Case study: Field house lobby
A project currently under construction provides an example of how lighting design does not have to sacrifice quality to meet energy codes.
A community college determined a need for an addition and major renovation of its existing field house. The state in which the project was constructed had adopted the 2015 edition of International Energy Conservation Code (IECC), which prescribed a building area lighting-power density (LPD) of 0.87 W/sq ft for education.
In addition, the project was pursuing U.S. Green Building Council LEED Silver certification under the 2009 edition, which required a building area LPD of 0.99 W/sq ft. However, achieving the minimum energy-performance LEED prerequisite was a challenge for a building in which the owner was keeping mechanical infrastructure and the existing façade in place. The design team selected an LPD target of 0.80 W/sq ft to achieve the various energy requirements.
A design focal point of the project was the building’s multistory atrium, which was to serve as both the main entrance and a gathering space. Based on the initial conversations with the architects during the charrettes at the beginning of the design, the lobby vision was that of an airy 2-story space, with gentile diffuse light throughout. The atrium’s light shelf and clerestory window, at 30 ft above the finished floor, allow natural daylight into the space from above. A design goal of the lighting team was to design a lighting system that would mimic the interaction of the natural light with the interior atrium geometry. The resulting 2-story lobby space exemplifies lighting quality in the built environment, as well as lighting energy management, by using the process and principles discussed earlier to achieve a code-compliant design.
The Illuminating Engineering Society (IES) recommends education lobbies to have an average of 10 to 20 fc. It also recommends 30 fc at reception desks. The lighting-power density (space-by-space method) for a lobby space under IECC 2015 is 0.9 W/sq ft. A direct way to meet these targets is by using downlights or pendant cylinders mounted in a grid layout to uniformly light the space. This simple design would allow the project to meet the energy and illumination requirements outlined above. However, it certainly would not achieve the lighting quality to the built environment that was expected in this project’s focal space.
Lighting design experience and the targets established earlier in the project for both the building as a whole and the individual spaces enabled the engineer to predict that the lobby, as an individual space, would exceed the IECC 2015’s space-by-space LPD requirement of 0.9 W/sq ft. Armed with the information, the engineer made adjustments and considered more aggressive LPD targets for the bulk of the field house spaces to make up for the expected overage at the lobby. By using the space-by-space method, the engineer chose to use tradeable allowances to offset the higher expected LPD of the lobby space. This method allowed the engineer to sum individual-space LPDs based on each space’s allowed wattage and compare them with the entire system-specified lighting power. Lobbies, feature walls, and points of interests are typical places to use extra wattage that can add value to the design, while back-of-house spaces may be better suited for fixtures that are more efficient and lend to a more straightforward lighting approach.
To complete the design intent and mimic the relationship of the lobby geometry with the natural light, the design team decided to use a cove light in the clerestory around the perimeter of the space. The challenge then was to select a fixture that would not only provide enough light on the lobby floor (10 to 20 fc average), but also wash the ceiling uniformly with light. Indirectly lighting the floor in a 30-ft-high space is not the most efficient method from an energy perspective. In fact, the cove fixtures ultimately accounted for 73% of the energy used in this lobby. The design decisions that followed were intentional moves to conserve power consumption while still meeting the design intent. (See the cove lights marked by the “A” in Figure 4.)
The lobby reception desk is an important focal point in the space. The IES recommends a higher illuminance of a 30-fc average on the surface of a desk, versus the 10 to 20 fc required on a lobby floor. To direct light specifically to the surface of the desk, a continuous run of recessed multiple fixtures were used. The recessed luminaires (30 ft above) uniformly illuminate the task-oriented desk by individually aiming each fixture. This lighting solution was a great choice because it helped limit the total lighting-power density by only placing light where it is necessary while maintaining the clean ceiling appearance. The overall lobby does not need 30 fc of illuminance; therefore, the designer relied on the cove fixtures to provide the lower amount of ambient light. Specific tasks are illuminated by more focused light sources. (See the recessed multiples over the reception desk in Figure 4 marked by the purple “B.”)
With a 2-story space and a large volume of human traffic comes a high demand for sound retention to manage acoustics. An architectural move to integrate the acoustic panels in the lobby into a feature wall naturally asked for lighting to respond. The acoustic wall spans from the floor below the lobby to the ceiling of the 2-story space above, spanning 38 ft top to bottom. Lighting from different angles with varied outputs and beam angles helps bring the feature wall to life (see the green “C” in Figure 4).
Decorative elements in the form of sculptural light pendants were applied near the entry portal. Although another higher use of power in the space, and the building overall, the luminaires purely serve a decorative function. IECC-2015 allows an exemption for decorative light sources, allowing their exclusion in the LPD calculation under section C405.4.2.2.1, Additional Interior Lighting Power. For a source to be considered decorative, the fixtures need to be controlled separately (C405.2.4) and must total no greater than 10.7 W/sq ft. As a result, the lighting control scheme identified these fixtures as independently controlled to help comply with code. These fixtures are just over 400 W each and fall under the LPD requirement of 10.7 W/sq ft or less as decorative luminaires (and they are controlled separately). (See the blue “D” in Figure 4.)
On the mezzanine level, there is a reading lounge and walkway. General lighting downlights accomplish a 10-fc average and add minimal wattage toward the building area method LPD. The decorative pendants over the sitting area provide illumination for the critical task in the defined area. (See the yellow “E” in Figure 4.)
Throughout the rest of the building, benchmark LPDs were used to determine progress and help steer the overall lighting-power consumption to account for weighted design decisions. By interfacing with the architectural team and using controls, specific illuminance targets, and critical luminaire evaluation, the lighting design realized an overall LPD of 0.78 W/sq ft, ultimately meeting the LEED 2009 requirement of 10% better than ASHRAE 90.1-2007. In addition, the lighting energy was managed to below the 0.80 W/sq ft goal. This allowed the engineers to achieve the energy-optimization credits sought by the entire design team.
Michael Brinkman is an associate and electrical engineer with CannonDesign where he concentrates on solving client challenges for health care, mission critical, and science and technology facilities. Owen Dalton is a lighting designer and electrical engineer with CannonDesign, and is a junior associate of the International Association of Lighting Designers. He focuses on developing lighting and control systems for the built environment .