Daylight harvesting is an integral part of sustainability as it enables building occupants to make use of natural daylight which reduces the demand of electrical lighting, reducing total energy consumption. Daylight metrics are tools to optimize daylight harvesting systems and estimate useable daylight in a given area. This article highlights a few daylight metrics available in the industry today and how they work.
1. Daylight Factor (DF)
Daylight Factor is the most dominant daylight metric used today due to its simplicity ¹. Daylight Factor is the ratio of the light level inside a building to the light level outdoors during an overcast sky condition. The equation below shows how it is calculated:
Using the equation above with an internal lux of 400 and outdoor lux of 20000 would equate the daylight factor to 2%. Currently, most rating tools adopt this metric for daylight assessment due to its simplicity while producing reasonably accurate results. Rating tools that use this metric are GBI, GreenMark and many others.
2. Useful Daylight Illuminance (UDI)
UDI is one of the three dynamic daylight metrics that we will be analysing. Dynamic daylight metrics are location-based and use actual weather data to produce hourly annual results (hourly daylight performance over the course of a year) ². As you can tell, dynamic daylighting metrics are much more accurate as it takes actual climate data into account in the calculations.
UDI measures daylight lux levels that are deemed ‘useful’ for occupancy use. The daylight illuminance are categorized into 4 ranges that are noted below ³:
- Daylight illuminances below 100 lux are considered insufficient for occupant use.
- Daylight illuminances between 100 to 500 lux are considered sufficient for use however may require artificial lighting for certain tasks.
- Daylight illuminances between 500 to 2000 lux are considered as desirable lighting levels for occupant use.
- Daylight illuminances above 2000 lux are not deemed ‘usable’ as it is likely to produce visual and thermal discomfort to the occupant.
Based on the information above, daylight illuminance that occur within the 100 to 2000 lux range are considered ‘useful’ to the space. UDI measures the percentage of space that receives daylight levels within this range for 50% of the total occupancy hours a year. It is recommended that at least 80% of the space receives useable daylight ⁴. This metric is a requirement in the Malaysian standard MS2680.
The above image is an example of a simulated room using UDI parameters where 81% of the room is deemed to receive “useable light”, for at least 50% of the total occupancy hours. Based on the UDI metric, the room receives sufficient daylight throughout the year and further design measures can be taken for areas in the room that receives too much or too little daylight.
3. Spatial Daylight Autonomy (sDA)
Spatial Daylight Autonomy (sDA) is a dynamic daylight metric that measures the amount of space that receives sufficient daylight. More specifically, it examines the percentage of space that receives 300 lux for at least 50% of the total operational hours measured at working plane ⁵. What sets sDA apart from other dynamic daylight metrics is the incorporation of window blinds. These blinds are set to close and open depending on the amount of direct sunlight the space receives. According to LM-83-12, blinds shall close whenever more than 2% of the space receives direct sunlight (1000lux), this is known as the 2% rule ⁶. sDA is used as a daylight scoring credit in the LEED v4 rating tool and requires at least 55% of the total space to receive 300lux and above for at least 50% of the total operational hours.
In the case above, 65% of the room receives 300 lux and above for at least 50% of the total operational hours. This implies that the room receives sufficient daylight for occupancy use throughout the year and complies with the minimum floor space required for sDA by LEED v4.
4. Annual Sunlight Exposure (ASE)
Annual Sunlight Exposure (ASE) is a dynamic daylight metric that measures the amount of space that receives too much daylight. ASE is made to complement sDA by providing an upper threshold for a building’s useable daylight levels. As you might have noticed, sDA only measures the minimum lux levels required for a space where such lux levels could be too high and produce glare and heat to the space. ASE measures the percentage of space that receives more than 1000 lux for at least 250 hours in a year. In order to design an efficient space in terms of lighting, achieving a high sDA to ASE ratio (high sDA and low ASE value) ensures that the space receives plenty of useable daylight so that occupants need not rely on artificial lighting and at the same time not turn on the air conditioning due to excess heat from daylight levels that are too high. According to LEED v4, to score credit points in this area, a space needs to maintain ASE of below 10%, which means less than 10% of the total space receives more than 1000 lux for 250 hours or more. Both sDA and ASE have to be met in order to score credit points for the daylighting criteria in LEED v4. As of today, LEED v4 is the only rating tool that adopts dynamic daylighting into its rating tool.
The image shows the same room with 8% ASE, which is below the maximum allowable ASE value of 10%. The simulation suggests that 8% of the room would receive daylight that is above the recommended lux levels (1000 lux) and would produce thermal discomfort and glaring. However, it meets the daylight requirement of LEED v4 which requires an ASE value of less than 10%.
As mentioned previously, sDA and ASE are used together as they provide the lower and upper limit of the daylight range that is deemed acceptable for occupant use. In the example used, 57% of the total space receives sufficient daylight and this value is calculated by subtracting ASE from sDA.
Conclusion
Each daylight metric serves to determine the percentage of space in the building that will receive sufficient daylight in order to aid architects and engineers to design buildings and artificial lighting as efficient as possible. The harvesting of daylight reduces the consumption of artificial lighting which in turn saves energy and electricity cost of the building, pushing the building a step further into being sustainable. Now that you’ve delved a little deeper into the world of daylighting, our next article will feature a real case study simulation of the mentioned daylight metrics to see the actual difference in results. Stay tuned!
References:
1) T. CK and N. Chin, Building Energy Efficiency Technical Guideline for Passive Design. Kuala Lumpur: Building Sector Energy Efficiency Project (BSEEP) Malaysia, 2013, p. 74.
2) “Measuring Daylight: Dynamic Daylighting Metrics & What They Mean for Designers”, Sefaira, 2018. [Online]. Available: http://sefaira.com/resources/measuring-daylight-dynamic-daylighting-metrics-what-they-mean-for-designers/. [Accessed: 17- Jan- 2018].
3) A. Nabil and J. Mardaljevic, “Useful daylight illuminance: a new paradigm for assessing daylight in buildings”, Lighting Research & Technology, vol. 37, no. 1, pp. 41-57, 2005.
4) C. Patherns, “EFA Daylight Design Guide”, Newcastle, 2014.
5) K. Van Den Wymenlenberg and A. Mahic, “Annual Daylighting Performance Metrics, Explained”, Architectural Lighting Technology, 2016. [Online]. Available: http://www.archlighting.com/technology/annual-daylighting-performance-metrics-explained_o. [Accessed: 17- Jan- 2018].
6) “Daylight Terms”, Daylight Deconstructed, 2018. [Online]. Available: https://blog.lightstanza.com/daylight-terms/. [Accessed: 17- Jan- 2018].
