In 2015, the Zero Carbon Hub1 indicated that 20% of the housing stock suffers from overheating. February 2018’s CIBSE Journal CPD article discussed some of the potentially serious consequences of overheating that relate particularly to the adverse effects on occupant health and wellbeing. The Committee on Climate Change2 notes that ‘people lack a basic understanding of the risks to health from indoor high temperatures and are therefore less likely to take measures to safeguard their and their dependents’ wellbeing’. Although interest in residential overheating has been popular, the challenges extend across the whole built environment, including commercial and institutional buildings. In 2016, Dr Angie Bone3 of Public Health England highlighted that 90% of existing hospital wards were overheating.
In equatorial, arid and Mediterranean climate zones, building technology has evolved to cope with the risks of overheating. These include the instinctive application of reflective roofs and walls, exterior shades, and low-emissivity window coatings and films to reduce or practically eliminate the adverse impact of solar gain. In temperate regions, such as the UK, the typical historical applications of solar control have principally been to cut problems from disability and discomfort glare, while also allowing the flexibility to benefit from useful solar gain during periods of cooler weather. In the past half-century, as building technology (and internationalisation) has enabled lighter-weight, highly glazed structures and – more recently – highly insulated, well-sealed constructions, the application of solar control has often become essential. The westerly facing aspect can suffer most notably, and is the greatest challenge, as the lower solar altitude during the afternoon can produce significant solar gain as the incident angle of the solar radiation (with vertical windows) increases transmission.
As outdoor temperatures rise, as a consequence of climate change, overheating is likely to become more common, so analysis is required at the design phase to ensure that appropriately holistic solar control measures and ventilation can be incorporated into the design. (This is discussed in the recently published CIBSE TM59 Design methodology for the assessment of overheating risk in homes.) The overheating report (as recommended by TM59) must produce dynamic modelling results both with and without blinds, indicating how significant they are in generating a ‘pass’ result. If blinds are part of the mitigation strategy, they must be allowed for in the model and then installed.
Determining the performance of solar control measures
The solar gain factor ‘g’ is used to express the proportion of heat gain into a space resulting from the total solar irradiance incident on the outside surface of the glazing. Practically, this has the same value as solar heat gain coefficient (SHGC), which is more popular in some areas of the world. Neither should be confused with the (formerly popular) shading coefficient, which is derived by comparing the solar heat transmittance properties of any glass with a clear float glass having a total solar heat transmittance of 0.87 (that is, clear float glass about 4mm thick).4 The solar gain factor includes the secondary heat transfer from the (warmed) glazing to the inside, so accounting for the heat transfer by convection and long-wave infrared radiation. (There is a more extensive explanation of the transmission properties of glass and glazing systems in Appendix 5.A5 of CIBSE Guide A 2015.)
Figure 1: The Shard, in London, has motorised blinds that are automatically controlled to track the angle of the sun
To account for temporal variations in performance, the total solar energy transmittance, gtot (formerly referred to as ‘effective g value’), is more useful in giving a more representative value for the performance of glazing and associated shading systems across the cooling season. In CIBSE TM37, g values and the various correcting factors are tabulated for a range of glass types (such as ‘low-e’ glass), blinds, louvres, overhangs and shades. It also provides correcting factors for moveable devices, based on the fraction of time that the shading is in place. All the data is related to the surface orientation, and corrections are given for different UK geographical locations. A gtot of 1 would indicate all incident radiation was being transmitted into the space and zero would be for a façade that totally reflected all incoming solar radiation.
The total solar energy transmittance can be calculated for combinations of glass, coatings, shades and blinds (as in the examples in TM37) so that the overall gtot can then be applied to determine the solar gain through the combined system. Solar and optical performance data of shading materials tested to European standards is available from the European Solar Shading Database at www.es-so-database.com
Methods of solar control
In a 2012 report on the performance of eight Danish passive houses, Larsen5 concluded that ‘it is important in the future to introduce the opportunity for active use of natural ventilation combined with external solar shading in our homes’, and again highlighted the importance of incorporation of a proper solar shading and ventilation strategy at the early design stages. Notably, it was highlighted that ‘thermal mass has a positive effect only as long as it is possible to cool the structure during the night hours… otherwise thermal mass may instead increase the overheating problems’. A recently completed overheating study6 by De Grussa, conducted in homes in Camden, London, indicated that thermal mass (even when combined with an appropriate ventilation regime) does act to adversely affect the occupied space, with the supporting data showing that the monitored rooms were 27°C even at 6am. Without shading, the maximum operative temperature was 47.5°C, but by applying external shading, operative temperatures were reduced by 10.6-17.9K, while with internal shading, these fell by 8.5-12.9K (at 95% confidence levels).
The wide variety of solar control glasses that are available are of two main types: absorbing glasses, which are body tinted; and reflective glasses, which have a specific surface coating. Reflective glasses are usually superior at rejecting incoming solar gain. Absorbing glass heats up more when sunlight falls upon it, and some of this heat can reach the inside of the building. Solar control films (for performance that approaches that of coated glass) can be added onto flat glazing. Although less durable than factory-treated glass, they can be retrofitted onto existing glazing systems.
As with fixed systems, both glazing and solar films will reduce useful winter solar gain and daylight. A US study7, undertaken in the 1990s, indicated that occupant satisfaction drops if the light transmittance of the window is less than around 25-38%. Dynamic solar shading could allow for more glazing to be used while maintaining light transmittance.
The most effective way to control overheating is to prevent sunlight from reaching the window, so static or dynamic external shading is particularly appropriate for heavily glazed buildings. Simple static overhangs can be highly effective at blocking high-angle summer sun, particularly on south-facing windows and, if properly fitted, do not obstruct their opening and retain a
full view. Problems from lower sun angles can be resolved with dynamic
Light shelves (an overhanging element installed part way up a window, typically just above an occupant’s head height), allows extra daylight to enter the space by reflection from the top of the shelf, passing through the glazing above it. Having an internal shelf as well as an external one helps control internal glare.
External horizontal slatted and screen roller blinds and other types of external controllable louvres can have the lowest solar gain factor of any system, as well as being able to maintain an outward view by using retractable blinds with occupant, or automatic, control. A more open-weave fabric blind can give a view out even when lowered (dependent on its colour), with glare from the sun controlled by selecting a screen material with a low visible light transmittance.
The solar performance of a mid-pane blind (located in between the two layers of glass) will be between that of an external and an internal system. Mid-pane blinds can be controlled from the inside of the building or through automatic control.
When implementing any mid-pane or external shading, maintenance and access for window cleaning should be carefully planned.
Figure 2: Sketch of the Shard’s ventilated and shaded façade (Source: BBSA)
Internal systems (such as roller blinds or louvres) can contribute towards solar heat control but tend to be less effective than external or mid-pane installations. The incoming solar gain will be absorbed by the shading device, and then part of the heat is convected or re-radiated into the interior space. A reflective coating on the outward face of the blind will reflect some of the solar radiation, usually transmitting less heat into the occupied space. Semi-opaque materials are available that control the solar gain while still allowing some daylight. Whatever the type of moveable blind, occupants must be properly educated in how to deploy them for best overall effect, so avoiding unwanted operational effects (such as excessive use of artificial lighting when daylighting could usefully be employed).
For new constructions and extensive renovations that include the replacement of the glazing (or where there are issues from external noise), a double-skin façade incorporating shading can offer an efficient option that effectively has an external shading system but which also benefits from the natural ventilation created within the façade.
CIBSE TM37 Design for improved solar shading control provides detailed guidance on the generic types of solar control methods summarised in this article.
The estimable and accessible guidance BRE 364 – Solar Shading of Buildings8 (and associated information papers) have recently been updated. As well as including a commentary on the credits that shading can contribute to environmental ratings, it provides a useful tabular summary of performance data for different shading systems. The table includes several parameters. However, in terms of ‘summer solar control’, external and mid-pane measures are clearly most likely to offer a suitable solution. (This does not exclude the need for a thorough life-cycle analysis, but it is a very useful starting point.) It notes that of the devices in the table, ‘only sophisticated external blinds systems are really effective at controlling both solar gain and sun glare.
Where both heat and glare are important, a hybrid approach is often best, perhaps with a brise-soleil, awning or window film to control summer heat, and internal blinds for glare’. Once again, the BRE guidance confirms the importance of solar shading being included at the design stage.
Some notable examples of applying solar control
The 2016 report9 by the National Energy Foundation (NEF) includes several independent case studies and analysis of the current and potential impact of solar shading in the UK built environment. As well as providing detailed information that is not necessarily in the other materials referenced in this article, it includes a study of the Shard, London (Figure 1), which is summarised here. The Shard has triple-skin glazing, with motorised open-weave roller blinds and a ventilated cavity between the inner and outer layers of glazing, as illustrated in Figure 2. The blinds are automatically controlled by a system that tracks the angle of the sun and solar gain throughout the year. The total solar energy transmittance of the double-skinned façade is approximately 0.12, and without dynamic solar shading the building could not have been built. The Shard is estimated to have equivalent annual CO2 emissions of 28.2kg·m-2, which would be around a third of the average (75kg·m-2) of the 50 new construction non-domestic buildings studied as part of Innovate UK’s Building Performance Evaluation portfolio.
Using blinds to reduce the solar gain means that glass can be more transparent (low-iron glass), improving daylighting and making the building lighter. The controls are designed to lower blinds when solar irradiance reaches 200W·m-2 (but can be adjusted if required).
The England Building Regulations Approved Document L2A requires a maximum gtot of 0.68 in summer – the Shard achieves 0.12.
For new UK buildings, where most blinds and shutters are within the Fixtures and Fitting (N10) section of RIBA’s National Building Specification (NBS), this can conspire to remove the correct integration of solar shading within the façade design. Solar shading should be considered at the design stage, and would potentially have more impact on the holistic energy assessment if it was part of the building services design responsibility. Since it is estimated that 80% of existing buildings will still be standing in 2050, there is also great scope for the adaptation of single and uncoated double-glazing systems in the building stock where there are issues with overheating, or simply to meet the imperative to reduce cooling loads, air conditioning costs and environmental impact.
© Tim Dwyer, 2018.
- Overheating in Homes – The Big Picture, Zero Carbon Hub, 2015.
- UK Climate Change Risk Assessment 2017 Synthesis Report: Priorities for the next five years, UKCCC, 2016.
- Presentation to the Good Homes Alliance, 25 November 2016.
- BS EN 410:2011 Glass in building. Determination of luminous and solar characteristics of glazing.
- Larsen, TS et al, The Comfort Houses: Measurements and Analysis of the Indoor Environment and Energy Consumption in 8 Passive Houses 2008-2011, Department of Civil Engineering, Aalborg University, DCE Technical Reports, No. 145, 2012.
- De Grussa ,Z et al, A Case Study assessing the impact of Shading Systems combined with Night-Time Ventilation strategies on Overheating within a Residential Property, 38th AIVC – 6th TightVent & 4th Venticool Conference, Nottingham, 2017.
- Boyce, P et al, Minimum acceptable transmittance of glazing, LR&T 27(3), 1995.
- Littlefair, P, BRE 364: Solar Shading of Buildings, BRE, 2018.
- Seguro, F, Solar Shading Impact, NEF on behalf of BBSA, 2016.
CIBSE TM37 Design for improved solar shading control 2006.
BRE 364 Solar Shading of Buildings – a very readable guide – updated in 2018.
For a complete description of glass factors, see BS EN 410:2011 Glass in building. Determination of luminous and solar characteristics of glazing.
The Society of Façade Engineering is the CIBSE society with a particular interest in solar control – www.cibse.org/Society-of-Facade-Engineering-SFE