Utilizing Dynamic Metrics to Balance Daylighting and Energy Performance

Utilizing Dynamic Metrics to Balance Daylighting and Energy Performance

By:  Daniel Overbey It is great to see the design profession becoming more sophisticated with energy modeling and design performance analysis. New tools and methodologies are allowing designers to go beyond crude rules-of-thumb and over-simplified calculations to make better informed design decisions. This is especially evident in the realm of daylighting analysis, where the emergence of dynamic metrics is enabling design teams to calibrate high-performance strategies with unprecedented ease. Daylight Metrics Defined Daylight Factor (DF) can be simply defined as the ratio of the internal light level to the external light level at the same location. Once a common metric, the DF has proven to be a poor predictor of actual performance due to its over-simplified calculation method. Its favor has been supplanted by more accurate metrics. Point-in-time simulations (e.g. illuminance at 3:00 p.m. on the equinox under clear sky conditions) can be useful for understanding particular scenarios, but the prospect of running an exhaustive set of simulations in order to fully assess all operating hours throughout the entire year can be quite cumbersome and is not realistic for project teams. Therefore, designers resort to strategic “spot checks,” which do not provide a complete picture of overall effectiveness of the daylighting strategy. Spatial Daylight Autonomy (sDA) is a metric that describes the annual sufficiency of ambient daylight levels in interior spaces. It is defined as the percentage of an area that meets a minimum illuminance level for a specified fraction of all annual operating hours. The sDA is expressed as a percentage of area. When LEED v4 calls out sDA300/50%, for instance, that specifies the percentage of area with at least 300 lux (28 fc) for at least 50% of all annual operating hours. Annual Sun Exposure (ASE) is a metric that describes how much of a space receives too much direct sunlight, which can cause visual discomfort (e.g. glare) and/or increase cooling loads. It is defined as the percentage of an area that exceeds a specified direct sunlight illuminance level more than a specified number of hours per year. When LEED v4 calls out ASE1,000,250, for instance, it is describing the percentage of the area that is exposed to more than 1,000 lux (93 fc) of sunlight for more than 250 hours per year (before operable blinds or shades are deployed to block the sunlight). Because sDA does not specify how much daylight is too much, designers really need to consider both sDA and ASE for a more balanced analysis. Ideally, designers will develop strategies to maximize sDA and minimizing ASE. LEED Daylight Credit as a Barometer of Evolving Daylight Metrics  In fact, the LEED v4 Daylight credit exemplifies the design community’s evolving sophistication regarding the definition of adequate daylighting. LEED NC-v2.2 permitted a calculation option for demonstrating a Daylight Factor (called ‘glazing factor’ in the Reference Guide) of 2% or a simulation option through which achievement of 25 footcandles over 75% of the regularly occupied floor area under clear sky conditions, at noon, on the equinox, at the 30 inch workplane height is verified through design performance modeling. (According to LEEDuser, the achievement rate for this credit is 35%.) LEED NC-2009 recognized the inadequacies of the Daylight Factor methodology and instead offered a relatively more complicated prescriptive option for compliance. In addition, the simulation option was revised to require that 75% of the regularly occupied floor area achieve between 10 and 500 footcandles under clear sky conditions, on the equinox, at both 9 a.m. and 3 p.m. (According to LEEDuser, the achievement rate for this credit is 20%, a considerable drop compared to NC-v2.2.) Both NC-v2.2 and NC-2009 require glare control devices to avoid high-contrast situations. LEED v4 brings a number of changes to the Daylight credit:

  • The prescriptive compliance path has been eliminated.
  • For the point-in-time illumination simulation option, the acceptable daylight level range has been revised to be 300 lux (28 fc) to 3,000 lux (279 fc). Also, the simulation must now utilize local TMY3 weather data to calculate the illuminance intensity.
  • A new simulation option incorporates spatial daylight autonomy (sDA) and annual sunlight exposure (ASE), based on computer simulations.

All versions of the Daylight credit contain a respective field measurement compliance option.

Daylight vs Energy

The emergence of design performance modeling tools – like Sefaira and AGi32 – will prompt designers to consider dynamic daylight metrics in concert with key energy metrics, like Energy Use Intensity (EUI), to balance daylighting effectiveness and energy performance. There is definitely a trade-off between daylight and energy. However, the emergence of dynamic daylighting metrics should help project teams to find the balance between daylighting and energy more effectively than ever before.

Comments are closed.