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Figure 1. Position of the sun in different seasons
This appendix explains some key information concerning the different types of radiation that have to be considered with regard to the performance of solar protection devices and the position of the sun. It also explains how a material behaves when it is affected by such radiation. The solar irradiance depends on the position of the sun in the sky, which can vary throughout the year and during the day. Figure 1 shows that the sun’s peak position in winter is lower than in summer. This gives an opportunity of harvesting large amounts of irradiance during the day through glazing in colder months.
When the sun irradiates a surface (glazing, fabric or a slat for example), incident radiation splits into three parts: transmittance, reflectance and absorptance.
These values are assessed for solar gain and visible light to determine the thermal and visual properties of the glazing combined with a shading device.
We are exposed to a large variety of radiation, either natural or artificial. Radiation has differing wavelengths and the “power” of a radiation is represented by its irradiance in W/m² (Watts per square metre). For a given wavelength, we talk about spectral irradiance and it is expressed as W/m²nm ((nm is nanometers- the measure of wavelength). Figure 2 shows the distribution of electromagnetic radiation depending on its wavelength. Based on its wavelength, radiation can be divided in two types:
Figure 2. Classification of various electromagnetic radiation depending on their wavelength
The sun’s surface produces an enormous amount of energy (66,000,000 W/m²) which is transmitted to the Earth through radiation. Only a fraction of this energy reaches the atmosphere (around 1,300 W/m²). Around 6% is reflected directly back into space and approximately 15% of this radiation is then absorbed by the atmosphere and emitted in all directions in the form of diffuse radiation. The remaining part (79%) on a clear day is directly transmitted to the ground through the atmosphere - see Figure 3.
As a consequence, the solar energy at ground level is much lower than at the top of the atmosphere. It is generally considered that the energy reaching the ground when there is a clear blue sky most of Europe is around 1,000 W/m². When considering a solar protection device, it is necessary to distinguish the three parts involved in the solar radiation - see Figure 4.
Figure 3. Solar Radiation
The actual values of the above are specific to a material and are affected by its type (fabric, metal, glass), the density or openness, and the colour. It also depends on the wavelength of the solar radiation. This is measured in a laboratory and as the amount of reflection and transmission will vary through the solar spectrum it is measured in nanometers (nm).
The three values for solar gain are taken from the data for the complete solar spectrum from 280-2,500 nanometers and are defined as:
The three values for visible light are taken from the data for the visible part of the solar spectrum from 380-780 nanometers and are defined as:
Figure 4. Spectral irradiance at the sea level for the solar spectrum
Figure 5. Behaviour of radiation in contact with a material
These are the specific material properties that are then used in calculations these then combine the material data of the blinds, awnings or shutters with glazing data for the type of glazing - see Appendix D.
A more detailed assessment of the methods and calculations can be found in Solar Shading for Low Energy Buildings (8). These characteristics are measured in accordance with the European Standard EN 14500 Blinds and shutters - thermal and visual comfort - Test and calculation methods.
Summer cooling with blinds and shutters
In hotter climates such as those found in southern Europe it is normal for the blinds or shutters to be closed in early morning and then the room is a cool refuge from the afternoon sun later in the day. It is a natural, efficient method of preventing a room from overheating without using any energy.
One of the traditional ways of minimising heat from the sun entering the interior is to plant trees outside windows. In the summer the leaves provide shade and in autumn the loss of leaves will allow the sun to penetrate and provide some natural winter heating even when temperatures outside are low. Trees can be a positive and effective environmental solution except for the fact that they need space and time to grow and so blinds and shutters are more practical options.
It gets hot in the summer in the rest of Europe as well and traditionally we have preferred large areas of glass in our buildings that need air-conditioning to cool the resulting heat gain in offices or alternatively fans in our homes rather than utilising passive cooling methods such as natural shading and ventilation. An office worker exposed to direct sunlight without shading can never feel comfortable even with mechanical cooling since their perceived thermal comfort is affected by the surrounding air temperature and the radiation from surrounding surfaces. The only way to become more comfortable is to change place or to sit in the shade, away from direct sunlight. As temperatures increase as predicted, it will become more of a problem.
Winter heat saving with blinds and shutters
A significant amount of heat can be lost from a building through its windows, especially during the cold winter months. Also in the winter it is important to maximise heat gain from the low angle sun. Blinds and shutters can help reduce the amount of heat loss as they can also be raised to maximise passive solar gains entering the building when needed. In contrast, solar protective glass will reduce solar gains year-round, even in the winter.
In commercial buildings this can mean that less heating is required in the morning to get the building to a comfortable temperature and in homes we will appreciate the lower heating bills. To achieve this, follow this best practice guide recommended by ES-SO.
Figure 6. The best practice guide on using solar shading to maximise energy savings. For the best results the system should be automatically controlled so that it is in the right position at all times of the year
In order to save energy, blinds and shutters just need to operated effectively. To achieve this, follow the best practice guide recommended by ES-SO.
What do you expect the blinds or shutters on your project to achieve? Do you know all of the benefits that blinds and shutters could provide for you? Which are the most important?
Blinds and shutters can perform many functions and this list is intended to help you identify the issues rather than direct you to a specific type of product. It is unlikely that a single product will achieve every requirement and in some cases two shading systems may be required. Therefore it is necessary to prioritise needs and check them when selecting your product.
|Degree of Importance
|Control of solar gain
|Control of natural daylight
|Control of energy costs
|Automatic control of solar heat gain
|Interface with building or home management system
|Master override of automated system and wind
|Minimal reduction of light levels on dull days
|Shading when area is unoccupied
|Alignment of automation with other building services
|Glare control for visual comfort
|Screen glare control
|Ease of operation
|Visual perception, allowing external vision
|Privacy during daytime
|Privacy during nighttime
|Manual override of automatic control
|Wellbeing and health
|Alignment of automated controls with other measures
|REGULATIONS AND STANDARDS
|Compliance with National Building Codes and Regulations
|Compliance with energy performance targets of building modelling software
|Planning Authorities requirements
|Energy performance and energy audits
|Help to reduce carbon emissions in buildings and improve our sustainability
|Co-ordination with decor
|Same solution for every elevation
|Symmetry of external appearance of shading
|BUILDING LIFECYCLE COSTS
|Manageable maintenance costs
|Designed for long life
|OTHER FUNCTIONS AND NEEDS
|Dim-out for projection and AV equipment
|External shaded area for outdoor living
|Flame retardant materials
|Reduce UV fading of furniture and decoration
Table 1. Checklist of requirements for solar control management
The European Solar Shading Association (ES-SO) in conjunction with its member partners have developed a database of shading material performance. The European Solar Shading Database (ES-SDA) includes independently validated energy performance data of blind and shutter fabrics and materials measured to European standards.
Figure 7. Example Energy Performance Indicator (EPI)
A user-friendly Energy Performance Indicator has been developed to be similar to the one used by the Glass and Glazing industry except that it combines shading with glazing.
The database enables the calculation of the performance of shading products in combination with the glazing (this combination is known as ‘complex glazing’). It takes the performance data for the glass and the shading product to enable the calculation for the actual combinations of gtot and U-values of glass and blinds or shutters.
To correctly calculate the energy performance of a blind, it is essential to do this in combination with the type of glazing (single, double, triple or low-e) and the blind’s position (external, internal or mid-pane). This is important because for each combination of glazing and shading system the results will be different. Consequently it is not a simple calculation and this is why ES-SDA is such a useful tool.
The output from ES-SDA shows the thermal and visual performance characteristics of a blind in combination with a particular glazing. However, to ascertain how these characteristics perform in a specific building it will be necessary to use a building modelling system. This is usually a computer simulation tool which creates a virtual building and calculates the energy requirements of the building. A building modelling system which accurately considers shading will be able to show the reductions of heating and cooling loads and electric lighting resulting from the installation of solar shading. One such example is Early Stage Building Optimisation (ESBO) developed by EQUA Simulation in Sweden (https://www.equa.se/en/es-so-esbo-order-form) To predict the energy performance of a building both with and without solar shading “reference” (typical) buildings are used and many aspects need to be considered such as:
Computer Building simulation should use solar shading performance data and apply them to the reference glazing combinations from ISO EN52022-1 or 3 in Europe and EN ISO15099 in some other areas. There are many building simulation tools that do not calculate to these standards and thus can give a completely unrealistic value for shading performance. It is essential to check the methodology being used by the programme to be certain that it does comply with standards.
|A. Clear single glazing
|B. Clear double glazing
|C. Clear Triple glazing
|D. Double clear glazing with
|E. Triple clear glazing with
Table 2. Reference glazing from ISO Standards
A calculation tool for office buildings Textinergie® has been developed by the French Association of Blind and Shutter Manufacturers, SNFPSA. For more information visit: www.textinergie.org
A holistic approach to low energy shading means solar shading should be considered with regards to other aspects of the build and therefore at the planning stage.
The g-value, also called Solar Factor, is the total solar energy transmittance through a building element. It is the sum of the solar transmittance, Ts, and the secondary internal heat transfer factor Qi. The latter term is the portion of heat absorbed by the window which is transferred to the inside of the building.
Figure 8. The difference between g-value and gtot
The symbol g is the solar factor of the glazing alone while gtot is the solar factor of the combination of a glazing and a solar protection device - see Figure 8. The g-value of glazing alone is determined by the calculation method given in EN ISO 50022/1. There are two methods for the calculation of the gtot of a blind and glass combination (also known as complex glazing).
A simplified method is given by EN ISO 52022/1 and a detailed method given in EN ISO 52022/2. The simplified method is considered adequate for basic energy calculations and this is the one used in ES-SDA described in Appendix D.
If we know the performance characteristics of the glazing and that of the blind or shutter being used, it might seem simpler to just add the two values together but that will not provide an accurate result. The reason for this is that solar radiation travels through the window and the blind (Figure 9). The sun’s rays are absorbed by objects and converted to heat.
Figure 9. Solar energy properties
This heat cannot entirely pass back through the solid glass because most of the heat becomes trapped between the two panes and some is absorbed by the panes of glass. That heat is also transferred by convection and some is re-radiated back through the second pane into the room, some remains trapped and some is radiated through the first pane to the outside (see Figure 10).
That is why we need the ES-SDA database to calculate the way this energy is reflected and transmitted. The simplified formulae for the calculations for external, internal and midpane blinds are specified in EN ISO 52022/1 and analysed in Solar Shading for Low Energy Buildings (13).
Figure 10. Solar radiation transmission and interaction with different object
There are two European standards that are used to compare the performance data for standard glazing types; EN ISO 52022/3 and EN 14501.
The industry standard for gtot comparisons is reference glazing C from EN 14501 which is for double glazing with low-e glass this has been selected as the minimum level of glazing for a new build design.
The g-value, also called solar factor, is the total solar energy transmittance through a building element. The symbol g is the solar factor of the glazing alone while gtot is the solar factor of the combination of a glazing and a solar protection device.
A U-value is a measure of thermal conductance which is the ability of a material to transfer heat. Closing a blind or shutter will help improve the U-value of a window and help to retain more heat during the heating season.
Many components of a building such as masonry, insulation materials, plasterboard, metals and windows have U-values. This measure is expressed as Watts per metre square Celsius W/(m2 oC) or Watts per metre square Kelvin W/(m2K); for example the typical U-value of a clear glass double glazed window is 2.9 W/(m2K).
Figure 7. Example Energy Performance Indicator (EPI)
Heat is conducted (lost) through a window in the heating season by (Figure 11):
Figure 12 shows that the U-value of glazing is improved by the fitting of blinds. (An example of an average air permeability blind would be an internal blind that has no openness factor and peripheral gaps of 20 - 80mm between the blind and the window frame.)
Table 3. Reference glazing U-values
The example also shows that the effect of the blind or shutter is more pronounced when it is combined with a window with low energy performance for example single glazing or first generation double glazing. An internal blind with average air permeability can reduce the heat loss through single glazing by almost 40%. The reason why we need to know the performance of blinds in combination with glazing is simply illustrated in the graph above. A blind installed with low-e glazing provides an improvement in U-value of 12%, still a significant improvement when you consider that low-e glazing already has a low U-value.
CIBSE Guide A (10) quotes that a conventional roller blind (unsealed) would have a U-value of 2.53 W/(m²K) when installed on double glazing. If the same blind was installed with a cassette, casing or channels on the same double glazing then the U-value would be improved by 25% to 1.9W/(m²K). A product such as an external insulated shutter which is fully sealed is likely to give a lower U-value and be more effective in reducing winter heat loss than an internal solution.
Figure 12. U-Value of blinds combined with different types of shading
EN standard 13125 (11) provides the methods and formulae for calculating different values. For more information see Solar Shading for Low Energy Buildings (12). U-Values for blinds combined with glass can be found on the ES-SDA database, see www.es-so-database.com
The U-value of glazing is always improved with shading.
Correctly specified blinds and shutters can significantly reduce the capital and energy running costs of HVAC systems. Comparisons of HVAC, lighting, glazing and shading systems are not straightforward as the systems should all interact with one another and are interconnected.
A positive design benefit for one aspect could negatively affect another. In addition, in office buildings the direct transmission of sunlight that can cause thermal discomfort and glare issues that will not be solved with a passive solar protective glass nor with a mechanical ventilation system. In this situation solar shading is the appropriate solution to provide comfort to the building occupants. The real savings in buildings will be reflected by improvements in productivity of the employees by providing a comfortable working environment.
Therefore a holistic approach combining all different aspects is essential. It is precisely this holistic requirement which means that solar shading should be considered at the planning phase of a building or building refurbishment. Solar control management using blinds and shutters must be a building services consideration.
Computer simulations carried out by REHVA and ES-SO considered a typical modelled office under two different scenarios:
The table below shows a comparison of the two model scenarios under Amsterdam’s climate conditions. The same models were used for Stockholm and Madrid climate conditions.
Both of these cities also showed a payback for the capital cost of the solar shading in less than a year. In table 4 the results are for Amsterdam results for other cities can be seen in Solar Shading for Low Energy Buildings¹.
The costs shown are those at the time of the study and are for illustration but the savings will be proportionate if applied to levels at the time of reading.
|Solar control glazing
|Low e-glazing with solar shading
|In this example the payback is less than one year
Table 4. Example investment and payback for solar shading versus low-e glazing.
The costs are those at the time of the study but will be proportionate at current rates.
The table shows that overall the payback period for the solar shading is less than one year. This is because the investment required for the shading is lower than the investment for the advanced solar control glazing and HVAC costs. This demonstrates that shading represents an investment rather than a cost by realising significant reductions in running costs (lighting, cooling and heating). In the Amsterdam model, a reduction of 20% is seen between the office without shading and the office with window blinds installed.
Figure 13. Solar control glazing with HVAC and extra investment required depending on the amount of glass used
The graph in Figure 13 shows the amount of investment required to install shading compared with the base case of investment for HVAC and solar control of glazing. This shows that when a building has a glazing percentage of greater than 50-60% (depending on the location) it requires no extra investment to install solar shading. This is because significant savings will be achieved through reduced running costs for lighting, cooling and heating and a smaller HVAC system can be specified saving additional capital investment.
As mentioned in Appendix A when solar gain passes through glass, it hits objects in the room and it changes from short wave radiation, which is mostly light, to long wave radiation which is mostly heat. As glass is opaque to long wave radiation, it does not allow it to pass through and so it becomes trapped in the room. This is known as the greenhouse effect.
Solar glass has been developed to reduce this effect but there are limitations on the ability of glass alone to reduce solar gain. Even the most effective glazing combination currently available cannot improve on a g-value of much better than 0.22 (that is 78% heat rejection) and to achieve that figure there has to be a compromise with a higher U-value and other effects.
For larger areas of glazing a g-value of 0.22 will not be sufficient for thermal comfort without additional measures such as mechanical cooling to reduce the heat gain.
Shading stops the solar gain before it reaches the glass and can improve the g-value of 0.22 by 10 fold to a Gtot of just 0.02. Admittedly this is with the shading in a closed position and so would be suitable for an occupied area in a low energy domestic property. For an occupied or working area shading that allows outward vision is more beneficial so a Gtot nearer to 0.10 or an adjustable blind in the working position at better than 0.10 should be considered. This is still a 100% improvement and in most situations will reduce the gain to adequate comfort levels.
It will also avoid the need for high specification glazing, enabling the façade to be configured more cost effectively for the lowest U-value. If the only option to meet building regulations is to reduce the area of glazing, consider dynamic shading as the solution. This is very effective and will allow large glazed areas to be used.
The Shard in central London is an excellent example of where glazing and shading combined allow a fully glazed building to pass building regulations and be comfortable to live and work in.
The double skin façade of this tallest building in Europe had a g-value of glazing of 0.68. The use of fully automated blinds improved this performance to a gtot figure of 0.12 (88% heat rejection).
Shading with blinds are the facilitator of large areas of glazing where heat gains without them would be intolerable.
There are many different technologies available to improve the energy efficiency of domestic buildings resulting in reduction in energy bills. This appendix concentrates on options that reduce the amount of heating a home requires and compares energy saving rates of different domestic technologies.
|Single glazing to
Table 5. Energy savings of home improvements - Source: BBSA and Energy Saving Trust
Technologies in this category include: loft insulation, cavity wall insulation, double glazing and blinds and shutters. All of these will improve the thermal performance of a building by reducing the amount of heat loss through the building elements such as walls, roofs and windows. The UK Energy Saving Trust estimates that 18% of all heat loss is through windows, 33% through walls and 26% through the roof.
The table above compares the indicative installed cost, annual savings, payback period and CO² savings associated with a range of domestic heat loss reducing products. The values are all based on a three bedroom semi-detached house with single glazing.
It is clear that double glazing is the most expensive of the products considered and therefore has the longest payback period. Loft insulation (0 - 270mm) shows the shortest payback period, highest annual saving and has a low initial cost. Installed on single glazed windows the blinds have a payback time of 5 years and can save around €120 per year on heating bills which makes them competitive with other domestic energy saving products in terms of recouping investment.
The values are those at the time of the study but are proportionate if updated to values at the time of reading.
Figure 14. Glazing and shading - better insulation
Blinds installed on single glazed windows are competitive with the other domestic energy saving products.
Traditionally air conditioning has not been used in domestic buildings as they tend to have smaller glazed areas than commercial properties where there has been more tolerance of excessive heat. The development of larger glazed areas has created rooms where the heat gains are intolerable without shading with blinds. The use of fans and, in particular, cooling systems is the least energy efficient option, as with commercial buildings, and the use of shading with blinds and shutters should be the first consideration.
Figure 15. Using external shading to reduce heat gain
In addition, the regulatory requirement for energy efficiency and near zero carbon emission buildings has resulted in a drive for more highly insulated and airtight dwellings, in both new build and retrofit.
Highly insulated and airtight low and zero carbon homes, often designed with large areas of glazing, mechanical ventilation and/or communal heating systems, have the potential to overheat throughout the year, not just in the summer months.
In rural and suburban locations it may be easy to use natural ventilation (e.g. window opening) to help cool dwellings, but in urban and deep-urban locations it is often not possible due to excessive noise, pollution and security concerns.
Overheating will also become an issue where cross ventilation is not possible in airtight houses and especially in single aspect apartments with little or no solar shading²
For effective energy savings, blinds and shutters should be considered at the building design stage. If shading is not considered when the area and type of glazing are decided, it is almost certain that the building will require larger than necessary heating and cooling systems.
The growth of air-conditioning has been identified in a study by the International Energy Agency³ as the greatest threat to our energy supplies. A study for ES-SO by the leading energy consulting group, Guidehouse⁴, analysed the difference between the continued growth of the use of air-con between 2021 and 2050 and the alternative of using automated external shading to ES-SO operational protocols. The cost difference for users in Europe was a staggering €288 billion.
The benefits of a solar shading system in terms of energy savings, the best use of daylight, improved indoor conditions and visual comfort can only be fully optimised if the system is automatically controlled. It will respond to the climatic conditions even when the occupant is absent, throughout the day and night not requiring any attention. Automation is needed to maximise the energy saving benefits.
Automated solar shading can operate on timers and with light and heat sensors that ensure the shading is operating in the most energy efficient way.
There are different control systems to suit a single blind, groups of blinds or whole building management systems. Yet even basic “plug and play” single blind systems have energy-smart functions. Sensors such as those measuring wind, light levels or temperature or the use of timer controls will ensure that the shading is in place to help control heat loss and gain.
For multiple windows there are two solutions. Either a basic system that can be expanded to enable multiple controls as a stand-alone solution, or a system that links into the building management controls.
All of the systems require sensors that will track the actual climatic conditions around the building. The most frequently used sensors are wind, outdoor temperature, rain, indoor temperature and occupancy detectors.
Figure 16. Schematic showing automated systems controls
The stand-alone system is a cost-effective solution for domestic and smaller commercial installations where all of the control functions are available.
Simple interfaces are available but to maximise energy efficiency a holistic approach to control is essential by linking all of the building services - lighting, heating, cooling and shading (Figure 16). That is why home automation for domestic dwellings and Building Management System (BMS) for commercial properties that function with a “bus” system are becoming increasingly popular. A bus is a network where all devices are attached directly to a line and all signals pass through all of the devices. Each device has a unique identity and can recognise the signals that are intended for it. The market offers a range of solutions but probably the most common are KNX and LON. All of these are based on an “open” data transfer language that is non-proprietary and allows different suppliers to connect to the bus.
This allows the user to create a control strategy to prioritise their particular requirements. The possibilities are almost limitless but early discussion between the system designer and the supplier of the shading system is essential so that the full benefits can be achieved. Today, computer technology and advanced software offer vast possibilities to interconnect systems in a building. However, that does not necessarily mean better energy efficiency or user satisfaction. It is easy to make the daily use of the system more complicated than needed.
Before designing a system it is recommended that some basic questions are considered:
A building services engineer and solar shading specialist will be necessary while a good dose of common sense will also be helpful.
Keep it simple when designing automated systems
A good quality internal environment has a positive effect[db6] [db7] on well being and comfort. That then improves health and productivity, this is visually demonstrated in the Velux video “the indoor generation”¹ It is widely acknowledged that the visual and thermal comfort of an internal environment will affect people’s relative comfort and consequently their work. This is visually demonstrated in the Velux video “the indoor generation” ¹
This can be true in schools, offices, factories and any other building where productivity can be affected by an uncomfortable working environment². Research from the University of California in Los Angeles found that employees of companies voluntarily adopting environmental practices and standards are 16% more productive than the average worker³.
Working in a comfortable temperature in a building can have a significant impact on employee productivity. However, building designers often do not consider thermal comfort as a priority and believe that it is more important to reduce construction and operating costs. But when the whole lifetime of a building is assessed, it is clear that employee’s salaries and associated costs are significantly higher than building operating costs. Add to that the cost of employee absence for the health issues that can be created, the benefit is obvious.
The flowchart below (Figure 17) shows that salary and employee costs typically account for around 80% of the total operating cost of a building. Increases in productivity will reduce the costs in this section due to increased efficiency and also reduced absenteeism.
If productivity is increased by 1% in an organisation, this equates to 0.8% of the total operating expense. This 0.8% is more than the total cost of energy for the organisation which is typically between 0.3 - 0.6% of the total costs.
This shows that even relatively minor decreases in productivity have a large economic impact and provides a valid case for organisations to invest in improving the quality of their indoor environments. Issues associated with incorrect indoor thermal conditions in a workplace include:
Figure 17. Typical operating cost of an organisation
Figure 18 shows the optimum temperature range for productivity for office based work. It shows that the ideal range is between 20-24°C. Above and below these temperatures there is a sharp drop in productivity as people become uncomfortably too hot or too cold, and find it difficult to concentrate . Productivity drops by more than 1% for every degree that the temperature is outside of the optimum range. The impact of this fall in productivity will be significant to the organisation. The costs of an effective solar shading system to help prevent such drops in productivity through temperature variation is typically a fraction of the lost productivity cost.
This offers a different perspective on the seemingly high capital cost of external shading for building control coupled with internal shading for user control. However, the capital payback is quick and the ongoing running costs for cooling will typically be significantly reduced. Where internal shading is the only option the payback on the additional investment using the most effective shading solution instead of a basic option is quickly recovered. The added value of a comfortable workforce is a welcome bonus.
People prefer daylight to electric lighting in buildings. It is clear that visual contact with the outside world affects people’s state of mind and it is proven that it can increase productivity as people will feel happier. In a study, by maximising the use of daylight without glare and providing daylight responsive lighting control, productivity in a workplace increased 3.75%. Dynamic solar shading that reflects and directs natural light will also have a significant effect on improving visual comfort and reducing lighting costs. In addition, many studies have demonstrated that solar-protective glass alone could not guarantee glare protection since it cannot reduce the luminance of the sky view or bright objects to sufficiently low levels.
¹ The indoor generation – Velux www.vimeo.com/270055806
² Wargocki P. and Wyon D. The performance of schoolwork by children is affected by classroom air quality and temperature. Proceedings of Healthy Buildings Congress 2006a, Lisbon Portugal.
³ Heschong L., Windows and Offices: A study of office worker performance and the indoor environment. Report prepared for the California Energy Commission, 2003a.
A double skin system consists of an external glazing, a ventilated cavity and an internal glazing. Overheating and excessive light transmittance can be overcome by shaded Double Skin Façades.
A double skin system consists of an external glazing, a ventilated cavity and an internal glazing. There are two types:
Naturally ventilated façades - composed of an external single layer of glass and an internal double glazing unit. The cavity between the two skins is naturally ventilated with outdoor air, which comes up through the base of the glazing and returns to the outside at the top.
Mechanically ventilated façades - composed of an external insulating glazing unit and an internal single layer of glass. The cavity between the two skins is ventilated with return room air which is extracted from the room at the base of the glazing and returned to the air-handling unit at the top.
All of these main issues can be overcome with a shaded DSF.
Glass is a static element in a building. It is static against the dynamics of the weather and static in the face of dynamic building loading and use, as the number and the needs of the people inside will change constantly.
Shading is an established technology that is not normally associated with energy efficiency and its energy saving benefits are generally not fully appreciated. Gtot solar gain rejection figures for shading between the façade are improved by the natural ventilation within the façade. This enables fully glazed areas that would not otherwise be achievable within the requirements of the regulations with single skin facades. With DSF systems with shading typically rejecting better than 88% of the solar gain.
The REHVA Guidebook “How to integrate solar shading in sustainable buildings“ ¹ shows that when the correct ventilation strategy is used, a blind placed between the outer and inner glazing of a DSF will have a similar effect in solar energy reduction as an external blind. This resolves the issue of interruption of the external appearance of the clean glazing lines and removes any weather protection concerns enabling the shading systems to be fully functional throughout the year.
The graph in Figure 19 shows that a DSF with integral blinds has almost the same net cooling demand as a standard façade with an external shading system. It also illustrates that a traditional façade with a standard internal blind typically has a much higher cooling demand.
Figure 19. Cooling demands with shading
In summary, some of the benefits of integrating blinds into DSF are:
Blinds in Double Skin Façades resolve the issue of interruption of the external appearance of the clean glazing lines and removes any weather protection concerns.
Blinds and shutters are important enablers of daylight as they regulate the flow of both direct solar radiation and diffuse radiation.
Blinds and shutters will not only improve the quality of an indoor environment and therefore productivity but they will also reduce the energy costs of an organisation through reduced heating and cooling requirement and reduced use of electric lighting.
Solar control management by blinds and shutters can harvest natural light which can help reduce the use of electric lighting. In a typical office building 30-40% of electric energy is spent on lighting so this represents one of the biggest uses of electricity in commercial buildings.
Natural daylight is far healthier than artificial lighting and is also a free energy source. Dynamic shadings are an important enabler of daylight as they regulate the flow of both direct solar radiation and diffuse radiation. The illuminance, or what is most commonly called brightness, is measured in lux. Illuminance levels by direct sunlight in summer are as high as 100,000 lux. For a general office environment (tasks such as computer work, writing, drawing) a recommended lux level at the work desk is 500 lux. Direct sunlight can also cause glare so attenuating and diffusing the incoming light reduces the chances of glare and brings light deeper into the space. This function is performed especially well by internal blinds.
An important aspect of visual comfort is glare control: making sure the bright sunlight is filtered enough so it does not cause annoying reflections on furniture, objects and computer screens.
Glare control is one of the characteristics of shading materials that the norm EN 14501 classifies to enable users to compare different solutions in an easy way. With the trend for larger areas of glazing glare has become a more significant issue
For the latest revision of the standard an elaborate study about daylight in buildings on which the updated classification is based was conducted by several research institutions across Europe who participated to develop a daylight glare probability index (DGP) using scientific data.
The study group wanted to find out in which situations people experience discomfort glare. This new methodology is an approach to consider both the amount of light falling on the eye and individual glare sources of high illuminance to estimate the percentage of persons affected by glare. This is when the light is not blinding but causes problems nevertheless like headaches and fatigue. Discomfort glare is however not directly measurable because it is a subjective rating.
From the results of the study a calculation software was developed to predict the daylight glare probability for different situations depending on the orientation of the façade, the amount of glazing and the viewing direction (parallel or diagonal to the window).
The daylight glare probability (DGP) predicts which percentage of people will be disturbed by glare. If the DGP is lower than 35% the glare is classified as imperceptible, above 40% it is classified as disturbing.
Glare: “condition of vision in which there is discomfort or a reduction in the ability to see details or objects, caused by an unsuitable distribution or range of luminance (the intensity of light emitted from a surface per unit area in a given direction), or by extreme contrasts.”
Discomfort glare: “glare that causes discomfort without necessarily affecting the view of objects.”
It is the ability of the solar protection device to control the luminance level of windows, doors and façades and to reduce the luminance contrasts between different zones within the field of vision.
Glare control is a function of diffuse light transmittance (τv, n-dif ) and direct light transmittance (τv, n-n), which are the two parts of the total light transmittance (τv, n-h )
The chart below shows the classification
|3< / <=6
|6< / <=10
|10< / <=15
|15< / <=20
|20< / <=25
|3< / <=5
|2< / <=3
|1< / <=2
|0< / <=1
These two components of Visible Light transmittance (Tv=Tvdir+Tvdiff) are the direct light transmittance which is related (but not equal) to the openness coefficient. The diffuse light transmittance relates to the colour (the darker the colour, the lower Tvdiff). The openness factor OF is an indicator of the amount of direct visible light transmittance
The Openness Coefficient is the ratio between the area of opening in the fabric and the total area of the fabric so, put simply, this is related to the number and size of the holes in the fabric.
Colour is another factor to consider when selecting for light control a white screen fabric with a low openness coefficient can perform worse for glare than a black fabric with a higher openness coefficient. Both factors are important and should always be considered together to make a good choice
Darker colours also provide a better outside view (with screen fabrics) and glare free environments making them more suitable for viewing computer and TV screens but they absorb more light and thus are less beneficial for control of solar gain.
Another option for user control of glare are venetian blinds that can be angled to reduce direct light transmittance
As well as controlling light levels, glare and heat there is another important consideration and that is the quality of the light. Solar shading combined with clear glazing can provide the full spectrum of visible daylight and maintain a high Colour Rendering Index (CRI). CRI measures how faithfully colours appear in natural light and artificial light.
Tinted glass designed to reduce g values affects CRI and using clear glass with dynamic shading will reduce gtot and maintain the quality of light at the same time.
“When designing for maximum daylight (and views), designers must evaluate and balance a number of environmental factors, including heat gain and loss, glare control, visual quality, and variations in daylight availability in different seasons and climates. Appropriate interior or exterior shading devices to control glare and reduce solar gain will help provide better visual comfort and reduce the need for additional cooling.”
World Green Building Council