Architectural Record BE - Building Enclosure

Top Five Tips for Successful Daylighting Design

Equip every project to meet its own unique performance goals
[ Page 2 of 5 ]        
Sponsored by Lutron Electronics
By Jeanette Fitzgerald Pitts

Preserve Outdoor Views

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 10th 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, manager of Building Sciences 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 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 of a lecture room interior.

Photo courtesy of Lutron Electronics

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.

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 effect 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 20 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 of a lecture room interior with solar shade fabric on the windows.

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.

Illustration of light entering a workspace and how it interacts with glass.

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.

 

[ Page 2 of 5 ]        
Originally published in Architectural Record

Notice

Academies