May 1, 2015
by Richard Wilson, B.Sc.
Skylights are effective for allowing daylight into buildings. However, this needs to be properly managed to ensure spaces are not flooded with too much daylight and the risk of glare is mitigated.
The amount of solar radiation coming through horizontal and inclined glazing is much greater than vertical façades, and this can cause significant heat gain issues. As a result, it is important the shading of skylights is addressed during the design process and an effective approach is taken.
The graphs in Figures 1 through 5 show the levels of incident solar radiation by elevation for a building in Indianapolis, Indiana. Each graph shows the amount of radiation by time of day and month.
On the north elevation (Figure 1), there is only background radiation, which remains at a broadly consistent level. The south elevation (Figure 2) is interesting—because the higher sun angles during the summer, the level of radiation on south-facing glazing is greater in the winter than the summer. The radiation levels on the east and west elevations (Figure 3 and 4) are almost mirror images of each other. The highest levels of incident radiation occur during the summer, in the morning on the east elevation, and during the afternoon on the west.
The amount of incident solar radiation, however, is significantly greater on roof glazing (Figure 5) for almost the whole year. It is noticeably more in the summer when heat gain is an issue and needs to be dealt with by the HVAC system. To get a more specific understanding of solar gains entering the building, based on the same area of glazing for each situation, it is possible to look at the average daily levels shown in Figures 6 and 7 (page 2).
From both the tabulated data and the graph, the varying impact of sun on the glazing can be seen. For the same area of glazing, during the summer, the incident solar radiation through a horizontal skylight is between two and three times higher than through vertical glazing.
The light levels inside the building also vary significantly depending on the glazing’s orientation, time of day, and day of the year. It is not as easy to undertake a comparison as is the case with direct solar gains. Figure 8, however, shows floor plans demonstrating the light levels at noon on June 21 for a space with south-facing glazing and one with a skylight. Both areas of glazing are the same. As can be clearly seen, using a skylight results in substantially higher light levels inside the space. In principle, this is good—but there is a risk of glare if this natural light is not effectively managed.
From the brief analysis, it is evident skylights can be a useful way of providing an abundant amount of natural daylight—particularly if above a central atrium—but they can also cause glare and major heat gain issues. However, when the solar gain through the glazing is controlled by a shading system, light levels can be moderated as required, and excessive heat gain can be mitigated, reducing the required HVAC system’s size. On the other hand, if the building has a heating requirement in the winter, certain shading systems can be retracted or alternatively set to a position to allow in free solar gains.
When considering skylight shading, the standard approach is to use a fabric system. Possible options include fixed fabric panels and suspended elements like kites or retractable fabric tension systems. Fixed panels have the benefit of being relatively low in cost and maintenance-free. Since they remain permanently in place, light levels will be reduced on cloudy days and at times when there is no sun. Rather than installing the panels directly underneath the glazing, they can be set some distance below to allow more light into the space.
Kites are also fixed elements, but can be a decorative feature in addition to providing shade. Kites will always be a compromise in as much as they only provide partial shading and significant amounts of sun penetration will occur. As a result, these systems can be ideal for public spaces, but may not be appropriate in working environments where more responsive light control is required.
Fabric tension systems offer the benefit of being able to be deployed when required, but retracted when there is no sun on the glazing. Additionally, versions of these systems can be installed either internally or externally. An exterior system provides additional solar control compared with an interior one, dealing with a large part of the solar energy before it comes through the glazing and into the building.
Fabric tension systems have a drive tube—normally housing a motor, although manually operated system can be used in some situations—onto which the fabric panel is installed, as well as a mechanism to provide tension to the deployed fabric. Springs are the most common method of achieving the required tension, but opposed synchronized motors are sometimes used as an alternative. Spring systems can have a separate spring tube or alternatively, a single tube incorporating both the motor and the spring tension mechanism.
Not all shading fabrics can be used under tension; most polyester-based products are insufficiently stable and will stretch. As a result, fabrics with a core glass fiber yarn are mostly used, although one polyvinyl chloride (PVC) polyester fabric works extremely well due to the way it is produced, which means it is stable under tension. The range of fabrics for exterior systems is not as great as is the case for interior ones—they do, however, use the same materials and have similar weave structures, but the base yarns are thicker.
As with a standard roller shade, various fabrics can be employed in skylight tension systems in terms of weave structure, openness factor, and color, giving different performance criteria regarding visible light transmittance (VLT), solar performance, and view-through. For a common weave structure and openness factor, lighter fabrics provide a greater level of VLT and have a better solar performance than darker-colored ones. However, view through the fabric will not be as good. Twill-weave fabrics incorporating different color yarn in the warp and fill directions can have a lighter side and a darker side. If the lighter side is installed to face the glazing, a reasonably good balance between solar performance and view can be achieved.
There is also an alternative to a standard tension system. This product uses a motorized drive system, but is made up of numerous fabric panels attached to a series of spring roller tubes. Two fabric panels are attached to each spring tube which means as the system is deployed—one is wound off the top of the tube while the other is wound off the bottom.
The spring roller tubes and the hem bar run in side guides, which also carry the drive cords that deploy and retract the system. The design means the system can achieve substantial draws and can cover any orientation of glazing—horizontal, vertical, and inclined without applying any tension load to the skylight structure. If the tracks are curved, barrel vaults can also be shaded. The system’s construction also means it can withstand substantial wind loads when installed on the exterior.
Further variations to the idea of a retractable fabric system include horizontal roman shades (sometimes referred to as ‘tenditos’) and sail shades. Sail shades are similar to a standard motorized roller shade, but instead of a spring tension mechanism, they use counterweights to provide some tension and enable the fabric panels to be deployed. With a standard fabric tension shade, the aim is for the fabric panel to be as flat as possible. With sail shades, however, a much more sculptural effect can be achieved with the amount of curvature of each panel being dependent on counterweight’s size. Given this curvature of the fabric panels, sail shades are generally only appropriate for larger atrium spaces. As can be seen from the image of the Peabody Essex Museum (Salem, Massachusetts.), there are relatively large gaps between adjacent panels and, as with kites, these systems are generally used in public spaces rather than in environments where occupants are working with computers.
As an alternative to fabric, louvered systems can also be used. The most flexible option is the non-retractable louver or rack-arm system. As the name implies, the louvers always remain above or below the glazing—they can be rotated, however, from the fully open position to fully closed. The system’s design means it can be installed on any type of glazing—horizontal, vertical, or inclined. It can also address almost any shape of glazing including rectangular, circular, and trapezoidal openings.
The system is made up of numerous rack arms. These are aluminium extrusions incorporating pivot arms and clips onto which the slats or louvers are connected. Each rack arm also incorporates a drive mechanism so when a number of them are connected together with a drive shaft, rotation of the shaft results in the slats being rotated from open to closed or vice versa. The slats themselves are relatively small profiles ranging in size from 50 to 145 mm (2 to 5 ¾ in.), which means the spacing between the rack arms is generally somewhere between 76 and 127 mm (3 and 5-in.)—depending on the slat profile selected and whether the installation is interior or exterior. Where possible, the rack arms are aligned with the skylight structure.
There is also an extruded aluminum profile that interlocks with the adjacent one to provide high levels of light exclusion. There will, however, be some light ingress around the system’s perimeter so the system will not achieve complete blackout.
The rack-arm system can be manually operated by means of a gearbox or crank handle, or alternatively, can be motorized. Motorized systems generally incorporate a tubular motor mounted onto one of the rack-arm profiles and connected to a reduction gearbox, which, in turn, is connected to the drive shaft. In standard configuration, the slats take approximately 15 seconds to move between the open and closed position allowing fine light control to be achieved. Due to this, the system is often used in museums and galleries and on other light-critical installations. In these situations, an automatic control system can also be specified to allow light levels to be maintained within a defined range.
In addition to extremely good light control, the system can also reduce the heat gain. If the system is installed on the interior, solar control depends on solar energy being reflected back through the skylight. A greater reduction in heat gain will be achieved, however, when the system is installed on the exterior. The system has been tested in a wind tunnel at speeds up to 160 km/h (100 mph) and has been installed on projects in locations such as the Caribbean and Hong Kong. There are, however, issues with ice due to numerous small, rotating components that can freeze up. As a result, the system can only be used on the exterior in certain locations in North America. In the northern United States, the system will be primarily installed on the interior.
As an alternative to the rack-arm system, larger-scale louvers can be used. These are generally produced from extruded aluminium, but other materials such as wood or glass can also be used. Moving to larger louvers means greater spans between supports. The system’s weight, however, will be much greater than the case with a rack-arm system. Whereas the rack arm system can weigh less than a pound per square foot and can be attached directly to the skylight, large-scale louvers will generally be fixed to the main building structure and may need additional steel framing to support the applied load.
These larger louvers can be fixed or operable. As with the rack-arm system, the louvers will not retract, but—depending on the drive mechanism’s design—can be rotated through 360 degrees if required. Since the system components are larger and more robust than the rack arm system, louver systems can also be designed to withstand all weather conditions—wind, snow, and ice—and exterior systems can therefore be used on projects throughout North America.
It is apparent many different approaches can be taken to shading a skylight. Some of the options blend in with the building architecture, whereas others can have a significant visual impact and become an important element of the building’s design. In all cases, however, properly designed skylight shading systems can make an important contribution to building performance, controlling both heat gain and natural daylight.
Richard Wilson, B.Sc., is a consultant to Draper Inc., and has been working with the company to introduce a range of exterior and specialty shading systems. He has been involved in the solar shading industry for more than 20 years. Wilson can be contacted by e-mail at email@example.com.
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