A New Methodology for Successful Daylighting Design  

Imagine a new commercial building situated on a rural stretch of vacant farmland. The envelope of the building is predominantly comprised of large windows on every exposure and boasts a window-to-wall ratio of 60 percent. The intention of the design was to maximize the penetration of soft, glare-free daylight as deeply into the space as possible, saving energy, and providing unobstructed views of the nearby lake and rolling hills that sit adjacent to the site. To that end, an automated shading system was specified to ensure that the shades were always lowered to the most advantageous position. They will be deployed to prevent bright and direct sunlight from penetrating the interior and raised when soft ambient light is available, allowing that usable daylight into the space. Overhead lights can be turned off or dimmed for much of the workday. A beige shade fabric that perfectly complements the interior furnishings was selected.

Upon visiting the occupied building one sunny morning, it is immediately obvious that something has gone horribly wrong. The windows on the eastern exposure are ablaze. The deployed beige shade is backlit to a blinding white by the rising sun, creating an uncomfortable glare across the entire interior window wall. The idyllic view beyond is obscured by the overwhelming brightness at the window. Occupants of the space mention that a similar phenomenon occurs on the western-facing facade as the sun sets.

This tale of window shade woe usually occurs for two reasons: the wrong type of fabric is unknowingly specified onto a project, or, if the right fabric is specified, a fabric that does not meet the specification is delivered and installed, something that happens surprisingly more often than one might think. In either case, the use of an inappropriate shade fabric can be an egregious offense, as it can dramatically disrupt the comfort, productivity, and energy consumption of the interior. This sentiment is echoed by Darrell Sawatzky, senior interior designer, LM Architectural Group, Manitoba, Canada: “Because of its role in daylight and thermal management, and its potential effect on building performance, I believe a shading fabric should be considered as an integral part of the building envelope, instead of a decorative window covering.”

The challenge for specifiers trying to select the right fabric is two-fold. Until recently, the process for identifying the best fabric for a project had not been simple or straightforward. Project objectives are often at odds with one another, like reducing glare, which requires that the shade provide an effective visual barrier between the exterior and the interior, and preserving an outdoor view, which requires that the shade remain as unobtrusive as possible. It is also unclear how various shade fabric characteristics contribute to the overall performance of the fabric, hindering a designer’s ability to tailor the specification of a shade fabric effectively to deliver the necessary performance.

Photo courtesy of ©Doug Scott 2014

Achieve successful daylighting design by looking beyond the color of the fabric and using performance-based criteria to select a shade.

This article will fill in the information gaps and share specifics on how to select a shade fabric that will meet the daylight management objectives of the space and explore options for ensuring that a conforming shade fabric is delivered and installed onto the project.

Photo courtesy of ©Doug Scott 2014

Different types of space have different daylight management needs in terms of the degree to which a shade must protect the interior from glare, preserve an outdoor view, and provide thermal regulation.

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.

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.

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.

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.

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.

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.

Shortcomings of the Typical Spec

Here is a very general description of how shade fabric is selected on a great majority of projects. As a fabric, it is often addressed during the furnishings and finishes stage of the job. A large fabric binder is placed in front of the design team, and the fabric for the space is selected based on the color that best complements the interior décor. Sometimes a specifier will select an openness factor based on a general rule of thumb, or the successes and failures learned from previous projects.

Unfortunately, the color and style of a shade does not dictate its ability to mitigate glare, preserve view, or promote daylight autonomy. This aesthetics-based selection process opens up the interior space to greater potential problems with glare and increases the likelihood that outdoor views will be unnecessarily compromised and energy savings limited. When color is the key consideration in selecting a fabric shade, specifiers are likely to get a product that looks good on the wall but may not manage daylight in an appropriate or satisfactory way. Performance and the fabric properties that drive performance, such as openness factor, visible transmittance, and solar reflectance, must be included in the specification process for optimum results.

Performance-based Specification Process

A more rigorous specification process will enable specifiers to better match the performance of the shade with the needs of the space and select a shade fabric that is aesthetically pleasing as well. The steps in this process are: prioritize performance goals, simulate fabric performance, identify and select an optimal fabric, and write a performance-based specification that communicates design intent.

Prioritize Performance Goals

In order to better match shade performance with the needs of the space, it makes sense to start by identifying the needs of the space. This is especially important when selecting shades because some of the performance goals are at odds with one another. Clearly identifying the priorities of each space enable a specifier to tailor the shade selection to deliver glare protection in spaces where vision-critical tasks are taking place and to allow a crisper view, and slightly less glare control, in social areas.

Typically, work areas place the highest importance on glare control and equal weight on daylight autonomy and view preservation. Transitional areas regularly prioritize daylight autonomy over view, identifying energy savings and a higher level of daylight exposure as the most important goal. Social areas often rank view over daylight autonomy, encouraging specifiers to find shades that preserve color and clarity in these more casual areas. However, goals are project-specific, and that is why it is important to evaluate the unique qualities and needs of each space before selecting a shade fabric.

Simulate Fabric Performance

Across the industry, shade performance simulations can be attempted in a number of ways, but most of them require that the fabric be selected, or at least significantly narrowed down, before the simulations occur. Some projects allow for mock-ups in the budget, which would include an opportunity to test the selected fabric on the mocked-up window. However, mock-ups often fail to capture the actual variability of daylight conditions that exist and may misrepresent the true performance of the shade over the year.

There are several daylight simulation programs that can be used to create virtual models of the building and gauge how daylight will behave in the space given certain material properties and weather data. These simulations are designed to model the daylight in the building one shade material at a time and often do not take into consideration the shade control, albeit automated or manual. Simulating multiple fabrics is a time-intensive process.

Ideally, before spending the time to input several types of fabric into a simulation or reaching the final stage of the mock-up, specifiers would have a tool to identify suitable fabric options for a specific project based on performance criteria.

New Shade Selection Wizard Now Available

A new, free simulation tool is now available to the design and specification community that can provide this performance-based, project-specific analysis in a quick and user-friendly way. This Web-based tool works as a shade selection wizard, where designers key in certain project-specific information, such as the location and facade orientations, window size, glass type, and even a few specifics on the interior layout and function of the space. The wizard then populates a selection of fabrics that will provide the necessary degree of glare reduction, daylight autonomy, and view preservation demanded by the space type.

A new, free simulation tool is now available to the design and specification community that can provide this performance-based, project-specific analysis in a quick and user-friendly way. This Web-based tool works as a shade selection wizard, where designers key in certain project-specific information, such as the location and facade orientations, window size, glass type, and even a few specifics on the interior layout and function of the space. The wizard then populates a selection of fabrics that will provide the necessary degree of glare reduction, daylight autonomy, and view preservation demanded by the space type.

Select an Optimal Fabric

This more rigorous specification process offers specifiers a way to balance aesthetic preference with performance. Once the fabrics that will perform as needed have been identified by the Web-based wizard, a designer can select the fabric color and style that will provide the best aesthetic complement to the space.

Write a Performance- Based Specification

As previously mentioned, a typical shade fabric specification identifies the color and, sometimes, the openness of the fabric. This relatively scant description can leave the specification open to an unsuitable product substitution that could jeopardize the success of the space. This happens for two reasons. First, an openness factor is not a universal metric, and specifiers can get hung up relying on the general approximation, often listed on the fabric cards, instead of the mean openness factor, which is a much more accurate measurement of shade openness. Second, two fabrics with similar perceived colors and similar listed openness factors can, and often do, have very different Tv values and, subsequently, perform very differently in the space. Specifiers should be wary of accepting a fabric as equivalent if its fabric properties have not been tested and if the fabric has not been proven to perform in a manner that is equivalent to the fabric that was originally specified.

Performance-based specifications include the mean openness factor, the Tv value, the Rs value, and the color. This would also be the place to include any certifications that may be required by code or to satisfy green building initiatives, such as PVC-free certification, GREENGUARD certification, or RoHS certification. The selection wizard not only helps in identifying appropriate shade fabrics, but it provides all of the information necessary to write a performance-based specification.

An Industry Secret: Rampant Deviation in Shade Properties

Until recently, a performance-based specification was regarded as sufficient to ensure a successful shading installation. It was assumed that if the specifier could look beyond color and identify an appropriate shade in terms of performance properties, then the right shade would be delivered to the project. New information has exposed a cringe-worthy lack of consistency and standardization in the shading industry that makes securing the right shade fabric more difficult than writing a great performance-based specification.

The Study

A recent manufacturer-led study tested more than 200 fabric samples from all of the major U.S. fabric suppliers in the industry. The test measured the openness factors and Tv values of the fabric and compared the actual values with the values promoted on fabric cards and other marketing materials. The deviation was astounding. Not only was deviation between the listed and actual values common, it was significant enough to materially affect the way that the shade would perform on a project.

If a fabric was found to have the openness factor and Tv value that it was purported to have, it would have been recorded on the crossbar, where the x-axis and y-axis intersect. The x-axis indicates the error in the openness factor. The y-axis represents the error in the Tv value. The rampant and varied results captured in the chart indicate that fabric standardization is an important, industry-wide issue.

Deviation in Openness Factors

Openness factors with a margin of error of up to 3 percent were found. Meaning that if a fabric with an openness factor of 5 percent was specified, a fabric with an openness factor between 2 and 8 percent could have been installed. Shades with openness factors that are higher than specified may expose a space to greater degrees of direct glare, while lower openness factors will compromise the available view.

Deviation in Tv Values

The study found fabrics where the actual Tv value was more than two times higher than the posted Tv value. This means that if a fabric with a Tv value of 7 percent was specified, it is possible that a fabric with a Tv value greater than 14 percent may have been delivered. This dramatic increase in Tv value could create a real diffuse glare problem, while significantly lower Tv values may limit the degree of daylight autonomy that a space is able to achieve.

Photo courtesy of Lutron Electronics Co., Inc.

The variation on this chart indicates that poor fabric standardization, in terms of openness factor and Tv value, is an industry-wide issue.

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