Daylighting Compliance for ECBC
What is Daylighting?
Daylighting is the controlled admission of natural light, direct sunlight, and diffused-skylight into a building to reduce electric lighting and saving energy. By providing a direct link to the dynamic and perpetually evolving patterns of outdoor illumination, daylighting helps create a visually stimulating and productive environment for building occupants, while reducing as much as one-third of total building energy costs.
What is Useful Daylight Illuminance (UDI) ?
Useful Daylight Illuminance (UDI) is the annual occurrence of illuminances that is within a “useful” range for occupants. Four UDI ranges are defined:
- Insufficient UDI (e.g. below 100 lux) represents lighting levels that are considered insufficient without electric lighting.
- Supplementary UDI (e.g. 100 lux – 500 lux) represents acceptable daylight levels to be integrated with electric lighting.
- Autonomous UDI (e.g. 500 lux – 2,500 lux) represents acceptable daylight levels. Electric lighting would not be needed for the majority of the day. Achieving a high percentage of Autonomous UDI represents a predominantly daylit space during occupied periods and glare is controlled.
- Exceedance UDI (e.g. above 2,500 lux) represents excessive amounts of daylight and a source of glare. This creates an expectation that blinds would be required.
ECBC Daylight Compliance
Useful Daylight Illuminance of spaces shall meet or exceed the daylight requirements provided for different building types. Compliance can be demonstrated either through Daylight simulation or by Manual daylight calculation method.
Following are the UDI requirements for different types of buildings:
1. Daylight Simulation Method
Illuminance levels for all spaces enclosed by permanent internal partitions (opaque, translucent, or transparent) with height greater or equal to 2 m from the finished floor, shall be measured as follows:
a) Measurements shall be taken at a work plane height of 0.8 m above the finished floor.
b) Period of analysis- 8 hrs per day, 8am to 5pm, that results to 2,920 hrs for all building types except schools. Schools’ period of analysis- 7 hrs per day.
c) Available useful daylight across a space shall be measured based on point-by-point grid values. UDI shall be calculated for at least one point for each square meter of floor area.
d) Fenestration shall be modeled with actual visible light transmission (VLT) as per the details provided in the material specification sheet.
e) All surrounding obstructions to be modeled if the distance of the building from obstruction is less than or equal to twice the surrounding obstruction.
f) Interior surface reluctance shall be modeled as per following:
BEE approved softwares for demonstrating Daylight compliance
UDI Analysis with a Daylighting Analysis Software
2. Manual Daylighting Compliance Method
This method can be used for demonstrating compliance with daylighting requirements without simulation. Daylight extent factors (DEF) mentioned in Table 4-3 shall be used for manually calculating percentage of above grade floor area meeting the UDI requirement for 90% of the potential daylit time in a year.
A 7,200 m2 four story office building in Delhi is trying to achieve ECBC level compliance. Building is oriented along east west axis. It has a rectangular layout (60 m x 30 m). Total built up area is distributed evenly across all floors above grade. VLT of glazing in all orientations is 0.39. Windows have light shelves and external shading devices with PF ≥ 0.4. Head height of fenestrations is 3.0 m. Length of glazing on the north and south facing façade is 45 meter and on the east façade, 25 meter. Table 4-1
This approach will be suitable for projects adopting the prescriptive compliance approach. From Table 4-3 determine the daylight extent factor (DEF) for the building. For a building located in Delhi (latitude > 15 degrees), with glazing of VLT ≥ 0.3, shading PF ≥0.4 shading and light shelves in windows, DEFs for windows in North = 3.5, in South = 3.0, in East = 2.1, and in West = 1.8. Head height is 3.0 m. There are no opaque partitions adjacent to the external walls and windows are arranged in a continuous strip.
4,346 m2 of area will meet the UDI requirements. This is 60.3 % of the total above grade floor area of 7,200 m2. Thus, the building will comply with UDI requirement.