Low-energy buildings

Lower fuel bills, more comfortable environments and reduced carbon emissions are just some of the benefits of low-carbon buildings.
Interior of St Luke’s School, Wolverhampton, designed by Architype

Buildings account for a major share of global energy use. Designing them to use less energy is key to reducing fuel bills, increasing energy security and building a low-carbon world. Design techniques include optimising the amount of natural light and the volume of air that flows in and out of the building, as well as using insulation and air-tightness to lock in heat during winter.

Key facts

  • Buildings accounted for about 43% of UK CO2 emissions in 2009.
  • Good design can cut CO2 emissions and energy bills by nearly two thirds.
  • Incorporating renewable energy can result in buildings producing no net CO2 in use.

Energy in buildings

Buildings are responsible for a substantial proportion of global energy use and CO2 emissions. For example, in the UK buildings accounted for around 43% of total CO2 emissions in 2009, with domestic buildings alone accounting for around 27%.

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. 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 building 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 50 to 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.

UK domestic energy consumption break-down (by final use). Source: BRE figures for 2001.

Infrared photograph on a winter’s day shows that a conventional office block (left) is losing far more heat than one designed to passivhaus standards (right). Source: Passivhaus Institut Damstadt

Low-energy building principles

Low-energy buildings use a mixture of passive techniques and active systems to deliver a comfortable environment with low energy use and associated greenhouse gas emissions. Passive techniques relate to the shape of the building and the materials that it is built with, while active systems use machinery to provide services to the building which minimise energy use. Incorporating 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 to meet the building demand.

It is possible to reach the point where a building produces net zero greenhouse gas emissions in use, known as ‘zero carbon in use’. Embodied energy can also be reduced by using low-carbon building materials and construction methods.

Low-energy building designs are developed by teams that include both architects and engineers. They work together to produce a design best suited to the local climate, internal comfort requirements such as light level and temperature range, how the building will be occupied, and other client requirements. Computer-based simulation tools are commonly used to model and improve building performance during the design phase.

Passive techniques

High levels of insulation and air-tightness are used to reduce heat loss from the building, and hence the need for heating during winter. Using heavyweight materials in the building construction creates ‘thermal mass’, which reduces temperature fluctuations by storing heat and releasing it later in the day, helping limit overheating during hot weather or times when there are a lot of people in the building.

GERES used low-cost, passive techniques to design these greenhouses, so that vegetables could be grown throughout the bitterly cold winters in the Himalayas

Buildings can be designed to make the best use of sunlight by orientating them relative to South and arranging 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. The design will also place windows to maximise daylighting, reducing 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, but it also transfers heat, 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 during cold weather, airflow is reduced to the minimum needed to provide fresh air.

Energy-efficient active systems

Mechanical ventilation with heat recovery uses the warm outgoing air to preheat incoming air, so reducing ventilation heat losses in winter. Energy-efficient artificial lighting such as high-frequency fluorescent or LEDs with daylight dimming minimises lighting energy use. In buildings where a lot of heat is needed year round, such as hospitals, combined heat and power (CHP) systems can be installed to 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. When cooling is unavoidable, for example in rooms with lots of IT equipment, alternatives to air conditioning can be used, such as Monodraught’s COOL-PHASE system. This uses a phase change material, which is cooled down at night using cold air from outside, allowing it to cool indoor air the following day.

Renewables

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 makes the adoption of renewable energy technologies more viable, because less capacity is required to meet the demand on site.

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 -25 degrees C.

Cross sectional drawing of St Lukes School. Source: Architype.

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.

Entrance on the North side of St Luke’s school, showing use of glazing to provide daylight, and timber cladding. Source Architype
The high level windows on the North of this classroom provide plenty of daylight, while shading the whiteboard below. Source: Architype
The extensive use of timber framing at St Lukes reduced embodied energy and carbon. Source: Architype
Ringmer Community College student Eco Reps encourage the energy-efficiency good practice in the management and operation of the school.
Ringmer Community College 6th form, with southfacing Cedar louvres screen to limit solar gain from high-angle summer sun. Source: Oliver Wilton

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 £1,029 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 energy 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.

Cost

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.

The future

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 a 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.

The PassivHaus design standard originated in Germany in the 1990s and there are now more than 10,000 PassivHaus buildings worldwide including homes schools and offices, mainly in Germany and Austria. The standard requires that buildings use no more than 15 kWh of space heating per m2 of floor area each year (note 7), compared with the current average of 139 kWh/m2 per year for a UK home (note 8). PassivHaus also targets 120 kWh/m2 per year total primary energy use (note 7) (this includes the energy used in electricity generation), compared with the current average of 278 kWh/m2 per year for a UK home (note 8). The key techniques used in PassivHaus designs are very high levels of insulation and air-tightness, and mechanical ventilation with heat recovery.

Target reduction in energy use standards for new homes in the UK, compared with 2006 Building Regulations. (note 9)