Daylighting in Buildings: Variation of the Daylight Factor with Distance from the Window and Window-to-Wall Ratio

Abstract

Bringing daylight into a building is one of the oldest aims of architecture andone of the most important for comfort and for saving energy, since rooms litby the sun need less electric light, but daylight does not reach evenly into aroom: it is bright near the windows and fades toward the back, and how much enters depends on how large the windows are. This study examines how daylight in a room varies with distance from the window and with the size of the windows, measured by the window-to-wall ratio. The daylight available at points across a room was expressed by the daylight factor, the proportion of the outdoor daylight that reaches a point indoors, and its variation was examined from the window to the back of the room for rooms with small, medium, and large windows, against the level usually taken as adequate for daylighting. The daylight factor was high close to the window and fell steeply with distance into the room, so that the area near the window was well daylit while the back of a deep room received little, the daylight factor there falling below the level considered adequate. Larger windows raised the daylight factor throughout the room, the room with the largest window-to-wall ratio having the highest daylight factor at every distance, so that increasing the window size extended the well-daylit zone deeper into the room. There was, however, a limit to this benefit, since very large windows admit heat and glare as well as light, so daylighting is a balance. The study shows that the daylight factor falls with distance from the window and rises with window size, which sets how deep into a room useful daylight reaches, and it discusses the implications for the daylighting of rooms. KEYWORDS: Daylighting, Daylight factor, Window-to-wall ratio, Naturallight, Building design, Fenestration, Energy efficiency, Architectural science,Visual comfort