Data-driven assessment of daylight metrics for building visual environmental quality

Evaluating building performance requires defining appropriate measures, timing, and interpretation methods to determine success or failure. One major challenge in effective daylighting is the varied definitions of daylighting performance among stakeholders, leading to fragmented assessment approaches. Historically, daylighting has been promoted for energy savings, but growing awareness of its health benefits, discomfort glare, and the need to minimize heating and cooling loads for Zero Net Energy buildings has broadened performance considerations.

Traditional metrics like the Daylight Factor (DF) have limitations due to their static nature and exclusion of dynamic conditions such as direct sunlight and weather changes. Climate Based Daylight Modeling (CBDM) offers a more comprehensive approach by using annual weather data to predict interior daylighting conditions, allowing for responsive design adjustments. Various CBDM metrics, such as Daylight Autonomy (DA) and Useful Daylight Illuminance (UDI) provide nuanced assessments of daylight performance, although differences between these metrics are not extensively discussed.

Previous studies have shown that the application and effectiveness of daylight standards and metrics are significantly influenced by specific geographical contexts. The variation in daylight hours across different locations is heavily impacted by latitude, which determines sunlight duration and elevation throughout the year. Such geographic differences introduce complexities in uniformly applying daylighting standards. Key components of daylight assessment include illuminance thresholds, time thresholds, analysis periods, and shading controls. The varying objectives in meeting these criteria result in different evaluations of a space’s daylight performance. For example, EN-17037 defines its analysis period as the top fifty percent of annual hourly values for diffuse illuminance, contrasting with other metrics based on the percentage of a fixed occupancy schedule.

This paper reviews the historical basis of current compliance methods for achieving daylit buildings, provides a comparative study of 35 global daylight metrics in 30 climates, and proposes a technical basis for the development of better daylight metrics. The assessment of daylight metrics involves several key criteria: examining the difficulty in meeting design targets across various building types and locations; analyzing the effectiveness of these metrics across different latitudes; evaluating the parity or agreement between different metrics; assessing visual discomfort metrics to address issues like glare and excessive brightness; and reviewing the impact of daylight metrics on their contribution to building energy savings by reducing the need for artificial lighting. The discussion reevaluates the diverse international definitions of ‘daylit’ and considers what is required to develop a new generation of climate- and human-responsive metrics.