Pattern 2: Window Area

The area of glass relative to wall area, or the window to wall ratio (WWR), is one of the key inter-relationships when designing daylit buildings. Generally the effective distribution of daylight from perimeter windows is a function of the head height of the windows. However, some themes emerge when we consider the position, configuration and overall window area.

Glazing in the upper portion of a wall tends to increase daylight illumination levels in the zone roughly twice the head height of the window, though its contribution is negligible at distances beyond this point. Additionally, when glazing is concentrated in a limited portion of the wall area, contrast between views to the exterior and the surrounding opaque wall can begin to detract from visual comfort. Glazing in continuous horizontal “bands” tends to provide more unobstructed views to the exterior.

Vertical windows tend to provide a more even distribution of daylight illumination across the floor area at lower glass areas (below 40% WWR). At glass areas above 40% WWR daylight distribution remains similar since the distribution of glazing on the wall is similar when the sill is set at 2’-6” or greater. Glass areas below 30% WWR deliver a daylight distribution that is limited to the immediate area (15’-0”) adjacent to the window.
Bear in mind that even with 100% glass area walls, the distribution of effective daylight to the interior is unlikely to exceed 25’-0” in multi-story buildings, even considering very optimistic assumptions with respect to ceiling height, interior surface reflectances, and furniture build out. Many designers working with deep floor plates will over-glaze the perimeter in an effort to drive light deeper into the building. However this commonly increases glare and contrast to a degree that blinds are deployed continuously to maintain visual comfort. This will defeat the purpose of the glazing and substantially compromise both daylight illuminance and views. A far better option is to glaze the building to provide even distribution of daylight across a realistic daylight depth at lower glass areas and to ensure that occupied areas are situated within reach of a daylight aperture. Rather than trying to “get more light into the building” a better strategy is to “get more of the building into the light.”

The case study patterns are based on the Banner Bank Building in Boise, ID. It includes a 40% window to opaque 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. Note that a 3’-0” plenum was included for structure and HVAC between floors in the calculation of window to wall area.