American Architectural Manufacturers Association

Objectifying the Subjective: In Pursuit of Metrics for Daylighting

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An interview with Lisa Heschong, Heschong Mahone Group

Lisa Heschong presented a roadmap for the development of daylighting performance metrics to the NRFC in November 2010. The following is a summary of her presentation and ensuing discussion.

In the context of green construction, building owners, designers and standards developers look increasingly to daylighting to provide natural interior illumination while saving money on energy costs and reducing a building’s impact on the environment.

Optimizing the trade-off between thermal conductivity (U-factor) and solar heat gain (SHGC), and minimizing air infiltration, have long been well-understood means for maximum thermal performance. However, daylighting is a relatively new performance parameter. In that context, ways to reduce energy consumption for artificial lighting and cooling system loads by integrating lighting controls with daylighting is now well established. A lesser-known and perhaps equally important role, however, is the profound effect daylighting has on worker productivity. And, quantifying that benefit as a guide to design has been more elusive.

Visual comfort is to daylighting as thermal comfort is to U-value and SHGC. Over time, evaluation techniques, performance metrics and rating systems for the first are evolving, as they have for the latter. New visual comfort performance metrics will likely feed continued code development and product performance improvement.

There have been numerous studies on human behavior in retail, academic and office environments under the influence of natural light. In general, occupants in daylit environments show improved mood, cognitive performance and memory formation. In retail settings, increased sales and more transactions are the measurable results. Daylit classrooms have been shown to be associated with a 20 percent improvement in test scores, and workers in daylit offices have shown seven percent faster workspeed and ten percent better cognitive performance.

In addition to daylight illumination, views also have an association with improved performance. In studies conducted by the Heschong Mahone Group (HMG), a call center exhibited 7-12 percent greater productivity where the best views were available. Similarly, office workers showed 10-25 percent better performance on mental acuity tests and experienced less fatigue. Students turned in 17-30 percent better math and reading test scores in classrooms with views. In another study by the Lighting Research Center (LRC), computer programmers exposed to daylight spent 15 percent more time on their primary task, while equivalent workers without exposure to daylight or views spent 15 percent more time talking on the phone or to one another.

Visit the following links to read more from Heschong's interview:

Psychological and Physiological Factors 
Codes and Standards
Daylight Methodology and Metrics
Residential versus Commercial Issues
Manufacturer Considerations

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