PERFORMANCE: Human Factors
Energy & Cost
Codes & Standards
Daylight is an important part of the desire for windows and has qualities that cannot be replicated by electric light. The changing direction, intensity, and even color of daylight are stimulating and connect people to the time of day and the natural world.
Daylight design is far more sophisticated than simply providing a window with a high enough visible transmittance. More daylight does not necessarily equate to better lighting conditions. It is a matter of balancing daylight admission with glare control, as well as providing uniform light distribution. Remember that the usability of daylight is dependent on the task. For some tasks, bright illumination improves visual acuity and glare is of little concern. For computer tasks, glare may be problematic and it is better to control illuminance levels.
Windows, of course, permit daylight to enter a space. The design of a window and choice of glazing can dramatically affect the quantity and quality of daylight in a space and how it is experienced. Most visual tasks require only 30–70 footcandles. There is great variability in daylight illuminance with different window types. For example, a reflective glazing has a very low VT and provides much less daylight than most other glazing types. The average annual daylight illuminance is linearly related to the product of the window-to-wall ratio and visible transmittance (VT*WWR). Larger windows with low-transmission glass can have the same average daylight illuminance as small windows with high-transmission glass. South-facing windows generally have more daylight levels than north-, east-, and west-facing windows because of direct sun. Higher latitudes have greater average daylight levels on the south than lower latitude locations. Shading devices reduce the amount of daylight in non-south-facing orientations. In reality, north-facing zones might even yield more useful daylight than other orientations since there is less need to deploy interior or exterior shades to control glare and direct sun.
The layout and shape of the interior space can determine the effectiveness of daylighting. The depth of daylight distribution is a function of window height relative to the working plane (floor or work surface). There are strategies to remedy the imbalance of equal distribution of daylight. For instance, light-colored surfaces within a room can provide good daylight distribution. Techniques, such as light shelves, can extend the daylight zone. If properly designed, light shelves reduce daylight illuminance near the windows and increase it further into the room. Introducing daylight from a second source—either a window on another wall or through toplighting—can also help balance daylight in a room, addressing the uneven distribution that occurs with sidelighting from one source.
Design Guidance from ASHRAE's Indoor Air Quality Guide