Solar controls and shading
Provided by Nick Baker: Research Associate, The Martin Centre, University of Cambridge
What, Why, When, How, Extras
Daylight and sunlight are a single source of natural energy which we need to allow into our buildings through glazed apertures, as daylight for visual tasks, and sometimes as thermal energy for useful heat gains. Windows, also provide a vital connection with the world outside. However, the energy density of daylight and sunlight varies over a huge range, and it has to be controlled to prevent overheating and glare.
All too often, the control defaults to on/off – that is: if the sun is causing overheating – blinds down and lights on. Artificial lighting is then being used when there is a surplus of luminous energy available. The problem is to modulate it, and possibly to re-distribute it spatially to provide a glare-free working illuminance for a minimum heat gain.
This is what solar control should address, and it is achieved by elements in or close to the window opening that reduce the overall transmission of the window by obstruction or reduced transmission through transparent or translucent layers.
It is important to understand that daylight (or sunlight) contains both visible and invisible radiation, in about equal proportion. When they are absorbed in the room, both contribute to the heating effect. Most shading controls do not separate the useful light from the unwanted invisible radiation. Only certain special selective glasses can do this.
Lighting energy forms a large proportion of energy consumption in buildings, so it is vital to use daylight whenever it is available. On the other hand, solar gains are the commonest cause of overheating in buildings, and if the building is air-conditioned, it creates a large cooling energy demand. Shading and solar control can resolve the challenge to get sufficient daylight into the building for the visual task, without the massive over-illumination, causing glare and unwanted heat gains.
Direct sunlight typically carries between 5 and 10 times more energy intensity than diffuse light from the sky – (between 300 and 800 W/m2); a window of 3m2 in an office for two people could easily generate 1.5 kW of heating effect. This is about 4 times the gains generated by the occupants and their equipment. Also, the room would be grossly over-illuminated with illuminances in the direct sun of 30,000 lux, 100 times higher than necessary!
In warmer countries, where the penetration of direct sunlight is almost always unwanted, traditional architecture often demonstrates elegant solutions – deep reveals overhangs, fins and louvres, and the correct use of them is often part of the unconscious culture. That they were part of the norm, before energy-rich mitigation in the form of air-conditioning became available, is a good indicator of their vital role in passive architecture.
Solar controls should be considered for all glazed openings exposed to direct sunlight. Solar control is particularly important on south to west-facing facades, since the solar gains will coincide with the hottest part of the day. Solar control is also vital for lightweight buildings with large areas of glazing. Ironically, the modernist buildings of the 60s and 70s were often like this, but didn’t incorporate the 'encumbrance' of shading on aesthetic grounds.
There are some mitigating circumstances where it may not be necessary, even for exposed glazing. North facing glazing, receives direct radiation in summer in the UK, but it is very oblique and glass has low transmission at large angles. Heavyweight buildings with small areas of glazing, and low internal gains may be able to cope with little other than curtains.
1. Solar control devices can have two functions:
Fig 1: A light shelf obscures sunlight to the front of the room whilst reflecting light to the back of the room. The unwanted energy is represented by the area between the two curves – i.e. the solar gain is reduced by at least a half without compromising the minimum daylight levels. (Click image to enlarge)
a. They reduce the total amount of radiation entering the room by reflection and absorption.
b. They improve the distribution of the light in the room
(See figure 1)
2. Types of shading:
Shading can be placed into 4 categories
Type A Retractable - shutters, roller blinds and louvres
A moveable device which can adjust the total transmission of the glazed opening by partial obstruction and or diffuse reflection, e.g. louvres or roller blinds. This allows the relatively fixed demand for light within the room to be matched to the widely varying incident radiation intensity. They may also improve the distribution of light within the room allowing a lower total, for a given minimum (at the back of the room). At times of low sky brightness they can be withdrawn from the aperture completely.
Type B Fixed redistribution devices – overhangs, lightshelves etc.
A fixed structure obscures part of the sky through which the sun passes, e.g. reveals, overhangs, fins and light shelves. It is selective due to the geometry of the device in relation to the facade and its orientation. Lower intensity and more diffuse light is allowed into the room. Even the simple overhang improves daylight distribution since at allows a greater proportion of ground reflected light to illuminate the ceiling and thence the back of the room. However, it also obstructs a brighter part of the sky in diffuse conditions, and since it is fixed, this has to be compensated with a larger glazing area.
Type C Fixed reduced transmission devices – fixed grids, perforated sheets, tinted, reflective and fritted glass.
This category is where the glazed opening is made to have permanently reduced transmission. This achieves no more than simply having a smaller opening in the first place since it cannot be modulated, nor is it selective in the part of the sky it allow light from. Although it can be found in the work of notable architects, it is not to be recommended.
Type D Selective high performance glazing.
A relatively recent development is glass that has a lower transmission for the invisible part of the spectrum, than for the visible. This has the effect of improving the luminous efficacy of the daylight, rather in the same way that light from a florescent lamp has greater luminous efficacy than a tungsten lamp. The use of this is beneficial, but on its own it cannot respond to the wide variations of illuminance from the sky. It is best to be used in conjunction with Type A or B. Note that conventional tinted and reflective glass are not selectively transmittive to the visible light.
3. Location of shading devices:
Three options are possible: external, internal and mid-pane; they all carry advantages and disadvantages.
The term greenhouse effect was coined long before the application of the term to global warming, and refers to the mechanism whereby short wavelength solar radiation (visible and invisible) enters the room through the glass, to which it is transparent. The radiation is then absorbed by the room surfaces warming them up, and re-radiates, but due to the relatively low temperature the radiation is of long wavelength, to which the glass is opaque. Thus the energy gets trapped behind the glass.
Fig 2: A comparison between external and internal shading. (Click image to enlarge)
External shading devices are the most efficient thermally because they intercept the solar energy before it has entered the room. Thus, even if energy is absorbed by them, it is not trapped behind the glass. They carry the disadvantage of having to be weatherproof and are more difficult to control from inside, figure 2.
Internal shading is generally much cheaper to install and is easy for users to control, but is less efficient, for reasons outlined above, see figure. It is also vulnerable to damage.
Mid-pane shading devices have become more popular as technical problems have been overcome, and now can be installed in sealed, gas-filed double glazing units. Control of the louvres can be achieved through magnetic linkages. Or, they may be installed in much larger non-sealed cavities found in so-called double skin buildings. They carry some of the advantages of both (the above), and are particularly effective in double skin buildings where they are protected, but the cavity is large enough to be independently ventilated, to remove any absorbed solar gains.
4. Related strategies and conflicts:
It is important that the design of shading devices is closely linked with the design of the daylighting. The window, shading devices and room system must be seen as a single system for delivering daylight, view, minimising unwanted heat gains in summer, and (possibly) maximising them in winter. It is quite likely that the window will also be expected to provide ventilation air, and shading devices may interfer with this as well. Figure 3 shows how different kinds of shading device affect view and ventilation. Figure 4 shows how different blind materials have quite different optical and thermal performance.
Fig 3: The impact of shading types on vision and ventilation. (Click images to enlarge)
Fig 4: The optical and thermal performance of different types of blind material. (Click images to enlarge)
We have already mentioned that shading devices can improve the uniformity of daylight distribution, which is an essential part of daylight design. Figure 1 shows the effect on the daylight profile of a light shelf – reducing the over-illumination at the front of the room, without diminishing it at the back.
There is growing evidence that view is a very important function of windows and this can be seriously compromised by fixed devices, in particular medium scale grids and perforated sheets. Bearing in mind that these are often the least effective ones – i.e. Type C, this is indefensible.
We have combined some of the many conflicting properties of different shading options in Table 1 . Note that orientation refers to the suitability for shading on a particular facade, and is related to the sun angle and selectivity of the geometry where it is present.
Finally, shading and solar control devices have a great potential for architectural expression, adding to the texture and modulation of the facade. They also have the potential (and should) respond to the orientation of the facade, thus visibly reflecting the building’s place in the natural world as well as its urban setting.
Table 1 - shading options