What is Daylight Autonomy (DA)?
What is spatial Daylight Autonomy (sDA)?
Daylight assessment through simulation is a well-known and the most accurate method used in the design stage. There are several criteria upon which the evidence of a well-daylit space is based upon, one of which, is Daylight Autonomy (DA) and it represents:
“The percentage of annual daytime hours that daylight levels exceed a predefined illumination level”
As an example, if a specific location in an office receives more than 300lux of light over 68% of annual daylighting hours, then the DA for that specific location is 68%.
Note that this metric doesn’t include the activity and actual demand for natural light. In other words, two buildings with similar geometries but with different activities will have the same DA values in a specific location. Although DA is a good metric, it is not sufficient, especially for commercial buildings; after all, what’s the use of daylight when no one is around to use it for day to day tasks? Hence, DA is good for residential buildings, but not so good for commercial buildings. And that’s where Spatial Daylight Autonomy (sDA) comes into action! sDA could be described as:
“The percentage of the floor area of a space, where daylight levels exceed a predefined illumination level, for a specific percentage of operation hours (i.e. workdays)”
For example, if 58% of the floor area in an office, receives more than 200lux across 70% of annual workdays, then the sDA for that office would be 58% (DA300lux[70%]=58%).
What are the differences between DA & sDA?
Comparing the above two metrics, we can conclude that they differ in three aspects:
1- Location: DA requires a specific test point, but sDA requires the whole space as a test location.
2- Annual hours: DA considers annual hours when the natural light is available, but sDA considers only the working hours regardless of daylight availability.
3- Outcome: aiming to meet a specific illuminance level, DA outcome is a percentage of annual hours, but sDA outcome is a percentage of floor area.
Do we undertake sDA assessment in Australia?
Green Star Rating is a sustainability rating system in Australia. If you are trying to achieve a Green Star 4-star rating or more, as evidence for a sustainable design, then you may need to achieve Green Star Credit Achievement for Light Quality. According to Green Star Buildings Submission Guidelines:
For non-residential buildings, at least 40% of the regularly occupied areas across the building must receive high levels of daylight with no less than 20% on any floor or tenancy (whichever is smaller)…
Daylight must be calculated using Daylight Autonomy
[Green Star Building Submission Guidelines, Version 1 (Revision A), July 2021, Page 85]
According to the guideline, high levels of daylight are deemed to have at least 160 lux due to daylight during 80% of the nominated hours. Also, according to US Green Building Council, one of the options to address a well-daylit space is through sDA assessment. The council requires a demonstration through annual computer simulations that sDA300/50% (300lux for 50% of operational hours) of at least 55%, 75%, or 90% is achieved.