Sun Screen

11 January 2010

Mitigating the effect of warm sun on buildings was the subject of a recent CIBSE seminar looked at retrofitting solar shading and the potentials for containing energy costs. Frank Booty reports

ENERGY BILLS ARE UPPERMOST IN MANY people’s minds currently. Buildings need to be heated in the cooler parts of the year to maintain occupants’ comfort levels and likewise cooled in the warmer months for similar reasons. There is a substantial
amount of older building stock, as well as new properties, where economical solutions are sought to solve problems caused by light and heat.
Solar shading is an effective and often cheap way of controlling the amount of light and heat admitted to a building. Indeed it can prove to be an effective energy saver all year round, decreasing costs of cooling in the warmer months and optimising natural daylight usage in the winter. It can also prevent glare from direct sunlight while making the most of daylight.
Paul Littlefair, associate director of BRE's Building Technology Group, says, “It boils down to a choice between air conditioning or solar shading – with both you have to spend to achieve savings. There’s a lot to play against – there is the growth in installations of air conditioning, climate change linked to warmer summers, meeting the
provisions of Building Regulations, controlling glare, and improving occupants’ comfort while maintaining productivity. With retrofitting solar shading the attraction is that it is a win-win situation. It is a lower cost to fit compared to air con, and you get energy savings as well.”
“Over 40 percent of the delivered energy in Europe is used in buildings, while in the UK twothirds of energy is used for space heating, and in commercial buildings 20 percent of the energy is used for lighting,” says Prof Michael Hutchins, visiting professor, London South Bank University. “Energy efficiency equally applies to walls, roofs, and windows, including glazing, shading, complex glass and frames.”
Standards
International standards loom large – there are ISO9050, EN410 for the ultra-violet, visible and near infra-red spectrum, and EN673 and EN12898 for the infra-red bands. There is TM37 from CIBSE – Design for improved solar shading control, and
European standards for blinds, shutters and complex glazing EN14500 and EN14501, and 13363-1 and 13363-2. The US meanwhile has such standards as ISO15099, and ASHRAE 74-88. There is a US window design tool at www.ibl.gov while Europe offers
www.windat.org, the British Blind and Shutter Association, BBSA (see www.bbsa.org.uk), and the Europe Solar Shading Operations or ESSO. That’s just for starters.
Hutchins continued, “Complex glazing systems bring positive benefits for heating and cooling in buildings. Glazing and blind selection criteria are based on a quantitative understanding of energy gains and losses. Key parameters for glazing are measured and calculated using international standard procedures.”
The energy performance certification of these products is, says Hutchins, “imminent” in
support of the EPBD (European Performance of Buildings Directive).
Shading
With solar trading there can be a trade off between controlling glare and cooling
temperatures and reducing the view and light for work through glass or window film, controllable shading or using the sun’s geometry. One approach is to deploy high performance, soft coat, low-emissivity film on double glazed windows, which offers low solar transference and high light transference. Deploying typical tinted glazing offers a low solar figure but an equally low light transference capability meaning occupants would need to have the lights on more often.
Controllable shading has various arguments for and against its usage. One key question is whether to opt for manual or automatic shading. People like to have control over shading. They want to take control of the space they are in; indeed it is psychologically good for them to do so. But full occupant control may not give optimum performance.
“People who are away from their workstations will come in – too late – and find the building too hot,” says Littlefair. “People still want control. So automatic back-up is a good idea. Shading coupled with an occupancy sensor which defaults to automatic control, having the blinds down to prevent overheating for example.”
Using the sun’s geometry is a valid technique enabling lots of daylight into a property with no overheating. According to Littlefair, one-tenth of the sky is where the sun is coming from; it means an overhang on the building is not shading much of the sky, but it is working at its best when the sun is high in the sky, and particularly on a south facing window.
There is a legal requirement for environments equipped with display screen equipment, namely that the windows “shall be fitted with an adaptable covering to attenuate the daylight that falls on the workstation”.
A typical bright sky is 10,000 cd/sq m (cd = candelas) while the sun is 20m cd/sq m. CIBSE recommends the light around an office should be 1,000 cd/sq m. An open-weave blind on a window will maintain a view out – the optimal open weave is 10 percent meaning the bright sky plus blind now exhibits 1,000 cd/sq m in the office (but the sun luminance is still 2m cd/sq m – which is too bright). Thus any shading maintaining a view
out is not going to control the sun. Venetian blinds and close weave blinds on south-facing windows constitute good practice. But the approach is less effective on east and west facing windows as the blinds need to be fully extended down. Vertical louvre blinds are regarded as too transmissive.
External shades can be a cross between roller types and awnings. Overhangs control the summer sun, but with the winter sun it is necessary to retrofit internal blinds. Rooflights cause glare and atria typically require rooflights with louvres, or baffles which are connected to a blinds system.
“People value windows and daylight, but they need glare control,” says Littlefair. “Remember that solar shading is a legal requirement for spaces with computer screens. Having user control is generally the best approach to take.”
There are four types of control blinds – screen, opaque, reflective, and decorative, According to Tom Brownson of London architects and designers Silent Gliss, specifying and planning what is required has to focus on the size and shape of the system required, which fabric to use and whether its width can be accommodated, and whether there is suitable safe access for servicing and maintenance. Control methods may be motorised – hardwired or wireless remote control, for example – and the blinds may be integrated to the building management system.
Keith Morrison, marketing and business director of glass processor Romag explained that selecting what glass to deploy involves investigating such characteristics as energy
efficiency, comfort, design and aesthetics, light transmission, condensation control, UV blocking, noise control and impact resistance/imposed loads. The types of glass used in buildings are:
● float glass, maybe with body tints
● reflective glass – pyrolitic coatings (hard) or sputter coatings (soft)
● low emissivity glass – hard and soft coats for insulation
● California series glass – sputter coated on film inside a laminate.
● Photovoltaic glass which can convert 13 to17 per cent of daylight is converted to electricity.
While it’s still early days for PV, PV glass provides solar shading as well as providing electricity while controlling solar gain.
● Thermotropic glass becomes opaque when the temperature reaches a certain point and provides solar shading.
● Photochromic glass utilises similar technology as ‘reactolite’ sunglasses. However when applied to shade of say 4x5.5m in size, the glass exhibits two different shades. Consequently there’s work still to be done.
● Electrochromic glass responds to an electric current passed through it. It tends to utilise LCD technology – when in normal state, the glass is opaque. Applying an electric current causes the crystals to align and means you can then see through the glass.
Only about 2 percent of installations in the UK are motorised limiting their Integration potential with other building control systems such as lighting, HVAC and BMS optimise the building performance. Energy savings could be achieved through the alignment of blinds, shadow management, glare control, privacy control, and temperature control.
● Frank Booty is a freelance writer