Last edited 12 Jul 2016

Passive building design

Designers tune the thermal characteristics of buildings so that they moderate external environmental conditions and maintain internal conditions using the minimum resources of materials and fuel.

Passive design maximises the use of 'natural' sources of heating, cooling and ventilation to create comfortable conditions inside buildings. It harness environmental conditions such as solar radiation, cool night air and air pressure differences to drive the internal environment. Passive measures do not involve mechanical or electrical systems.

This is as opposed to 'active' design which makes use of active building services systems to create comfortable conditions, such as boilers and chillers, mechanical ventilation, electric lighting and so on. Buildings will generally include both active and passive measures.

Hybrid systems use active systems to assist passive measures, for example; heat recovery ventilation, solar thermal systems, ground source heat pumps and so on. Very broadly, where it is possible to do so, designers will aim to maximise the potential of passive measures, before introducing hybrid systems or active systems. This can reduce capital costs and should reduce the energy consumed by the building. However, whilst passive design should create buildings that consume less energy, they do not always produce buildings that might be considered 'sustainable' as sustainability is dependent on a range of criteria, only one of which is energy usage.

Passive design can include:

NB Passive solar design is an aspect of passive building design that focusses on maximising the use of heat energy from solar radiation.

Passive design can include consideration of:

In its most simple form, a shallow building orientated perpendicular to the prevailing wind with openings on both sides, will allow sunlight to penetrate into the middle of the building and will enable cross ventilation. This should reduce the need for artificial lighting and may mean that cooling systems and mechanical ventilation are not necessary. In taller buildings, stack ventilation can be used to draw fresh air through a building, and in deeper buildings atriums or courtyards can be introduced to allow light into the centre of the floorplan.

However, difficulties arise, for example; when buildings have cellular spaces that block the passage of solar radiation and air, or where site constraints create complex massing or mean that windows cannot be opened because of noise or air quality issues. This can lead to the introduction of more complex passive measures such as; trombe walls, solar chimneys (or thermal chimneys), solar stacks, acoustic louvres, thermal labyrinths and so on.

The situation is complicated further by different climates, changing seasons, and the transition from day to night, so that passive design may have to allow different modes of operation, sometimes rejecting external inputs and expelling the build up of internal conditions, whilst at other times, capturing external inputs and retaining internal conditions.

Typically these variations can be dealt with through measures such as shading, shutters, overhangs and louvres that allow low-level winter sun to penetrate into the building, but block the higher summer sun. Thermal mass can be used to store peak conditions during the day and then to vent them to the outside at night. Even deciduous trees can be beneficial, their leaves shading buildings from summer sun, but then allowing the solar radiation to penetrate through their bare branches during the winter.

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Additional complexities can be introduced by internal heat loads such as people and ICT equipment and by occupancy patterns. In a 9 to 5 office with a moderate amount of installed equipment, it may be possible to use thermal mass to store heat loads during the day and then to vent these and cool the thermal mass when the building is unoccupied at night. This may not be possible with a building such as a hospital that is continuously occupied.

Considering all these issues early in the design process, so that they can be incorporated into the fundamental design of the building, requires close working across the entire design team. The historic model, where the architect designed a building and then a structural engineer made it stand up and then last of all a services engineer made it comfortable, is unlikely to achieve a satisfactory result.

Passive design measures can require occupant involvement, for example to open windows, turn out lights, adjust louvres and so on. This requires education so that occupants are able to understand the building and to operate it efficiently. Occupant behaviour is often cited as one of the prime causes of the 'performance gap', that is, the difference between the expected and actual energy consumption of completed buildings.

As well as reducing energy consumption, adopting passive design strategies can help building ratings across standards such as PassivHaus, BREEAM, the Code for Sustainable Homes and LEED.

NB The urban heat island effect, is an effect found in urban environments where the predominance of hard, heat absorbing surfaces results in a higher ambient temperature than in rural environments. It has been found that simply selecting lighter coloured materials that reflect solar radiation rather than absorbing it can significantly reduce urban temperatures and so the need for active systems to provide cooling.

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