Simulation-Based Review of the Ubiquitous Window-Head-Height to Daylit Zone Depth Rule-Of-Thumb


C. Reinhart. Ninth International IBPSA Conference, 2005, pp. 1011-1018

Review by Scott Schuetter, Energy Center of Wisconsin: 

In this paper, Reinhart explores the validity of various daylighting rule of thumb (DRT) design methodologies by checking their accuracy against Radiance-based models in Daysim. Specifically, the paper investigates the rule that a sidelit daylit area is defined by a depth equal to some multiple of the window head-height or ceiling height. A range of definitions exist, but the majority specify a ratio of depth to height between 1.5 and 2.0.

A multitude of models were created representing 5 climate zones, four facades, office or classroom occupancy, high or low visible transmittance of glazing, and the presence or lack of blinds, among others. The daylit depth was calculated by finding the depth at which the space's daylight autonomy was equal to 50%. Daylight autonomy was defined as the percentage of occupied times that a task-specific minimum illuminance was provided by only daylight. The study found good agreement between the DRT and the simulation results. Reinhart concludes that "in a sidelit space with a standard window and venetian blinds, the depth of the daylit area usually lies between 1 and 2 times the size of the window-head-height." Further, in spaces that do not have blinds, this ratio can be as high as 2.5.