As buildings account for almost 40% of the total primary energy use in Europe, pressure has grown to make them more energy efficient. The savings potential is huge. It is not uncommon today to have buildings that consume a total energy of more than 250 kWh/m² per annum, whereas some state-of-the-art technology buildings show figures well below 100 kWh/m² per annum.
To assess the energy performance of buildings, countries in Europe use software tools for dwellings and for non dwellings. Both tools are used to demonstrate compliance with the Building Regulations that are derived from the European Energy Performance of Buildings Directive (EPBD), but unfortunately do not currently allow adequate credit for solar shading measures.
Several European countries are working on legislation limiting maximum building energy use to 50 kWh/m² per annum. At the same time, Passive and Active House technology is gaining market share and the European Parliament has stated that all new buildings should be near zero energy buildings from 2019 (public) and 2021 (dwellings) onwards.
“As buildings account for almost 40% of the total primary energy use in Europe, pressure has grown to make them more energy efficient. The savings potential is huge. It is not uncommon today to have buildings that consume a total energy of more than 250 kWh/m² per annum, whereas some state-of-the-art technology buildings show figures well below 100 kWh/m² per annum.”
Controlling the entry of light and solar heat has a considerable beneficial effect on the energy needs of a building. It is a key element for improving the daylight and energy efficiency management of existing buildings and for optimising the low-energy designs of new buildings. Solar shading, although a proven technology, is still under-utilised although it has a major impact on the reduction of energy consumption of buildings whilst simultaneously improving the thermal and visual comfort of the occupants.
However, solar shading is only one element of the building envelope, along with glazing, window frames, walls, doors, roofs and floors. When we think of energy saving we tend to think of insulation. Solar shading in these terms can also be considered as the insulation of the transparent parts of a building - the glazing.
Indeed, solar protection devices enable us to adjust the properties of windows and façades to weather conditions and, most importantly, the needs of the occupants. Effective management of these systems can maximise the solar heat gains in the winter, so reduce the heating loads, and minimise heat gains in the summer to reduce cooling loads.
“Solar shading is the insulation of the transparent parts of a building.”
Shading can reduce heat gain, reduce heat loss, control light and provide visual comfort.
Since 2008 multiple reports are emerging in Northern, Mid and Eastern Europe demonstrating that low energy buildings are sensitive to overheating. See on ES-SO homepage In a report by the NHBC Understanding Overheating - Where to start (2012) amongst the multiple causes are doubleglazed windows with low-e coating that prevents the heat from escaping. The report furthermore states that houses with unshaded west-facing glass will suffer from higher levels of solar gain during the warmer parts of the day due to the glass coatings.
Prof Heiselberg from Aalborg University in Denmark highlights in his work on passive cooling project (part of the EU Venticool) that modern air-tight and well-insulated buildings have an increased need for cooling, not only in the summer, but throughout most of the year. He states that indoor temperatures that are too high is the most common reported problem in post-occupancy studies, even during the heating season.
And it is not just homes that are prone to overheating. Lord Krebs of the Climate Change Commission highlights a recent study headed by Prof Alan Short of Cambridge University. It states that types of hospital ward that are vulnerable to overheating currently make up 90% of the total stock by floorspace. The report goes on to state that one solution is external shading devices as retro-fitting air conditioning would be uneconomic.
In both homes and hospitals, solar shading is identified as a key, passive solution to overheating. The World Green Building Council report Health, Wellbeing and Productivity in Offices - The next chapter for green building (2014) identifies four drivers for green buildings that are conducive to healthy, productive occupiers:
- Good design (such as passive solutions, shading, and natural ventilation where possible)
- Good construction (new technologies, innovation, smart controls)
- Good behaviour (appropriate clothing, adaptability and engagement with systems)
- Good location (enabling low carbon commuting and easy access to services and amenities)
It is very telling that in this global report on improving the internal environment in offices, the first driver identified is good design utilising passive solutions like shading. It should be noted here that it is almost impossible to cool down a person subjected to overheating by direct sunlight by just mechanical ventilation alone as the thermal comfort depends on the ambient temperature and the temperature of surrounding surfaces.
Solution exterior and interior shading
Solar shading is a broad term used to cover techniques that limit the entry of excessive solar energy. These techniques include shading using fixed awnings or brise soleil all the way through to fully automated blinds and shutters. In the heating season, solar shading can also be used effectively to retain the desired heat as it reduces the thermal conductance of the glazing and therefore acts as an insulator.
Weather conditions such as light and heat change constantly in the course of one day. That is why, in the context of this guide, there is an emphasis on automated systems which help us to benefit from optimal indoor conditions.
When designing a new building or preparing works to an existing one, it is necessary to take into consideration all of the characteristics of solar protection devices in order to select the correct product and façade management strategy. Shading products have an impact on the solar heat transmittance, daylight transmittance and also on the insulation of the façade. Consequently it is necessary to identify the product that will best balance these effects depending on the building properties, its location and orientation.
This guide is intended to provide the basic knowledge of how solar shading characteristics are evaluated and what properties are involved in the transmission of the solar radiation in relation to the whole energy balance of the building. It is based on calculation methods taken from European Standards.
Examples of simulations highlighting the impact of solar shading on the total energy loads of buildings are shown in Appendix - D (Data and calculation tools).
Selecting the right shading system is an energy as well as a design decision.
There is no straightforward answer to this question.
The sun gives us both daylight, essential for health and well-being, and free passive solar heat energy. The optimal shading solution should:
- Stop or minimise overheating in the summer
- Allow passive solar heating in the winter
- Optimise daylight utilisation in the building
- Provide visual contact with the outside
In addition solar shading should prevent glare problems which are common typically in office buildings and where seating and view positions are fixed.
The main energy cost associated with dwellings is heating, whereas in commercial buildings it is cooling and lighting.
As the position of the sun moves conditions inside the building change throughout the day and also throughout the year. The choice of shading should be determined not only by the building’s orientation and location, but also by the building type and the activities that take place inside. A building with high internal heat gain, like a highly glazed office, will typically require more shading than a domestic building.
Different types of glass and frames will exhibit a different performance when shading is added. The size of a window and the amount of the glazed area will affect the choice of the shading. It should be noted at this point that the selection of the right shading device is an energy affecting decision as well as a design decision.
To help you select the most appropriate shading for your requirements use Appendix - C (Checklist of requirements of solar control management) to prioritise your needs.
Providing clear energy ratings has proved to be a very effective in improving the energy consumption of many products such as white goods. Producing a simple rating system for shading to cover all of the variables would be difficult to achieve. Ideally we need to know how the shading will perform in conjunction with the different types of glazing also taking into consideration the orientation of the building and its specific location.
The purpose of a typical A - G rating is to allow a simple comparison of products in an example situation and to provide an easily understandable label. The A - G rating for tyres for example does not determine the exact
performance level. It is a tested measure indicating the rating that could be achieved based on typical test conditions.
ITRS, the German Solar Shading Trade Association working with the German laboratory ift Rosenheim has developed a simple rating system for shading products when combined with glazing.
A labelling system is also being developed for construction materials to meet The Ecodesign of Energy Related Products Directive 2009/125/EC, a framework directive primarily focusing on energy in use.
This Directive sets minimum requirements for certain energy consuming products and one of the first to be developed will be for windows combined with shading products. This is currently at the consultation stage but it is unclear whether there will be an outcome.
Until then an Energy Performance Indicator (EPI) can validate the information provided by manufacturers. It confirms that performance data has been calculated correctly using the ift Rosenheim rating system with peer reviewed validated data. (See Appendix - D (Data and calculation tools) and the ESSDA database)
The performance figures for solar shading must be calculated to European Standards. (See Appendix - D (Data and calculation tools) and section 4.0 of the guide) For a detailed assessment of building performance site specific computer modelling should be used.
The science of dynamic shading is simple - to keep the heat out in the summer and keep the heat in during the winter. However, calculating the effects is not so simple. Make sure that the data you are using is correct.
To understand solar gain we need to understand how the sun’s rays work. Solar radiation contains a wide range of wavelengths – short wavelengths mainly correspond to visible radiation whilst long wavelengths are perceived as heat. Glass allows most of the shortwave radiation (light) to pass through it but will not transmit
most of the longwave (heat). So the sun’s rays pass through the glazing as shortwave, hit objects in the room such as walls and furniture which absorb the radiation and re-radiate it back as longwave heat.
That heat cannot pass back through the glass as this is opaque to longwave radiation. However, this is absorbed by the glass causing a warming effect internally. This same phenomenon happens in the atmosphere and is known as the Greenhouse effect. See Appendix - A.(What is solar gain)
Blinds, awnings and shutters can help prevent excessive solar gain by blocking some of the incoming solar radiation.
External blinds and shutters are very effective as they prevent the radiation from even reaching the window as the heat is convected to the outside air.
Although less effective, internal blinds also reduce solar gain. This is especially true of fabrics that have a reflective finish facing the window which will effectively reject some of the incoming shortwave radiation, therefore not allowing it to be absorbed and turned into heat. For a more detailed assessment of the effects of solar gain see Appendix - A (What is solar gain?).
When considering the performance of solar shading devices there are three important measures to understand: U-value, g-value and Tv.
KEY PERFORMANCE MEASURES OF SOLAR SHADING DEVICES
U-value is a measure of thermal conductance which is the ability of a material to transfer heat.
g-value is the measure of the total energy transmitted through glazing when exposed to solar radiation.
gtot - is the measure of the total energy transmittance of the glazing in combination with the shading when exposed to solar radiation.
Tv is the measure of the proportion of visible light transmitted through the shading material.
These values must be always measured in conjunction with the glazing.
In effect this is a measure of solar heat gain.
Also called Solar Factor, g-value is the measure of the total energy passing through glazing when exposed to solar radiation. It is the sum of two values; the solar transmittance, Ts, which is the heat absorbed by the glass and re-radiated as longwave inwards and the secondary internal heat transfer factor Qi.
When the g-value of the glazing is combined with the value of the shading, this is known as the gtot value.
The lower the g-value or gtot, the lower the heat gain. The value of gtot is between 0 and 1, where 0 equates to no radiation being transmitted into the room and 1 means all radiation (100%) is transmitted. So a gtot of 0.25 (25% heat gain, 75% heat rejection) reduces heat gain three times more effectively than a gtot of 0.75 (25% heat rejection). External shading helps to significantly reduce gtot values and has a much more significant impact on gtot than internal shading.
For energy calculations it is necessary to use the combined figure of the glass and shading and not just the shading alone. A detailed assessment is shown in Appendix - E (g value calculation).
U-value is a measure of thermal conductance which is the ability of a material to transfer heat by conduction, convection and radiation. All components of a building have U-values - e.g. masonry, insulation materials, plasterboard and windows.
The lower the value, the slower the heat loss through the material. Therefore a material with a low U-value is a good insulator. The U-value of glazing is always improved by installing blinds or shutters. For a more detailed explanation see Appendix - F (U value calculations).
As with g-value for energy calculations, it is necessary to take the combined figure of the glass and shading and not just the shading alone.
Blinds and shutters can prevent excessive solar gain by blocking some of the incoming shortwave solar radiation and retaining heat when necessary. This is why it is important to know the gtot and the U-values.
The lower the g-value or gtot, the lower the heat gain.
The U-value of glazing is always improved by the installation of blinds or shutters.
This measure refers to the fraction of visible light transmitted into a room. As with the g-value we have to consider the measure of the glass in combination with the shading device. The value is between 0 and 1, where 0 means no light is transmitted and 1 means all (100%) visible light is transmitted. A Tv value of 0.25 means 25% of light is transmitted.See Appendices A (What is solar gain?) and N (Light and glare control) for more information.
This database includes energy performance data of blind and shutter fabrics and materials independently validated to European standards. The database calculates the energy performance of blind and shutter products when used in combination with reference (typical) glazing as defined in the standards ISO EN52022-1. All calculations are also performed in accordance with the relevant European Standards.
The process of independent peer review and use of European Standards employed by ESSDA is identical to the glazing industry database (1) and is a robust and effective way of ensuring the integrity of the database (2).
- Total solar energy transmittance, gtot (amount of heat gain)
- Thermal transmittance, U-value (amount of heat loss)
- Visible light transmittance, Tv (amount of daylight)
A detailed assessment of the calculation methods and data required is shown in Appendix - D (Data and calculation tools).
Installation of blinds, shutters and awnings needs to satisfy a wide range of requirements and there has to be a balance in fulfilling those requirements. Designing for the best energy saving is unlikely to produce the most satisfactory solution.
For example, one of the most effective energy saving solutions is an external blackout roller blind. On the ESSDA database this will probably show one of the best gtot performances, better than 0.05 (95% heat rejection), and good U-values as well but would it work in practice? Almost certainly not, with no view through and no daylight transmission, resulting in extra energy cost for artificial lighting. This is an extreme example but it shows the need to choose the most suitable, practical solution that best reflects the requirements of the occupants and the building.
So when selecting the right shading solution we need to consider what is best meets the needs of the building’s occupants. The creation of a checklist of the requirements is necessary to recognise and prioritise the most important criteria. See Appendix - C (Checklist of requirements of solar control management) for an example.
The completion of this checklist may lead to the same conclusion as several scientific studies conducted by the Fraunhofer Institute of Building Physics (3) amongst others. These show that ideally a window requires both internal and external shading as the solar heat gain may be desirable in winter while the sunlight could cause glare. In this instance, the interior shading would be used to manage glare but still admit some free passive solar heat into the building. The exterior shading would be used to reduce solar radiation in the
summer resulting in a reduction in overheating and the need for artificial cooling.
Effective shading can allow highly glazed buildings to be built respecting the latest building regulations.
Solar shading typically installed in commercial buildings can have a significant impact on the size or even the requirement for air-conditioning systems. The savings in terms of installation, running and maintenance costs of air conditioning will far outweigh the cost of a typical solar shading installation, especially if full lifecycle costs are considered.
Selection of solar shading should always be one of the first steps in the design of HVAC systems. It will prevent unwanted solar heat from entering the building and so avoid the need for additional cooling to remove this heat, saving energy and money. This is particularly important in highly insulated and airtight buildings such as near zero energy (NZEB) buildings.
In winter when the shading is raised the free heat from the sun is desirable as it reduces the building’s heating costs.
Appendix - G (Commercial building shading cost-benefit analysis) shows a cost-benefit analysis from computer simulations of a typical office building using solar glass without shading compared to double low-e glass with shading. Not only is the capital cost lower but there is a continuing payback from reduced running costs. In fact effective shading allows highly glazed buildings to be built in compliance with the latest building regulations.
Moreover significant productivity performance improvements can also be achieved with a controlled internal environment and an analysis can be seen in Appendix - L (Productivity and internal environment).
“Selection of solar shading should always be one of the first steps in the design of HVAC systems.”
Olli Seppannen, REHVA
Typically domestic buildings do not have mechanical cooling so the greatest energy savings will come from heat retention. However, as the housing stock becomes better insulated and air-tight overheating is increasingly becoming an unintended consequence even during the heating. Shading and shutters help to provide a more comfortable environment without the cost of artificial cooling.
As significant carbon dioxide emissions come from domestic buildings (4), improving the energy efficiency of the housing stock by improving heat retention will significantly reduce national CO2 emissions.
“The current development in building energy efficiency towards nearly zero energy buildings represents a number of new challenges to design and construction of buildings. One of the major challenges is the increased need for cooling in these highly insulated and airtight buildings”.
Per Heiselberg, Aalborg
Whilst by 2020 all new buildings are expected to be nearly zero energy only 1-1.5% are replaced every year (5).
In both new build and renovation, better insulation of the opaque (solid) parts of the building envelope is what is normally thought of first alongside reducing air leaks through windows and doors. What is often overlooked is the serious risk of overheating in the summer.
Windows are typically one of the weakest elements of all the building components. Windows are static whereas the position of the sun and weather conditions vary continuously.
Shading reduces or eliminates the need for active cooling in summer conditions by controlling the amount of solar energy entering through the windows. Correctly managed solar shading also allows harvesting of free solar energy in the winter season and offers additional insulation to the transparent parts of the building structure which helps to reduce heat loss in cooler weather and particularly at night time.
Solar shading will also control daylight admittance, reduce glare and improve visual comfort. When seeking to limit solar gain, the provision of adequate level of daylight needs special attention. An effective way to control the quantity of light transmitted through windows is by using blinds. For more details see Appendix - N (Light and glare control).
The development of the PassivHaus concept has created requirements for shading that have not previously been considered in traditional building design.
The PassivHaus principle encourages high levels of insulation and large areas of glazing to maximise winter heat gains. However, these positive heat gains have a detrimental effect especially in the spring and the summer when they become excessive, causing overheating. This could be reduced by a passive fixed systems to reduce high angle peak solar gain. However, this only solves part of the problem as in the winter months, when the sun’s path is lower heat gain will still be an issue particularly on east façades. Also the high level of insulation of this new building technology is likely to cause overheating unless a more effective external solar shading system is integrated into the design.
In order to achieve the best results it needs to be moveable allowing it to be lowered for cooling and raised for heating depending on the season. Several post-occupancy studies of high performance buildings report high temperatures as one of the most frequent problems of PassivHaus buildings (6).
The PassivHaus principle encourages high levels of insulation and large areas of glazing to maximise winter heat gains. However, heat gains have the opposite effect in the spring and summer when they become excessive, causing overheating. External blinds or shutters can be an effective solution.
For extensive commercial renovation that includes the replacement of the glazing or in high traffic zones,
consideration should be given to a double skin façade
that incorporates shading within the building envelope. This is an effective option that has the benefits of the external shading system utilising the natural ventilation created within the façade. The design of The Shard in London was only possible with highly effective automated shading within the façade that enabled the g-value of the glazing alone to be reduced from 0.68 (32% heat rejection) to a gtot figure of 0.12 (88% heat rejection). More detailed analysis of Double skin façades can be found in Appendix - M (Double skin façades) or in REHVA Guidebook Solar Shading - How to integrate solar shading in sustainable buildings (7).
The optimum energy saving benefits of solar shading will only be achieved if the system is controlled to react to changing outdoor conditions. The most effective way, especially for commercial buildings, is an automatically controlled system. It will operate effectively and react to the environmental conditions regardless of the occupants’ presence, ensuring the energy balance of the building is maintained. Control of the internal environment can be achieved with simple stand-alone systems or large scale systems integrated into the building or home management system. Indeed dynamic shading could even be operated with active solar energy since shading will typically be needed when the sun is shining. Automated solar shading systems will maximise energy savings and improve internal comfort and accommodate the varying needs of the occupants. Automation will also protect external shading from damage from high winds or extreme weather conditions. Careful consideration must be given to the selection of an automated system. For more detailed information please see Appendix - K (Types of automated control).
Behavioural change is a key part of the EU strategy to reach environmental and energy saving targets.
With energy prices set to increase even further in the future, the only way for home owners and businesses to save money on energy is to change the way it is used - that is behavioural change. When energy was cheap and plentiful there was no incentive to consider how energy was used and how it could be saved.
By adopting best practice when using shading, occupants will quickly realise how they are saving energy and money. Occupants’ understanding how the system works is essential for reaping its full benefits.
The World Green Building Council report Health, Wellbeing and Productivity in Offices - The next chapter for green building (2014) states that; “True sustainability is found at the ‘sweet spot’ of good design (passive solutions such as shading, orientation and natural ventilation), good technology (including air-conditioning, automation, and temperature control) and good behaviour (clothing, acceptance of wider temperature ranges and familiarity with systems). Often, good behaviour can be the most elusive to achieve and the hardest to maintain.”
Advice on understanding the benefits of, and applying, behavioural changes is shown in Appendix - B (Behavioural change).
Designers, architects and engineers, now more than ever, should consider the performance characteristics of solar shading, rather than their decorative value, at the early design stage of the building in order to satisfy low energy building requirements.
Solar shading is increasingly seen as an essential building service and one that positively affects the energy balance of the building when controlled and used correctly.
Unfortunately in many countries shading is only considered as the decoration at the window. It is important for that but it typically masks the positive impact correctly specified solar shading systems can have on façades and building services such as reducing the capital cost, running and maintenance costs and the effective control of the internal environment.
“The change towards an ecological design in the fields of urban planning, agriculture, manufacturing, and energy systems as well as architecture will require a major change in how we think and so changes in education at all levels.”
Prof. David W. Orr
In order for blinds, awnings and shutters to improve the energy performance and comfort of buildings they need to be correctly specified, installed and used. Automation will ensure the chosen solar shading devices provide the optimum benefit but manual systems can also be highly effective.
The power consumption of automated systems is minimal in comparison to the energy they save and with latest developments in solar power technology they could even be operated by free solar energy.
When considering the performance of a solar shading system, it is important to remember:
- Blinds and shutters should be considered at the design stage of the building as they will affect other building services. This early design stage thinking also ensures a better architectural integration of the shading device with the façade design
- Blinds and shutters must always be considered in combination with glazing to determine energy saving calculations
Solar shading will have a beneficial impact on the specification and will result in reducing the costs of:
- Artificial lighting
- Heating systems
- Cooling systems
- Mechanical ventilation
Solar shading systems can be installed externally or internally or within the glazing itself. In each position the performance characteristics will be different. Similarly, the type of glazing, colour, material, fitting, operation and position of the blind, awning or shutter will all affect performance.
With the ever increasing costs of energy and stricter control on building design and use, solar shading has an important part to play in managing the energy costs as well as improving the thermal and visual comfort of the occupants of a building. The available solutions are many and varied so expert advice should be sought from a ES-SO Association Member
The conclusion of several scientific studies is that ideally a window should have both internal and external shading. Internal shading is better for glare control while external shading is better for solar gain control.