Daylighting Rule of Thumb for Room with Glazing Transmittance Variation

Abstract

Since antiquity, architects have been using simple guides or easy to use calculation methods called rules of thumb for predicting daylighting performance inside buildings. These rules of thumb in daylighting continue to develop as new knowledge in science and technology evolves. In architectural practice, daylighting rules of thumb have often been expressed in terms of the percentages of window area to floor area. Such rules can be found in architectural literatures as well as in building regulations in various countries. The percentages of window glazing area to floor area suggested in the literatures and building codes often range from 10% to 35% depending on the specified visual tasks and the illumination characteristics desired. One of the most frequently cited daylighting rules of thumb is for 20% glazing to floor area allocated for habitable room or space. These rules of thumb are usually based on the standard clear glass with transmittance value of approximately 0.8 to 0.9. However, many recent buildings in tropical countries have glazing transmittance lower than this value for glare control and privacy purpose. In order to address this issue, daylighting simulations were conducted using AGi-32 to determine the effects of different glazing transmittances on indoor daylight performances and to modify existing rule of thumb. Based on the simulation data and analysis, a well-known daylighting rule of thumb by Littlefair is modified to cater for multiple glazing transmittances. This new rule of thumb presents an easy to use calculation to estimate daylight factor for interior based on glazing transmittance value.