A New Methodology for Successful Daylighting Design

Selecting fabrics for performance shading
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Sponsored by Lutron Electronics Co., Inc.
Jeanette Fitzgerald Pitts

Defining Performance Goals for Shade Fabrics

Across the spectrum, shade fabrics provide varying degrees of glare mitigation, daylight management, outdoor view preservation, and thermal regulation. Selecting the right shade for a project requires that specifiers establish the degree to which they need a shade to deliver each of these four performance objectives. Quantifiable metrics can be helpful tools when trying to define how a shade fabric needs to perform in a space. Here is an overview of the four most common reasons that shades are specified on a project and a few metrics that can be used to define the level of performance desired.

Glare Reduction

Solar shades are often used to mitigate glare that can be caused by a direct view of the sun or intense sunlight streaming through the windows on a bright day. Shades manage these glare conditions at the window by diffusing, reflecting, or absorbing light, delivering a less intense, more usable level of daylight into the interior.

In terms of the level of glare prevention required in a particular space, different types of space tolerate different levels of potential glare. Spaces where vision-critical tasks are performed, such as office spaces, conference rooms, and classrooms, have a low tolerance for glare because it would disrupt the purpose of the space. Transitional spaces, such as hallways and stairways, can accept a higher level of potential glare, as long as it doesn’t create a safety concern. Lobbies, break rooms, and other social spaces also have a higher threshold for potential glare because a brighter atmosphere would not negatively impact the casual interactions that occur there.

The Daylight Glare Probability (DGP) estimates the likelihood that an average occupant in a space would begin to experience glare from daylight.

Photo courtesy of Lutron Electronics Co., Inc.

The Daylight Glare Probability (DGP) estimates the likelihood that an average occupant in a space would begin to experience glare from daylight.

This reference to a low, moderate, and high tolerance for glare can be quantified by a metric that has been gaining momentum in the industry: the Daylight Glare Probability (DGP). The DGP estimates the likelihood that an average occupant would begin to experience glare from daylight, taking into account the potential brightness, contrast, light levels, and field of view. DGP values can range from less than 20 percent to 100 percent; however, good design practices are typically focused on the DGP values that range from 35 to 50 percent. A DGP value of 35 percent or less is considered an acceptable upper limit for vision critical spaces, like office spaces and classrooms, which have a low tolerance for glare. At this level, glare would be identified as unnoticeable by most occupants in the space. DGP values between 35 and 40 percent represent spaces where glare may register as noticeable for many occupants in the space but not uncomfortable. This range is commonly used in transitional and social spaces where tolerance for glare is more moderate. DGP values greater than 40 percent categorize areas where occupants would begin to feel uncomfortable in the space. Beyond 45 percent, the glare probability in the space is considered critical and should be better managed.

Daylight Autonomy and Energy Savings

Today, windows can offer so much more than an outdoor view; they can offer an alternative light source. The practice of reducing electric light levels when daylight is present is becoming more and more commonplace as building codes, such as the ANSI/ASHRAE/IESNA Standard 90.1-2010, now require the inclusion of daylighting controls in many of the interior areas that receive daylight. The return can be significant. Lighting energy savings generated from these daylighting practices can meet or exceed 60 percent. Regardless of the lighting energy benefits of advanced daylighting, there are significant gains in occupant comfort and well-being due to increased daylight availability.

New daylighting goals attempt to do more than coordinate the presence of electric light and daylight; instead, designers are attempting to use daylight as the exclusive light source for the space. This design objective is referred to as daylight autonomy (DA). Automated shade systems help projects attain greater levels of DA because designers can incorporate more windows into the envelope of a building and maximize the amount of daylight allowed onto the floorplate without exposing the interior to excessive glare or heat gain.

Spatial Daylight Autonomy (sDA) measures the percentage of the work area where daylight contributes at least 30 foot-candles (fc), which is the light level recommended for office spaces by the IESNA, for 50 percent or more of the work hours. As the sDA values of a design rise, more and more of the interior space is exclusively lit by daylight for at least 50 percent of the workday.

Whether a shade is able to prevent glare or provide a clear outdoor view or reduce solar heat gain is determined, in large part, by the openness factor, visual transmittance, and solar reflectance of the shade fabric.

Photo courtesy of ©Doug Scott 2014

Whether a shade is able to prevent glare or provide a clear outdoor view or reduce solar heat gain is determined, in large part, by the openness factor, visual transmittance, and solar reflectance of the shade fabric.

Preserve View Clarity

One of the unique benefits of using solar shades, rather than blinds and other window coverings, is that solar shades enable occupants to enjoy the outdoor views, even when the solar fabric is deployed over the window to prevent glare. It is important to note that the clarity of view available through the fabric can vary greatly from one shade to the next. Through one fabric, a person may be able to experience an outdoor view that is crisp in object identity and deep in color. Through another, the view may seem slightly muddied or muted. Another fabric may provide very limited outdoor views, where a person can see only general outlines of the cityscape or landscape on the other side of the window.

Until recently, no metric that defined the clarity of view existed to help a design team specify a shade fabric on a project. The View Clarity Index (VCI), created in partnership with top researchers from Purdue University, ranks view clarity from 0 to 100 percent. A value of 100 percent means that the fabric causes no perceivable interference with exterior views. A value of zero indicates that no view is visible through the fabric.

The VCI index defines view clarity across a spectrum that considers the visibility of both the shape and color of exterior objects. A VCI rating of 25 percent indicates that the outlines of buildings and skylines are somewhat visible through the fabric but object details and color differentiation may be severely limited. At a 50 percent VCI, most of the objects on the exterior are recognizable, although the edges are blurred and colors are visible but washed out. At 75 percent VCI, objects can be seen in much greater detail and occupants are able to experience the true palette of the surrounding colors.

Thermal Management

Thermal management is a primary concern on facades that receive direct sun and do not have window glass that is designed to minimize solar heat gain. Solar shades can create an effective barrier between the glass and the interior space, reflecting solar energy off of the exterior face of the fabric and back into the atmosphere, before it can heat up the workspace. Although the glass may trap some of the reflected radiation in the building, fabric shades improve both the real and perceived thermal performance of the space. Shades significantly improve the impression of thermal comfort, which is a person’s perception that they are surrounded by the right air temperature, and shades reduce the amount of energy necessary to keep the building cooled to the desired temperature. In fact, realized energy savings for cooling a building with high solar reflectance shades have been shown to range from three to 22 percent, depending on facade design and properties.

 

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Originally published in Architectural Record.

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