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Daylight Zones: Sidelighted Spaces

When designing an energy-saving daylight harvesting control system, a critical decision is to establish lighting control zones, identifying lighting loads to be separately controlled. Before this decision can be made, however, we must first determine the daylight zones.

A daylight zone, also called the daylight area (expressed in square feet), is defined by the ASHRAE/IES 90.1-2010 energy standard as “the floor area substantially illuminated by daylight.” In other words, it should consistently receive significant quantities of daylight during the day.

By identifying daylight zones, the lighting control system designer identifies areas where daylight harvesting control is appropriate. The designer can then make further decisions about how many control zones are appropriate for the given daylight zone, and their configuration.

In this and a second article to be published next month here at, we will examine methods for establishing daylight zones based on prevailing energy codes and standards. This month, we will cover the most common type of daylighting—sidelighting, or daylight entering a space through vertical fenestration such as windows. Next month, we will cover toplighting (e.g., skylights) applications.

Energy Codes/Standards And Sidelighted Daylight Zones
Daylight zones are increasingly being determined by codes and standards, however, notably the ICC’s 2009 International Energy Conservation Code (IECC) (model energy code), ASHRAE/IES 90.1-2010 (model energy code), ASHRAE 189.1-2009 (model green building code) and California’s Title 24-2008 (state energy code). These codes and standards all require that daylight zones be established adjacent to sidelighting apertures, and general lighting in these zones be separately controlled from other lighting. Regional and national design firms working in multiple jurisdictions and project types may find themselves determining daylight zones using up to four or more definitions that have many similarities but also significant differences.

Basically, each code or standard defines the dimensions of a daylight zone, and then identifies elements in the space that could limit these dimensions, such as tall obstructions (e.g., walls) and other daylight apertures. Additionally, some codes and standards recognize a difference between primary and secondary sidelighted daylight zones, with control in primary zones typically being mandatory, and control in secondary zones being encouraged through power credits.

Below is a short reference to how these codes and standards establish daylight zones in sidelighted spaces (vertical glazing below the ceiling), which, depending on the code or standard, may be called daylight areas. Note that window height (WH, also called window head height) is formally defined in ASHRAE/IES 90.1-2010 and ASHRAE 189.1-2009 as the distance from the floor to the top of the glazing. Also note that the main limitation to the daylight zone is the presence of some type of permanent partition, which may be defined slightly differently across the four codes and standards.

Here we see these rules presented visually in an example space. In each case, the width of the daylight zone is limited by the wall located 1 ft. to the north of the window. In each case, the depth is limited by the wall located southeast of the window, which is located at a distance that is less than one window head height deep into the space.

Some codes and standards, notably the ASHRAE/IES 90.1-2010 energy standard and California’s Title 24-2008 energy code, also establish secondary daylight zones. In the case of sidelighting applications with vertical fenestration such as windows, these are daylight zones extending deeper into the space, controlled separately from primary daylight zones and other general lighting in the space. This level of control is not mandatory but instead encouraged through the use of power credits—that is, a multiplier increasing available watts for the controlled lighting load. If you save energy, the code/standard says, you can have a more power.

As the below graphic illustrates, in each case, the secondary sidelighted daylight zone should be the same width as the primary zone, and another window head height in depth, with the same limitations in regards to 60-inch or taller permanent partitions.

Once the daylight zones are established in a space, we can then decide whether daylight harvesting control is warranted, how many control zones we will need (including what loads will be covered by each controller), and what control method or methods—switching, dimming, etc.—we will use. Each of these areas may be covered by separate code/standard requirements.

Next month, we will examine how to set up daylight zones in toplighted spaces.

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