A Guiding Light

SHUTTERSTOCK / BILLION PHOTOS

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) publishes free design guides for buildings that achieve major energy reductions without compromising quality. The latest guides go even further, targeting net-zero buildings. The guides' actionable recommendations, which rely on practical design and technology, provide deep energy savings without compromising light levels and comfort.

The first publications in the series cover K-12 school buildings (January 2018) and small to medium office buildings (June 2019) that are new and approached as a retrofit. The K-12 school buildings include all sizes and types. The office buildings include spaces ranging from roughly 10,000 to 100,000 square feet with a height of less than 75 feet.

Strategies for achieving net zero include plug-load control, sizing and designing HVAC systems, reducing and eliminating thermal bridging, and optimizing lighting, among other techniques. The lighting heavily relies on daylighting, LED sources and controls.

K-12 buildings

The goal here is to minimize energy consumption to the practical limit without compromising light levels and comfort. Specifically, a lighting power density (LPD) of 0.4 watts/square feet (W/sq.ft.) and 0.45W/sq. ft. respectively for primary and secondary school buildings. The guide provides background information and general instructions applicable across the building before providing more specific guidance for each major space type.

The lighting design should begin with ensuring appropriate minimum maintained light levels on horizontal and vertical surfaces. Avoid glare by selecting luminaires that balance proper shielding of the light source with efficacy and by testing how lighting interacts with room surfaces. Luminaires should have a dimmable driver and operate across the dimming range without objectionable flicker confirmed by a mock-up.

Care should be taken to achieve a uniform light distribution with appropriate brightness and contrast. Otherwise, lighting can be used for wayfinding and highlighting objects of interest, such as school logos and trophy displays. Lighting should be controlled by a combination of automatic and manual devices.

In classrooms, the guide recommends 40 horizontal foot-candles and 15 vertical ones for appropriate illumination for students, for students with special needs in general classrooms, and for nighttime adult learning.

For classrooms with ceiling heights above 9½ feet or with daylighting, consider placing a series of pendants—preferably semi-indirect or direct/indirect in light distribution and spaced for uniform light levels across the space—parallel to the window wall. Below 9½ feet, 2×4 “volumetric” luminaires can be specified. Manual controls are recommended for the entrance and teacher area, the latter providing at least two scenes—general teaching and A/V modes—and potentially integrated with any automatic window shading. Otherwise, dual-technology, auto-on to 50% occupancy sensing, and daylight-responsive dimming controls are recommended for this type of space.

Small to medium office buildings

The goal is an LPD of 0.4W/sq.ft. for the building and as low as 0.31W/sq.ft. for open office areas. The same care must be taken for visual comfort. This design guide further calls for light-colored finishes to maximize internal light distribution, specifically 80–90% reflectance for ceilings, 50% or more for walls and 20% or greater for floors. Workstation partitions parallel to windows should be at least 50% translucent or limited in height to 36–42 inches. Task lighting should be installed depending on daylight availability and automatically controlled.

Let’s look specifically at open offices, which represent about half of all interior floorspace in this building type. The target ambient light level is 25–30 average maintained foot-candles, and the target task level on the desktop is about 50 foot-candles. The LED lighting installed in this space should feature 125 lumens per watts efficacy, 50,000+ hour life (L70) 80+ color rendering index with sufficient red saturation, dimmable driver and 5+ year warranty. The guide states one 25W luminaire per 80 square feet of area supplemented by task lighting as needed. Adjacent feature walls and corridor file areas can be highlighted with wall-wash luminaires.

All lighting should be automatically turned off when not needed and reduced based on sufficient daylight. Luminaire-level lighting controls are recommended for maximum response and energy savings, especially with wireless communication for existing buildings; they install like typical luminaires. Networking the controls enables sensor data to flow to a central point for analysis to optimize energy savings.

Download these guides from the ASHRAE website, ashrae.org.

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