Rating System

LEED BD+C: New Construction

Rating System Version

v2 - LEED 2.1

Inquiry

For LEED Credit EQ 8.1, we are attempting to demonstrate that we achieve daylighting for 75% of the occupied spaces in a school. The classrooms are simple, rectangular shapes, with windows on one wall and tubular skylights toward the interior of the room. Using the method outlined in the Reference Guide, we can achieve the 2% daylight factor if our skylight openings equal 12 square feet. Because tubular skylights are more efficient at bringing in light, the actual opening is smaller and thus the calculation does not yield the 2% daylight factor. We used Lumen Micro to simulate the daylighting and can demonstrate that the distribution of light is more even, with the tubular skylights, to that with the larger conventional skylights. Our question has to do with the calculation of the daylight factor. As stated in the Reference Guide, the daylight factor is the ratio of exterior illumination to interior illumination. 1. The exterior illuminance for our site on a clear day is 4,736 footcandles. A 2% daylight factor would require 95 footcandles across the interior grid. 2. The tubular skylights have similar light levels and more evenly distributed light than larger conventional skylights that qualify using the method outlined in the Reference Guide. Nonetheless, they do not meet the 2% daylight factor due to the high exterior illuminance. 3. Using a lighting simulation program, the rooms with north windows yield lower light levels than the rooms with south windows, as would be expected. This difference is not considered using the Reference Guide method. Again, the grid points do not come close to 95 footcandles, however over 90% of the room has light levels of 25 footcandles or higher, the proposed level in LEED NC 2.2. As per the CIR of 4/19/2004, we would only count the portion of the room that achieves 25 footcandles or more. Would these calculations successfully demonstrate meeting the requirements for the daylighting credit?