Design Condition Information

Window Area / Window-to-Wall Ratio

Flush-mounted, non-operable windows were modeled in the exterior wall of each perimeter zone office. Five window sizes were modeled with a fenestration window-to-wall area ratio (WWR) (which includes the area of the whole window with frame), of 0.15, 0.30, 0.45, and 0.60, 0.75, where the wall area was defined as the floor-to-floor exterior wall area and the floor-to-floor height was 12 feet. The head height of the framed window was set flush with the ceiling at 9 feet for all window areas except WWR=0.15.

Daylight Controls

Recessed fluorescent lighting systems were modeled with a lighting power density of 1.2 W/sf throughout the building. Heat from the lighting system was apportioned to the interior space (60 percent) and to the unconditioned plenum (40 percent). If no daylighting controls were specified, the lighting was assumed to be at 100 percent power, and governed, as in the daylighting case, by the occupancy schedule.

For continuous dimming, the overhead lights dim continuously and linearly from (maximum electric power, maximum light output) to (minimum electric power, minimum light output) as the daylight illuminance increases. The lights stay on at the minimum point with further increase in the daylight illuminance. The lowest power the lighting system can dim down to, expressed as a fraction of maximum input power (see figure below taken from the EnergyPlus InputOutput Reference Guide).

For stepped dimming, the electric power input and light output vary in discrete, equally spaced steps. The number of steps is defined by the fraction of the lights that are on versus daylight illuminance (see figure below taken from the EnergyPlus InputOutput Reference Guide).

The design work plane illuminance level of 50 footcandles is recommended for performing typical office tasks by the Illuminating Engineering Society. The recommended illuminance level in modern buildings has been changing. The current recommendation is for 30 footcandles where there is intensive visual display terminal (VDT) use and 50 footcandles for filing, intermittent VDT use, and private offices. The reduction in the recommended light level where VDT use is intensive is a response to the reflection problems with cathode-ray terminal (CRT) based VDTs. It is generally assumed that higher light levels would be preferred if there were not problems with the VDTs. The recent increase in availability and popularity of flat screen liquid crystal display (LCD) VDTs is likely to change lighting practice and recommendations as these screens are much less prone to veiling reflection problems. The recommendations may return to 50 footcandles, even for intensive VDT use.

Interior Shades

The windows were modeled with and without an interior shade. The shade was manually operated where it was drawn down completely by the occupant for daylight hours if direct sun or glare was present. The shade was deployed if the heat gain per square foot of window area from direct (beam) solar radiation transmitted through the window exceeded 30 Btu/hr-sf or if the daylighting glare index computed using the Hopkinson Cornell-BRS formula exceeded 22 ("just uncomfortable," maximum recommended for general office work). With the shade drawn, the visible transmittance of the glazing is reduced by 65 percent (translucent light white drape) and the solar heat gain coefficient (SHGC) by 40 percent.

The shade was modeled as a planar, ideally diffuse, translucent layer parallel to the glass layers and interior to the window. The effectiveness of a shading system to moderate heat flux through a fenestration system varies with the type of window it is combined with, solar incident angle, and other factors. However, the present state of available measured data on shading devices does not permit greater accuracy in the calculation of heat flow through shaded fenestration and prior measurements predate modern developments in fenestration systems.

Exterior Shades

Fins, overhangs, and a setback window were modeled as opaque, non-reflective surfaces. These obstructions block diffuse light from the sky and direct sun but reflect no light from the ground.

Setback. The window is recessed from the exterior plane of the facade, so that a 1-foot-deep overhang, fin, and sill shade the glazing.

Overhangs. Overhangs were modeled at the depths of 2' and 4'. The width of the overhang was made the same width as the office module (not the window): 10 feet. The overhang height was set so that its lower surface was flush with the top of the framed window opening.

Fins. A single case was modeled for fins. The fin depth yields an azimuthal cutoff angle of 26.6° for shading the full window width ("100 percent-shade" condition) and 45 degrees for shading half the window width ("50 percent-shade" condition). The fin was made the full height of the window and placed flush with the left and right edges of the framed window opening.

Overhangs and Fins. When the overhang and fins were combined, the overhang width was the width of the window. The overhang depth was 4'.

Windows

The double glazed types were modeled in aluminum thermally broken frames with a U-factor of 1.0, and the triple and quad glazed types were modeled in an high-performance aluminum frame with a U-factor of 0.35.