Page 322 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

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shading devices, high performance glazing and automatic photocell-based controls. Predict
daylight factors via manual calculations or model daylighting strategies with a physical or
computer model to assess footcandle levels and daylight factors achieved.

USGBC Rating System (LEED 2009)
The requirements for the updated LEED 2009 version of IEQc8.1 can be summarized by
the following four options.
Requirements LEED 2009
LEED 2009 OPTION 1—SIMULATION (This was previously Option 2 under LEED 2.2 with some
modifications.)
LEED 2009 IEQc8.1 Option 1 requires a computer simulation demonstrating that at least
75 percent of regularly occupied areas achieve a minimum daylight illuminance of 25 footcandles
(fc) and a maximum daylight illuminance of 500 fc at 30 inches above the floor level in a clear sky
condition at both 9 a.m. and 3 p.m. on September 21st (March 31st is given as an alternative in
the Reference Guide). The grid lines used in the simulation must not be more than 5 feet apart.
LEED 2009 OPTION 2—PRESCRIPTIVE (This prescriptive option replaces the previous
Calculation Option #1 in LEED 2.2.)
LEED 2009 IEQc8.1 Option 2 separates all daylit areas into two different types of zones, one for
side-lighting and one for top-lighting. These should be separately analyzed and then summed to
determine compliance. A summary of the requirements can be found in Table 6.8.1.
LEED 2009 OPTION 3—MEASUREMENT
LEED 2009 IEQc8.1 Option 3 is the same as LEED 2.2 IEQc8.1 Option 3.

Side-Lighting Daylight Zones Top-Lighting Daylight Zones
The product of the visible light Skylights with a minimum T of 0.5 should cover
vis
transmittance (T or VLT) between 3 and 6% of the roof area.
vis
and the window to floor area ratio (WFR)
must be between 0.150 and 0.80 for all
window areas >30 in above the floor.
The floor (bay) area used for each window Skylights should not be separated by >140% of
is defined by the bay width and twice the the ceiling height.
window height into the room.
The ceiling cannot obstruct into the area The daylit zones under a skylight are its plan view
under a line drawn from the height of the plus in every direction the lesser of 70% of the
window-head to a distance two window-head ceiling height, half the distance to the nearest
heights away from the plane of the window skylight or any opaque partition farther away
for any portion of the window or the floor area from the plan edge than 70% of the distance
included in the WFR for that particular bay. between the top of the partition and the ceiling.
Provide sunlight redirection and/or glare Any skylight diffuser must have an ASTM D1003
control as needed. tested haze value >90% and not be in direct line
of sight.
The LEED 2009 Reference Guide provides
some typical visible transmittance values
as 0.86 for N/S and E/W bays and 0.45
for corner bays.

TABLE 6.8.1 LEED 2009 IEQc8.1: Daylight and Views—Daylight Option 2. Prescriptive Requirements