Daylight Autonomy 101  

Throughout history, daylight has been considered in the design of the built environment. Before electric light was invented, in the late 19^th century, daylight was the primary light source available to illuminate the interior of buildings, schools, and residences. The houses of Ancient Rome were commonly found to have been planned around a courtyard, with surrounding rooms positioned so that the available daylight could penetrate deeper into the space. A guiding principle of Michelangelo’s iconic Laurentian Library, built around 1550, was to maximize the presence of daylight from both the northern and southern exposures in the reading room. In the mid-1800s, the one-room schoolhouses found throughout the United States relied on large windows to provide the teacher and students with enough light for their lessons.

Today, while buildings could meet their illumination needs entirely with electric light, the inclusion of daylight in the interior is credited with several important benefits. Daylight exposure has been linked to improvements in employee productivity and student performance, and even the regulation of a person’s circadian rhythm, the internal clock of the body which drives the sleep/wake cycle and has a powerful impact on general well-being. Beyond the touted health benefits, using daylight instead of electric light to illuminate interiors could save an enormous amount of energy, making it a good-for-the-body and good-for-the-planet lighting solution.

The key challenge lies in the fact that daylight is a dynamic light source. It comes in many forms, and its presence changes almost hourly. Daylight is affected by the time of the year and the weather conditions of the day. Fortunately, the advancements in daylighting technology and design know-how made in the past decade now make it possible to use the presence of daylight as an illumination source. Some designers are even seeking to use daylight as the exclusive light source for much of the day. This design objective, referred to as daylight autonomy, requires the careful reconsideration of many of the early design decisions made on a project and the selection of the right daylight management system to create a comfortable interior space, largely lit by the sun.

Introducing Daylight Autonomy

Today, designing a space to meet specific daylight-related objectives is a common practice. The usual daylighting goals include achieving some predefined daylight illuminance level on the workplane or at the floor, incorporating some measure of glare control, or delivering a daylight zone of a certain size. Achieving daylight autonomy essentially requires a project to achieve all of the above and more.

The 10^th edition of The Lighting Handbook, published by the IES (Illuminating Engineering Society of North America, formerly IESNA), defines daylight autonomy as “the percentage of the operating period (or number of hours) that a particular daylight level is exceeded throughout the year.” It is a dramatically different way to think about and measure the presence of daylight in a building. “One advantage of using daylight autonomy to quantify daylight availability in a building, is that the daylight autonomy calculations take climate into account, an aspect that previous metrics for quantifying daylight had not included,” explained Jack Bailey, Partner at One Lux Studio in New York City. “This metric could benefit architects and owners significantly. Architects can use daylight autonomy analysis to evaluate different design alternatives to determine which concept provides more usable daylight in the interior, and owners will know, definitively, that their building is making good use of daylight. It also provides a consistent metric for comparing the performance of different buildings for building codes and green building initiatives.”

It should be noted that at this moment achieving daylight autonomy in a building is not required by any international, federal, state or local building code. “It was included in the first public draft of the International Green Construction Code (IgCC), but was removed in favor of a simpler metric,” explains Bailey, who served on the committee that wrote the IgCC. “Nor is daylight autonomy analysis required for a project wishing to achieve LEED or any other type of green building certification,” he continues, “however, daylight autonomy is recognized as an option for achieving the daylighting credit in LEED v4.”

Metrics for Measuring Daylight Autonomy

Where daylight autonomy is the goal, it is critical to understand that a high daylight autonomy value does not necessarily create a productive, comfortable, energy-efficient daylit space. The DA value of a given point in a space represents the percentage of annual daytime hours that daylight levels exceed a predefined illumination level. When the defined threshold is the recommended illumination level for the space, the DA value indicates the amount of time that the interior space could be exclusively illuminated with daylight. For example, consider a location in an office space where the target daylight level is 30 footcandles (DA30) and the DA value is 87 percent. This means that area of the office receives at least 30 fc of daylight for 87 percent of the workday, enabling the overhead electric lights to be turned off a substantial amount of time every day.

While the DA metric indicates the amount of time every day that predefined minimum daylight levels have been achieved, it does not consider whether the available daylight is, in fact, too bright to use in the space, without a daylight management mechanism. In order to achieve a more complete picture of the presence of daylight in a space and ensure that the design adequately protects the interior from glare, there are a few additional metrics that should be considered.

Spatial daylight autonomy (sDA) refers to the percentage of floor area where 30 fc is achieved for at least 50 percent of the workday. Higher sDA values indicates that a larger interior space receives at least 30 fc of daylight for at least 50 percent of the workday. Generally, sDA is calculated using a daylight simulation tool that computes the daylight levels in the space for every hour of the year. Although there are many DA metrics, sDA has become the most common due to its inclusion in LEED v4 and the WELL Building Standard.

There is a metric that identifies the presence of extremely bright or direct daylight in the interior space. The annual sunlight exposure (aSE) metric identifies the percentage of the floor area that receives intense daylight, exceeding 100 fc, for more than 250 work hours every year. This metric is included as a design consideration in both the LEED v4 rating system and the WELL Building Standard. For example, to achieve the LEED v4 Daylight credit, a project must have a maximum aSE value of 10 percent.

Beyond identifying the exposure to intense daylight that a space receives, there is also a metric that measures the amount of useful daylight that a space receives. The idea of useful daylight revolves around the level of daylight that can be introduced into a space without causing glare or disrupting the visual environment. That range of useful, glare-free daylight is generally considered to be between 10 and 200 fc at the workplane. Useful daylight illuminance (UDI) refers to the percentage of work hours where the illuminance from daylight in a space is between 10 and 200 fc.

There is also a metric that measures the perception of glare. Daylight glare probability (DGP) quantifies occupants’ perceived glare from daylight. It is calculated by evaluating the entire visual field of a potential occupant, taking into account the light intensity, size of the glare source, contrast, and its position in the field of view. DGP is generally considered the best metric to evaluate glare potential today.

The Benefits of Daylight Autonomy

In many instances, energy savings is achieved by sacrificing something else that is deemed valuable, such as occupant comfort. One of the reasons that daylight autonomy is such an attractive design objective is that it manages to deliver significant energy savings, without negatively impacting occupant comfort or the functionality of the space. In fact, it may improve occupant comfort by improving access to outdoor views and ensuring that occupants are exposed to optimal amounts of usable daylight throughout the day, which improves productivity and satisfaction, while protecting the space from direct or overly bright light that can cause glare and discomfort. Daylight autonomy creates energy savings by being smarter about the inclusion and management of daylight and by eliminating excess lighting energy that is essentially unnecessary.

The Obstacles to Achieving Daylight Autonomy

If there is a down-side to daylight autonomy, it may be that, especially as a relatively new design objective, daylight autonomy can be challenging to achieve. It requires the ongoing, optimal management of a very dynamic light source and, as such, achieving daylight autonomy is very dependent upon selecting the right daylight management technology for the project. Setting a project up to achieve daylight autonomy may also require designers to retool their traditional approaches to a new project as it relates to the planning of the building envelope and interior. In addition, evaluating the potential DA factors of different concepts requires the use of complex software programs.

The dynamic nature of daylight. At the center of the daylight autonomy challenge is the dynamic and powerful nature of daylight. Daylight levels can range from a few footcandles on an overcast day to over 8,000 footcandles on a clear, sunny day. It can arrive at the window in many forms: streaming directly from the sun, gently diffused through the clouds, or harshly reflected off of a surrounding structure. And it arrives, in some form or another, every day, although its angle and position will vary as the earth orbits the sun.

The potential intensity and daily presence of daylight require that, if daylight is allowed into a building, it must be effectively managed or it can wreak havoc on the visual environment. Some problems commonly experienced as a result of mismanaged or uncontrolled daylight include: glare, hot spots, and thermal heat gain. These problems can cause larger issues of occupant discomfort, loss in productivity, loss of usable interior space, and energy waste.

Selecting the right technology. It is often prudent practice to design a building to be functional in the worst-case scenario. As it relates to the lighting system, the worst-case scenario would be something like blackout or midnight conditions, where zero daylight is available and all of the illumination must be provided by the electric lighting system. In terms of a daylight management system, the worst-case scenario would be that the building was subjected to intense, direct, glare-creating, and unusable daylight all day long, so the daylight management system would need to be deployed to either block or diffuse the daylight all day long. Fortunately, the conditions of an actual day rarely mirror those defined in a worst-case scenario.

It is the technology of the daylight management system that enhances or limits the building's ability to allow useful daylight into the interior when it is available. Automated shading systems automatically raise and lower throughout the day to maximize the useful daylight allowed into the building. A manual shade requires that a person manipulate its position and may not be regularly raised when useful daylight is present. Equipping a building to maximize the presence of useful daylight in a space is a critical step in achieving daylight autonomy.

A new approach to design. The daylight autonomy of a building is affected by variables of the building envelope as well as the interior layout and furnishings. “One of the challenges in achieving daylight autonomy is that it requires the design team to consider how the massing, siting, and orientation of the building impact the availability of daylight in the interior floorplate,” explains Bailey. It also requires that many of the elements of the interior space be reconsidered in terms of how it affects daylight penetration and daylight management. This includes the layout of the interior space, the placement and selection of cubicle walls, and even the interior color and finish.

Evaluating alternative designs requires a specialist. Perhaps the greatest challenge for early adopters trying to achieve daylight autonomy is the complexity of the software programs currently available to help designers evaluate the DA factors of their various concepts. “The complex software often requires that a lighting designer with a specialized knowledge of these programs be included in the design team. The need for this specialized expertise is one of the factors currently limiting the inclusion of daylight autonomy in many of the building codes or wider adoption of this metric in green building programs, because it does not seem reasonable to mandate that a professional with this capability be required on every project, regardless of scale,” says Bailey. “I'm confident that a more out-of-the-box software solution will be available soon, but it's just not there yet.”

Designing to Achieve Daylight Autonomy

The daylight autonomy a building can achieve is affected by so many variables, it is critically important that the goal of daylight autonomy be identified and actively considered as early in the project as possible. Decisions about the shape and position of the building, interior layout, furnishings, and type of daylight management system specified are all critical components in the amount and penetration of usable daylight that is ultimately found in a building throughout a year.

Building Envelope

Many designers begin creating the conceptual design for a building by considering the general shape and mass the building will have, also known as massing, selecting the location on the site that the building will occupy, referred to as siting, and determining how the building will be oriented on the site in relation to the sun.

The initial decisions made with regard to the massing, siting, and orientation of the building all dramatically impact the type of daylight that is available to the project and the potential access that daylight will have to the interior space. For example, the deeper the floorplate of the building, the more challenging it becomes to achieve target levels of daylight illumination in the more central spaces. In terms of orientation, in the northern hemisphere, it is generally accepted that northern exposures offer the best access to glare-free, ambient light throughout the day, whereas eastern exposures are often subjected to intense and glare-causing daylight as the sun rises, and western exposures experience direct sunlight exposure as the sun sets.

Interior Layout and Furnishings

“Interior space planning is another opportunity to optimize daylight autonomy, especially in office space,” explains Bailey. “Designing private offices with solid walls around the perimeter of the building used to be common practice, but that approach blocked daylight from penetrating deeper into the building. To improve the daylight autonomy of an office building, private offices are strategically placed in more central locations and equipped with glass office fronts, to provide occupants with access to daylight and views,” he adds. Designing a circulation space around the perimeter of an open office area is another way to improve the potential daylight autonomy of the space, by creating a buffer zone to help limit the potential of glare on the workplane. Lighter colors and lower cubicle walls are two examples of interior furnishings that are often specified to help daylight reach deeper into the space.

The Need for Daylight Management

Achieving daylight autonomy requires more than filling a space with daylight, it must be filled with “usable daylight.” As a reminder, usable daylight is defined as daylight within the range of 10 fc and 200 fc that will not disrupt the visual environment or cause glare or hot spots in a space. This range of usable daylight represents a pretty select segment of the daylight that could be available at the window of a building throughout the year, especially considering that daylight can range from a few footcandles on a cloudy day to over 8,000 footcandles on a bright, sunny day. Intense daylight exposure can trim away the usable space in a building by making areas too bright, too glaring, or too hot to use for parts of the day. Avoid the potential pitfalls of daylight exposure by equipping a building to manage the daylight as it enters the space and protect the interior from the bright, direct daylight that can destroy the visual environment and undermine the daylight autonomy of the space.

Here's an example of how a space may be negatively affected if it does not have a way to manage the daylight streaming in through the windows. Consider a section of an open office bay, positioned on the perimeter of the building where one wall is completely exposed to direct daylight.

On a sunny day, the daylight level may reach up to, or over, 8,000 footcandles at its most intense. Today, glass used in Class A office space may have a visual transmittance value (or Tv) of 0.65, which indicates that 35 percent of the available footcandles are reflected back out into the atmosphere and 65 percent of the visible energy is transmitted through the glass and into the building. In this case, 5,200 fc of the 8,000 fc of daylight would enter the open office space, well outside the range of usable daylight.

This scenario was simulated to evaluate the percentage of working hours where the presence of daylight exceeded the usable range of 200 fc on a sunny day. The simulated open office space is 30 feet by 30 feet, the windows are 8 feet tall and the working hours were defined as 8 a.m to 6 p.m. The simulation used real NYC climate data and assumes that no immediate surrounding buildings are present that would cast shadows onto the facade. The DA plots illustrate the percentage of time, during those predefined working hours, that the anticipated daylight level will exceed 200 fc.

The results were shocking. Without any way to manage the available daylight beyond the glass, portions of the office space were too bright to occupy comfortably throughout the entire workday. Even more surprising was that in the examples with eastern, southern and western exposures, over half of the office space was uncomfortably bright for over 50 percent of the workday. In the open office area with northern exposure, nearly a third of the office space would be potentially uncomfortable for half of the working hours. That is a significant amount of usable square footage to lose on a sunny day.

Selecting the Right Daylight Management System

Luckily, designers have many options for managing the daylight that penetrates into a building's interior. There are permanent exterior and interior shading devices, like fixed overhangs and high-performance glazing, which provide constant glare control, but are limited in their ability to offer any variety in the amount of daylight protection they provide. As a result, these systems treat daylight on an overcast day and daylight on a sunny day as if it had equal opportunity to create glare, which it does not. In an attempt to protect the interior from damaging daylight, designers may, inadvertently, minimize the amount of usable daylight that is allowed into the work space, reducing the level of daylight autonomy that the building could achieve.

A shading system is a great example of a daylight management solution that offers enough flexibility to mitigate glare and heat gain when the outside daylight is intense and unusable, but maximize the penetration of usable daylight, when it is available. This variable level of daylight control makes a shading system a great tool to help a building reach its goal of daylight autonomy, without risking daylight exposures that would make the spaces unusable or uncomfortable. When selecting the right shading system for a project, the fabric is the key to mitigating glare and unusable daylight and the controllability is the key to achieving daylight autonomy and greater energy savings.

Specifying fabric. It is a common practice to select the fabric for a shading system based on its color, instead of based on how the shade will need to perform in terms of daylight management. While color can impact the view provided to the outside when the shades are deployed, for example, darker fabrics can provide a crisper view than lighter alternatives, selecting a shade based exclusively on its color compromises the ability of the shading system to prevent glare and maintain an optimal visual environment in the building. It can also negatively impact the aesthetic appeal that the shade color was intended to have in the first place. If the window is too bright to comfortably look at, no one will be able to appreciate the carefully selected color of the shade.

A shade manages solar energy in three ways: reflect it, absorb it or transmit it into the interior. The key to glare mitigation is limiting the visible transmittance of the daylight through the shade and into the visual environment, so that it stays within an acceptable brightness. In terms of achieving daylight autonomy, the daylight passing into a space should not exceed 200 fc in intensity.

In order to specify a shade fabric that will function as needed, it is important to understand that the shade does not work alone. Both the window glass and shade fabric impact the total visual transmittance of daylight together and must be considered in tandem to create a glass/fabric system that appropriately manages the available daylight throughout the year.

The glass used in most buildings today has a visual transmittance of 65 percent, allowing 65 percent of the light to pass through the glass and into the building. Fabric shades are most commonly available in visual transmittances ranging from 3 percent to 30 percent. If a building has standard, double-paned windows with a 65 percent transmittance, then on a sunny day where 3,000 fc of daylight is available, 1,950 fc will pass through the glass. If a shade fabric with a 10 percent visual transmittance is specified on these windows, then only 195 fc, of the 1,950 fc that were transmitted through the glass, will be ultimately transmitted into the interior space. A daylight level of 195 fc supports the design goals of achieving daylight autonomy and maintaining a comfortable visual environment. The shade fabric keeps the space comfortable and usable, even on a sunny day.

Typically, windows with higher visual transmittance values should be paired with fabrics that have lower transmittance values. Spaces that receive direct sunlight, such as the direct early morning light that an eastern exposure receives, should keep the combined transmittance of the glass and fabric to less than 10 percent. Areas without the threat of direct sun exposure can benefit from shades with higher transmittances to maximize the potential level of daylight autonomy it could achieve.

Specifying Controllability: Manual vs. Automated Shading Systems

Specifying the way that the shading system is controlled can have a powerful impact on the level of daylight autonomy that a building is able to achieve. The controllability of shading systems can be divided into two categories: manual and automated. While the shades in either system can be made from identical fabrics and similarly positioned at any height, a person must physically deploy or retract the shade fabric of each shade in a manual system. Automated shades are programmed to move into their different positions throughout the day in response to a pre-determined schedule or in response to sensors that measure the intensity of the daylight at the window. No manual manipulation is necessary to ensure that glare conditions are being prevented and usable daylight is allowed in.

Manual Shading System and Daylight Autonomy

The primary challenge that a building equipped with a manual shading system will face, when attempting to achieve daylight autonomy, is reliably letting in diffuse, ambient daylight, when available. People are relatively proficient in closing the shades to relieve a space from glaring or uncomfortable conditions, however, they are not as diligent at opening the shades back up, when the daylight transforms from direct to diffuse. It is quite normal for a shade to be pulled down to block harsh, direct light and then left down for days, months or longer. A window with a manual shade deployed over it, day after day, protects the space from glare, but it limits the usable daylight allowed into the interior and significantly reduces the space's ability to exclusively illuminate the space with daylight.

Automated Shading Systems and Daylight Autonomy

An automated shading system is a powerful tool for a building trying to achieve daylight autonomy. These systems are dedicated to managing the dynamic and ever-changing nature of daylight, every day, all day long. The automated shading systems use the solar path of the sun as it arcs over the building to determine the optimal position of the shade and continuously adjust the position to accommodate the changing solar angles. The adjustments reliably block direct sunlight, while allowing usable daylight into the building. These systems also have sensors placed near the windows that can detect the level of daylight in the space and adjust accordingly, keeping shades deployed on overly bright days and retracting shades on overcast days to allow the soft, diffuse daylight into the building.

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 early morning light, while the shades on the northern, southern and western exposures may be entirely retracted, to let in the gentle morning light. As the morning progresses, the shades on the eastern exposure may lower to a full closed position, while the shades on the northern and southern exposure deploy to one-fourth of the way down the window, and the shades on the western façade remain fully open. As it approaches noon, the shades on the eastern exposure retract to halfway position, while the shades on the southern exposure are deployed to the halfway position and the western exposure shades lower to cover the top fourth of the window. In early afternoon, the shades on the eastern exposure are retracted completely, letting in the available afternoon light and the shades on the southern and western sides deploy further to block the direct light as the day progresses. At sunset, all of the shades are fully open, allowing occupants to enjoy the soft light of early evening. Automated shades move silently and automatically through all of these position changes, without requiring any manual manipulation. It should also be mentioned that automated shading systems move shades to precise, aligned levels along a façade, maintaining the curb appeal of the building, while maximizing the daylight autonomy of the interior.

“Our firm was recently hired to compare the daylight autonomy that could be achieved with manual shades and automated shades on a large, corporate campus,” Bailey said. “Our analysis concluded that automated shades significantly increased the DA factor of the building and provided additional energy savings. The owner also installed daylight responsive controls throughout the project which reduced the building's overall dependence on electric energy.”

When deciding which shading system to specify, the most common challenge to selecting an automated system is the cost, however, the attitude is changing as the benefits of daylight exposure and daylight autonomy become more widely accepted and valued. As Jack Bailey experienced on a recent project, “The owner of this particular project knew how much they were spending to install glass curtain walls around the buildings. The added cost of an automated shading system was not significant when compared to the cost of the windows. From the owner's perspective, this automated system allowed the building to make better use of the windows and provided access to improved daylight and views for a small up-charge. It made a lot of sense to them.”

Compare and Contrast the Performance of Manual and Automated Shading Systems

These shading systems can deliver significantly different results in terms of the amount of interior space that can be exclusively lit throughout the work day by daylight and the energy savings that can be generated.

Compare Useful Daylight Zones

The useful daylight zone refers to the area of a building that achieves its useful daylight illuminance level at least 90 percent of the working day. It identifies the square footage of a space that could be almost exclusively illuminated by comfortable, glare-free daylight for much of the day. The type of daylight management technology used on a project has a profound effect on the size of the useful daylight zone created in the space. This was a lesson LMN Architects learned when evaluating various daylight technologies for its office renovation.

In 2014, LMN Architects undertook a renovation of its Seattle-based office. It was looking to maximize the presence of natural light in the workspace while maintaining employee comfort and saving energy. The firm ran daylight simulations to model the performance of the space when equipped with different daylight management technologies. The simulations compared the useful daylight zones achieved with light shelves and an automated shading system. The results were surprising.

The daylight simulation with the light shelf revealed high (more than 200 fc) potentially glare-causing daylight levels around much of the perimeter of the office. In the simulation, the areas receiving more than 200 fc of daylight are illustrated in yellow, and the area in red indicates a space that is always within the useful daylight range of 10 to 200 fc. The light shelf would need to be supplemented with manual shades so that occupants could work comfortably in the space. Modeling the space with a light shelf and manual shades that would overwhelmingly be left in the down position would have dramatically reduced the size of the daylight zone and the presence of daylight in the interior.

When the team modeled the use of automated shades in its space, it found that it was able to create a large useful daylight zone, free from the overly bright, glare-causing daylight that plagued the light shelf scenario. The team also ran a simulation to consider the benefit of combining light shelves with automated shades, but it created only a slight improvement and was a significantly higher cost. Ultimately, LMN installed automated shades (with local sensors) throughout the entire office.

Compare Energy Savings

With daylight harvesting products now being required in skylit or daylit areas, the increased presence of daylight in a space can immediately generate energy savings. As automated shades are able to more reliably allow greater amounts of usable daylight into a space, the systems can also deliver greater energy savings when compared to manual shades.

A recent study, completed in collaboration with Lutron Electronics and Purdue University, compared the energy savings that could result from both shading systems. An energy simulation of a perimeter private office with a lighting power density of 0.9W/square feet, standard, clear, double-pane glass, and a shade fabric with 5 percent transmittance was conducted. The study averaged the results of spaces with 20 percent, 40 percent and 60 percent window-to-wall ratios. Manual shades were defined as closed shades for the study. The team concluded that an automated shading system was able to reduce the electric light use in the private office by 83 percent, when compared to the amount of electric light used if the windows were fitted with manual shades.

In projects of any scale or application type, the effective incorporation of daylight is steadily becoming a more and more common design goal. Armed with more advanced technology and daylighting design know-how, designers today are able to adequately illuminate a space using daylight exclusively. Achieving daylight autonomy saves energy and creates a more satisfying and productive atmosphere for building occupants, which are just two examples of how a daylit workspace works harder. And with automated shading systems, no one has to lift a finger.

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. For help with shading design, visit www.performanceshadingadvisor.com www.lutron.com