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

The Solution: Define an Acceptable Tolerance

Today, there is no industry standard for what constitutes an acceptable deviation or tolerance from published fabric property values. This acceptable tolerance range for openness factors and Tv values would need to maintain design intent for glare control, daylight autonomy, and view preservation, ensuring that as long as the properties of the fabric fell within these limits, the shade performance in the space would not be materially compromised.

A series of simulations and studies were conducted by Purdue University to determine an acceptable tolerance range for openness factors and Tv values. The goal was to identify a range of deviation in these property values that would yield an unperceivable change in glare control. Specifically, researchers were looking at how much the openness factor and Tv value could change without increasing the DGP values of the original design by more than 5 percent.

The study identified the recommended acceptable tolerance of the mean openness factor as ±0.75 percent. This means that a fabric specified with a desired openness factor of 3 percent could actually have a mean openness factor between 2.25 percent and 3.75 percent and perform materially the same.

The recommended acceptable tolerance for the Tv value of a shade, as determined by the study, depends upon the Tv value. When the Tv value is less than or equal to 5 percent, the acceptable Tv range is ± 1 percent. When the Tv value is greater than 5 percent, an acceptable Tv range is ± 20 percent of the Tv value. So if the Tv value is specified at 4 percent, then a fabric with a Tv value between 3 percent and 5 percent would be acceptable. If the specified Tv value is 10 percent, then the acceptable Tv range would be between 8 and 12 percent.

Test the Acceptable Tolerance

Consider the New York City-based, west-facing curtain wall, where the specifier selected a shade with a 1 percent openness factor and a 7 percent Tv value. The recommended acceptable tolerances for openness factor and Tv value would recognize a shade with an openness factor of 1.7 percent and a Tv value of 8.4 percent as falling within allowable limits. Running a simulation of this fabric, in this space, the DGP remains at the lowest levels and glare would be considered unnoticeable by the majority of occupants. This fabric, although its fabric properties are not exactly what was specified, maintains the intent of the original design.

Specify with Acceptable Tolerances

In light of these recent industry findings, including the recommended acceptable tolerances for both openness factor and Tv value, the written performance specification is considered a prudent step in protecting the design intent of the space.

Another Solution: Specifications Grade Fabric

Perhaps a better solution is to manufacture shade fabrics that perform as promoted, and as specified, with openness factors and Tv values that fall within the identified acceptable tolerance limits. That is the idea behind the specification (spec) grade fabrics now launching onto the market.

Spec grade fabrics are fabrics that the manufacturer guarantees will meet the specified performance. The purpose of these spec grade fabrics is to give specifiers confidence that the fabric delivered to the job site will meet their design intent. To that end, spec grade fabrics adhere to rigorous manufacturing protocols, testing, and measurement standards, and are supported with meticulous documentation.

Consider the following as an example of the process used to deliver a single roll of certified specification grade shade fabric to a plant for assembly: A cut of shade fabric is a very long and continuous piece of fabric comprised of a single color and openness factor. Fabric rolls are smaller subsets of the longer cuts and it is the fabric rolls that are delivered to shade assembly plants. For a single roll to be certified, the entire cut that the roll came from must be tested and found to be within the tolerance limits to a degree that is statistically significant. Every compliant roll must arrive at the assembly plant with the documentation that validates that the entire cut, from which the roll was produced, was found to be within the acceptable tolerance limits. Samples taken from each certified cut must be measured by a spectrophotometer for openness factor and visible transmittance according to the measurements standards EN14500:2008 and ASTM 903. These processes and protocols were designed to provide 95 percent confidence that 95 percent of all of the fabric from certified rolls within certified cuts will meet the performance requirements on the date of manufacture.

There is a standardization problem in the shade industry right now. Carefully selected and specified fabrics can, and do, arrive on-site with openness factors and Tv values well outside of any acceptable limit. These rampant deviations can wreak havoc on the interior space exposing occupants to glare and poor view preservation, limiting a project’s ability to achieve daylight autonomy, and undermining the original intent of the design in the first place. Luckily, with tighter, performance-based specifications and specification grade fabrics, designers have the tools to ensure that the right fabric makes it onto their projects. Then they can sit back and enjoy the glare-free, beautiful views they’ve created.

Jeanette Fitzgerald Pitts has written dozens of continuing education articles for Architectural Record covering a wide range of building products and practices.


Lutron Electronics
Lutron Electronics Co., Inc., headquartered in Coopersburg, Pennsylvania, designs and manufactures energy-saving lighting controls, automated window treatments, and appliance modules for both residential and commercial applications. Its innovative, intuitive products can be used to control everything from a single light, to every light and shade in a home or commercial building www.lutron.com


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Originally published in Architectural Record.