boots and  raincoats

•     Large pantry, well ventilated and on the north side of house, to provide
      ample cool food storage reducing the need for refrigeration; hopefully filled
       next autumn with sacks of potatoes and strings of onions.

•     Utility room, to liberate living space and provide an ’airlock’ to the back
      door; fitted with re-used kitchen units, and ceiling-mounted laundry drying

•     A large living/dining space in addition to existing lounge, with significant
       solar gain (with large silver birch to south offering summertime shading)
       and a shared kitchen with new efficient appliances (small fridge,
       dishwasher) but no freezer.

•    Thermal Insulation: All the new construction is done to much higher
      standards of thermal efficiency than building regulations require, especially
      around spaces that will be kept at a comfortable ambient temperature.
      Existing external walls are lined with insulation.  

•    Acoustic insulation to party walls (using some rigid foam, and some dense
      mineral wool) and between floors (mineral wool), additional layer of plaster
      board on ceilings, acoustic matting under woollen carpets)

We retained the front living room and two of the existing bedrooms with additional thermal insulation added to the inside, leaving the appearance of the front of the house unchanged.

(This includes a few measures not completed by 11 September2009)

•    Ground floor, where new, is solid concrete slab with 100mm of rigid foam
      insulation, with underfloor heating in kitchen/living/dining area, solar gain
      from the south and thick terracotta tile flooring to increase thermal mass.

•    (The front of the house is suspended timber floors, to be insulated at some
      later date.)

•    New walls along the northern side of the house (ie utility, pantry and porch
      that will have no dedicated heating) have a cavity of 100mm filled with
      mineral wool insulation; in the new walls at the back of the house (SE
      facing, main living/dining area that will be occupied much more and heated
      with underfloor heating) the cavity is 150mm, with 100mm rigid foam

•    Existing external walls are lined with rigid foam and plasterboard – 25 to 50
     mm of insulation, depending on location and practicality (as general rule,
     bathrooms and bedrooms have maximum insulation, the stairwell has less)

•    Existing party walls have also been lined with 50mm of insulation, to reduce
      noise transmission.

•    Intermediate floor voids are filled with mineral wool to reduce sound
      transmission between rooms.

•    The main roof was stripped: the original clay tiles re-used, over a new
      breather membrane and battens; 50mm rigid foam insulation between the
      rafters (100mm deep) and multi-foil insulation beneath rafters; then
      battens and plasterboard.

•    Large south-facing box dormer added (providing a flat roof that could carry
      solar water heating panels when funds allow).

•    New roof to rear extension similar except slightly deeper rafters, and
      multi-foil over rather than under rafters.

•    Windows specified to be timber, with U value of better than 1.5; (Actual
      u-values not yet provided by supplier). The rooflights in the main room are
      triple-glazed, with U-value of 1.1 and the other rooflights (loft and utility
      room) have u-value of 1.3.

•    We re-used doors as much as possible (buying new pine ones where
      regulations required fire doors). We customised the exterior door to utility
      room with a 10mm layer of insulation within the lower panel, and
      double-glazing above; and the pantry door also has a 10mm layer of
      insulation added within it.

•    Redundant chimney construction removed from ground and first floor rear
      rooms. Tops capped with ‘bonnets’.

•    The existing bathroom was displaced by the new staircase to loft, so the
      original tiny third bedroom was partitioned to create shower room, and hot
      water tank/airing cupboard

•    Large airing cupboard with 210 litre hot water cylinder and pipework.  The
     cylinder has two coils, suitable for use with solar water heating

•    Additional bathroom created in loft, to make it easier for five people sharing
      the house.

•    Ground floor wc (unheated) is lined with ‘Sempatap’ insulation as an earlier
      DIY experiment. There is minimal space and comparatively low performance
      required.  It gives a poor finish requiring lining paper and re-decoration.

•    Front living room heated solely by woodburner. This requires a couple of
      hours to be fully effective, but is then capable of far more heat than
      required to heat that room, and so we tend to leave the door open to
      allow heat to percolate through rest of house.

•    New condensing gas boiler feeds radiators in bedrooms, towel-rails in
     bathrooms and underfloor heating in kitchen/dining area (which we expect
     to be the most-used living space)

•    New kitchen fitted with energy-efficient appliances (and no freezer).
•    The pantry has an insulated draught-proofed door and low-tech manually
      controllable air vents, on the cool side of the house.

•    The utility room is fitted with the old kitchen cupboards, and a ceiling hung
      clothes drier.
•    Self-built garden workshop and office. Totally wooden -  cedar shingles on
      the roof, UK-sourced cedar cladding on the walls. Floors made of scaffold
      boards; otherwise all framing, windows, doors etc are reclaimed materials,
      including a complete garden shed incorporated as one wing. Insulated with
      hempwool. Invaluable workspace and storage while house in chaos.

•    Eco-paints used throughout – lovely to work with, no vapour, great


We found an architect and a builder sympathetic to what we were doing, but neither were as experienced or knowledgeable about thermal or acoustic insulation as we would have liked. We have had to assert the importance of this repeatedly.  None of the subcontractors or labourers were as alert as we would like to cold-bridging.  Both design and execution have led to more cold bridging and leakage than we would like; reliance on steel beams for structural strength (required by the structural engineer) was particularly regretful.  Fortunately, the steel beam in the rear room could be placed completely within the room so it is included in the insulation envelope. 

It was horribly difficult and time-consuming researching different materials and goods and assessing their comparative merits and expense.

We employed others to do the work, and attempted to foresee and plan fully, but lots of details had to be thought about throughout the build and our most skilled householder was on site every day also carrying out parts of the work.

Even reusing and reclaiming as much as possible (doors, floorboards, sundry joists and rafters and about 1000 bricks – ‘imperials’, still available…) the project produced far more debris than one might expect: clay dug from foundations, rubble from removal of old extension; old plaster from walls and ceilings, bags, scraps, painted & wormy wood, packaging. It has been piled five foot high in the front garden several times over.

Every stage took longer than anticipated, and often cost more.  We are looking forward to finishing.


By my (inexpert) calculations, heat loss through the fabric of the house will be halved, (Specific Heat Loss for fabric of the house was 300, and when work is complete will be 150) and the building will comfortably accommodate 4 or 5 people rather than 2 or 3, so the per capita emissions will have been cut by about 75%. I’ve not attempted to quantify solar gain, and we don’t know yet where the rainwater butts will fit… )

This was all done whilst retaining the conventional appearance of the front of the house; using mainstream building supplies, and within a tight budget. 

We decided that embodied energy is far less significant than lifetime energy use of the dwelling and the existing aesthetic did not lend itself to strawbale, timber frame, cob etc.

We learnt (again, for my part) not to attempt to renovate a house while living in it!

To be done:

September 2009 – still finishing off the main project, to be followed by decoration.
Soon - a bicycle shelter in the front garden

Another year - Install solar water-heating, insulate under front half of house, assess remaining heat leakage.

Main products and suppliers

•    Rigid foam insulation(polyisocyanurate/urethane/phenolic?  50mm boards for
      roof, 100mm boards for rear wall cavity and under floor, 25mm and 50mm
      thick, backed onto plasterboard, for existing walls,  by Kingspan and
      Celotex, supplied through Buildbase and through Minster Insulation (Didcot

•    Sempatap insulation (10mm thick, flexible rubber roll), through MGC Ltd, by

•    Triso super 10 insulation to line all roofs.
•    Mineral wool batts 100mm for cavity walls to side extension.
•    Mineral wool rolls 100mm for floor voids and stud walling.
•    Isonat hemp insulation for shed from ‘Green Building Store’.

•    Magnet supplied stairs, windows, velux rooflights (triple glazed ones took
     two weeks longer than normal to arrive) and fully glazed external doors.

•    Howdens supplied utility room external door and internal solid pine doors
     (which we discovered on delivery were made in South Africa) .

•    Buildbase were used for most basic supplies, and for Lindab guttering.
•    Wickes was where we found cork tiles (laid on bathroom floors)
•    B&Q are a short walk away and so were very handy for small items
•    Tileright in Kidlington sourced the terracotta floor tiles

•    MagmaTech provided excellent service, with very prompt delivery, of cavity
     ties for brickwork. These were one of the few specialised products we used
     – ceramic rather than steel, reducing the cold-bridging that conventional
     ties create

•    Eiko and Biopin paint, emulsion and eggshell. Easy to use, very little odour,

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Design - AHG