Key issues of solar shading

In many ways effective solar shading is more important than ever before. Today's buildings are better insulated and often contain more equipment, creating even greater internal heat gains.

The use of air conditioning and comfort cooling is also increasing rapidly (by 6·5% annually in the UK), so there are important energy savings to be had from controlling solar heat gains. The widespread use of computer screens in all types of buildings, not just offices, means that glare from windows has a vital impact on productivity and comfort.

To an extent legal standards are beginning to catch up with this trend. The HSE regulations^{1, 2} for areas with display screen equipment require that "windows shall be fitted with a suitable system of adjustable covering to attenuate the daylight that falls on the workstation". The recently issued Building Regulations consultation document³ contains a new proposal for Part L to include a measure to avoid solar overheating.

The draft legislation suggests that "Those spaces that rely on natural ventilation should not overheat when subject to a moderate level of internal heat gain and those spaces that incorporate mechanical cooling do not require excessive cooling plant capacity".

However, evidence from studies of recent buildings suggests that solar shading has yet to receive the attention it deserves. It is often included as part of the fit-out, along with carpets and the tea machine, or alternatively as a retrofit measure once the building has already begun to perform badly.

Shading design, unlike lighting design or hvac design, is not seen as a distinct discipline. Apart from manufacturers there are few who would see themselves as competent in this area.

Key solar shading issue

A recent BRE design guide⁴ sets out some of the principles of shading design. The guide proposes that solar shading must be considered at the architectural design stage.

If this is done, shade elements can enhance the appearance of the facade, and emphasise the overall architectural concept of the building. Some of the most effective forms of solar control are difficult or costly to install in retrofit, and they may not tie in with the original design of the facade.

Shading should protect from overheating and glare, and provide privacy. Not all of these issues may be important in any one building, but if they are it is essential to address them. This may require a combination of devices: for example advanced 'heat mirror' glazing to control overheating coupled with internal blinds to prevent glare and provide privacy.

Shading systems also need to be able to take maximum advantage of natural light and heat. For maximum energy efficiency, shading should be sufficiently flexible to use the available solar gain and daylight on dull days to reduce the loads on heating and artificial lighting systems. This will involve adjustable shading devices.

Consideration should duly be given to other techniques to maximise daylight. Some shading types diffuse incoming sunlight, though this can sometimes be uncomfortably bright. Other devices redirect incoming sunlight, such as the light shelf (figure 1).

Some solar control elements are designed to block the areas of sky where the sun is, but admit light from other parts of the sky. Finally, spectrally selective devices are being developed that allow more daylight to pass through than the other parts of the solar spectrum. Some 'heat mirror' type glasses have this property.

Shading elements should be considered in conjunction with the building's natural ventilation strategy. For example in the UK windows generally open outwards, which means that external solar shading, and the building facade, must be designed accordingly. Choosing sliding windows can get round this. Centre hung windows can also pose problems, interfering with internal blinds when they are opened.

The choice of an appropriate shading control is also important. In a recent EPSRC project, BRE and Brunel University investigated this issue. The project concluded that the choice of automatic or manual control will depend on the needs of the space and its occupants.

Predominantly manual control is best in owned or shared spaces like offices. People may raise the blinds themselves if there is a good view out, and the blinds are easy to operate. Automatic resetting of blind position could be carried out at night, because occupants tend to review their blind position at the beginning or end of the working day anyway.

The use of mid-pane blinds can lower heat losses and the automatic lowering of these blinds at night could save energy.

Given a combination of shading devices, occupants used those that were easiest to operate, even though they restricted daylight and view. Predominantly automatic control is appropriate for unowned and managed spaces like circulation areas and foyers. Time delay or solar reset algorithms are appropriate to avoid over frequent operation. Manual override should operate quickly and reliably. There should be a long time interval (maybe a day) before the system reverts to automatic control.

Planning for maintenance is another vital area. All forms of solar control will require some maintenance even if it is only occasional cleaning. Appropriate maintenance, particularly of moveable systems, can be highly cost-effective as it can prevent premature system failure.

The Shade project

A new project, Shade (Promoting solar shading and intelligent facades), was launched last month at the RIBA and at the UK Blinds and Shutters exhibition. It has the objective of encouraging solar shading as a core design strategy.

The study aims to raise awareness and overcome barriers to the effective use of solar shading, and  provide information and training on selecting, commissioning and managing an effective solar shading system.

The project is sponsored by DETR through its Partners in Innovation Programme. Project partners are BRE, the British Blind and Shutter Association (BBSA), Pilkington, Southwark Energy Agency (SEA), publishers Construction Research Communications (CRC), Buro Happold and the Royal Institute of British Architects (RIBA).

Key outputs from the project include:

• presentations to businesses and free energy audits in Southwark

• an award for students on facade design

• a major conference/seminar incorporating demonstrations of innovative systems, details of new research findings and material on intelligent facades.

• courses, on selecting and designing shading systems, and control of shading devices

a seminar for the shading industry

• BRE Information Papers on shading selection, and the control of moveable devices.