Daylight Simulation Services (sDA / ASE)

Globally recognized for its profound impact on human health, natural light reinforces circadian rhythms and significantly boosts occupant productivity. From a sustainability perspective, it reduces reliance on electric lighting, directly contributing to energy conservation.

Within the LEED v4 rating system, optimal daylight performance is not just encouraged—it is rigorously quantified through the EQ Credit: Daylight. For project teams aiming for high-performance certification, understanding the nuances of daylight simulation is essential.

Understanding the LEED Daylight Criteria

The LEED EQ Credit: Daylight offers projects up to three points (depending on the specific rating system and option pursued). To earn these points, project teams must quantitatively demonstrate that interior spaces achieve a balance: receiving sufficient natural light without introducing visual discomfort.

The primary compliance path involves sophisticated annual computer simulations utilizing two critical metrics developed by the Illuminating Engineering Society (IES): Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE).

The Core Metrics: sDA and ASE Explained

Successful daylight simulation requires a "Goldilocks" approach: finding the sweet spot between too little light and too much glare.

1. Spatial Daylight Autonomy (sDA)

The measure of sufficiency

sDA calculates the percentage of the regularly occupied floor area that receives enough daylight to work by without electric lighting. Specifically, it measures if an area meets or exceeds a minimum illuminance level (e.g., 300 lux) for a specified fraction of operating hours (e.g., 50% of the time) annually.

LEED Targets (BD+C):
2 Points: sDA (300/50%) ≥ 55%
3 Points: sDA (300/50%) ≥ 75%

2. Annual Sunlight Exposure (ASE)

The measure of potential discomfort

ASE is the companion metric designed to flag glare risks. It reports the percentage of the analysis area exposed to excessive direct sunlight (exceeding 1,000 lux) for more than 250 hours per year.

LEED Limit:

To comply, ASE (1000, 250) must generally remain below 10% of the regularly occupied floor area.

How is Daylight Simulation Performed?

This is not a simple estimation; it requires rigorous, performance-based analysis using specialized simulation software.

Weather Data

Hourly time-step analysis based on local, site-specific weather files (e.g., TMY).

Grid Precision

Calculations on a precise grid (max 2 feet / 600 mm square) across occupied areas.

Work Plane

Analysis set at a standard work plane height of 30 inches (760 mm).

Geometry

Must include all permanent interior obstructions that might block or redirect light.

Glare Control: A Critical Design Requirement

Because a high ASE value indicates a risk of visual discomfort, LEED places a heavy emphasis on glare mitigation. Even with good simulation numbers, functional design is mandatory.

LEED requires the provision of manual or automatic glare-control devices for all regularly occupied spaces.

Acceptable Devices

Interior window blinds, shades, curtains, or movable exterior louvers that allow occupant control (manual override is required for auto-systems).

Unacceptable as Sole Control

Fixed architectural elements—such as dark glazing, exterior fins, or patterned frits—are factored into the simulation math, but they do not count as sufficient glare control devices on their own.

The Synergy with Views and Energy Modeling

Daylight simulation does not happen in a vacuum; it influences other critical LEED credits.

Relationship with View Quality

Strategies that increase daylight (like larger vision glazing) often support the EQ Credit: Quality Views. However, there is a trade-off. Glazing selected for daylighting must preserve a clear image of the exterior. Heavy frits or patterns used to control sDA/ASE might render a window ineligible for the Views credit if they distort the color balance or obstruct clarity.

Integration with Building Energy Modeling (BEM)

Daylight results directly inform the EA Prerequisite: Minimum Energy Performance.

The BenefitEffective daylighting lowers electrical lighting loads via daylight sensors.
The RiskHigh solar heat gain from excessive glazing can increase cooling loads.

Therefore, architectural and lighting choices must be evaluated holistically. The goal is to maximize daylight gains without undermining the building's overall energy efficiency.

Conclusion

Achieving high marks in daylight simulation is a testament to an integrative design process. It confirms that a project delivers superior indoor environmental quality for its occupants while robustly supporting broader energy conservation and climate goals.

Illuminating Performance:
A Technical Guide to Daylight Simulation in LEED v4