Daylight Autonomy 101

Specifying useful daylight deeper into the interior
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Sponsored by Lutron Electronics Co., Inc.
Jeanette Fitzgerald Pitts

Beyond improving the human experience, effectively incorporating daylight in the interior, or daylighting, can dramatically improve the operational performance of the building and create energy savings. In 1997, the Illuminating Engineering Society of North America (IESNA) published a guide entitled Daylighting Design: Smart and Simple in which it posited that a building with a 25 percent window-to-wall ratio could realize a lighting energy savings of roughly 30 percent by reducing electric light levels when sufficient daylight levels were available.

Both daylighting know-how and technology have dramatically improved over the past decade, advancing the role and the potential benefits that daylight can now deliver to the built environment. Daylight harvesting, the practice of reducing electric light levels when daylight is present, is becoming more and more commonplace and is now, in fact, required by ANSI/ASHRAE/IESNA Standard 90.1 – 2010 and the California Building Efficiency Standards. Some designers are now interested in taking daylight availability to the next level and using it, when possible, as the exclusive light source for a space, in much the same way it was used in the one-room schoolhouses of the past. The phrase for this new design objective: daylight autonomy.

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 10th 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

The measurement of daylight autonomy (DA) is the percentage of working hours that a particular daylight level is exceeded throughout the year. The DA value represents the percentage of the workday that a space could be exclusively illuminated with daylight. There are a few other metrics that can be considered along with the DA value to paint a more complete picture of the presence of daylight in a building throughout the day. Continuous Daylight Autonomy (cDA) gives partial credit to hours where daylight is present, but cannot completely achieve the target illuminance level. Spatial Daylight Autonomy (sDA) refers to the percentage of floor area where 30 footcandles (fc) is achieved for at least 50 percent of the workday.

There is a key term used in the definition of daylight autonomy, and some of the supporting daylight autonomy metrics, that warrants additional attention to ensure that the full concept of daylight autonomy is truly understood.

Defining Useful Daylight

Where daylight autonomy is the goal, it is critical to understand that not all daylight is created equal. As the definition of daylight autonomy implies, there is useful daylight and daylight that is not usable or practical for the interior environment. Useful daylight describes the daylight in the interior space that does not cause glare or discomfort to building occupants.

In an office setting, the range of useful daylight illuminance is generally considered to be between 10 fc and 200 fc at the workplane. This aligns with recommendations developed by the IES that define the optimal light levels for various visual tasks. The IES recommends that office buildings maintain 30 fc at the workplane in private offices, open office spaces, and conference rooms. Lower light levels are recommended for circulation areas and higher levels are recommended in areas where reading and studying will be the primary task. There is even a metric, entitled Useful Daylight Illuminance (UDI), which refers to the percentage of work hours where the illuminance from daylight is between 10 fc and 200 fc.

A 2003 study conducted by Heschong Mahone Group linked daylight exposure in the workplace to improved employee productivity and satisfaction.

Photo © Bruce Damonte

People can experience visual discomfort when direct sunlight or overly bright sky conditions are present in a workspace due to the intensity and contrast they create. Two metrics that have been developed to help designers identify and protect a space against this potentially destructive and glare-creating daylight are referred to as Max Daylight Autonomy (maxDA) and Annual Sunlight Exposure (aSE). The metric maxDA measures the percentage of work hours where daylight levels provide 10 times the necessary levels of design illuminance. The metric aSE measures the number of hours per year where the space receives direct sunlight. This direct or bright, potentially glare-causing sunlight may be considered unusable daylight, whereas the diffuse daylight that fills the sky on a cloudy day is generally considered usable.


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Originally published in Architectural Record