A comparison of four daylighting metrics in assessing the daylighting performance of three shading systems

The assessment of the daylighting performance of a design solution is a complex task due to the changing nature of daylight. A few quantitative metrics are available to designers to assess such a performance, among them are the mean hourly illuminance (MHI), the daylight factor (DF), the daylight autonomy (DA) and the useful daylight illuminance (UDI). Each of these metrics has a purpose, a set of criteria and limitations that affect the outcome of the evaluation. When to use one metric instead of another depends largely on the design goals to be achieved. Using Design Iterate Validate Adapt (DIVA) daylighting simulation program, we set out to examine the performance behavior of these four metrics with the changing dimensions of three shading devices: a horizontal overhang, a horizontal louver system, and a vertical fin system, and compare their performance behavior as the orientation changes of the window to which these devices are attached. The context is a typical classroom of a prototypical elementary school. Our results indicate that not all four metrics behave similarly as we vary the size of each shading device and as orientation changes. The lesson learned is that not all daylighting metrics lead to the same conclusions and that it is important to use the metric that corresponds to the specific goals and objectives of the design and of the daylighting solution. The UDI is the metric that leads to outcomes most different than the other three metrics investigated in this paper.