The following are the prescriptive requirements for daylighting devices (clerestories, horizontal slats, and light shelves) that qualify for PAFs and for daylighting in large, enclosed spaces.
§140.6(a)2L|topic=2. Reduction of wattage through controls.
Certain design features and technologies can increase the daylighting potential of spaces. Some of these design features and technologies may be used in conjunction with automatic daylighting controls to receive PAFs from Table 140.6-A or as a performance compliance option in the performance method.
A careful analysis should be performed to avoid glare when including daylighting devices in a design. For example, specularly reflective (e.g., polished, or mirror-finished) slats may redirect sunlight and cause uncomfortable glare. Since that is not the only consideration to make when considering daylighting design features, a careful daylighting analysis should be performed on a space-by-space, project-by-project basis.
The daylight dimming plus off PAF and institutional tuning in daylit areas may be added to any of the daylighting design PAFs to create a combined total PAF.
In addition, the horizontal slat PAF can be added to the clerestory fenestration PAF if the requirements for both are met.
In the performance method, a variety of control strategies is available in the compliance software to take advantage of further savings.
At permit application, use form NRCC-LTI-E to document daylighting device PAFs.
Section 140.3 has prescriptive requirements for building envelopes, including daylighting for large, enclosed spaces directly under roofs. Lighting installed in these spaces is required to comply with all lighting control requirements, including the automatic daylighting control requirements. Mandatory daylighting control requirements are covered in Section 5.4.4 of this chapter.
For projects that comply with the prescriptive daylighting requirements by installing daylight openings in large, enclosed spaces directly under roofs, the daylit areas may require automatic daylighting controls. However, for projects using the performance approach, it is possible to displace the daylighting openings and daylighting controls with other building efficiency options
The prescriptive daylighting requirements for large, enclosed spaces apply to both conditioned and unconditioned nonresidential spaces that meet the following qualifying criteria:
1. Directly under a roof.
2. Located in Climate Zones 2 through 15.
3. Have a floor area greater than 5,000 sq ft.
4. Have a ceiling height greater than 15 ft.
EXCEPTIONS:
1. Auditoriums, churches, movie theaters, museums, and refrigerated warehouses.
2. Enclosed spaces having a designed general lighting system with a lighting power density less than 0.5 W/ft2.
3. In buildings with unfinished interiors, future enclosed spaces in which there are plans to have one of the following:
a. A floor area of less than or equal to 5,000 sq. ft.
b. Ceiling heights less than or equal to 15 feet. This exception shall not be used for S-1 or S-2 (storage) or F-1 or F-2 (factory) occupancies.
4. Enclosed spaces where it is documented that permanent architectural features of the building, existing structures, or natural objects block direct beam sunlight on at least half of the roof over the enclosed space for more than 1,500 hours per year between 8 a.m. and 4 p.m.
In Climate Zones 2 through 15, enclosed spaces larger than 5,000 sq. ft. shall have at least 75 percent of the floor area within the primary sidelit daylit zone or skylit daylit zone.
Enclosed spaces that are required to comply must meet the following prescriptive daylighting requirements:
1. A combined total of at least 75 percent of the floor area, as shown on the plans, shall be within the skylit daylit zone or primary sidelit daylit zone. The calculation of the daylit zone area to show compliance with this minimum daylighting requirement does not need to account for the presence of partitions, stacks, or racks, other than those that are ceiling-height partitions. The envelope may be designed before there is any knowledge of the location of such obstructions, as is often the case for core and shell buildings. Thus, the architectural daylit zone requirement of 75 percent of the enclosed space indicates the possibility of the architectural space being mostly daylit.
The daylit zone and controls specification in §130.1(d) describe which luminaires are controlled. The obstructing effects of tall racks, shelves, and partitions must be taken into consideration when determining the specifications. The electrical design will likely occur later than the architectural design; thus, planning for these obstructions can be built into the lighting circuiting design. With addressable luminaires, the opportunity is available to the contractor to incorporate the latest as built modifications into the daylight control grouping of luminaires according to unobstructed access to daylight.
2. The total skylight area is at least 3 percent of the total floor area in the space within a horizontal distance of 0.7 times the average ceiling height from the edge of the rough opening of the skylights; or the product of the total skylight area and the average skylight visible transmittance is no less than 1.5 percent of the total floor area in the space within a horizontal distance of 0.7 times the average ceiling height from the edge of the rough opening of skylights.
The above two requirements are represented by the following equations.
Equation 5-1
Equation 5-2
Definitions of the above equation terms:
Skylight Area = Total skylight area on the roof
Daylit Zone under skylights = Total floor area in the space within a horizontal distance of 0.7 times the average ceiling height from the edge of the rough opening of skylights
VT = Visible Transmittance
3. General lighting in daylit zones shall be controlled in accordance with §130.1(d).
4. Skylights shall have a glazing material or diffuser that has a measured haze value greater than 90 percent, tested according to ASTM D1003 or other test method approved by the CEC.
Skylights must also meet the maximum glazing area, thermal transmittance (U-factor), solar heat gain coefficient (SHGC), and visible transmittance (VT) requirements of §140.3(a). Plastic skylights are required to have a VT of 0.64 and glass skylights are required to have a VT of 0.49. Currently, plastics are not accompanied by low-emissivity films, which transmit light but block most of the rest of the solar spectrum. As a result, there is no maximum SHGC for plastic skylights. Glass skylights are required to have a maximum SHGC of 0.25. With a maximum SHGC of 0.25 and a minimum VT of 0.49, glass skylights must use low-emissivity films or coatings that have a high light-to-solar gain ratio.
5. All skylit daylit zones and primary sidelit daylit zones shall be shown on building plans.
The total skylight area on the roof a building is prescriptively limited to a maximum of 5 percent of the gross roof area (§140.3[a]6A).
Example 5-15: Using Skylights to Meet the Daylighting Requirement for Large Enclosed Spaces
In buildings with large, enclosed spaces that must meet the minimum daylighting requirement, the core zone of many of these spaces will be daylit with skylights. Skylighting 75 percent of the floor area is achieved by evenly spacing skylights across the roof of the zone. A space can be fully skylit by having skylights spaced so that the edges of the skylights are not farther apart than 1.4 times the ceiling height. Therefore, in a space having a ceiling height of 20 feet, the space will be fully skylit if the skylights are spaced so there is no more than 28 feet of opaque ceiling between the skylights.
Example 5-16: Large, Enclosed Spaces in a Warehouse Building
Question
For a 40,000 sq. ft. warehouse with a 30-foot ceiling (roof deck) height, what is the maximum skylight spacing distance and recommended range of skylight area?
Answer
The maximum spacing of skylights that results in the space being fully skylit is
Maximum skylight spacing = 1.4 x ceiling height + skylight width
Spacing skylights closer together results in more lighting uniformity and, thus, better lighting quality (at an increased cost because more skylights are needed). However, as a first ,approximation one can space the skylights by 1.4 times the ceiling height. For this example, skylights can be spaced 1.4 x 30 feet = 42 feet. In general, the design will also be dictated by the size of roof decking materials (such as 4’ by 8’ plywood decking) and the spacing of roof purlins so the edge of the skylights line up with roof purlins. For this example, we assume that roof deck material is 4’ by 8’ and skylights are spaced 40 feet on center.
Each skylight is serving a 40-foot by 40-foot area of 1,600 sq. ft. A standard skylight size for warehouses is often 4’ by 8’ (displacing one piece of roof decking). The ratio of skylight area to daylit area is 2 percent (32/1,600 = 0.02). Assuming this is a plastic skylight with a VT of 0.65, the product of skylight transmittance and skylight area to daylit area ratio is (0.65)(32/1,600) = 0.013 = 1.3 percent. An 8 ft. by 8 ft. skylight (two 4 ft. by 8 ft. skylights) installed on a 40-foot spacing would yield a 2.6 percent product of skylight transmittance and skylight area to daylit area ratio. With 64 square feet of skylight area for each 1,600 square feet of roof area, the skylight to roof area ratio is 4 percent, which is less than the maximum of 5 percent allowed by §140.3(a).
An alternate approach would be to space 4 ft. by 8 ft. skylights closer together, which would provide more uniform daylight distribution in the space and produce closer to the desired minimum VT skylight area product. Taking the product of the skylight VT and the skylight area and dividing by 0.02 (the desired ratio) yields the approximate area the skylight should serve. In this case, with a VT of 0.65 and a skylight area of 32 square feet, each skylight should serve around (0.65*32 /0.02) = 1,040 square feet. A 32-foot center to center spacing of skylights results in 32 x 32 = 1,024 square feet of daylit area per skylight.
For a 4 ft. by 8 ft. plastic skylight with a visible light transmittance of 0.65, the product of skylight transmittance and skylight area to daylit area ratio is
(0.65) x (32/1,024) = 0.0203 = 2.03 percent.