This article covers domestic and international guidelines for daylight-related metrics in schools, beyond the default sDA (300lux/50%) and ASE (1000, 250) calculated in the 3D Analysis tool. We employ sDA because it is the most widely used metric, does a good job of understanding daylight quality for both residential and commercial buildings, and is location/context specific.
Daylight Analysis Metrics
Daylight is essential to connect building occupants with the outdoors, reinforce circadian rhythms, and reduce the use of electrical lighting. The usual daylighting goals include achieving some predefined daylight illuminance level on the work plane or at the floor, incorporating some measure of glare control, or delivering a daylight zone of a certain size. Achieving daylight autonomy essentially requires a project to achieve all of the above and more (source).
Daylight Autonomy (DA) is defined by the IES (Illuminating Engineering Society of North America) as “the percentage of the operating period (or the number of hours) that a particular daylight level is exceeded throughout the year.” The most widely accepted method to measure annual daylight autonomy is Spatial Daylight Autonomy (sDA). When combined with Annual Sunlight Exposure (ASE) it provides a clear understanding of how much beneficial daylight is entering into a space. In general, designers should strive for high sDA values while implementing strategies to lower ASE values, and strive to lower ASE values without sacrificing sDA.
Some other daylight-related metrics which are not calculated by cove.tool include:
Illuminance is a photometric term that quantifies light incident on a plane or a surface and can include contributions from electric light and daylight. LEED v4.1 requires demonstrating illuminance values between 300 lux and 3,000 lux (28 ft.cd – 279 ft.cd). Both horizontal and vertical illuminance targets are recommended to ensure adequate illumination and safety for occupants of various ages.
Useful daylight illuminance (UDI) refers to the amount of daylight that is useful to the occupants i.e. the level of daylight that can be introduced into space without causing glare or disrupting the visual environment. The acceptable level is typically 500 lux – 2,500 lux, also referred to as Autonomous UDI. Achieving a high percentage of Autonomous UDI represents a predominantly daylit space during occupied periods and glare is controlled. It implies that electric lighting would not be needed for the majority of the day.
Daylight Factor (DF) is the ratio of the illuminance at a point on a plane in a room due to the light received from a sky of assumed or known luminance distribution to that on a horizontal plane due to an unobstructed hemisphere of this sky. Direct light is excluded from both values of illuminance, so a cloudy sky is modeled. It has limitations as DF calculations provide the same results regardless of the time of day, orientation, or location. DF is expressed as a percentage. Generally considered an outdated standard.
Daylight Saturation Percentage (DSP) is a modification of Useful Daylight Illuminance (UDI) that increases the thresholds to 430 to 4,300 lux (40 ft.cd - 400 ft.cd). It goes further to penalize grid point annual hour values above 400 footcandles by forcing them to be subtracted from the grid point annual hour values within the range of 40-400 footcandles. This metric was developed by CHPS in 2006.
Daylight glare probability (DGP) quantifies occupants’ perceived glare from daylight at a point-in-time, position in space, and occupant view angle. It is calculated by evaluating the entire visual field of a potential occupant, taking into account the light intensity, size of the glare source, contrast, and its position in the field of view. Expressed from 0 to 1, it ranges from Imperceptible Glare (<0.35), Perceptible Glare (0.35-0.4), Disturbing Glare (0.4-0.45) to Intolerable Glare (>0.45).
Green Building Standards and Daylight for Schools
Various standards use a combination of daylight availability, glare control, and daylight controls (automatic and manual) to quantify daylight within a space. This table outlines the thresholds identified by a few building standards and green building certifications. Metrics with the asterisk (*) symbol are currently not possible within the 3D Analysis tool.
LEED v4.1 - Schools
An average value of at least 55% (2 pts), 75% (3 pts), 40% (1 pt) for the regularly occupied floor area
ASE (1000, 250)
No more than 10% of the area can receive more than 1,000 lux (93 fc) for 250 hours each year
WELL Building Standard v3
Average value of at least 55% at least 55% for regularly occupied space
ASE (1000, 250)
No more than 10% of the area can receive more than 1,000 lux (93 fc) for 250 hours each year
An average of 175 lux (16 fc) or more measured on the horizontal plane 0.76 m (30 inches) above the finished floor
Title 24 - 2019 Building Energy Efficiency Standards
Daylit Zones* (Skylit, Primary sidelit, Secondary sidelit)
75% of the floor area within the primary side-lit daylit zone or skylit daylit zone
US - Collaborative for High-Performance Schools (CHPS)
Daylight Excess* (Annual DA and DAmax)
Illuminance target multiplier of 10x (i.e., a 30fc classroom target would use 300fc for Daylight Excess)
Spatial Daylight* Saturation (sDS)
Greater than 50 to 90% sDS 300/50%
UK - Education Funding Agency (EFA)
Useful Daylight Index* (UDI)
An average of 80% UDI-a (100 to 3,000 lux)
Daylight Autonomy* (DA)
Minimum 50% of the time for 50% of the working plane, with a target illuminance of 300 lux
Daylight Factor* (DF)
Average 1.5 to 2.25 (depending on latitude) for 60 to 80% area
A ratio of at least 0.3, at least 80% of the room has a view of the sky from desk or tabletop height, and room depth criteria to be satisfied
Daylight illuminance* - average and minimum
At least 300 lux for 2000 hours per year or more, and at least 90 lux for 2000 hours per year or more
This section breaks down the details in each of the standards mentioned before, with their specific requirements.
01// LEED v4.1 - Daylight
As part of the Indoor Environmental Quality section, LEED v4.1 has three options for measuring daylight:
Option 1. Simulation: Spatial Daylight Autonomy and Annual Sunlight Exposure - Perform annual computer simulations for spatial daylight autonomy300/50% (sDA300/50%), and annual sunlight exposure 1000, 250 (ASE1000,250 ) as defined in IES LM-83-12 for each regularly occupied space.
Option 2. Simulation: Illuminance Calculations - Perform computer simulations for illuminance at 9 a.m. and 3 p.m. on a clear-sky day at the equinox for each regularly occupied space. Demonstrate illuminance levels are between 300 lux and 3,000 lux at both 9 a.m. and 3 p.m
Option 3. Measurement - Measure illuminance in each regularly occupied space. Healthcare projects should use the regularly occupied spaces located in the perimeter area determined under EQ Credit Quality Views. Achieve illuminance levels between 300 lux and 3,000 lux.
02// WELL Building Standard v3 - Daylight modeling
This feature requires that individuals inside a building receive ample exposure to natural sunlight and allows designers versatility for a variety of layouts and daylighting designs.
62: Daylight modeling - Part 1: Healthy Sunlight Exposure
sDA - at least 55% of the space receives at least 300 lux (28 fc) of sunlight for at least 50% of operating hours each year.
ASE - no more than 10% of the area can receive more than 1,000 lux (93 fc) for 250 hours each year.
53. Visual Lighting Design - Part 5: Visual Acuity for Learning
The ambient lighting system at desks in classrooms for the specified age groups meet the following requirements:
Early education, primary and secondary schools, and adult education for students primarily under 25 years of age: Able to maintain an average of 175 lux (16 fc) or more measured on the horizontal plane, typically 0.76 m (30 inches) above the finished floor. The lights may be dimmed in the presence of daylight, but they are able to independently achieve these levels.
03// California – Title 24 - 2019 Building Energy Efficiency Standards
These standards are applicable to non-residential buildings throughout California, including schools and community colleges. 2019 Nonresidential Compliance Manual: Chapter 5 – Indoor Lighting.
Daylit Zones are defined as a region of space considered to be close to a source of daylight such as a window, a clerestory, a roof monitor, or a skylight, where luminaires can be dimmed or switched in response to available daylight. The three different types of daylit zones are Skylit, Primary Sidelit, and Secondary Sidelit.
188.8.131.52 Prescriptive Daylighting Requirements
In climate zones 2 thru 15, enclosed spaces larger than 5,000 sq ft. shall have at least
75 percent of the floor area within the primary sidelit daylit zone or skylit daylit zone.
The total skylight area is at least 3 percent of the total floor area in the space within a horizontal distance of 0.7 times the average ceiling height from the edge of the rough opening of the skylights.
04// US - Collaborative for High-Performance Schools (CHPS)
Daylight Excess is a measure of excessive daylight and reports the fraction of annual hours that exceed a certain illuminance threshold at a given point (typically a work-plane point). The maximum acceptable illuminance threshold for spaces with critical visual tasks is considered to be a multiplier of the target illuminance, typically 10x (i.e. a 30fc classroom target would use 300fc for Daylight Excess).
EQ 11.0 – Daylighting: Glare Protection - Optimize daylight while preventing glare by controlling direct sunlight ingress
EQ 11.1 – Daylight Availability - Ensure quality daylighting designs
05// UK - Education Funding Agency (EFA)
Useful Daylight Index (UDI) defined as the annual occurrence of illuminances across the work-plane that is within a range considered ‘useful’ by occupants. The output specification sets a minimum target of an average of 80% UDI-a (100 to 3,000 lux)
Daylight Autonomy (DA) is the amount of time a space can expect to reach a target illuminance level on the working plane. This criterion is aimed at delivering an energy-efficient space. It sets a minimum target DA of 50% of the time for 50% of the working plane, with a target illuminance of 300 lux.
Under the Hea 01 Visual Comfort, the standard outlines criteria for glare control and daylighting. The potential for disabling glare has been designed out of all relevant building areas using a glare control strategy, either through building form and layout or building design measures. Daylighting has the following requirements:
Daylight Factor - Average daylight factor required by latitude (degrees) ranges between 1.5 to 2.2% based on location, for 60% (1 credit) to 80% (2 credits) minimum area (m²) to comply
Daylighting uniformity - A uniformity ratio of at least 0.3 or a minimum point daylight factor of at least 0.3 times the relevant average daylight factor value. At least 80% of the room has a view of the sky from desk or tabletop height (0.7m). In addition, room depth criterion d/w +d/HW < 2/(1-RB) needs to be satisfied.
Illuminance requirements - Average daylight illuminance (averaged over entire space) of at least 300 lux for 2000 hours per year or more. Minimum daylight illuminance at the worst lit point of at least 90 lux for 2000 hours per year or more. Both criteria need for 60% (1 credit) to 80% (2 credits) minimum area ( m² ) to comply.
While spatial daylight autonomy (sDA) and annual solar exposure (ASE) are most commonly used, some standards utilize other metrics to quantify daylight quality for schools. Metrics are likely to evolve as new building regulations become more stringent on energy consumption and considerate about occupant well-being. The building owner and occupants should be made aware of the energy and cost implications of the different measurements and of complying with specific rating systems. By prioritizing occupant visual comfort, the appropriate metrics for a project can be identified.