Low-energy buildings use passive techniques, such as optimal solar gain, and advanced active systems, such as mechanical ventilation with heat recovery, to create comfortable internal environments that have low energy demand and associated greenhouse gas emissions. Some low energy buildings use renewable energy generation to further reduce or eliminate their greenhouse gas emissions in use.
- Buildings account for about 45% of UK CO2 emissions.
- Good design can cut CO2 and energy bills by nearly two thirds.
- Incorporating renewable energy as well can give buildings that produce no net CO2 in use.
Read on for more information about low energy buildings, or go to our database for films and case studies of Ashden Award winners who use them.
Energy in buildings
Buildings are responsible for a substantial proportion of global energy use and CO2 emissions. For example, in the UK buildings account for around 45% of total CO2 emissions (note 1) with domestic buildings alone accounting for around 27% (note 2).
Energy is used in buildings to deliver comfortable conditions for occupants and to power appliances: this is known as ‘energy-in-use’. Energy is also used in building materials and during construction: this is known as embodied energy’. Currently the energy-in-use over the lifetime of most UK buildings greatly exceeds their embodied energy. For a typical existing three-bedroom detached house in the UK, it is estimated that energy-in-use overtakes embodied energy within two to five years (note 3). Thus over a 60-year building life the energy-in-use will account for between 92 and 97% of the lifetime energy requirements, and is therefore the first target of low energy design.
The UK and much of Northern Europe have relatively mild summers and cool winters. Most buildings require significant active heating in winter and limited or no active cooling in summer. For example, an average UK home consumes around 60% of its total energy use on space heating. Nearer the equator, the climate generally becomes warmer and the emphasis shifts from keeping warm in winter to staying cool in summer.
Low energy building principles
Low energy buildings use passive techniques relating to the design of building form and fabric and energy-efficient active systems, in order to deliver a comfortable environment with low energy use and associated greenhouse gas emissions. Incorporation of renewable energy generation on site can reduce emissions further. However, low energy use should be the first priority, since this is the cheapest way to cut greenhouse gas emissions. In addition, low energy use makes the adoption of renewable energy technologies more viable because less capacity is required.
It is possible to reach the point where a building produces net zero greenhouse gas emissions in use, known as ‘Zero Carbon in use’. Building materials and construction methods can also be specified that reduce embodied energy.
Low energy building designs are developed by a team including architects and engineers. They work together to produce a design best suited to the local climate, the internal comfort requirements like light level and temperature range, the way in which the building is occupied, and other client requirements. Computer-based simulation tools are commonly used to model and improve building performance during design development. A range of passive techniques, active systems and renewables are incorporated as appropriate to the project.
High levels of insulation and air-tightness are used to reduce heat loss from the building, and hence the need for heating during winter. Thermal mass, using heavyweight materials in the building construction, damps temperature fluctuations by storing heat and releasing it later on in the day and can help to limit overheating during hot weather or periods of high occupancy.
Optimal solar gain orientates the building and arranges windows and other apertures to allow sunlight in at cooler times of year, but to limit direct sunlight penetration during summer when it can cause overheating. Design for day lighting shapes and arranges windows and other apertures to give optimal levels of daylight inside a building and so minimises the need for artificial lighting. The use of high efficiency, low emissivity glazing allows high levels of daylight in whist reducing heat losses through windows.
Air flow through buildings is necessary to provide fresh air. It also provides convective heat transfer, which may or may not be desired. Passive design can use wind-driven and stack-driven natural ventilation to provide cooling in summer without the need for air. conditioning. To minimise heat losses in winter, airflow is reduced to the minimum needed to provide fresh air.
Energy-efficient active systems
Mechanical ventilation with heat exchange preheats incoming air with warm outgoing air and so reduces ventilation heat losses in winter. Energy-efficient artificial lighting such as high-frequency fluorescent with daylight dimming minimises lighting energy use. Combined heat and power (CHP) systems generate electricity on-site, and use the heat produced for space and water heating. This makes them more efficient than typical electricity generation systems where the heat is wasted.
Renewable heating systems including biomass boilers, active solar water heating and ground source heat pumps can be used to supply heating and hot water needs with reduced greenhouse gas emissions. Solar photovoltaics can be mounted on or integrated in the building roof, to provide renewable electricity. Wind turbines can be included within the site as another means of providing renewable electricity. Reduced energy demand also makes the adoption of renewable energy technologies more viable because less capacity is required.
How low energy building techniques are used
Ashden Award winner GERES incorporated many passive techniques into the design of greenhouses for the bitterly cold but sunny climate of Ladakh. The greenhouses are constructed using locally-available materials as far as possible. They are aligned East-West with tough, transparent agricultural plastic on the South face to maximise solar gain: a blanket is pulled over the plastic for night-time insulation. The North, East and West walls are made from mud and stone with high thermal mass to store heat during the day. The walls incorporate cavities insulated with straw or sawdust to cut heat loss, and the roof is thatched. Opening the door and vents controls ventilation. With these passive techniques, vegetables can be grown throughout the winter even when the outside temperature falls to -25degreesC.
Ashden Award winner Architype is a UK based architectural practice which uses passive design, active systems and renewables to reduce energy-in-use, as well as trying to cut embodied energy. It currently designs to the PassivHaus standard (see box). St Luke’s Primary School, designed by Architype, opened in 2009. This uses passive techniques including day lighting, optimal solar gain (windows and roof lights that let in sunlight when the sun is low in winter but block it in summer), high levels of insulation and airtightness, and natural ventilation (adjustable louvres and stacks) for cooling in summer. Energy-efficient active systems are also incorporated, including a biomass wood-chip boiler that supplies the underfloor heating. It is predicted to use less than half of the statutory energy targets. Sustainably sourced timber was extensively used in its construction, including a prefabricated timber frame, untreated Douglas Fir wall cladding and Cedar roof shingles. This reduced embodied energy compared with other common forms of construction such as concrete frame, and further reduced greenhouse gas emissions as the timber ‘locks up’ CO2 for the life of the building (around 1.8 tonnes of CO2 for every tonne of timber used, source: TRADA).
Another important aspect to reducing building energy use is to inhabit and operate the building efficiently. Ringmer Community College, an Ashden Award winner, is a secondary school in East Sussex that is proactive in reducing its energy use via good operational practice, with the active involvement of the school’s two hundred student Eco Reps. The school also champions low energy building design and Eco Reps were actively involved in the design of its new 6th form building, completed in 2008. The building has passive features including optimal solar design, natural stack ventilation and high thermal mass. A ground source heat pump provides low carbon heating and hot water.
What are the benefits of low energy buildings?
Low energy buildings cut greenhouse gas emissions but bring other benefits too. They are often more comfortable than conventional buildings, because rapid temperature variations and draughts are minimised. Daylight brings psychological benefits.
Reduced energy-in-use cuts energy bills. For example, the average UK household in 2010 paid £1029 per year in energy bills (£397 for electricity and £632 for gas) (note 4). Bills for a PassivHaus home would typically be around 35% of this, saving nearly £700 per year. Such savings reduce financial exposure to the volatile fuel markets, and can bring households out of fuel poverty.
Income can be generated if the building incorporates any of the renewable electricity technologies (such as photovoltaics) that qualify for the UK renewable electricity Feed-in-Tariff.
The additional cost of constructing a low energy building over a conventional one depends on the specific design and the features it incorporates. There are current examples of low energy buildings delivered within conventional budgets via good design and the prioritising of low energy use. At present constructing a ‘Zero Carbon in use’ building can typically add between 5% and 30% (note 5) to construction costs depending on the building type and other details. When very low energy buildings become more widespread then this premium will diminish.
Most countries are planning how to reduce their fossil fuel reliance and associated greenhouse gas emissions. For example, the UK Climate Change Act of 2008 is targeting a 34% reduction in greenhouse gas emissions by 2020 (compared with 1990 levels) and an 80% reduction by 2050. New buildings have been identified as the sector where radical cuts can be achieved. The UK targets are for all new homes to be net Zero Carbon in use by 2016, and all new non-domestic buildings by 2019. As energy-in-use decreases, attention will shift to reducing the energy embodied in the construction of new buildings.
However, it is estimated that up to 87% of the homes that will exist in 2050 have already been built (note 6), so it is also crucial to retrofit existing buildings with energy efficiency measures.
- View our low energy buildings photo collection on flikr
- Good introduction to the basics of building environmental design including
1. Carbon emissions - growth in a low carbon economy, Richard Miller
2. New-Housing Energy Saving Trust note regarding the Nottingham Declaration on Climate Change
3. Sustainable Homes: Embodied energy in residential property development, 2000
4. Department for Energy and Climate Change (DECC) Quarterly Energy Prices: September 2010
5. Report on carbon reductions in new non-domestic buildings, UK Green Building Council
6. Home Truths: a low-carbon strategy to reduce UK housing emissions by 80% by 2050, Brenda Boardman, University of Oxford, 2007
7. Good source of basic information on PassivHaus
8. AECB, The Sustainable Building Association
9. Sustainable New Homes – the road to zero carbon, DCLG, 2009