Building a greener future


In December 2006, the Department for Communities issued a consultation paper titled Building a Greener Future: Towards Zero Carbon Development. It included proposals to tighten Part L of the current Building Regulations so that from 2016 new homes would emit zero carbon, with the ambitious proposal that all new non-dwelling buildings will emit zero carbon by 2019. This was mainly introduced in order to reach the Governments targets to reduce emissions as a result from human actions. These targets have to be reached by 2050 as these targets are legally binding under the Climate Change Act. "The UK's 21 million dwellings are responsible for 27% of CO2 emissions, consume half of water supplies and produce 8% of waste" (Communities and Local Government, 2008) With more than 200,000 houses planned to be built every year to house a growing population, 'sustainable homes' seem to be the only answer. The complex energy saving technology employed in such houses often claim to be able to perform against the Government's targets, however sometimes they are unable to perform due to their complexity, constantly changing markets and compatibility with other technologies.

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Established in 1975, the Stewart Milne Group is one of the UK's largest independent homebuilders which is renowned for its innovation of timber system technologies. In response to the Government's aspirations, the Stewart Milne Group designed the Sigma Home. It has been constructed as a three storey townhouse, arranged over four floors to minimise the buildings footprint whilst still maximising living accommodation. It has an open plan layout which makes effective use of natural light and also allows for the building to be flexible in offering various layouts from family home, house share or even office space. The Sigma Home took just 8 weeks (compared to the normal 24 weeks) to build due to the use of offsite technologies including the closed timber frame walls and the pre-fabricated bathroom pods. In order to maintain the compliance for Level 5, the Sigma Home offers triple glazed windows, low energy appliances and micro renewable technology. Water saving devices have been used throughout to achieve the 80 litres of water usage per person per day required for Code 5. Fabric insulation at 0.15w/m2k provided by the wall, floor and roof elements with high performance timber windows and an air tightness range of 1-3m3/m2/h means that the construction is "ten times better than current Building Regulations requirements" (Stewart Milne Group, n.d.) A solar chimney on the roof positioned above the staircase optimises the passive ventilation by using the stack effect in the stair case enabling air to be drawn throughout the home for cooling. Lighting, hot water and space heating are provided by solar thermal, photovoltaic roof panels and roof mounted wind turbines.

In December 2007, the Stewart Milne Group commissioned the Oxford Institute for Sustainable Development and Architecture (OISDA) to evaluate and monitor the Sigma Home whilst it was occupied by a family of four. They occupied the house for four two week periods, one in each season of the year. The home was lived in as normal as possible with the family partaking in typical daily activities. The occupants experiences were recorded using several research methods including; videos, interviews, log sheets and thermal comfort surveys. OISDA monitored the temperature, humidity, indoor air quality, energy, gas and water consumption and window/door openings. This meant that a better understanding of how occupants respond to low energy appliances and complex energy saving technology could be ascertained and used for future research and design.

The energy performance of the Sigma Home is an improvement upon conventional houses, but failed to achieve the predicted energy performance set out in the design stages of the project. It was suggested that improvements need to be made in the air tightness and the fabric of the construction through heat retention in order to improve on the energy performance. Thermal images were produced during the test on the property whilst the family were living there. These revealed that the "heat loss parameter was approx 40% worse than the predicted design stage assessment" (Stewart Milne Group, 2009.) As the building is open plan it is difficult to lock air into particular rooms and with warm air rising, heat would inevitably be lost through window and door fittings and the roof. One solution would be to improve the air tight seals around window and door fittings as well as installing an air membrane in the roof to reduce heat loss.

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However, the property does maximise the use of solar gain by employing large areas of glazing, in particular on the ground floor at the front of the building. But, by having so much glass in the building conflicts with interests of privacy as one of the living areas is at the front of the property. The glass is triple glazed; it is unlikely that solar gain would influence the temperature within the building. This technology means that artificial lighting is unlikely to be used during daylight hours, however would only be really effective if the property was south facing. The Sigma Home is east facing so would not receive the maximum amount of daylight all day. When artificial lighting is required in the building, low energy light bulbs are used throughout. However, in some areas of the property these bulbs are not appropriate. They are generally slow to respond and in rooms where there is little solar gain such as the bathroom, by the time the bulbs have responded and are able to emit sufficient light, the user has most probably turned the light off.

Quite a high proportion of the electricity used in the Sigma Home is generated by micro renewable technology. Wind turbines are located on the roof of the building and generates "little effective electricity" (Stewart Milne, 2009.) This technology is not suited to the low rise urban location of the Sigma Home where the building is shielded from the wind by surrounding buildings, thus only able to generate very little electricity. There used to be three wind turbines on the roof, but ironically strong winds brought down one of the turbines and have damaged another. This raises a Health and Safety issue with this technology especially if this prototype is to be rolled out on a larger project. The height of the property also suggests that the occupier of the property would be unable to maintain and repair this technology which would require the expertise of specialist contractors.

Photovoltaic panels have been installed on the roof and the south facing gable wall and average at generating 27kWh/m2/year over the year. The photovoltaic's are beneficial in the way that they generate a high percentage (approximately 50%) of the electricity used within the building. However, the cost of installing photovoltaic technology and maintaining it can often outweigh any benefit. Also the panels performance are often dependant on the amount of sunlight they have access to. Since installation of the panels on the south facing wall, the Barrett Green House has been built adjacent to the Sigma Home. This severely reduces the amount of sunlight able to be absorbed by the photovoltaic's and also reduces the accessibility to the panels for maintenance too.

Solar thermal panels are also located on the roof. The energy from the panels preheats the water for heating and domestic use. The solar thermal technology is adequate for its purpose and is regarded as one of the most successful and cost effective sustainable technologies. However, the particular product present at the Sigma Home has been discontinued from the market which raises issues associated with future maintenance and replacement. It seems that products on the sustainable technologies market are consistently changing and are likely to become outdated quickly and newer and more efficient technologies are adopted for other projects, leaving earlier products redundant. Solar thermal technology does have its limitations though. Often sufficient heat storage is not installed meaning it is difficult to synchronise the output with the occupier's needs, which may include heating the building in summer to use the energy from the panels.

Underfloor heating technology has been adopted throughout the property. The heat is supplied from a low NOx gas boiler that is capable of supplying up to 14kW, however the average demand would only be approximately 4kW. Underfloor heating tends to have a time lag in responding to the occupant's desired temperature and occupants may constantly adjust the thermostat because of this and use more energy that is required. By using this technology too also means that maintenance and replacement would be problematic by having to lift the flooring. Low water content radiators may have been more preferable to heat the building instead of underfloor heating. Low water radiators require very little energy to heat the small amounts of water within the radiator; however, this would mean that permanent fitted radiators would reduce the flexibility of the building.

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Rooms generally retain their heat due to the fabric and materials of the building. By using heavy panels and tiles on blockwork would retain heat which is a positive in some aspects, however this temperature may not be comfortable in bedrooms. Louvered and meshed ventilation panels in windows are often adopted to improve the ventilation of the property day or night whilst maintaining security. These technologies are not adopted in the Sigma Home, however the designers have envisaged ventilation by constructing a balcony for the master bedroom on the third floor which would provide some shading. By opening the balcony doors in the master bedroom, internal heat gains. The positioning of the doors is effective in the master bedroom as it means warm rising air from the building can leave via the balcony doors. The living room on the first floor has been designed to overhang the living/ dining area on the ground floor which would provide shading on sunny days. Throughout the property there are LED room temperature panels which tell the occupier the room temperature and the need (if any) to provide ventilation or heating. The only risk of overheating in the building aside from solar gains is from internal heat gains such as heat generated from household appliances. The Sigma Home aims at minimising internal heat gains by using low energy electrical goods and low energy lighting. By using low energy appliances and lighting, it lowers the need to control overheating and to ensure that the occupants do not require additional air conditioning which would use more energy.

The Stewart Milne Group did experiment with the use of phase change materials in the Sigma Home. Energain (manufactured from Dupont) uses wax that melts at approximately 26°c and acts as a form of latent heat storage that absorbs heat from the building as it starts to overheat. As the temperature then lowers, Energain changes its property from a liquid to a solid again and releases the stored heat back into the room. Unfortunately the wax is flammable so the product needs to be protected by plasterboard which raises questions of its effectiveness and its ability to be used in sustainable homes if the Sigma Home is a prototype to be rolled out on larger projects.

If the building does overheat, a solar stack has been installed at the top of the building and its main function is to allow warm air to ventilate out. The stack sits above the stairwell and takes in warm air as it rises. A heat sensor opens and closes a vent at the top of the stack to manage the optimum temperature in the building. However, the stack can act as a funnel which channels external noise to ventilate down in the property. As the M25 and M1 are all close the noise can be quite irritating. The many windows located within the property also reduces the risk of overheating. It was established though, that only 4 out of the 25 windows were used consistently whilst the building was being tested. This could then reduce the amount of windows required within the property which would be addressed in the design stages of further builds. A mechanical ventilation with heat recovery system has been installed in the Sigma Home which is required for Code 5 and 6 houses. Extractors for the mechanical ventilation systems have been fitted in wet areas such as the bathroom. They extract warm, moist air to the heat recovery unit where the heat is transferred to warm the fresh incoming air, which is then supplied at low pressure to habitable rooms through vents.

The water usage is well within the Code for Sustainable Homes limits for Code 5 of 80 litres per person per day. The building operates a grey water recycling unit which takes waste water from taps and the showers and cleans and filters it for re-use in flushing the toilet. The Ecoplay system is self monitoring and stores the grey water for a maximum of 24 hours before being removed from the system to ensure hygiene standards are maintained in the property. Some of the electrical appliances have water usage limiters on them, to ensure the water usage for Code 5 is also maintained. The dishwasher uses 10 litres of water per cycle, the washing machine uses 40 litres. One negative of using water usage reduction technology is that often occupants need to use more water to fulfil activities within the home such as flushing the toilet. Depending on the load of washing, more water may be required too. However, if the occupants exceed using 80 litres of water per person per day, the Sigma Home would lose its Code 5 status.

It is clear that properties such as the Sigma Home have a reputation of using complex and specialist technology. Such technologies should be explained to the occupants before the handover to ensure that the occupants are aware of what the buildings technology is capable of and how to minimise energy use. Simple but detailed explanations and diagrams should be available in the building with an aftercare hotline, to ensure occupiers are capable of using the technology. After receiving feedback from the investigation, Stewart Milne (2009) have now developed the KISS approach "Keep It Simple Stupid" and also believe that home maintenance agreements could be compulsory in the future, which would enable occupants to become familiar with the technology.

Using the Sigma Home research project to inform the next stage of research and development, Stewart Milne Group has now embarked on creating the Sigma II Build System. This is designed to learn from the past of the Sigma Home and to further the requirement for low energy homes, low carbon homes whilst providing a solution that is easy to build, maintain and use by occupants.

BRE (2009) believes that "achieving a low energy use society will depend on consumers themselves changing the way they use their homes to make a positive impact on reducing their energy consumption."

Code for sustainable homes



  • Communities and Local Government
  • Stewart Milne Group (n.d)Advancing Tomorrows Home
  • Stewart Milne 2009 the future of low energy, carbon neutral homes.


The experimental family commented that there was a lack of drying facilities within the property which would then increase energy usage by using a tumble dryer. Their recommendations were to optimise the space at the top of the property. As the property is four storeys high and heat rises, this would be suitable, however carrying wet washing from the kitchen on the ground floor to a room on the fourth floor is not practical.

It would also be sensible to have designed the bedrooms so that they would have been on the ground and first floors. This would mean that the bedrooms would be kept at a comfortable temperature. This would mean the heat would rise to the upper floors and the contact with the ground would also improve its coolness. However, this arrangement of rooms is not conventional as this would mean bedrooms would be next to the kitchen and as the property is tall it would be unacceptable for the kitchen to be further up in the property.