From 10% to 20%: Influence of Annual Sunlight Exposure Threshold on the Acceptability of Daylighting Performance under Different Sky Conditions

The Leadership in Energy and Environmental Design (LEED) v4 daylight credit criteria were adjusted, and the Annual Sunlight Exposure (ASE) threshold was increased from 10% to 20%. The ASE < 20% threshold was also implemented by the Illuminating Engineering Society in the recent lighting measurements standard in 2023. However, the implications of this change across diverse sky conditions remain unclear. Simulations for a typical office space across 32 cities (15°N to 65°N) considered various sky conditions, encompassing 360° orientations and 5%–100% window-to-wall ratios (WWRs). Implementing the ASE < 20% threshold increased accepted daylighting cases from 21% to 52%. Spaces with WWRs between 85% and 100% and oriented between 140°S and 230°S now consistently achieve acceptable daylighting performance in all tested cities, compared to those oriented between 150°S and 210°S, when ASE is ≤10%. The new threshold resulted in greater sensitivity to the sky conditions, aligning more closely with the city’s latitude. Applying both the old and new thresholds shows that spaces with WWRs less than 20% do not achieve acceptable daylighting, regardless of orientation or location. The ASE < 20% appears to favor cities at lower latitudes, with a 168%–260% increase in accepted cases for cities closer to the equator (latitude 15°N to 35°N), compared to a 125%–150% increase for cities further north (latitude 45°N to 65°N). This is related to the spatial Daylight Autonomy limitations imposed by the sky conditions of the northern locations. This disparity suggests that the LEED daylight credit criteria may be more difficult to achieve in higher latitude locations. It is suggested that the LEED credit criteria could be reconsidered in the future for those locations so that they become more sensitive to the differentiated sky conditions in various latitudes.