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

Exploring Shade Fabric Properties

There are four key fabric properties that determine how a shade will perform in a space; they are: openness factor, visible light transmittance, solar reflectance, and color. These properties are interrelated, and any manipulation affects the degree to which a fabric can mitigate glare, promote daylight autonomy, preserve outdoor views, and manage thermal heat gain.

This table illustrates the relationship between the three key fabric properties and the most common daylighting performance goals of a space.

Photo courtesy of Lutron Electronics Co., Inc.

This table illustrates the relationship between the three key fabric properties and the most common daylighting performance goals of a space.

Openness Factor (OF)

The openness factor (OF) of a shade fabric refers to the amount of direct light that is able to pass straight through the shade. While this is affected by the unique angle of the light rays that strike any particular window, making the longitude and latitude of the project something to consider during specification, the openness factor, as a fabric property, refers to the percentage of normal or perpendicular light that would be allowed to pass directly through the shade. For example, in a scenario where a window is exposed to primarily perpendicular sunlight, if a shade has an openness factor of 5 percent, then 5 percent of the available sunlight will pass directly through the shade and into the interior. The remaining 95 percent of the light that contacts the shade will be diffused, absorbed, or reflected.

Visible Light Transmittance (Tv)

Visible light transmittance (Tv) refers to the total amount of light allowed to move through the shade and into the space. It accounts for both direct and diffuse light energy. The Tv value of a shade is primarily affected by the physical openness in the weave and the color of the shade but is also affected by the shape, opacity, and specific weave pattern.

Dark fabrics absorb more of the available daylight than light-colored fabrics, ultimately affecting the amount of light available to pass through the shade. When openness factors are equal, a dark-colored fabric will have a lower Tv value than a light-colored shade fabric.

Solar Reflectance (Rs)

Solar reflectance (Rs) refers to the percentage of the total solar radiation that is reflected off of the exterior face of the fabric and, ostensibly, back outside. Solar reflectance values are determined, in large part, by the color of the exterior face of the fabric. Dark colors absorb more of the available light energy and, therefore, offer lower reflectance values. Lighter-colored fabrics reflect more of the light energy and provide higher solar reflectance values. For example, a standard black solar screen fabric will typically offer an Rs value in the neighborhood of less than 10 percent, where a white fabric can deliver Rs values of 50 percent. An Rs value greater than 30 percent will provide some protection from solar heat gain, while an Rs value of 50 percent or greater provides good solar protection.

Color

Shades today are available in a wide range of colors and styles: neutrals, bolds, and pastels, with patterns, textures, or graphics. Designers can find a shade fabric that will complement any type of interior decor. One of the pitfalls of specifying the right shade for a project is choosing a shade solely because of its color or style, without considering how the shade will manage the daylight in the space.

Putting it All Together: Fabric Properties and Performance

It is easy to see why shade specification can be tricky. Between the different, often conflicting, performance goals and the multiple fabric properties that impact a fabric’s ability to perform, the design and specification industry needs a little guidance on finding the right fabric for the job. The above table provides a general guideline for matching fabric properties with performance. Of course, performance goals will need to be prioritized on a project-by-project basis, so the selected shade provides a performance blend that matches the unique needs of the space.

Glare Mitigation

If protecting an interior space from glare was the absolute goal of most designs, then more buildings would be built without windows, but they are not. This is because providing access to exterior views and creating energy savings by using daylight, instead of electric light, to illuminate a space are also considered important design goals that improve the environment of the interior. This requires specifiers to engage in a balancing act for better design. For example, a low openness factor will provide good direct sun control and glare mitigation, but it will significantly reduce view preservation. A shade with a low Tv value will provide good diffuse daylight control, limiting the potential glare caused by light that is diffused by the shade, and better view preservation, but it significantly restricts the amount of light that penetrates through the shade, limiting the level of daylight autonomy, and energy savings, that can be achieved.

Daylight Autonomy

The only way to achieve daylight autonomy, where parts of the interior are capable of being illuminated exclusively by daylight for much of the workday, is to allow a sufficient amount of daylight into the interior. Using a fabric with a high visible transmittance increases the amount of total light, both direct and diffused, allowed through the shade and into the building.

The shade selection wizard enables specifiers to compare the performance of different fabrics on their unique projects.

Photo courtesy of Lutron Electronics Co., Inc.

The shade selection wizard enables specifiers to compare the performance of different fabrics on their unique projects.

View Clarity

View clarity can be predicted as a function of Tv and openness factor. Darker fabrics with higher openness factors generally achieve higher clarity scores, followed by dark-colored fabrics with low openness factors. Light-colored fabrics typically receive the lowest view clarity scores.

Thermal Management

Dark shades with low Rs values absorb and reradiate solar heat and do not provide significant protection from solar radiation. Lighter-colored shades often offer a higher Rs value, but outdoor views aren’t as clear. Although much of shade selection is a balancing act, manufacturers were able to develop shades that improved thermal management without negatively impacting view preservation. Dual-sided fabrics were introduced to offer a significantly improved Rs value, often above 50 percent, which dramatically improves the heat rejection of the fabric, without sacrificing the clarity of the objects or colors seen on the other side.

 

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Originally published in Architectural Record.

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