LEED and Daylighting

The LEED rating system of the U.S. Green Building Council (USGBC) has always recognized the significance of daylighting in buildings, and has listed it as a credit option under the general category of Indoor Environmental Quality.

Daylighting has also been linked to the support of other prerequisites and credits such as Minimum and Optimized Energy Performance, Quality Views, and Interior Lighting. LEED v4 contains significant updates and changes to the process of demonstrating how daylight is effectively used in buildings. The first distinction is that different point levels are available based on the particular building type or LEED program being used (i.e. LEED for Healthcare, Schools, etc.). The second distinction is that the previous prescriptive compliance-path option has been eliminated, meaning that computer simulation, calculation, or measured data are required in order to illustrate the performance benefits of daylight in a building.

LEED v4 still encourages building designers to maximize daylighting by setting daylighting goals, considering site and massing opportunities, and configuring the building for the best exposure to natural daylight. Credit is based on the availability of daylight to the regularly occupied spaces, which are distinguishable from other spaces such as utility rooms, rest rooms, or storage locations. Glare-control devices (manually operated or automatic with a manual override) are required for all regularly occupied spaces.

In order to demonstrate and document how daylight performs in a building seeking LEED certification, there are three compliance paths available to design teams. One of these three available options must be selected and used as follows:

Computerized Simulation

This first compliance-path option is new for LEED v4 and allows the use of an Illumination Engineering Society (IES) based computerized daylight simulation utilizing hourly and annual daylight data.

The simulation program must be able to generate spatial daylight autonomy (sDA) ratios. The sDA is a metric describing annual sufficiency of ambient daylight levels in interior environments. It is defined as the percentage of an analysis area that meets a minimum daylight illuminance level for a specified fraction of the operating hours per year. Essentially, that means the computer program must determine the simulated daylight levels across a standardized grid series of points located no further apart than 2 square feet and at a work plane height of 30 inches. The light must be simulated in the computer model at different times of the day and of the year. It is measured in lux value, with a minimum threshold of 300 lux of daylight necessary in order to be considered effective. The light level must also be present for at least 50 percent of the hours between 8 a.m. and 6 p.m. local time at each measurement point. This threshold level is abbreviated sDA300/50% and is the first basis for earning daylighting credit under this compliance path.

The more measurement locations that meet or pass this threshold, the more credit points that can be earned. For most buildings, if the simulation shows that 55-74 percent of the locations in regularly occupied spaces meet or exceed the sDA300/50% threshold, then two points can be achieved. If 75 percent or more is demonstrated, then three points can be earned. For healthcare projects, the basis of the simulation is focused upon perimeter areas of the building, as defined within LEED for Healthcare. In these cases, a total of 75 percent of the measurement locations meeting the sDA300/50% threshold can earn one point, while a total of 90 percent of the measurement locations meeting the sDA300/50% threshold can earn two points.

In addition to the sDA portion of the simulation, a second criteria must be met—namely that of determining whether the daylight is causing unwanted glare. To make that determination, annual sunlight exposure (ASE) is measured across the same grid locations as the sDA simulation. Under the LEED guidelines, a maximum of 1,000 lux of daylight is permitted without the use of any operable shades, blinds, etc. The time portion of this measurement is not a percentage, but a stated number of hours over the course of the year, which LEED defines as no more than 250 hours annually. Thus the value ASE1,000/250 is the upper limit for demonstrating usable daylight in the occupied spaces of the building. In order to earn the daylighting credit points, the ASE threshold must not be breeched for more than 10 percent of the area illuminated by daylighting.

The process of performing this type of computerized simulation may require specialized software as well as the assistance of a daylighting professional. There are commonly five steps involved. First, the needed simulation inputs must be identified based on the building design, including its location, geometry, permanent light obstructions, glass and glazing specifications, surface reflectances of interior finishes, etc. With this information in place and the grid of measurement locations identified, the next step is to run the sDA simulation for the regularly occupied spaces. The processes detailed in IES publications and standards must be followed appropriately. Once the results are obtained, the third step is to evaluate the sDA values for compliance against the credit requirements. If the percentage does not meet the targets necessary to earn credits, then some aspects of the building design will need to be revised to improve performance. The simulation and evaluation steps are then repeated as needed until sDA targets for minimum daylight levels are attained. Determination of ASE is the fourth step and may require minor adjustments to the sDA simulation model, again according to IES guidelines. When ready, the ASE simulation can be performed and the outputs generated. The final step is to evaluate that the ASE upper limit is not breeched for any more than 10 percent of the areas being daylit. Otherwise, further iterations of design revisions, simulation, and evaluation will be necessary.

Photo courtesy of Guardian; Photo credit: Vos Glass

The Gordon Food Service Headquarters building uses high-performance glass to achieve daylight, energy efficiency, and other LEED benefits.

Illuminance Calculations

The second compliance-path option is also based on a computer simulation, but requires calculations only for specific dates and times. It is similar to previous versions of LEED, but LEED v4 now requires that site-specific daylight illuminance values form the basis of the simulation along with local climate and weather files. Under this compliance path, a space is considered daylit if the simulation shows an illuminance level between 300 and 3,000 lux on a clear-sky day.

The weather data should be selected from the clearest sky conditions within 15 days of September 21 and within 15 days of March 21 (the two equinox dates). The average hourly values of the two dates shall be used for two simulations—one run for 9 a.m. and the other for 3 p.m. The results must present the percentage of regularly occupied floor area, which meets the daylight criteria within the range of lux values stated above. For all buildings, if 75 percent of the floor area meets this calculated level, then one credit point is earned. If 90 percent of the floor area is demonstrated as daylit, then two credit points are earned. Note that this compliance path does not provide an opportunity to earn three points as in the other two paths.

The process of using this compliance path requires three basic steps. First, as with the sDA simulation, all of the simulation inputs related to the building design, materials, and surfaces needs to be collected and input along with the local climate and weather files. Once ready, then the next step is perform the two point-in-time simulations for the 9 a.m. equinox conditions and the 3 p.m. equinox conditions. When the results are determined, then the final step is to evaluate the floor areas where illuminance compliance is demonstrated and where it may not be. If the total percentage of qualified daylit floor area is less than the target for LEED credit, then further iterations of the design will be needed and the simulation/evaluation steps repeated until the goal is achieved.

Light Measurements

The third compliance-path option is intended for completed new buildings or existing buildings which are being renovated. Rather than a computer simulation, this path draws upon measurements from light meters within the building. The daylighting requirements are similar to those of the second compliance option, necessitating illuminance levels of 300 to 3,000 lux taken on a given day between 9 a.m. and 3 p.m. Yet instead of requiring equinox measurements, this path permits measurements from any two days of regular operation during the year, as long as the days are five–six months apart. In order to earn two credit points, at least 75 percent of the regularly occupied floor areas must meet the daylighting criteria (only one point awarded in healthcare). If 90 percent of the floor area meets the criteria, then three points can be earned (two points in healthcare).

The process for using this compliance-path option involves three basic steps. First, preparation is needed involving the establishment of a grid of measurement locations for all regularly occupied spaces based on IES standard guidelines. This grid is typically marked on a floor plan. The spaces must be accessible, and all furnishings must be in place. Electrical lighting must be off during the measurements. Next, the actual measurements are recorded based on stated IES procedures. Note that measurements must be taken on days of regular operation, not during breaks or normal shutdown times. All measurements should be taken at the standard 30-inch height above the floor. In the event that measurement can’t be completed in one day, the following day may be used as long as it is during the same 9 a.m. to 3 p.m. time frame. Finally, the results are evaluated. If the building falls short of the target, then modifications will be necessary in advance of further measurement. After the first compliant measurement set, a second set of measurements will be needed five to six months later. The two sets of data are then averaged to demonstrate compliance.