Daylight analysis is a key aspect of sustainable building design, ensuring adequate natural light in indoor spaces, reducing dependency on artificial lighting, and enhancing occupant comfort and productivity. Various metrics are used globally, and different green building certifications prescribe specific methods to evaluate daylight performance.
Key Daylight Metrics
1. Illuminance Method (IGBC Requirement)
- Definition: Measures the lux levels at the work plane (0.75 m above floor level) under standard sky conditions.
- Application in IGBC:
- For regularly occupied spaces, a minimum of 110 lux for 90% of occupied hours is required.
- Compliance can be demonstrated via simulation (climate-based sky model) or on-site measurement.
- Advantage: Simple, practical, and aligns with functional daylight sufficient for building occupants.
2. Daylight Autonomy (DA)
- Definition: Percentage of occupied hours when illuminance exceeds a threshold (commonly 300 lux).
- Strength: Climate-based, accounts for dynamic daylight availability.
3. Continuous Daylight Autonomy (cDA)
- Definition: Continuous Daylight Autonomy (cDA) is a refined version of Daylight Autonomy (DA). While DA counts only the hours when illuminance on the work plane meets or exceeds a threshold (e.g., 300 lux), cDA gives proportional credit for hours when the daylight is below the threshold but still contributes partially.
4. Useful Daylight Illuminance (UDI)
- Definition: Percentage of time when daylight illuminance falls within a useful range (100–2,000 lux).
- Application:
- Helps assess both sufficiency and glare/overexposure.
- Increasingly used in GRIHA daylight studies.
5. Spatial Daylight Autonomy (sDA)
- Definition: Percentage of floor area that achieves ≥300 lux for at least 50% of annual occupied hours.
- Application in LEED, WELL:
- sDA ≥ 40%, 55% or 75% earns points.
- Strength: Evaluates usable daylight across large areas, not just single points.
6. Annual Sunlight Exposure (ASE)
- Definition: Percentage of floor area receiving >1000 lux of direct sunlight for more than 250 occupied hours annually.
- Application in LEED:
- ASE ≤ 10% is required to minimize glare and overheating.
- Role: Complements sDA to balance daylight quality and comfort.
Daylight Metrics in Green Building Certifications
- IGBC: Uses Illuminance Method (110 lux requirement on workplane, 90% occupied hours).
- LEED (v4 & v5): Uses sDA (≥40%, 55% or 75%) and ASE (≤10%).
- GRIHA: Uses illuminance levels and daylight sufficiency targets.
- WELL: Focuses on sDA, glare control to improve occupant health and visual comfort.
Conclusion
While international standards such as LEED and WELL emphasize climate-based metrics (sDA, ASE, DA, UDI) GRIHA standards and IGBC specify the Illuminance Method as per its guidelines for practical and consistent assessment. By adopting these metrics, projects can optimize daylight performance, reduce energy consumption, and enhance occupant well-being, while also earning valuable points in green building certification systems.