Architectural Record BE - Building Enclosure

Managing Heat and Light Through Exterior Shading Systems

The advantages of solar control from the outside
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Sponsored by Draper, Inc.

Choosing the Correct Exterior Shading

With a number of different shading options available, it is important to get an understanding of specific project requirements to determine the most appropriate system. The following factors should be considered.

The Building Location and Orientation of the Glazing

The shading requirements for east, south, and west facades vary because of the different sun angles that must be addressed.

Geographical Location

Just as important as building and glazing orientation is the actual geographical location of the building. In Miami, for example, the sun is almost vertical in the sky at midday on June 21 when the peak sun angle is just over 86 degrees. Even in winter, the sun angle in Miami is almost as high as the peak summer sun angle in Helsinki, Finland. In Miami, therefore, relatively short projections can provide effective shading, particularly on the south elevation. In places such as Helsinki, however, fixed projections will not be very effective and a much more adjustable system will be required to deal with the varying conditions and low sun angles. In summer in Helsinki, the sun sets almost due north; in the winter, however, it doesn't set very far to the west. Retractable shading systems are therefore appropriate for the west elevation and it may also be necessary to provide some shading on north west (or even north) elevations to control glare.

In addition to latitude and longitude, it's also important to understand the context of the building. In an urban environment, overshadowing from surrounding buildings could have a significant impact and may mean that certain parts of the building that should be exposed to the sun are, in fact, permanently shaded. The effect of the building being close to water might also need to be considered. For example, light reflected from water can cause glare issues that can be effectively addressed by a fabric system but not by a venetian blind or horizontal louvers, which will allow reflected light to pass through the system to the interior.

Climatic Conditions

All shading systems have to be suitable for the prevailing climatic conditions. Fixed systems need to withstand maximum wind, snow and ice loads whereas operable systems such as exterior roller shades and venetian blinds need to be retracted in adverse weather conditions. Depending on the wind and ice conditions, however, certain operable systems may not be the best choice; if shades need to be operated frequently, a better choice might be a fixed system or one that is adjustable but not retractable.

Building Heating and Cooling Requirements

A building's heating and cooling requirements can also influence the choice of shading system. Exterior shading systems don't discriminate between different times of the year—if they are installed in front of the glazing, they will reduce the amount of heat gain entering the building. If a building has heating requirements in the winter months, exterior venetian blinds or roller shades may be retracted and left in this position during this period to allow sun and the associated heat gain into the building. This is a free source of heat and will reduce energy requirements and costs. If this is the case, however, interior glare control systems, such as roller shades, will almost certainly be required.

Building Aesthetics

With certain building designs, large scale louvers may be used to provide a strong aesthetic look. In other situations, shading systems might be on the interior or in a double façade to allow the building to have a clean, glazed appearance. Projecting systems, like brise soleil systems, are often used to shade south elevations—they don't work well with low sun angles on the east and west elevations but may be still used to provide a consistent look to the building. In this case, however, it's important that the client and the design team understand that the shading systems will not function very effectively and that a secondary interior system will almost certainly be required as well on these elevations.

Performance Requirements

Naturally, performance requirements must be considered and depending on the level of solar control required, one system may be more appropriate than another. The shading systems are just one item that will impact the building and façade performance. Other factors will include the extent of the glazing by façade, the type of glass selected and the wall construction. The required performance contribution from the shading will therefore vary depending on the design decisions that have been taken. In certain situations, the shading system will only need to make a relatively small contribution to the overall performance and limited usage of fixed exterior louvers might be appropriate. In other situations, an adjustable system that can provide effective solar control throughout the year may be required.

Specific Options—Exterior Venetian Blinds and the Rack Arm System

The final sections of this article will look in more detail at two of the available exterior shading systems: exterior venetian blinds and the rack arm system.

Venetian Blinds

Exterior venetian blinds operate in a similar way to interior ones in as much as they can be raised and lowered. The slats can also be tilted, which provides an additional level of controllability compared to a roller shade system. It also means that the systems can be used on any façade as the slats can be adjusted to provide solar control whatever the sun angle. As highlighted earlier, this system has a solar heat gain coefficient of less than 0.10 and can therefore prevent more than 90 percent of the solar energy from passing through the glazing and being converted into heat. Venetian blind slats are available in a variety of sizes from two to six inches and are of a larger size than the systems that have historically been used in the US.

Image courtesy of Draper, Inc.

 

The accompanying chart, which was produced by software, shows the sun exposure of west facing glazing on a building in Indianapolis. The vertical axis of each graph shows the time of the day and the horizontal one shows the months of the year. Areas on the graph that are blue indicate no sun on the glazing; areas that are yellow indicate full sun.

As expected with a west facing elevation, the sun is on the glazing during the afternoon and there is more sun exposure in the summer than in the winter. As can be seen, a brise soleil system is not very effective, even with a relatively large projection, as the lower sun angles pass below the shading system and onto the glass. The graphs on the right hand side of the illustration show a venetian blind with the slats horizontal and then tilted up to 45 degrees. The improving performance is very noticeable and by tilting the slats further to the closed position—which is in fact a rotation through approximately 70 degrees—it is possible to deal with the late afternoon / early evening when the sun is low in the sky.

There are various slat options as well as perforation alternatives. Slats can be fully perforated, highway perforated (where the central strip is left unperforated), half perforated, or solid. Perforated slats will allow a view through to the outside, even when the blinds are in the closed position. The blinds can, however, be horizontal or only partially tilted during most of the day, and are only generally closed during the early morning or late afternoon when the sun is low in the sky. In these cases, perforation can cause glare issues and, as a result, the need for perforation should be carefully considered. If the decision is made to go with perforated slats, it is preferable to use ones with a low openness factor. Perforated slats are also more expensive than a solid-slat system, as an additional production process is required.

 

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Originally published in Architectural Record

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