Planning for effective daylighting

Improving codes and evolving metrics
Spatial daylight autonomy (sDA) can help achieve maximum daylighted floor area. The building and climate data is combined, and a simulation is conducted. If 55 percent of the building hits the target daylight level, it is sufficient—however, the target preferred by designers is 75 percent. Then, a co-metric called annual solar exposure (ASE) is conducted that intends to determine whether illuminance levels are high enough that shading will be needed, or low enough they will be considered acceptable.

“If too high, we redesign and re-run both metrics to achieve the best balance between shading and daylight,” explained Tanteri. “It is a means to optimize performance.”

“Trying to combine a glare-controlled environment with flexibility is what most clients want,” said George Loisos, LEED AP, and principal of Loisos + Ubbelohde (Alameda, California).

Controlling light through the window is the ultimate challenge; in addition to integrating electric lighting with daylighting as day time transitions to night time. How do these elements work together? Implementing a proper control system is the key to how electrical lighting responds to daylight.

In the U.S. Green Building Council’s (USGBC’s) LEED program, view is considered nearly as much as daylight. Therefore, to take advantage of both, lighting designers may use a simple approach often referred to as a ‘dual-function’ window. The top of the window is for daylight and the bottom of the window—up to about 2.1 m (7 ft)—is for view. Considering shading from the outset is critical to both daylight and view function. Generally, it is difficult to create a shade-free environment without numerous design elements.

Shade positions on the window—correlated to the sun’s position in the sky, the building location, orientation, and geometry—are automatically raised and lowered at New York Times building in New York City. The system contributes up to 70 per cent in energy savings. Photo © PDK Commercial Photographers

Additionally, there are code aspects. Certainly, energy and building codes have great influence
on the design process and the authority having jurisdiction (AHJ) will be scrutinizing plans for adherence to the regulations. In addition to LEED, many regulations will be relevant, such as:

  • energy performance simulation tools;
  • ASHRAE 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings; and
  • ASHRAE 189.1, Standard for the Design of High-performance Green Buildings Except Low-rise Residential Buildings.

“There may be either performative (a space that can handle glare) or architectural parts of the building, such as a brise soleil, that can do some of the work,” explained Loisos. “The main issue is static solutions usually cost a lot in both space and money and use up a lot of light.”

If the responsibility of raising and lowering the shades is left to the occupant closest to the window, the concept of daylighting simply cannot work. The occupant will be bothered by the sun’s glare or warmth, and will potentially pull the shade down. This ultimately defeats the whole design.

“Our experience has demonstrated manual shade control is a terrible way to control light as the failure mode is shades down, which will control glare, but not raising the shades means one loses a lot of light,” Loisos said. “But, it does not become a problem if someone is willing to automate them.”

With an automated shading system—such as the one installed in the New York Times Headquarters in 2007—up to 70 percent of lighting costs can be saved. The system adjusts to various shade positions on the window, which are correlated to the sun’s position in the sky, and the building location, orientation, and geometry.

When the sun is bright, the shades are lowered to increase occupant comfort. But, when the windows are in shadow, the shades are raised to maximize both daylight and view.

The system calculates the sun’s angle on each window in every zone. The program adjusts the shade bands to one of the specified positions on the window to manage the distance direct sunlight can enter a space, and the Btu load on the glass.

Meanwhile, three roof-mounted radiometers monitor the sky conditions in real time. The system uses proprietary algorithms, which translate raw solar-sensor data to determine whether the sky is clear or cloudy. When clear, the shade band positions are adjusted according to the sun’s angle in the sky; on a cloudy day, the shades are raised.

These systems also reduce solar heat gain, which decreases the building’s air conditioning needs. Additionally, the system alleviates brightness and glare, which provides a more comfortable environment, especially for those using computer screens.

“The New York Times building is a project that I’m glad happened, because it brought a lot of attention and a lot of information filtered through the lighting community,” Tanteri stated. “Many white papers, articles and presentations followed that conveyed a lot of real experience with daylighting. It received this massive amount of exposure, generated dialogue, and brought attention to the concepts involved.”

Fabrics are also a critical aspect of the design process. Since openness and reflectance are the two most important properties for view and shading.

“As a rule a rule of thumb, you want the higher reflectance (i.e. lighter) on the outside to reflect heat and the lower reflectance (i.e. darker) facing in to promote view,” Tanteri explained. “Then, you want an openness factor that provides the desired view, but at the same time is capable of minimizing the brightness of the sun if it was in view. There is no ideal fabric.”

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