The sDA Metric: Is this a sufficiently daylit space?
Welcome to the reboot of our series on Spatial Daylight Autonomy (sDA), where we will be discussing sDA as a new, annual metric for a more accurate measure of daylight. In Part 1 of our series we will explain what an annual metric is, the difference between how much daylight a space is getting versus the portion of daylight that is usable, and why this distinction is important.
An annual metric is a function of hourly simulation results across an entire year in conjunction with climate analysis. The climate analysis is based on typical meteorological year (TMY) climate data. This type of continuous (and intuitive) analysis of a space can remove the uncertainties found in analyses that only evaluate a single point in time by accounting for hourly and seasonal changes in daylight availability and sun angle. LEED v4 prioritizes annual metrics for daylight credits, which is an improvement over the 2 solar positions simulated for LEED 2009 and the single ratio used for Daylight Factor. Although, LEED v4 does still offer a second option of 2 solar positions, excluding blinds, if sDA scores are too low.
While there are multiple annual metrics, sDA is the first one to use hourly daylight measures in conjunction with manual blind operation use. As pictured below, blinds are used by building occupants to control glare and maintain visual comfort, so acknowledging occupants’ influence on available daylight is crucial for calculating an accurate score.
By not accounting for blind use, annual metrics other than sDA are overestimating the amount of usable light and underestimating energy consumption. The image below shows that even with available daylight, electricity is often used in place to avoid direct sunlight. This not only increases use of electricity, but it also negates window benefits by allowing heating and air conditioning to escape when we’re not using the windows for daylight.
If your software doesn’t show blinds operating, your score is inaccurate. It is necessary to use a dynamic software simulation that accounts for how occupants may interact with their daylit environment: after all, what do lighting levels mean without considering the people experiencing them?