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Daylight Zones: Toplighted Spaces

Last month at, we published an article about how to establish daylight zones in sidelighted (windowed) building spaces. We looked at an industry rule of thumb and then how the latest generation of energy codes and standards address it.

In review, 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.

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 article, we will examine methods for establishing daylight zones in toplighted building spaces, such as spaces with skylights, roof monitors and clerestories.

Rule of Thumb for Toplighted Daylight Zones
For toplighted spaces, a rule of thumb is that a daylight zone can be established as the skylight length or width plus 1/2 the ceiling height on each side, and a second zone as the skylight length or width plus the ceiling height on each side.

Image courtesy of the Lighting Research Center.

If a space is uniformly lighted using skylights as shown below, with properly spaced skylights covering about 3-5% of the floor area, the entire space may be considered a daylight zone suitable for daylight harvesting control.

Image courtesy of Acuity Brands Controls.

Energy Codes/Standards And Toplighted Daylight Zones
As stated in last month’s article, daylight zones are increasingly being determined by codes and standards, notably the 2009 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 around toplighting apertures, and general lighting in these zones be separately controlled from other lighting. The dimensions of the daylight zone are defined and adjusted based on elements in the space that would limit daylight availability, such as tall obstructions (e.g., walls and stacks).

Codes and standards may recognize one or more of the following types of toplighting:

Let’s begin with skylights. Below is a short reference to how the latest generation of codes and standards establish daylight zones, or daylight areas, in spaces toplighted using skylights. Note that CH = ceiling height (floor to ceiling), and OH = obstruction height—the height of permanent obstructions (to daylight distribution) such as walls and permanent storage stacks.

The below graphic illustrates the daylight zoning requirements. In each case, a wall restricts the daylight zone in the north because the distance between the skylight and the wall is less than the ceiling height (IECC 2009) or 0.7*CH (ASHRAE 189.1-2009).

ASHRAE/IES 90.1-2010 and Title 24-2008 use a different system that addresses how likely the permanent partition will block the daylight. If the distance between the skylight and the obstruction is less than 0.7*CH and greater than 0.7*(CH–OH), then the front of the partition (facing the skylight) marks that boundary for the daylight zone.

Further, IECC 2009 and ASHRAE 189.1 reduce the given daylight zone dimension to 1/2 the distance to the nearest skylight or vertical fenestration, while the daylight zone in ASHRAE/IES 90.1-2010 and Title 24-2008 is reduced by the outermost boundary of any nearby primary sidelighting zone or roof monitor daylight zone.

Let’s look at a sample problem:

We have 10 skylights that are each 18 ft. from each other, providing illumination in a space with a ceiling height of 20 ft., and bounded on all sides by ceiling-height walls 10 ft. away. Under IECC 2009, what are the dimensions for each skylight daylight zone?

Under IECC 2009, 1/2 distance (D) between skylights (9 ft.) < CH (20 ft.) and D to any nearby partition, so the zone around each skylight would be the skylight length or width + 1/2 D between it and its nearest skylights (9 ft.). The exception is the dimension facing the walls, which would be limited by the distance between the skylight and those walls (10 ft.), as D to these nearby partitions < CH (20 ft.). Now suppose we added a 15 ft. tall warehouse stack 10 ft. from the edge of a skylight. Under ASHRAE/IES 90.1-2010 and Title 24-2008, would that stack limit that skylight’s daylight zone?

The distance between the skylight and the stack (10 ft.) > [0.7*(CH-OH)] (3.5 ft.) and < 0.7*CH (14 ft.), so the stack would limit the boundary of the daylight zone extending toward the stack to 10 ft. In this scenario, any obstruction over roughly 5 ft. in height would impose a limitation on the dimensions of the daylight zone. Now suppose the northern row of skylights is 20 ft. from a windowed wall with a primary sidelighted daylight zone of 15 ft. Under IECC 2009 and ASHRAE/IES 90.1-2010, would there be any limitation to the daylighting zone?

Under IECC 2009, the answer would be yes. The windows are 20 ft. away from the skylight, and 10 ft. < CH (20 ft.) and there is no intervening partition, so the daylight zone would be halved to 10 ft. Under ASHRAE/IES 90.1-2010, the answer is also yes. The windows are 20 ft. away, so the outermost edge of the primary sidelighted daylight zone is 5 ft. away. As 5 ft. < (0.7*CH, or 14 ft.), the northern daylight zone for these skylights would extend 5 ft. toward the windows. What if, for theoretical purposes, we add a ceiling-height vertical obstruction 2 ft. away from the skylight? Would the daylight zone be limited to 2 or 5 ft.?

Anytime the limitations are in conflict, common sense dictates that the closer one applies, in this case the obstruction 2 ft. away, as it would block any light from extending the extra 3 ft.

Roof Monitors And Clerestories
Now let’s move on to roof monitors and clerestories, which are recognized by ASHRAE/IES 90.1-2010 and ASHRAE189.1-2009.

ASHRAE/IES 90.1-2010 specifically recognizes roof monitors, which it defines as “vertical fenestration integral to the roof.” Below is a summary of the ASHRAE/IES 90.1-2010 requirements:

The below graphic illustrates the daylight zoning requirements.

Let’s look at a sample problem:

We have a roof monitor with a width of 10 ft. and a monitor sill height of 20 ft., meaning the daylight zone would extend 20 ft. from the vertical glazing. A 14-ft.-tall vertical obstruction, placed 15 ft. from the monitor, protrudes into the daylight zone, and the outermost edge of a primary sidelighted daylight zone is 18 ft. away from the glazing. Under ASHRAE/IES 90.1-2010, what are the dimensions for this daylight zone?

The daylight zone would normally be 10 ft. wide and extend from the glazing 20 ft. (the MSH), or an area of 200 sq.ft. However, the primary sidelighted daylight zone is 18 ft. away, which is less than the MSH, so that would limit the zone to a depth of 18 ft. The distance to the vertical obstruction (15 ft.) > MSH – OH (6 ft.) and < MSH (20 ft.), so it too would limit the zone to a depth of 15 ft. along the front face of the obstruction. Basically, in this scenario, any obstruction over 5 ft. in height would impose a limitation on the dimensions of the daylight zone. Finally, ASHRAE 189.1 recognizes clerestories, roof monitors and clerestory roof monitors. In review, ASHRAE/IES 90.1-2010 defines clerestories as “that part of a building that rises clear of the roofs or other parts and whose walls contain windows for lighting the interior.” Below is a summary of the ASHRAE 189.1 requirements.

The below graphic illustrates the daylight zoning requirements.

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.

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2 comments to Daylight Zones: Toplighted Spaces

  • It is refreshing to note that daylight is highly an issue looking into conserving low energy use in buildings.

    However we must consider the effect of the U-Value of the materials i.e the glaze materials we are using in respect of the cooling sytem.

    Would this not affect the cost we are trying to reduce in terms of energy consumption?

    Thank You



    1609 BOOTH LANE
    ALVIN, TX 77511

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