Daylighting can be a major energy-efficiency asset when combined with automatic lighting controls. For this reason, skylights are required in by the energy code for enclosed spaces greater than 2,500 square feet and meeting the following criteria (2015 IECC 402.4.2):
- Located directly under a room with not less than 75% of the ceiling area with a ceiling height greater than 15 feet, and
- The space is used as an office, lobby, atrium, concourse, corridor, storage space, gymnasium/exercise center, convention center, automotive service area, space where manufacturing occurs, nonrefrigerated warehouse, retail store, distribution/sorting area, transportation depot or workshop
The total daylight zone under skylights shall be not less than half the floor area and shall provide one of the following:
- A minimum skylight area to daylight zone under sky-lights of not less than 3 percent where all skylights have a VT of at least 0.40 as determined in accordance with Section C303.1.3.
- A minimum skylight effective aperture of at least 1 percent, determined in accordance with the following equation:
Skylight Effective Aperture = 0.85 (Skylight Area) (Skylight VT) (WF) / (Daylight zone under skylight)
Skylight area = Total fenestration area of skylights
Skylight VT = Area weighted average visible transmittance of skylights
WF = Area weighted average well factor, where well factor is 0.9 if light well depth is less than 2 feet, or 0.7 if light well depth is 2 feet or greater
Light well depth = Measure vertically from the underside of the lowest point of the skylight glazing to the ceiling plane under the skylight
Exceptions to this requirement include
- Spaces where the designed general lighting power densities are less than 0.5 W/ft2.
- Areas where it is documented that existing structures or natural objects block direct beam sunlight on at least half of the roof over the enclosed area for more than 1,500 daytime hours per year between 8 a.m. and 4 p.m.
- Spaces where the daylight zone under rooftop monitors is greater than 50 percent of the enclosed space floor area.
- Spaces where the total area minus the area of day-light zones adjacent to vertical fenestration is less than 2,500 square feet, and where the lighting is controlled according to Section C405.2.3.
The threshold for skylights was reduced from 10,000 square feet in the 2012 IECC to 2,500 square feet in the 2015 IECC, which can quickly come into play with the open floor layouts that are commonly constructed today.
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