Exterior Venetian Blinds: The most effective way of providing solar control?

All images courtesy Draper Inc.

By Richard Wilson, B.Sc.
When considering solar control, most think of reducing heat gain. There is no question this is a significant part of what an effective exterior shading system should achieve.

However, energy from the sun also includes the visible spectrum so the control of natural daylight is also important. An effective solar control system appropriately deals with heat gain while allowing as much natural daylight into the building as possible without causing glare issues for occupants. By controlling heat gain, reductions in the size and running costs of the HVAC system can be achieved. Making good use of natural daylight also allows artificial lighting requirements to be reduced, resulting in further cost savings during the building’s life. Effective solar control through exterior shading is therefore an approach that should be actively considered as part of the design process.

Controlling heat gain does not necessarily mean preventing it from getting into the building throughout the year. During the summer, there is no question the key aim will be to minimize heat gain. As a result, the shading system will need to prevent direct sun from passing through the glazing and into the building. (Figure 1 shows an atrium with blinds open and closed.) Depending on the building’s location and design, it might also be appropriate to minimize heat during the winter months. In buildings requiring heating during the winter, however, it may be better to allow solar energy into the building as it is a free heat source. It will, nevertheless, still be necessary to include an interior shading system to control natural daylight and prevent glare, which can be a particular problem with low winter sun angles.

A high-performing shading system should be capable of addressing numerous key requirements, such as:

effective shading—provide complete shading when there is sun on the façade;
adjustability—be capable of dealing with the varying sun angles over the course of the day and the year;
light control—permit as much controllable light into the building as possible; and
flexibility—allow heat gain into the building during the winter months, if required.

Numerous different exterior shading devices are available, including brise soleil systems, fixed vertical and horizontal louvres, and operable systems such as roller shades.

Figure 2 shows the performance of numerous shading options assessed by reference to these criteria to determine which system is the most effective. This demonstrates that operable systems allow the shading to be adjusted to address the changing sun conditions. Exterior roller shades and venetian blinds can also be retracted when there is no sun on the façade or when it would be beneficial to allow solar gain into the building. The additional adjustability of venetian blind slats means they can provide even more responsive shading than roller shades.

The relative effectiveness of different exterior shading systems can be easily demonstrated by using software. A west-facing office in Indianapolis was modelled and the sun exposure of the glazing was analyzed by means of a sun exposure graph. The graph shows the time of day on the vertical axis and the months on the horizontal one. Blue areas on the graph indicate there is no sun on the glazing—either because there is no sun on the elevation or because the glazing is fully shaded. Yellow areas indicate full sun (i.e. no shading), and the scale on the right hand side of each graph shows the extent of shading between these two extremes (Figure 3).

Since the model has a west-facing elevation, all the sun exposure is in the afternoon; further, since the sun sets earlier in the winter, exposure is much greater during summer months. Incorporating a range of exterior shading devices into the model gives the results seen in the graphs within this article.

Using a brise soleil system achieves only limited shading. When sun first comes onto the façade, it is high to the south and passes between the first louvre and the glazing. However, because of the sun’s angle to the glazing the impact of this is insignificant. As the sun starts to move to the west, but is still relatively high in the sky, almost complete shading is achieved. As the sun moves further west and the angle reduces, it passes underneath the brise soleil system, and the amount of sun on the glazing increases (Figure 4).

Using horizontal louvres, the amount of shading achieved increases noticeably. The extent of the shading can be modified (resulting in more or less shading) by changing the size of the louvres, the spacing between them, the angle at which they are set, or a combination of all three (Figure 5).

Generally, vertical louvres should be used on east and west elevations. However, as demonstrated by the data, they are ineffective, particularly during the summer, whether they are perpendicular to the façade (as analyzed) or set at an angle to the glazing. The systems will prevent solar gains from entering the building in the winter (when it might be beneficial to allow them in) but not during the summer when solar control will certainly be required (Figure 6).

Exterior roller shades’ performance primarily depends on the fabric selected. This influences the amount of light passing through the fabric, the view through it, and the solar performance. The shading achieved will be consistent over the year. Given there is some thickness to the fabric, however, shading will be slightly greater at high sun angles than at lower ones (Figure 7).

The graphs in Figure 8 clearly demonstrate the effectiveness of exterior venetian blinds. The slats can be tilted from horizontal through approximately 70 degrees to the closed position. When closed, there is horizontal cut-off (i.e. no sun penetration), even when the sun is on the horizon. When an automated control system is used to operate the blinds, numerous intermediate slat angles are set up to provide optimal solar control while allowing ambient light into the building.

As can be seen, when the blind is deployed and the slats are horizontal, full shading is achieved for a large part of the afternoon. Later in the day, when the sun is lower in the sky, the slats can be tilted to continue to provide effective shading. During the summer, they probably do not need to be fully closed during the normal working day as they will provide full shading until 6:00 p.m. or later when still partially open.

Not only do venetian blinds provide shade, but they also significantly reduce the heat gain in the building.The lowest solar gains occur when exterior venetian blinds are used. The analysis, however, is based on the slats being set at a partially closed position—if an automated sun tracking control system was used and the slat position was automatically adjusted to prevent any direct sun penetration, even better performance would be achieved.

The analysis clearly shows exterior venetian blinds provide the most effective and responsive solar control, particularly when combined with a sun-tracking control system. The systems are specifically designed for exterior use and can operate effectively for many years with minimum maintenance. However, when determining whether they are appropriate for a project, numerous things need to be considered.

Building esthetics
Exterior venetian blinds will have an impact on a building’s appearance. This will certainly be the case when the blinds are deployed. However, they can also have an impact when retracted. If side-guide extrusions are used (rather than the more discrete 3-mm [1/8-in.] diameter side-guide wires used on most applications), the vertical lines of the façade will be accentuated. The façade design might incorporate pockets into which the blinds retract. Alternatively, formed or extruded aluminum head boxes can create a strong horizontal feature.

Slat options range in sizes from 50 to 150 mm (2 to 6 in.). Moving to the larger end means a greater spacing between adjacent slats and greater transparency when the slats are in the open position.

Weather conditions
With all exterior shading systems, it is necessary to consider the prevailing weather conditions and their impact on system performance. Venetian blinds need to be retracted if the wind speed is too high. A control strategy sometimes used is to move the slats to the open (i.e. horizontal) position at a certain wind velocity, and then retract them at a second, higher, wind speed. Moving slats to the open position allows the wind to pass through them and for the wind pressure to quickly equalize on both sides of the blind. If this is the case, the blinds will remain stable in windy conditions and will not act as a distraction to building occupants.

The maximum acceptable wind speed will be influenced by numerous factors including:

local wind conditions—unusual wind patterns can potentially reduce the maximum wind speeds at which the blinds can be used;
distance of the blind from the glazing—if the blind is set some distance away from the glazing, it will be more affected by the wind and will need to be retracted at a lower wind speed; and
height of the blinds—taller blinds take longer to retract and are therefore more prone to gusting. As a result, they will need to be retracted at a lower wind speed than would be the case with standard sized systems.

The impact of wind also means exterior venetian blinds cannot be installed on the upper floors of high-rise buildings due to the increased wind speeds at higher levels. As a result, exterior venetian blinds tend to be employed on buildings up to 12 floors. With taller buildings, the blinds will have to be installed on the interior or inside a double façade.

In addition to wind, it is also important to consider the impact of snow and ice. If the venetian blinds are automated and only deployed when there is sun on the glazing, this should not be an issue as they will be retracted when there is risk of snow and ice. Providing the blind pockets or head boxes are watertight, the blinds will be protected from rain and snow and can be deployed when there is sun on the façade, even when temperatures are near or below freezing.

As previously mentioned, however, if a building requires heating during the winter, it might be appropriate to leave the blinds retracted during this period and allow solar gain into the building. 
If this is the case, interior roller shades will generally be installed to provide light and glare control as required.

Integration with the façade
As with all exterior shading systems, it is important to look at the integration of the blinds with the façade at an early stage during the design process. If the head boxes and guide wire brackets are going to be attached to the curtain wall, the connection methods and applied loads must be discussed with the curtain wall contractor to ensure no problems arise. This is also the case if the systems are going to be connected to a cladding system. It will also be necessary to look at the routing of electrical cables to ensure the proposed approach is in accordance with the National Electrical Code (NEC) as well as local code requirements.

Control methodology
Using exterior venetian blinds to minimize the solar heat gain inside the building means the HVAC system’s size can be reduced. This is only possible, however, when the blinds are deployed when there is a risk of excessive solar gain. Given this is the case, the use of a fully automated control system should be carefully considered. This ensures the shading system is as effective as possible—it means, however, building occupants have no control of the blinds. This is generally not a problem if the reasons for the blinds and how they respond to the sun conditions are understood. When required, some manual control can be given, but this should be set up in a way that does not compromise building performance.

Possible options include:

manual override to tilt the slats but not retract the blinds;
full manual override and blinds revert to automatic operation at set times—possibly at the end of the morning and the end of the working day; and
a limit to the number of times the blinds can be manually overridden during the working day with a reset to ‘automatic’ during the evening.

Maintenance and cleaning
There are often concerns about the reliability of operable systems on the exterior of a building and the need for regular maintenance. The requirements are limited, although it is always recommended exterior venetian blinds are inspected on an annual basis. 
It is also recommended slats are cleaned as part of the window-washing procedures.

Motors are installed inside the venetian blind head rail and do not require lubrication. The blinds should, however, be operated to ensure they are running correctly and to verify there are no signs of wear or damage to the system. Side-guide wires (if used rather than side-guide extrusions) should also be checked and re-tensioned if required.

In this author’s opinion, exterior venetian blinds are the most effective system available for the shading of vertical glazing. They may not be appropriate for all projects, but can provide a level of flexibility, control of heat gain, and control of natural daylight unsurpassed by any other shading system. Although exterior venetian blinds are relatively new to the North American market, they have been used in Europe for more than 50 years demonstrating the durability and reliability of these systems. The ongoing use of exterior venetian blinds on new projects shows that they continue to be seen as an effective way of achieving solar control.

Richard_WilsonRichard 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 rwilson@draperinc.com.

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