Daylight design offers an opportunity to control the use of natural light in and around buildings. Defined as a daylight design strategy, we can design glazing and its ability to provide light into a building.
Within the National Construction Code, daylight requirements within Health and Amenity - FP4.1 provisions for natural lighting are now mandatory for Class 2, 3 or 9 buildings, or a Class 4 part of a building.
The Performance Requirement for FP4.1 mandates:
“Sufficient openings must be provided and distributed in a building, appropriate to the function or use of that part of the building so that natural light, when available, provides an average daylight factor not less than 2%”.
In other words, all windows in Class 2, 3 or 9 buildings, or a Class 4 part of a building must provide natural light that is fit for purpose and guided by a minimum of 2% average daylight factor for both Verification and Deemed-to-Satisfy methods.
What is the Average Daylight Factor (ADF)?
An average daylight factor is a design metric that sets a minimum quality of light required by the code. It remains one of the most widely used methods of establishing static daylight compliance and rating tools performance (Green Star, BREEAM, LEED, Green Mark, etc.) globally, but this is changing for more dynamic daylight metrics.
Today, the average daylight factor is a very simple design metric, as a simple ratio of the internal and external illumination levels within a room. Stipulated within FV4.3 Verification of suitable provision of natural light, an average daylight factor is defined by the following equation:
As per the above, the equation uses the area of the windows (W) and walls (A) and adds window transmittance values (T) and surface reflectance values (R) (to reflect light from internal surface areas). It then aims to account for external structures that may obscure the so-called view of the sky (θ) and therefore the transmittance of natural light and applies an angle to account for such an obstruction.
What is important to note in the equation is the lack of any information related to the sun, thus the application of the average daylight factor is insensitive to orientation and climate. It is also important to note that the stated average daylight factor intent is for it to be used in hand calculations only.