Top Five Tips for Successful Daylighting Design  

Good daylighting design is difficult. Even the premise of successful daylighting seems to border on the nearly impossible: design a human-centric building to manage the largest and most dynamic energy source in the universe so that the structure will protect occupants and the interior from experiencing glare and solar heat gain throughout the day, while maximizing the amount of ambient daylight allowed onto the floorplate, when it is available.

Despite its challenging nature, good daylighting design is in demand. In fact, daylight and views are the most requested amenity within the workplace.¹ This trend is motivated, in no small part, by the fact that study after study, beginning with the influcntial 2003 work conducted by the Heschong Mahone Group², continue to prove that daylight benefits both the operational bottom line of the built environment and the people in it. Daylight can save energy and reduce operating costs, because electric lights can be dimmed or turned off when sufficient daylight is available. Access to daylight and views also improves the health, well-being, and productivity of the people inside. If a space is designed to accomplish work, educate, or heal, then bringing daylight into the space has been shown to make people work harder, learn more quickly, and heal faster. These results have left building owners of all types clamoring for daylight inclusion.

Photo courtesy of Lutron Electronics

While every project is unique, there are five best practices that can reliably position a space to achieve daylighting success.

In response to all of this daylighting enthusiasm, the market has been flooded with new technologies and solutions designed to successfully incorporate daylight into the built environment. But not all of these systems are created equal, and designers face the monumental task of sifting through the mountain of options to match the unique needs of their project with the right daylight management solution.

Unfortunately, there are a lot of misconceptions in the industry that often lead designers to select the wrong daylighting system or write an insufficient specification, leaving angry building owners to deal with subpar daylighting performance. Common mistakes include: specifying static daylight solutions to manage a dynamic daylight source; selecting products that rely on unreliable manual manipulation; and basing material selection solely on aesthetics, without considering performance.

While every project is unique, differing not only in the type of daylight available at the site but also in the specific daylighting objectives that must be met, there are some best practices that can reliably position a project to achieve daylighting success—a more comfortable, productive, sustainable workplace. Incorporating these five tips into a project’s design will enable a space to maximize the presence of ambient daylight when it is available, preserve the view to the outdoors, and protect the interior space from glare and solar heat gain. Here they are.

Photo courtesy of Eric Laignel for Perkins+Will

There are four primary daylighting objectives: preventing glare, preserving outdoor views, reducing electric light use, and mitigating solar heat gain.

Tip #1: Define and Prioritize Daylighting Performance Goals

With daylighting design, the best place to begin is to identify how the owner would like to see the building use daylight to keep occupants engaged and motivated³, while also saving energy once the project is completed. There are four primary daylighting objectives. They are: preventing glare, preserving outdoor views, reducing electric light use, and mitigating solar heat gain. Identifying which objectives pertain to a particular project and prioritizing them is an important step in defining the necessary performance of the selected daylighting system.

Prevent Glare

Preventing glare and excessive brightness from destroying the balance of the interior visual environment is perhaps the most critical task that daylight management systems must accomplish. Glare occurs when an element in the visual field, either a light source or the reflection of a light source, is significantly brighter than the surrounding visual atmosphere. When the difference in illuminance levels within the visual field is too great, the eye cannot effectively adapt to the extreme brightness in its view and discomfort occurs in the form of headaches and eye strain, which lead to fatigue. The key to preventing glare in a space is to keep the illuminance ratios of the interior balanced within the scope of what can be effectively used and managed by the human eye. In RP-5-13: Recommended Practice for Daylighting Buildings, the IES recommends that, “illuminance ratios of task to surrounding areas be kept below 1:3 for adjacent areas (30-degree cone) and 1:10 for remote areas (60-degree cone).”

Photo courtesy of Lutron Electronics

Different types of space can tolerate different levels of potential glare. Lobbies and social spaces may prioritize access to views over glare control.

Maintaining this illuminance ratio is a constant problem when daylight is allowed into the interior because the sun is a powerful light source and daylight has the potential to become incredibly bright. The intensity of daylight can range from 500 to 2,000 foot-candles (fc) on an overcast day, and direct sunlight can reach up to 10,000 fc when the sky is clear. In a typical office setting, the range of useful daylight levels is considered to be between 10 fc and 200 fc at the work plane. This range of daylight supports the balanced illumination ratios recommended by the IES and will not disrupt the visual environment or the people working there. Looking at the range of possible daylight intensity and the useful range of daylight that is acceptable at the workspace, some type of daylight management system will be required to reduce daylight levels sufficiently at the window on even overcast days. Systems that only control direct sun are insufficient for providing the comfort necessary for most office tasks. This highlights the need for window coverings beyond fixed shading devices, such as overhangs and fins.

Glare control is the most common objective of a daylighting system. However, 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 function of the area. 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.

When determining the goals of the building, be sure to evaluate glare-reduction needs in individual spaces. This will impact the type of daylight management system that is right for the project and enable the designer to offer a more tailored solution that best fits the needs of every space.

Be careful using metrics that only evaluate the presence of direct sunlight or do not incorporate the use of dynamic daylighting systems in their analysis, such as the annual sunlight exposure (ASE) metric used in the Leadership in Energy and Environmental Design (LEED™) rating system. Their limited scope is not meant to adequately account for all glare factors that can significantly reduce comfort and productivity.

Preserve Outdoor Views

Americans spend approximately 90 percent of their time indoors.⁴ Research suggests that those who have access to outdoor views are more satisfied and have a higher sense of wellbeing at work.⁵

Offering people in the interior of a building a connection and view of the outdoors has been shown to improve their mood, satisfaction, and well-being. It is also a design criteria included in green building rating systems, such as the LEED rating system. Unfortunately, intense daylight or bright sky conditions can disrupt these beneficial outdoor views because when it is too bright to look comfortably at a window, it is also too bright to enjoy the view beyond the window.

Many daylighting systems are ill-equipped to simultaneously manage daylight intensity and provide a great view to the outdoors. Most dynamic daylighting systems can be moved into position to block direct or intense sunlight, with some view obstruction, and then retracted to allow occupants to enjoy outdoor views when the daylight conditions are appropriate. The ability to manage intense daylight while preserving outdoor views is a key differentiator in the daylighting systems currently available and, if this is an important goal, it should be identified early to inform the selection of the right daylight management system.

Reduce Electric Light Use

Daylighting, also referred to as daylight harvesting, refers to the practice of reducing electric light levels when daylight is present. The potential savings can be significant. In the Daylighting section of the Whole Building Design Guide (www.wbdg.org), Gregg Ander, FAIA, writes, “For many institutional and commercial buildings, total energy costs can be reduced by as much as one-third through the optimal integration of daylighting strategies.”

On the spectrum of the energy savings that can be achieved through daylighting, from moderate to magnificent, there is a new design objective that has recently emerged, where the goal is not to use daylight as a supplement for electric light, but to use daylight exclusively, in lieu of electric light, to illuminate the building for a portion of the workday. This lofty design objective is called daylight autonomy (DA). The 10^th edition of The Lighting Handbook, published by the IES, defines daylight autonomy as “the percentage of the operating period (or number of hours) that a particular daylight level is exceeded throughout the year.”

In terms of designing for daylight-responsive energy savings or daylight autonomy, it is important for a designer to understand the performance goals of the building’s daylight harvesting efforts. Achieving higher and higher degrees of energy savings will require that the building maximize the presence of usable daylight allowed into the interior so that electric lights can be dimmed or off as often as possible. This functionality has control implications that will need to be considered when selecting the daylighting system that is right for this project.

“Dynamic daylighting systems are the only way to simultaneously optimize for glare control and daylight availability,” explains Brent Protzman, Director of Building Science & Standards at Lutron Electronics. “Passive, stationary systems are incapable of meeting the continuous variability of daylight conditions, and this limits their ability to take full advantage of useful daylight.”

Mitigate Solar Heat Gain

Infrared and near-infrared radiation are heat sources that provide no value in daylight harvesting. Solar heat gain occurs when direct sunlight, which contains a significant amount of infrared radiation, passes through a daylighting aperture and is absorbed by the interior, heating it up. This absorption can happen at the window or deeper into the floorplate, where the direct beam radiation is absorbed by the furnishings and occupants with which it comes into contact. Solar heat gain can increase the demand on the HVAC system as it adds heat to the environment, and it can compromise the thermal comfort of the occupants in the building.

The Need to Prioritize

Daylighting objectives are generally at odds with one another, which is why it is important to prioritize and consider the unique needs of different types of space within the project. Identify areas where glare control is critical and areas where achieving a greater degree of daylight autonomy or preserving an outdoor view may be more important. Functional productive spaces, such as office spaces and conference rooms, may prioritize glare control over view preservation, where social and transitional areas, such as hallways and cafeterias, can enjoy greater levels of daylight without disrupting the purpose of the space. Once the priorities have been established, it is easier for a designer to select a daylight management system that meets these space-specific needs.

Tip #2: Specify Interior Shades for Daylight Management

There are three types of daylighting technologies regularly considered for projects today. They are a louvered system, electrochromic (EC) glass, and interior solar shades. Each has its unique strengths and limitations in terms of daylight management, but interior solar shades are best equipped to juggle the many, and somewhat conflicting, daylighting deliverables, as well as design and life-cycle costs, demanded in commercial and institutional spaces.

Photo courtesy of Eric Laignel

Interior solar shades are best equipped to juggle the many, often conflicting, daylighting deliverables demanded in commercial and educational environments.

Louvered System

A louvered system combines angled slats with solid blades and open space to direct daylight. When fully closed, these systems effectively block intense daylight from entering the space. When partially open, the system uses angled slats to direct daylight away from the building, toward the ceiling or floor or deeper into the interior. Louvered systems can be added onto a building facade, incorporated into an environmental wall, or applied at the interior window. Exterior louvered systems most often run vertically along the building structure and are constructed of substantial materials like concrete and metal. The blades in interior louvered systems, like venetian blinds, often run horizontally and are made from metal, wood, or plastic.

While the approach to daylight management is the same between interior and exterior louvered systems, the system costs are significantly different. Exterior louvered systems are structural components that must be custom designed for every project. Interior louvered systems are regarded as a somewhat typical window treatment.

Electrochromic Glass

Electrochromic (EC) glass is an IGU that can be electronically tinted. It differs from a standard IGU in that a special coating, made up of multiple layers of ceramic material, is applied to the inside, or cavity facing, surface of the exterior pane of glass. Applying a low-voltage direct current to the coating causes it to gradually tint, providing an increasingly more effective barrier to light penetration and solar heat gain, while preserving outdoor views. The effect is easily reversed, untinting the glass and returning it to its highest transmittance state. It can be used for windows, skylights, and curtain walls.

EC glass offers multiple tint options that range from clear to fully tinted. These tint options enable EC glass to tailor the level of direct light and heat control it provides to best match the exact daylight conditions or support the unique visual needs of the task at hand. It also tends to take several minutes to transition to a different state -- sometimes requiring additional interior shading that can be moved to reduce glare instantly.

In terms of cost, EC glass is a substantial structural component of the building facade that requires design and performance considerations beyond the way the technology manages daylight. Design, installation, and the material cost of EC glass make it one of the more expensive daylight management solutions.

Interior Solar Shades

Interior solar shades use a woven fabric to diffuse, reflect, or absorb the light at the window. This woven filter uniformly manages daylight across the window pane and creates a softer, more usable level of daylight from what is available.

Performance Comparison

Louvered systems, EC glazing, and solar shades manage daylight very differently, and their ability to balance glare control, view preservation, and daylight autonomy varies dramatically as well. Each system is also differently equipped to combat solar heat gain.

Glare Control

In terms of preventing glare, the only way to effectively reduce the intensity of daylight allowed into the interior, when direct sunlight (10,000 fc) is at the window, is to create a solar barrier between the outdoors and the interior that will block or filter the intense daylight. When fully closed, the louvered system creates a complete light barrier at the window. Whenever louvered systems are somewhat open, they will allow some degree of unfiltered daylight into the space. This can create striations of extremely bright slivers of light next to shadow. This pattern of stark contrast can be visually disrupting and may disrupt the balance of illuminance ratios within the field of view. Electrochromic glass tints to its lowest transmittance level to reduce the amount of daylight allowed to pass through it, but EC glass is limited in its ability to diffuse the glare that can be caused when the orb of the sun is in the field of view. The extreme tinting required to achieve sufficiently low transmittance levels can also affect the color of light in the interior space, giving daylit spaces an unnatural hue or giving the exterior an unnatural night appearance during the day. It can also take a long time for EC glass to achieve a sufficient tint level. Depending upon the size of the panel, it can take up to 30 minutes to reach its lowest transmittance level. In many applications, this delay is poorly suited to protect interior spaces from direct sun that may suddenly appear from behind a cloud.

In contrast, solar shades apply a uniform filter across the window that creates a softened and continuous shadow in the space. In addition, shades with low transmittance levels are able to restrict the amount of light coming through the window and effectively diffuse the image of the sun’s orb, reducing occupant exposure to glare, even as the sun travels through the field of view.

View Preservation

The daylighting goal of view preservation can have two meanings. On one hand, it can refer to the idea that, in the absence of glare-causing conditions, the daylighting system retracts to allow unobstructed views out of the window or daylighting aperture. This relates more to the control system driving the daylighting technology and not to the physical daylighting device so it will be addressed in the next section. As it relates to the daylighting technology of a louvered system, EC glass, or solar shades, view preservation describes the ability to see the coveted outdoor view, while the daylighting technology is in place to combat glare. Only interior solar shades are capable of providing critical glare control, diffusing the view of the sun, and preserving the outdoor view at the same time.

Photo courtesy of Lutron Electronics

Solar shade fabric filters and diffuses the sunlight passing through it, while allowing the eyes to look beyond the woven thread to enjoy the outdoor view.

The solar screen fabric filters and diffuses the sunlight passing through it, while allowing eyes to look beyond the woven threads and into the outdoor environment. Even when the fabric is lowered, building occupants can enjoy landscape and city views. Louvered systems, whether exterior or interior, are incapable of providing glare control and view preservation at the same time. Exterior louvered systems are always in place, although their position may change, creating a constant visual interruption of the outdoor view. Interior louvers or vertical blinds can be retracted, but when they are deployed to prevent bright sunlight from entering the space, they also disrupt the view to the outdoors or eliminate it entirely. When louvers, interior or exterior, are fully closed or closed beyond the cutoff angle, it is impossible to see the cityscape or outdoor environment on the other side. It is possible to see through EC glass when it is tinted, but, at this time, when the tint gets very dark, it discolors the surrounding landscapes within view.

Mitigate Solar Heat Gain

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. As previously mentioned, the key to thermal management is the control of infrared radiation contained in direct sunlight.

Exterior louvers, when they are positioned correctly, are a highly effective solution for mitigating solar heat gain, primarily because the infrared radiation is reflected away from the building before it enters the physical structure. Interior louvered systems, on the other hand, manage the infrared radiation once it has moved into the interior. Although they may direct daylight up toward the ceiling or down toward the floor, keeping it out of the eyes of people in the space, louvers do little to manage the component of infrared radiation in the daylight that is readily absorbed by the ceiling tiles or the carpet, heating up the interior.

The ceramic material in EC glass is able to modulate the transmission of near-infrared radiation through the glass. The technology is able to effectively reduce the solar heat gain coefficient (SHGC) of the pane of glass, which means that it controls both the radiation allowed to pass through the glass into the interior and also the amount of solar radiation absorbed by the glass and re-radiated into the interior space. EC glass, in its most tinted state, can reduce the SHGC to 0.09, meaning 9 percent of the total solar radiation that contacts the glass is transmitted inside. However, EC glass tints considerably to control solar heat gain, it can affect the amount and color of light in the interior space.

Image courtesy of Lutron Electronics

Solar shades create an effective barrier between the glass and the interior space, reflecting the solar energy off of the exterior face of the fabric and back into the atmosphere.

Solar shades 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 enters the workspace and heats it up. Realized energy savings for cooling a building where windows are equipped with shading fabric have been shown to range from 3 to 22 percent, depending upon the facade design and fabric properties. As an added benefit, shades significantly improve the impression of thermal comfort, which is a person’s perception that they are surrounded by the right air temperature.

Tip #3: Choose Automated Controls

The type of control used by a daylighting system refers to how the physical technology (i.e., louver, electrochromic tint, fabric solar shade) is moved into and out of position. There are three categories of control from which specifiers have to choose: manual, motorized, and automated. The type of control selected for a project has a profound effect on the overall performance of the daylighting system.

Manual Control

Manual control requires that a person physically manipulate the position of the daylighting device. This traditional control approach has serious limitations in the level of daylighting performance it is able to provide because the positioning technology it relies upon is often unreliable. Energy savings, access to outdoor views, and comfort are readily compromised if the shade, louver, or electrochromic tint is not at its optimal position at any time throughout the work day. Unfortunately, the optimal position for these daylighting technologies changes constantly, and most of the people charged with the manual manipulation of a daylighting device are not interested in or available to tweak its position multiple times a day.

In determining the successful daylighting performance of a system, dynamic positioning is of paramount importance. Eastern exposures receive direct sunlight and may have a view of the sun’s orb during the morning hours, requiring that daylighting devices be lowered in the morning to manage glare and raised in the afternoon to allow ambient daylight inside. Western exposures have an opposite schedule, accessing soft, ambient light in the morning and receiving direct beam sunlight, and a direct view of the sun, as the sun sets in the afternoon. The sun’s daily path across the sky is not the only consideration for successful daylighting. Clear, sunny days can create overwhelming brightness at the window, regardless of the position of the sun, whereas, overcast days offer the softer, diffuse daylight that can be used without threat of glare from sunrise to sunset.

Unfortunately, positioning deficiencies are inherent in a manually operated system because effective daylighting is not top of mind for most employees in a building, nor would the employer want it to be. Manual operators are more likely to deploy a daylighting device to address discomfort (i.e., glare or heat) and then leave it in place, reducing the amount of ambient daylight allowed into their space. For example, it is common for people to completely pull down manual shades to control glare conditions they may be experiencing and then leave the shades down for days, weeks, or months. This single, static position severely limits the daylight and views benefit that can be realized in this space.

Motorized Technology with Manual Control

Motorized control is manual control at the touch of a button instead of the pull of the chain. Users can move single shades or banks of coordinated shades into preselected positions from keypads placed on the wall or by using wireless remote controls. Motorized control offers a clear aesthetic improvement over manual control in that the position of adjacent shades, or blinds, can now be consistent, creating a cleaner look. In terms of controllability, the different positions programmed onto the keypad may encourage users to select different positions beyond open and closed, which will allow some daylight into the space, even when the shades must be deployed to reduce the daylight level at the window or manage a direct view of the sun’s orb, but the system still requires manual manipulation throughout the day to create the balance of blocking glare-causing light and maximizing the usable daylight allowed into the space, when it is available. The manual aspect of a motorized system means that its performance can be extremely limited depending upon the attention the system is given by its handlers.

Photo courtesy of Lutron Electronics

Motorized control is manual control at the touch of a keypad or touchscreen, instead of the pull of a chain.

Of course, moving from manual to motorized also affects the total cost of the system. Exterior louvers are typically the most expensive solution, requiring custom system design and regular maintenance to address the issues caused by exposing a motorized system to the outdoor elements. While interior louvered systems do not battle wind, rain, or snow, they can be more expensive than expected because they require the use of two motors. One motor tilts the blades at the desired angle when the blind is deployed. The other motor provides the lift that retracts the louvered system when soft, ambient daylight is available. Motorized solar shades require only one motor to deploy and retract the fabric as needed, offering a more cost-effective motorized daylight management solution.

Automated Control

Automated controls are designed to precisely position the daylighting device for optimal daylighting performance. These systems use the known solar path of the sun to determine the optimal position of the shade or blind, or degree of tint on the glass, to manage glare caused by a direct view of the sun and continuously adjust to accommodate changes in solar angles. These systems can also have small wireless sensors placed near windows that detect the level of daylight entering a space and adjust the control device accordingly. This sensor placement enables the system to appropriately respond to overly bright days or overcast days.

Photo courtesy of Lutron Electronics

Automated shades move into precise and optimal positions throughout the day to protect the interior from glare and solar heat gain, while allowing usable daylight into the space when it is available.

Here is an example of how an automated shading system in a glass-clad building may function throughout the day. As the sun rises, the shades on the eastern exposure may deploy to a position halfway down the window to block the direct view of the sun from the workplane. The shades on the northern, southern, and western exposures may be entirely retracted to bathe the interior in gentle morning light. As the sun climbs higher in the sky, the shades on the eastern exposure may rise slowly, while the shades on the southern exposure deploy one-fourth of the way down the window, and the shades on the western and northern facades remain fully open. As it approaches noon, the shades on the southern exposure lower to the halfway position and the shades on the western facade lower to cover the top fourth of the window. In early afternoon, the shades on the eastern exposure are retracted completely, letting in the ambient afternoon light, and the shades on the southern and western sides deploy further to block the direct view of the sun and the direct sunlight it delivers. At sunset, all of the shades are fully open, allowing occupants to enjoy the soft light of early evening. All of this active daylight management occurred without anyone touching a button.

Lighting Energy Savings Comparison: Manual vs. Automated Shade Controls

Having a system that can maximize the presence of ambient daylight in a space also enables the lighting system to maximize the energy it can save when daylight is present. A study completed in collaboration with Lutron Electronics and Purdue University evaluated the difference in lighting energy savings that could be achieved with shading systems using manual and automated controls. The studied space was a perimeter private office with a lighting power density of 0.9 watt per square foot and standard, clear, double-pane glass. The same shading fabric was also used in both energy simulations, leaving the type of control used to manipulate the daylighting system as the only variable. Manual shades were defined as closed shades for the study. The singular position, although not ideal, was thought to more closely resemble actual manual shade operation than scenarios where manual shades are repositioned multiple times throughout the day. The team concluded that an automated shading system was able to reduce the electric light use in the private office by 83 percent more than the same space when the windows were outfitted with manual shades. For more information about this study on energy savings click here or visit www.performanceshadingadvisor.com/LutronResources/pdf/energySavings.pdf.

Tip #4: Consider More Than Color When Selecting a Solar Shade Fabric

As a fabric, the shade fabric is often selected during the furnishings and finishes stage of the job. Oftentimes a large fabric binder is placed in front of the design team, and the shade fabric is selected based on the color and style that best complements the interior décor. On some occasions, openness may be decided based on loose rules of thumb. Unfortunately, this aesthetics-based approach does not take into consideration the fabric’s ability to control glare, preserve view, allow diffuse daylight into the space, or mitigate solar heat gain. When color is the key consideration in selecting a fabric, specifiers are likely to get a product that looks good on the wall but may not provide satisfactory daylighting performance. Before specifying a fabric, be sure to match the fabric properties—openness factor, visible transmittance, solar reflectance, and color—to the daylighting management needs of the space.

Openness Factor

The openness factor (OF) of a shade fabric refers to the amount of light that is able to pass straight through the shade. If a shade has an openness factor of 5 percent, then 5 percent of the sunlight that transmits through the window at an angle that is perpendicular to the pane will pass directly through the shade and into the interior. The remaining 95 percent of the light that contacts the shade fabric will be diffused, reflected, or absorbed.

Visible Light Transmittance

Visible light transmittance (Tv) refers to the total amount of light allowed to move through the shade and into the space. It includes both direct and diffuse light energy in its calculation.

Image courtesy of Lutron Electronics

Visible light transmittance (Tv) refers to the total amount of direct and diffuse light allowed to move through the shade.

The Tv value of a shade is primarily affected by the physical openness in the weave and the color of the shade, but it is also influenced by the shape, opacity, and specific pattern of the weave. 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 often have a lower Tv value than a light-colored shade fabric. However, there can be significant performance differences between fabrics of the same color. Be sure to review the Tv value of a fabric when specifying solar shades to better understand how that product will perform in the space.

Solar Reflectance

Solar reflectance (Rs) refers to the percentage of the total solar radiation that is reflected off of the exterior face of the fabric and back outside. Solar reflectance values are determined, in large part, by the color or coatings on 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 shade 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. As a general rule, Rs values greater than 30 percent will provide some protection from solar heat gain, while an Rs value of 50 percent or greater provides good thermal protection. Remember that there are ways to have fabrics with a high reflectance exterior and dark interior color. This includes dual-sided fabrics, metal-backed fabrics, and fabrics with special coatings.

Color

Solar shade fabrics 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 décor.

Matching Fabric Property and Performance

When attempting to identify which fabric properties will achieve the prioritized design objectives, it is important to acknowledge that the fabric properties are interrelated. Here is some guidance on how these different fabric properties work together to control glare, allow ambient daylight into the space, preserve outdoor views, and mitigate solar heat gain.

Image courtesy of Lutron Electronics

This table illustrates how daylighting objectives can conflict and underscores the importance of prioritizing project goals to select the right fabric.

There are web tools available that have been designed to consider conflicting daylighting objectives and identify suitable fabric options based on the various properties of the fabric and the desired performance specifications.

Image courtesy of Lutron Electronics

The Performance Shading Advisor (shown here) is an example of a web tool that can balance daylighting objectives and help designers select fabrics that will perform as needed.

Fabric Properties to Control Glare

Solar screen fabrics with different openness factors are often specified throughout a project to best match the need for direct sunlight control on each facade and elevation. East and west-facing facades, particularly with clear glass, that experience direct sun exposure during sunrise and sunset are often matched with solar shade fabrics that have an openness factor of 3 percent or less (recommended: 1 percent). The smaller openness factor creates a finer filter across the window pane and effectively diffuses even the orb of the sun when it is in view. Fabrics with openness factors of 4 percent or less (recommended: 2 percent) are typically placed on southern-facing windows that have a direct view of the sun. Windows with northern exposures are often outfitted in fabric shades with larger openness factors, allowing more of the readily available ambient light into the space.

A shade with a low Tv value will also provide good diffuse daylight control and limit the potential for the fabric to become overly bright when managing intense daylight conditions. Remember that white fabrics, even with low openness factors, become a very bright glare source when in contact with direct sunlight.

Fabric Properties to Maximize Daylighting

If the design objective is to maximize the amount of glare-free daylight that is allowed into a space in order to achieve the greatest degree of energy savings possible, then select a shade with the highest Tv value that still maintains glare control. The higher Tv value will increase the amount of light, both direct and diffuse, that is allowed through the shade fabric and into the space.

Fabric Properties for View Preservation

The crispness and clarity of view afforded through a shade fabric can be predicted as a function of the Tv value and openness factor. Darker fabrics with higher openness factors generally achieve a higher degree of clarity, followed by dark-colored fabrics with low openness factors. Light-colored fabrics typically provide the most interference with outdoor views, offering slightly muddied or muted versions of the surrounding colors.

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) ranks view clarity from 0 to 100 percent. A value of 100 percent means that the fabric causes no perceivable interference with exterior views. At 50 percent, most of the objects on the exterior are recognizable, although the edges are blurred and colors visible, but washed out. A value of zero indicates that no view is visible through the fabric.

Fabric Properties for Thermal Management

Manufacturers have developed shades that improve 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 heat rejection of the fabric, without sacrificing the clarity of the objects or colors seen on the other side.

Photo courtesy of Lutron Electronics

Clearly identifying the daylight management needs of a project and then selecting interior solar shades with automated controls and a shading fabric with the right daylight properties equips every space for daylighting success.

Tip #5: Select Specification-Grade Fabric

Unfortunately, new evidence has revealed that even if a specifier selects fabric with the properties that should manage daylight as desired, it is highly likely that a fabric with different properties will end up on the job site. Combat this industry-wide standardization problem with specification grade fabrics: fabrics that guarantee their delivered product will match the specified performance.

The Study

A recent manufacturer-led analysis 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 the fabric cards and other marketing materials. The deviation was astounding. Not only was a difference between the actual values and the listed values common, the difference was enough to materially affect the way that the shade would perform.

Deviation in Openness Factors

Openness factors with a margin of error of up to 3 percent were found. This means that if a fabric with an openness factor of 5 percent was specified, a fabric with an openness factor of between 2 and 8 percent could have been installed. Fabrics with openness factors that are higher than specified may expose a space to greater degrees of 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 amount of energy savings able to be realized on a project.

The Solution: Specification-Grade Fabric

Specification-grade fabrics, or spec-grade fabrics, are fabrics that a manufacturer guarantees will meet the specified performance metrics. The purpose of these spec-grade fabrics is to give specifiers the 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.

Good daylighting design is difficult. It requires the management of a dynamic and intense source of light and heat, the satisfaction of often conflicting project goals, and the ability to balance the need to allow daylight into a space, while preventing glare and solar heat gain. There are five tips for achieving successful daylighting on a project: 1) define and prioritize daylighting performance goals; 2) select interior solar shades; 3) choose automated control; 4) consider more than color when specifying a solar shade fabric; and 5) select specification-grade fabric. Applying these five tips to a daylighting project will not change the fact that good daylighting design is difficult, it just won’t be difficult for you.

End Notes

¹ Future Workplace. (2018) workplacetrends.com/wp-content/uploads/2018/08/The-EmployeeExperienceFINAL08-072.pdf

² Heschong Mahone Group (2003). Windows and Offices: A Study of Office Worker Performance and the Indoor Environment. Detailed Report. Fair Oaks, CA.

³ Browning, B., Cooper, C. (2015). The global impact of biophilic design in the workplace. Interface. Retrieved from: www.interface.com/US/en-US/campaign/positive-spaces/Human-Spaces-Reporten_US

⁴ www.epa.gov/indoor-air-quality-iaq/inside-story-guide-indoor-air-quality

⁵ Farley, K. M., & Veitch, J. A. (2001). A room with a view: A review of the effects of windows on work and well-being.

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

Lutron Electronics, headquartered in Coopersburg, Pennsylvania, designs and manufactures energy-saving light 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