Pattern 3: Section Depth

When describing daylight potential in buildings, “section depth” refers to the overall distance between perimeter windows and the innermost wall of an occupied space. In multi-story buildings that are illuminated solely from vertical glazing at the perimeter, the section depth is often the most crucial determinate of whether occupants have access to daylight and views. In general, buildings that exhibit a narrow section depth provide a far higher percentage of daylit area than buildings with deep perimeter to core depths. At common ceiling heights (9’-0” t0 12’-0”), it is nearly impossible to provide effective daylight distribution beyond 25’-0” from a window wall if it is lit exclusively from one side. In contrast, many speculative office buildings are predicated on a minimum 40’-0” “leased span” which implies that the occupied area consists of a 40’-0” minimum section depth, and often a far deeper perimeter to core dimension. This configuration virtually ensures that less than 50 percent of the floor plate will be effectively daylit.

Beyond simple horizontal daylight illuminance on a workplane, the section depth of a space contributes greatly to visual comfort. Specifically, the relationship of the brightness of the perimeter windows to the “back” interior wall is crucial. Since the human eye tends to adjust to the brightest surfaces it’s visual field, a space that is bright at the perimeter, and much darker toward the interior can feel like a cave, even when electrically illuminated at common interior light levels. This “cave effect” is common in buildings with deep section depths, especially when they are over-glazed at the perimeter. Conversely, in spaces with section depths that are not much more than two and a half times deeper than the head height of the perimeter windows, daylight is able to wash the “back” wall, enabling balanced brightness across the building section.

In side-lit buildings, depending on orientation, the perimeter zone can be subject to large fluctuations in illumination and direct sunlight penetration. One approach to managing this is to pull workstations from the perimeter and creates a narrow aisle way where direct sunlight can be present without impacting critical visual task areas. This strategy has been termed creating “glare storage” by some designers.

In short, buildings with a narrower section depth place their occupants in closer proximity to windows and therefore within reach of a potential daylight aperture. This creates greater opportunity for lighting power savings, and more uniformity of daylight illuminance, subsequently increasing the likelihood of visual comfort.

The case study patterns are based on the Banner Bank Building in Boise, ID. It includes a 40% window to exterior wall ratio with a window head height at 9’-6”, a sill height at 3’-0” and a ceiling height at 10’-0”. Interior reflectances are roughly 80%, 50%, and 20% for ceiling, walls, and floors, respectively. The overall section depth in the “as-built” condition is 26’-0”.