Achieving daylighting credits for LEED
Learn the steps to achieve points for the U.S. Green Building Council LEED EQc8.1: Daylight and Views – Daylight credit.
- Understand the U.S. Green Building Council LEED criteria for daylighting credit.
- Learn to use modeling software to conduct a daylighting analysis.
- Learn to document and use the results of an analysis.
The use of naturally lighted spaces is a valuable strategy for achieving energy savings and earning U.S. Green Building Council (USGBC) LEED credits. Beyond energy savings, careful use of daylighting can contribute to occupant well-being and productivity. Consequently, architects and engineers are encouraged to improve their ability to apply daylighting strategies to building design solutions.
The USGBC, which administers the LEED green building rating system, has several design elements to achieve points. Daylighting is one of them. The primary standard of reference, of course, is the LEED 2009 Reference Guide for New Construction and Major Renovations.
Under LEED EQc8.1: Daylight and Views – Daylight, the intent is to provide building occupants with a connection between indoor spaces and the outdoors through the introduction of daylight into the regularly occupied areas of the building.
At the risk of making it sound too simple, daylighting design to achieve this credit point for LEED can be boiled down to three steps:
1. Understanding the criteria
2. Using the software
3. Documenting the results.
There are four options available:
Option 1: Simulation
Option 2: Prescriptive
Option 3: Measurement
Option 4: Combination of any of the above methods.
The goal for any of these options is to achieve daylighting in 75% of the regularly occupied spaces.
This article focuses on Option 1, the simulation method, defined by the USGBC as follows:
- Demonstrate through computer simulations that 75% or more of all regularly occupied spaces achieve daylight illuminance levels of a minimum of 25 fc and a maximum of 500 fc in a clear sky condition on Sept. 21 at 9 a.m. and 3 p.m.. Areas with illuminance levels below or above the range do not comply However, designs that incorporate view-preserving automated shades for glare control may demonstrate compliance for only the minimum 25 fc illuminance level.
- View-preserving shades are not part of the study in this article.
Understanding the criteria
Computer software: Physical models are an accurate way to evaluate the effects of daylighting and can be a cost-effective method of analysis. However, software programs are increasingly used to simulate and analyze daylighting effects.
Several daylighting software analysis programs can address factors such as window area, room floor area, window geometry, window height, and the visible light transmittance of glazing and factors such as orientation, room cavity ratios, visible angle to the sky, latitude designation, and interior room reflectance resulting in development of more sophisticated design strategies that demonstrate LEED compliance.
In general, the USGBC does not endorse one software brand over another. Any software that can simulate based on the requirements listed in the rating system is acceptable. The rating system does require that the simulation be done under clear sky conditions. Some of the software products for daylighting analysis are AGi32, Radiance, Lumen Designer, and Daysim.
Regularly occupied spaces: According to USGBC, "Regularly occupied spaces are areas where one or more individuals normally spend time (more than one hour per person per day on average) seated or standing as they work, study, or perform other focused activities inside a building." Some examples are private and open offices, conference rooms, break rooms, and lunch rooms.
"Non-regularly occupied spaces are spaces that occupants pass through, or spaces used in pursuit of focused activities for less than one hour per person per day (on average)." Examples are hallways and foyers.
"Non-occupied spaces are defined as spaces designed for equipment and machinery or storage with no human occupancy except for maintenance, repairs, and equipment retrieval." Examples of these include mechanical rooms, electrical rooms, storage areas, and restrooms.
Sky conditions: In 2003, the International Commission on Illumination (CIE) adopted 15 standard skies that cover a broad spectrum of the usual skies found in the world. Each sky represents a unique sky luminance distribution, which is the most effective way to classify the 15 CIE Standard Skies. Most software brands use Illuminating Engineering Society (IES) and CIE-accepted sky models to calculate the sky’s distribution in the daylight calculations. LEED requires calculations to be done on a clear sky condition (see sidebar “Things to keep in mind” linked below).
Illuminance: Output of the simulation model shall show the illumination level range of 25 to 500 fc on a clear sky condition on Sept. 21 (Equinox), 9 a.m. and 3 p.m. Footcandles shall be measured at 30 in. above the floor (desk height). (In the past, LEED required daylighting calculations to be done at noon, but now they are done at 9 a.m. and 3 p.m. when solar positions are “lower” in the sky, which help projects claim credit point because daylight should penetrate more deeply into the building.)