Achieving daylighting credits for LEED
Using the software
Software used in this example is AGi32 by Lighting Analysts. The application of daylighting in AGi32 allows you to consider the effect of sun, sky, and ground as a light source in the environment. The daily rotation of the earth as well as its location in its orbit about the sun produces a predictable amount of sunlight at a given location. As the sunlight enters the atmosphere, a portion of the light is scattered by the atmospheric conditions. AGi32 uses many categories of sky to emulate these conditions: clear, partly cloudy, and overcast, plus 15 other general sky conditions described by the CIE. LEED 2009 requires calculations to be done under clear sky conditions.
For interior applications, the daylight will need to be “let into” the space by creating transition openings such as windows and daylight openings in the room. Exterior surfaces such as overhangs, fins, etc., that obstruct and/or reflect light into the interior environment will also be accounted for.
The building used in this analysis is a prototype office building with regularly occupied spaces having south facing (and some east facing) windows. Simulation methods generally favor south facing rooms. Using computer programs in the early stages of the integrated design process helps the designer to account for factors that influence daylighting, such as location and climate, surroundings, orientation, opening position, and size.
The Revit-based floor plan should be extracted and exported to CAD format, then imported to AGi32.
According to the AGI website, to consider the effect of daylighting in interiors, surfaces imported into or created within AGi32 must be designated appropriate surface types specifically for daylighting analysis. The following transition types are modeled with planar objects and must be located in the model to provide for daylight insolation into the interiors. These transition surfaces model the glazing portions of windows and skylights. Each planar surface represents the entire glazing assembly (glass surfaces and airspace within), using the visible transmittance (VT) value found on the glazing label. This value is the transparency specified in AGi32.
- 20: Daylight transition glass (transparent)
- 21: Daylight transition glass (diffuse)
- 22: Daylight transition opening.
Room surfaces also can be designated as a transition surface to model a window wall.
As listed above, LEED requirements for daylight spaces include a criterion that 75% of the work plane illuminance should fall between 25 and 500 fc. Our next step will place an illuminance grid and will include a parameter to show the percentage of points in this range. All of the assessment results in this study were conducted on an analysis plane positioned 30 in. above each finished floor.
We can now set the following daylight parameters for our calculations:
- Sky conditions: Clear sky
- Date and time: Sept. 21, 9 a.m. (do similar calculation for 3 p.m.)
- Location: Chicago, IL, USA
- True north = 90 (the windows face south)
- Latitude = 41 deg north; longitude = 88 deg west.
Now we are ready to run the computation.
Documenting the results
Computation results in iso-footcandle (or contour) plan shows curves or lines along which the footcandle values are constant. The plan shows these values for each regularly occupied room.
A calculation summary schedule displays the maximum and minimum values for each calculation grid, allowing you to show the spaces that comply. If you specify an illuminance grid to include the percentage points in a range, and define the range from 25 to 500 fc, you can display this information in the calculation summary schedule as well. The daylight summary schedule can also be displayed, showing the daylight parameters, site latitude, longitude, and other relevant data.
Ensure the documentation includes a narrative explaining why and what rooms are selected as regularly occupied spaces. Do not include non-regularly occupied areas. Provide drawings that show illumination simulation results and graphics. Here we need to show the output of the simulation model. Ensure parameters are spelled out: footcandles measured at 30 in. above the floor; daylighting at 9 a.m. and 3 p.m. on Sept. 21, under clear sky conditions; and following the minimum 25 fc to maximum 500 fc range.
LEED provides an opportunity to earn an additional point by exemplifying performance under the “Innovation in Design” category. Here, we need to achieve the same illuminance requirements for 95% of regularly occupied spaces under the same conditions as before.
The documentation and narrative for this prototype building are as follows:
- An assessment for daylighting performance and probability of achieving LEED daylight and views credit EQc8.1 was performed for the prototype building in Chicago. The daylighting analysis was carried out using AGi32 lighting simulation software developed by Lighting Analysts in the United States and is considered one of the most recommended and accurate tools available for both daylight and artificial lighting analysis.
- The daylighting performance of the prototype building did meet the requirements to achieve EQc8.1. The overall daylighting performance is good with balanced daylight distribution. From the daylighting analysis results shown in this report, 85% of the regularly occupied space floor area is 25 to 500 fc illumination level at 3 p.m. and 75% at 9 a.m.