Practical and Policy-Relevant Performance Metrics for Complex Fenestration Systems

The selection of a fenestration system for a building is critical, as it impacts energy performance, occupant comfort, and ambiance of a space. Complex Fenestration Systems (CFS) address these criteria using a wide variety of novel technologies but are difficult to define or be characterized. Existing metrics for fenestration systems are unable to reveal the dynamics or degree of variety over climate conditions or time of year that define CFS because they rely on a single and arbitrarily-defined set of environmental conditions to calculate. Although the optical characteristics of a CFS can be predicted using its Bi-Directional Transmission Distribution Function (BTDF) – a mathematical dataset that describes the angular distribution of light flux as it passes through a material – this information is too abstract to be meaningful to the building industry. A set of metrics that uses the BTDF in an intuitive way could allow the performance and physical characteristics of these technologies to become more accessible, ultimately allowing the various benefits of daylighting to be realized. The proposed approach offers a solution to this problem by using an annual climate-based methodology to provide a comprehensive evaluation of a system by incorporating three of the most relevant performance aspects: energy efficiency, occupant visual comfort, and ability to view through. Three metrics, the Relative Energy Impact (REI), the Extent of Comfortable Daylight (ECD), and the View Through Potential (VTP), were derived from these three criteria to express, in relative terms, a façade’s contribution to building energy use, the fraction of time and space for which it achieves comfortable daylight conditions, and the degree of transparency as it relates to an occupant’s view through the façade, respectively. These metrics are intended to exist as a mechanism by which manufacturers can evaluate and compare façade systems, provide high-level intuition of relative performance for designers and contractors, and enable the balance of performance objectives based on user preference. In order to successfully implement these metrics, a simple and repeatable calculation process was identified first through a series of sensitivity analyses compromising on relevance or accuracy, and then by defining input conditions that are able to reduce calculation or simulation time substantially. Using both approaches, each of these metrics was further and applied to five sample façades that cover a broad range of Complex Fenestration System types, including a validation study for the VTP metric. A visual representation of this information in a condensed format was then investigated so as to allow straightforward comparisons amongst systems and a synthetic understanding of their performance. A graphical, label-like structure could indeed provide an initial suggestion for the use of these metrics in the rating and standard-setting environments.