Sunshade considerations

Passive sunshades help save energy and improve worker comfort. They also can reduce capital investments in electrical and air conditioning equipment and enhance a building's appearance.

10/01/2009


        Shielding windows from the direct rays of the sun is nothing new. Sunshades in one form or another have been around for centuries. So what has changed to bring them into the spotlight? An increased desire to save energy, protect the environment, and create more sustainable buildings.

         

        Sunshades are made of strong, durable screens; they are strategically installed to block out the sun's glare and heat in the summer, yet admit the sun's energy in the winter. Consider the following benefits:

         

        • Sunshades can help create dramatic savings in air conditioning requirements. In addition, smaller (less expensive) air conditioning units can be specified.

        • Sunshades enable building occupants to use more natural, and less artificial light. They eliminate the need for interior window treatments or expensive, tinted or darkened glass, all of which increase artificial light requirements.

        • Because there is more natural light, worker comfort and productivity are enhanced.

        • Once installed, passive sunshades are virtually maintenance-free.

        • Sunshades lower peak electrical demand. Smaller (less expensive) electrical switch gears can be installed because the building requires less power.

        • Sunshades produce continual savings year after year with little or no additional expenditures.

        INCREASED DAYLIGHTING

        The amount of energy required to provide artificial lighting can be reduced further when exterior sunshades are coupled with interior light shelves to bring more natural daylight deeper into the interior space.

         

        Fixed exterior shades have traditionally been installed to project 3 to 4 ft from the head of a window. Moving an exterior sunshade down approximately one-third of the height of the window and introducing an interior element (a light shelf) of approximately the same projection enables the building to achieve the same amount of shading as shaded glass; however, light can now be harvested through the upper third. This light is then reflected off the top of the interior light shelf, and is “pushed” deeper into the interior space.

         

        Studies show that using daylighting to reduce task or general lighting can produce energy savings as high as 80% during daylight hours. Bringing natural ambient daylight deeper into a building's interior also can have positive benefits on the building occupants. Studies show that increased daylight helps patients recover faster, improves test scores for children, and leads to more purchases in stores.

         

        ORIENTATION AND LOCATION

        There are three basic types of exterior sunshades: cantilevered, horizontal line, and vertical line. Geographic location and building orientation will determine which type to use when designing a sunshading system.

         

        Cantilevered sunshades are most effective on southern elevation during the midday hours when the sun is at its highest point in the sky. These systems are most often comprised of a series of slats or blades, available in many styles that provide a visually appealing application. The slats/blades allow for wind, and in some cases snow, to pass through. A suspended system distributes the load from the exterior sunshade to the building structure.

         

        Horizontal line sunshades are most effective when used on tall expanses of glass or on curtain walls where attaching a series of cantilevered sunshades on top of each other is not practical.

         

        Vertical line sunshades are most effective on east and west elevations to block the low sun angles in the early morning and late afternoon. Typically, a hollow extruded shape sets either perpendicular to the building or at a slight rotation to maximize solar protection while providing occupants with the maximum amount of visibility to the outside.

         

        Many factors must be taken into account when designing a sunshade system. AMCA Publication 504-08, External Shading Devices in Commercial Buildings—The Impact on Energy Use, Peak Demand, and Glare Control offers guidance. In particular, Part 6.0, Sunshade Selection, lists the following “key factors to aid in selection:”

         

        • Geographic location (latitude and longitude)

        • Building exposure to be shaded (south, east, or west)

        • Time of year for complete or partial shading

        • Critical time of day to be shaded

        • The sun's position, azimuth, and altitude based on the location.

        Consider the following example:

         

        The sun's apex in Miami on June 21 at solar noon is just about 84 deg above the horizon. In Manhattan on June 21, the sun's altitude is about 72 deg. A simple 3 ft 0 in. cantilevered sunshade on a south-facing widow would be very effective on a typical window in either location.

         

        However, if we look at Dec. 21 using the same sunshade, while still effective in Miami, it would block only 50% of the window in Manhattan from receiving direct sun at its most effective time of the day (noon). A more effective approach in Manhattan would be to use a horizontal line sunshade, dropping its front edge to block the sun from some of the lower sun angles. This would help minimize solar heat gain as well as glare.

         

        Elsewhere in the country on June 21, the sun rises in Houston at 6:21 a.m. with an azimuth of 62 deg/9 ft/56 in., or almost 30 north of due east. At 9 a.m., the sun's position is 31 deg/37 ft/3 in. in altitude, with an azimuth of 79 deg/18 ft/46 in. The sun's angle of attack is still north of due east, but it is not until just before 11 a.m. that the sun is striking the building from south of east.

         

        The same scenario is played out all across North America. With these lower sun angles, the traditional cantilevered sunshade cannot block direct sun and minimize glare. Vertical fins become a much more effective solution for east and west elevations. The designer can use the size, spacing, and rotation of the fins to optimize their effectiveness for these low sun angles.

         

        In summary, when properly applied, sunshade systems enable buildings to use less energy and bring in more natural light, which benefits the occupants. In addition, they visually enhance a building's appearance.

         

         

         

        Author Information
        LYNCH is technical sales manager of Construction Specialties Inc., Cranford, N.J.

        Sunshades and LEED points

        Under the U.S. Green Building Council's LEED v3.0 there are three areas where passive solar controls can be used to help achieve credits:

         

        • Under Energy and Atmosphere Credit 1 Optimizing Energy Performance (p. 270)—The use of solar shading can help reduce energy consumption through reduced HVAC capital cost as well as operating cost.

        • Under Material and Resources Credit 4.1 Recycled Content (p. 369)—Most architectural sunshades are aluminum, a highly recyclable material.

        • Under Indoor Environmental Quality Credit 8.1 and 8.2 Daylight and Views (p. 554)—Here, designers are specifically asked to consider adding interior light shelves, exterior fins, louvers, and adjustable blinds as daylighting options to control glare.

         



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