Page 324 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

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In either version, all the options require having 75 percent of the regularly occupied
floor areas in the building meet a minimum daylight illumination level. However, there
are some differences in how the 75 percent is determined. The applicable regularly
occupied floor areas can be segregated into different modeling units. They can be in
modeling units of “rooms” or “areas.” For the modeling options in LEED 2.2, most
small rooms are taken as one unit, and large rooms can be segregated into applicable
areas. In LEED 2009, the prescriptive modeling option uses special areas defined by the
window locations and sizes. The square footage of all the modeling units that meet the
minimum daylight illumination level can be used for the 75 percent determination. For
the third option, the one based on measuring illumination levels, the applicable regularly
2
occupied floor areas are segregated into 10-ft by 10-ft grids. All the 100-ft squares that
meet the minimum daylighting levels count toward the 75 percent determination. Only
the calculation model in LEED 2.2 will be explained in greater detail here, as the second
model requires more complex simulations and the third option requires actual
measurements after construction. In LEED 2009, the prescriptive option (LEED 2009
Option 2) methodology is summarized in Table 6.8.1.

LEED 2.2 Option 1 (Calculation) The first option gives a simple method that can be used
in the design phase to quickly evaluate options that may or may not meet the credit
criteria. As can be seen in the LEED-NC 2.2 Requirements, it is based on determining if
the glazing factor for at least 75 percent of the regularly occupied areas meets a minimum
value of 0.02 (2 percent). The equation given in the Requirements section is based on the
window’s location (height), angle (geometry factor), and a minimum visible transmittance
T based on the window location. Together, these parameters can be combined to give a
vis
combined daylight factor for each window type i, denoted by CDF . Typical minimum
i
values for each combined daylight factor as specified by the parameters set in LEED-NC
2.2 are listed in Table 6.8.2. Note that each window type may vary, and the manufacturers’
information should be used for the final calculations.
By using these factors, the area of each window type, and the areas of the regularly
occupied spaces, compliance can be calculated using the following definitions:

DC Daylighting compliance for the project (in percent)
GF Glazing factor of regularly occupied room area j
j
RA The floor area of regularly occupied room area j
j

Glazing Type Combined Daylighting Factor CDF
i
Sidelighting (2.5–7.5 ft 0.20
above the floor)
Sidelighting (>7.5 ft above 0.20
the floor)
Toplighting, vertical 0.50
Toplighting, sawtooth 0.83
Toplighting, horizontal 1.25

TABLE 6.8.2 Typical Minimum Combined Daylighting Factors (CDF)
i
LEED 2.2