Advancing the Daylighting Discussion

Explore the scientifically proven advantages of automation in daylight management
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Sponsored by MechoSystems
Jeanette Fitzgerald Pitts
This test is no longer available for credit

Code-Mandated Daylighting and Daylight Harvesting

Effectively using daylight to enhance the interior environment for the health and productivity of building occupants and to reduce the energy use of the building is not just a good idea anymore—it is now required by code in many places throughout the United States.

LEED

For years, the U.S. Green Building Council (USGBC) LEED green building rating system has included credits focused on the inclusion of daylight and views in sustainable design. Although LEED is a voluntary green building program, many state and local governments are adopting LEED requirements into their building codes, making green building mandatory.

Compliance with LEED building practices has been a requisite for federal buildings for quite some time. In fact, the General Services Administration (GSA) now requires that the new construction and substantial renovation of any federally owned facilities meet or exceed standards for LEED Gold certification, an increase from the LEED Silver certification that was previously mandated.

It should be noted that the GSA recently recognized the Green Globes 2010 program from the Green Building Initiative as an acceptable third-party certification system that can be used to verify the construction or renovation of high-performance federal buildings.

Written with the intent to “connect building occupants with the outdoors, reinforce circadian rhythms, and reduce the use of electrical lighting by introducing daylight into the space,” the Daylight credit in LEED v4 offers up to three points for achieving a certain level of daylight illumination (between 300 and 3,000 lux) throughout a percentage of the regularly occupied areas and requires that the design provide either manual or automatic glare-control devices in those regularly occupied areas, as well.

Effectively using daylight to enhance the interior environment is now required by code in many places throughout the United States.

Photo courtesy of MechoSystems/Carlos Rivera

Effectively using daylight to enhance the interior environment is now required by code in many places throughout the United States.

ANSI/ASHRAE/USGBC/IES Standard 189.1-2014

The ANSI/ASHRAE/USGBC/IES Standard 189.1: Standard for the Design of High-Performance Green Buildings (ASHRAE 189.1) is a model code that is comprised of minimum requirements designed to improve the environmental and health performance of building sites and structures. ASHRAE 189.1 is a compliance option of the International Green Construction Code (IgCC). Some federal buildings may require compliance with ASHRAE 189.1 or with IgCC and these standards have been incorporated, in part, by several building authorities in jurisdictions across the United States.

As it relates to daylighting, one interesting and unique aspect of ASHRAE 189.1-2014 is its recognition of automated solar shades as an exception to the requirement for the shading of vertical fenestration by permanent projections on the west, south, and east in certain climate zones. There are additional criteria that must be satisfied by the automated shading system, including the coverage and solar reflectance of the interior shading device, the inclusion of localized manual control for temporary override, and acceptance testing and commissioning. Manual blinds and shades do not satisfy the criteria because a user can leave them in any position, at any time of day, making the actual performance of the manual shade system difficult to predict. Because of the potential variability in position, manual shades and blinds must be modeled to reflect the worst-case scenario, which, when considering the impact of solar heat gain, would be completely retracted. Automated solar shading systems are programmed to precisely position shades to protect the interior against solar heat gain, when it exists as a potential threat, and can, therefore, contribute toward the modeled energy performance of the building.

Daylighting in the Energy Codes: ANSI/ASHRAE/IES 90.1 and IECC

While both IgCC and ASHRAE 189.1 contain minimum sidelighting effective aperture values to ensure that daylighting is included in the design of office spaces and schools, the energy codes, such as the International Energy Conservation Code (IECC) and ANSI/ASHRAE/IES Standard 90.1 (ASHRAE 90.1), typically regard any fenestration as a potential threat to the efficiency of the building envelope. This has changed recently, as the ASHRAE 90.1-2010 included a requirement that certain types of stores, big box applications in certain climate zones, incorporate a minimal horizontal fenestration area. Recent versions of IECC and ASHRAE 90.1 have also increased their requirements for daylight-responsive controls in areas that are sidelighted or toplighted, recognizing the significant energy savings that can be created when electric light levels are reduced when daylight is present.

Daylighting Challenges

The incorporation of daylight in a space can provide impressive benefits to the health, productivity, and efficiency of the built environment, but it is not without risk. Glare and solar heat gain can disrupt the interior environment, dashing the very health, productivity, and efficiency gains that the daylight was brought into the space to provide. Automated shading systems offer designers a way to maximize the presence of soft, useful daylight in the interior, while protecting it from glare and solar heat gain.

Daylighting Challenge: Prevent Glare

Glare is a function of excessive contrast in luminance levels being present within a field of vision and the strain that it puts on the eye to attempt to adapt to two very different luminance levels at the same time. It occurs when an element in the visual field, either a light source or a reflection of a light source, is significantly brighter than the rest of the visual environment. While the human eye is equipped to adjust to bright light conditions and low light conditions, the physiological change is uniform across the entire eye, making it ill-equipped to accommodate irregular bright spots within the visual field. Unable to manage the presence of these hot spots effectively, the pupil of the eye will dilate and constrict repeatedly to try to match the light catching needs of the space, ultimately causing headaches, eye strain, and fatigue.

An expansive motorized shading system manages glare, controls solar heat gain, and adds to the decor of the lobby in the Aria resort and hotel in City Center, Las Vegas.

Photo courtesy of Carlos Rivera

An expansive motorized shading system manages glare, controls solar heat gain, and adds to the decor of the lobby in the Aria resort and hotel in City Center, Las Vegas.

The Recommended Practice for Daylighting Buildings (RP-5-13), published by the IES, identifies two types of glare: disability glare and discomfort glare. Disability glare, also called veiling glare, occurs when light interferes with the performance of the visual task by either reflecting off of the surface where the visual task is taking place or compromising the contrast on the work surface, making it difficult to see. An example of disability glare is when bright light sources or bright windows reflect off of computer screens or glossy magazine pages, creating distracting bright spots that interfere with a person’s ability to clearly see or read the contents on the screen or page. Discomfort glare occurs when a light source within the field of view is significantly brighter than the visual task, causing discomfort.

The key to controlling or preventing glare conditions from occurring is to actively manage the brightness of the daylight at the window and the intensity of the daylight that is allowed to penetrate the interior space. The ultimate goal is to keep the luminance levels on the interior balanced and within the scope of what can be appropriately managed by the eye. The IES recommends, in RP-5-13, “that luminance ratios of task to surrounding areas in the field of view be kept below 1:3 for adjacent areas (30-degree cone) and 1:10 for remote areas (60-degree cone).”

There are many factors that affect the brightness of daylight at any given window, including the time of the day, the time of the year, window orientation, and the weather of the day. In order to create an interior visual environment that maintains the IES-recommended luminance levels, the design of the space and the automated shading system will need to consider the intensity of the daylight at the window, the solar angle of the sun in relation to a specific window, and the daily sky conditions. When the sun is not in direct view, the illuminance value of the sky ranges from 5,000 to 15,000 lux (roughly 500 to 1,500 fc) depending upon whether the daily conditions are cloudy or clear. When the sun is directly visible, illuminance levels can reach up to 50,000 or 100,000 lux (about 5,000 to 10,000 fc).

The IES also makes recommendations on the optimal light levels that should be provided in certain types of space or to accommodate certain types of visual tasks. In office buildings, the IES recommends that a private office and an open-plan office be designed to maintain 400 lux (40 fc) at the work plane and 300 lux (30 fc) at the work plane in a conference room.

In order to put the idea of a luminance ratio into context, consider a small private office, with a window, that has been lit to provide 400 lux (40 fc) on the work surface with electric lighting. To adhere to the IES-recommended ratio for task and adjacent area lighting, the luminance ratio should be kept below 1:3. If the lighting system is designed to provide 400 lux (40 fc) at the work plane, the daylight that reaches this area should not exceed 1200 lux (120 fc), and the daylight levels in the remote areas of the office should not exceed 4,000 lux (400 fc). Unfortunately, these maximum illumination levels can be easily reached and exceeded, even on somewhat overcast days, and if the window has a direct view of the sun at any time, the potential illumination levels, and potential for glare, will be much higher.

An automated shading system can dramatically reduce the amount of available daylight that is allowed into the small private office, protecting the balance of the visual environment. The shades lower to shield occupants from a direct view of the sun when the orb is visible through the window or to further diffuse the ambient daylight on an excessively bright day. The shades can rest in various positions—one-third of the way down the window, half-way down the window, three-fourths of the way down the window—offering various degrees of daylight penetration that are specifically tailored by the automated system as to not overload the office in daylight.

The sun’s position is radically different throughout the year, automated shading systems consider the exact location of the sun to precisely position shades and protect the interior from direct beam radiation. 
Metric for Measuring Potential Glare

Image courtesy of MechoSystems

The sun’s position is radically different throughout the year, automated shading systems consider the exact location of the sun to precisely position shades and protect the interior from direct beam radiation. Metric for Measuring Potential Glare

There is a metric to help designers identify the likelihood that an average occupant in a space would begin to experience discomfort as a result of the daylight conditions. This daylight glare metric is called daylight glare probability (DGP). DGP values range from less than 20 percent to 100 percent. A DGP value of 35 percent or less indicates that occupants in the space would find any existing glare to be imperceptible. When the DGP reaches 40 percent, glare is perceptible. A DGP of 45 percent or more indicates that the average person would be disturbed by the glare condition of the space.

In order to quantify the potential for glare, the space must be considered in terms of the position of the occupant in relation to the sources of light and illuminated objects. Then the luminance ratios within the field of view for that particular position must be evaluated and the probability that the average person would find it objectionable identified. The reference method for glare assessment is to generate a high dynamic range (HDR) image for every daylight hour of the year from a specific vantage point. Daylight simulation software can also assess a DGP for an established viewpoint, but beware that these calculations can be time-consuming to execute.

Daylighting Challenge: Solar Heat Gain

Roughly 50 percent of the solar radiation emitted by the sun is long wavelength infrared radiation. While not part of the visible light spectrum, this solar energy becomes radiant heat when it is absorbed. Solar heat gain in a building occurs when direct beam solar radiation, which contains a significant amount of infrared radiation, is absorbed by or passes through a window or a skylight or any type of daylighting aperture and is then absorbed by the interior, heating it up. Solar heat gain can increase the demand on the HVAC system in order to remove the extra heat and can compromise the thermal comfort of occupants in the building.

Over half of the solar energy emitted by the sun is outside of the visible light spectrum. This near infrared and infrared radiation contributes significantly to solar heat gain when absorbed by an interior.

Image courtesy of MechoSystems

Over half of the solar energy emitted by the sun is outside of the visible light spectrum. This near infrared and infrared radiation contributes significantly to solar heat gain when absorbed by an interior.

 

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Originally published in Architectural Record
Originally published in May 2016

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