5.5 Other Prescriptive Daylighting Requirements – Daylighting, Daylighting Devices, and Secondary Sidelit Daylit Zones

This section contains daylighting requirements that are in addition to the mandatory automatic daylighting controls covered in Section 5.4 of this chapter.

They include the prescriptive requirements for daylighting in large enclosed spaces, for daylighting devices (clerestories, horizontal slats, light shelves) that  are qualified for PAFs, and for automatic daylighting controls in secondary sidelit daylit zone

5.5.1          Daylighting Devices (Clerestories, Horizontal Slat, Light Shelves) – Daylighting Design Power Adjustment Factors (PAFs)

Certain design features and technologies have the capacity to 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 (PCO) in the performance method.

A careful analysis should be performed to ensure the avoidance of glare issues when including daylighting features in the design. An example where caution should be taken is specularly reflective (e.g. polished or mirror-finished) slats. These slats may redirect direct beam 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.

For the PAF, 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 PAFs are met.

For the PCO, a variety of control strategies is available in the compliance software to take advantage of further savings.

For the PAF, at permit application, use form NRCC-LTI-E.

5.5.2          Minimum Daylighting Requirements for Large Enclosed Spaces

§140.3 has prescriptive requirements for building envelopes, including minimum daylighting for large enclosed spaces directly under roofs. Lighting installed in spaces complying with these prescriptive envelope measures are also required to comply with all lighting control requirements, including the mandatory and prescriptive lighting control requirements.

The mandatory daylighting control requirements are covered in Section 5.4.4 of this chapter.

If one prescriptively complies by installing daylight openings in large enclosed spaces directly under roofs, the daylit areas could have electric lighting systems with high enough lighting power to trigger the mandatory requirements for daylighting controls. However, if one complies using the performance approach, it is possible to displace the daylighting openings and daylighting controls with other building efficiency options      Large Enclosed Spaces Requiring Minimum Daylighting – Qualifying Criteria

The minimum prescriptive daylighting requirements for large enclosed spaces apply to both conditioned and unconditioned nonresidential spaces that meet the following qualifying criteria:

1.  Space is directly under a roof.

2.  Is located in climate zones 2 through 15.

3.  Has a floor area greater than 5,000 ft².

4.  Has a ceiling height greater than 15 ft.


1.  Auditoriums, churches, movie theaters, museums, or 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 ft2.

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 daytime hours per year between 8 a.m. and 4 p.m.      Prescriptive Daylighting Requirements

In climate zones 2 thru 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.

For large enclosed spaces that are required to comply, following are details of the minimum 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 high partitions. The design of the envelope may be developed before there is any knowledge of the location of the partial height partitions or shelves as is often the case for core and shell buildings. Thus the architectural daylit zone requirement of 75 percent of the area 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 while determining the specifications. There is a greater likelihood that the electrical design will occur later than the architectural design and thus greater planning for these obstructions can be built in to 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 can be translated and 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 a Commission approved test method.

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 minimum VT of 0.49 and a maximum SHGC of 0.25, glass skylights must utilize 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). If one fully daylights the space with skylights and the skylights meet the prescriptive requirements of 64 percent visible light transmittance, a minimum skylight area of at least 3 percent of the roof area is needed to optimize energy cost savings according to several simulation studies.2.


Example 5-4

Methods for buildings with large enclosed spaces in compliance with the minimum daylighting requirement

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 further 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 5-5

Large enclosed spaces in a warehouse building

A warehouse with 40,000 sq ft. area and a 30-foot tall ceiling (roof deck). 

Maximum skylight spacing distance and recommended range of skylight area.

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 – but at an increased cost since more skylights are needed. However, as a first approximation one can space the skylights 1.4 times the ceiling height. For this example, skylights can be spaced 1.4 x 30 = 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 on 40 foot centers.


Each skylight is serving a 40 foot by 40 foot area of 1,600 square foot. A standard skylight size for warehouses is often 4’ by 8’ (so it displaces one piece of roof decking). The ratio of skylight area to daylit area is 2 percent (32/1600 = 0.02). Assuming this is a plastic skylight and it has a minimally compliant visible light transmittance 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

This is shy of the 2 percent rule of thumb described earlier for the product of skylight transmittance and skylight area to daylit area ratio. If one installed an 8 ft. by 8 ft. skylight (two 4 ft. by 8 ft. skylights) 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 (SRR) is 4 percent which is less than the maximum SRR 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 could more closely approach the desired minimum VT skylight area product. By 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*32) = 1,024 square feet of daylit area per skylight.

For the minimally compliant 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.


Example 5-6

Methods for complying with the mandatory daylight control requirements for a space with HID lighting

The Standards require that automatic daylighting controls shall provide functional multi-level lighting levels having at least the number of control steps specified in Table 130.1-A.

A space with HID luminaires that are greater than 20 watts, shall have a minimum of 1-step between 50 percent and 70 percent.

This can be achieved in one of the following ways, using:

A. Continuous dimming - Here the photocontrol gradually dims all luminaires in the daylit zone in response to the available daylight.

B. Stepped dimming - Here the photocontrol signals the stepped dimming ballast to reduce power in incremental steps such that there is one control step between 50 percent and 70 percent as noted above.


Example 5-7

Complying with the 150 percent of the design illuminance daylighting requirement

When the illuminance received from the daylight is greater than 150 percent of the design illuminance (or nighttime electric lighting illuminance), the general lighting power in the daylit zone must reduce by a minimum of 65 percent.

For example, a space has 500 watts of installed lighting power in daylit zones. The design illuminance for the space is 50 foot-candle (fc). When the available daylight in the space reaches 75 fc (i.e. 150 percent of 50 fc), then the power consumed by the general lighting in the daylit zones should be 175 watts or lower.

Without checking all points in the daylit zone served by controlled lighting, verifying that the requirements are met at a worst case location far away from windows or skylights is sufficient. This location is called the “Reference Location”


Example 5-8


Draw the daylit zone for two roof top monitors with four 4-foot long windows projecting over a 10-foot tall roof. The two monitors are 13.5 feet apart.


Standards currently define skylights as glazing having a slope less than 60 degrees from the horizontal with conditioned or unconditioned space below. Because rooftop monitors have a slope greater than 60 degrees, they are therefore considered to be windows.



5.5.3          Prescriptive Automatic Daylighting Control Requirements in Secondary Daylit Zones

The daylighting control requirements for secondary daylit zones are not mandatory but prescriptive.

All luminaires providing general lighting that is in, or at least half of the luminaires are in, a secondary sidelit daylit zone as defined in §130.1(d)1C, and that is not in a primary sidelit daylit zone shall comply with the following:

1.  The general lighting shall be controlled independently from all other luminaires (including those in the primary sidelit daylit zone, the skylit daylit zone and lights that are not in daylit zones) by automatic daylighting controls that meet the applicable requirements of §110.9.

2.  The general lighting shall be controlled in accordance with the applicable requirements in §130.1(d)2 (see Section 5.4.2 of this chapter).

3.  All secondary sidelit daylit zones shall be shown on the plans submitted to the enforcing agency.


1.  Luminaires in secondary sidelit daylit zone(s) in areas where the total wattage of general lighting is less than 120 watts.

2.  Luminaires in parking garages complying with §130.1(d)3.