The daylit area – Correlating architectural student assessments with current and emerging daylight availability metrics
Highlights
► New method for testing daylight availability metrics against building occupant assessments. ► Method was tested as a classroom exercise by 60 architectural students. ► Daylight autonomy reproduced the student assessments more reliably than other metrics. ► Method has substantial educational value as a teaching exercise for architectural students.
Introduction
What is daylighting, why are we pursuing it, and what is a well daylit space? The answers to these questions are complex and subjective. A rather unambiguous response to the first question is that daylighting describes the controlled use of natural light in and around buildings. Several drivers exist for why one might want to implement daylighting. A starting point for most explorations on daylight is that there must be a certain, program-specific amount of daylight available within a space for the space to be called daylit (daylight availability or sufficiency1). To balance occupant comfort and energy concerns, this amount should neither be too low nor excessive. Another frequently voiced requirement is the ability of building occupants to adapt the (day)light in a space to their programmatic needs. In classrooms and office-type environments – where occupants do not typically have the freedom to adjust their position, and have rather stringent visual comfort requirements – occupants usually have access to movable shading controls to adapt the indoor environment to their needs. In public spaces, such as atria, occupants can adapt by moving around the space. The combination of daylight availability, occupant comfort and energy efficiency leads to a more specific definition of daylighting: A daylit space is primarily lit with natural light and combines high occupant satisfaction with the visual and thermal environment with low overall energy use for lighting, heating and cooling [1]. The three categories are linked. For example, when blinds are lowered to avoid discomfort glare, the interior daylight availability is reduced, the electric lighting may be switched on and heating or cooling loads my change accordingly.
Thinking of daylighting in terms of three linked but separate design objectives (appropriate light levels, occupant comfort and building energy use) can help designers to work on one objective at a time. In order to do so, metrics are required to reliably evaluate the design intent of each category. The objective of this paper is to test the first category, i.e. how contemporary daylight availability metrics compare with occupant evaluations of a daylit space.
Section snippets
Review of daylight availability metrics
A variety of daylight availability metrics based on rules of thumb and computer simulations have been proposed in the past. The most common rule of thumb used to rate daylight availability in a sidelit space is the window-head-height rule of thumb. The rule relates the distance from floor to the head of a window to how far “adequate, useful and balanced daylight enters the spaces for most of the year” [2]. A simulation-based validation study of this rule of thumb for unobstructed facades
Student assignments
During spring 2011 the first author taught two semester-long graduate-level classes to architectural students, a required introductory class on Environmental Technologies in Buildings (6205) and an elective class on Daylighting Buildings (6332). Enrollments for the classes were 45 and 15 students, respectively. There was no student overlap between the two classes. Course 6205 was concerned with basic phenomena of heat flow, lighting and acoustics whereas the primary focus of Course 6332 was the
Assignment results
15 students enrolled in GSD course 6332 completed their assignments on February 14, 2011, a mostly sunny day. On April 4, 2011, a mostly overcast day, 45 students enrolled in GSD course 6205, repeated the experiment. All participants were asked to make their recordings between 11 am and 2 pm on the given day. Submitted assignments were then digitized and the daylit area boundaries were traced and compiled using the Adobe Creative Suite software [20]. After importing all vectors into Rhinoceros,
Discussion
What can the reader learn from these results? First, for the investigated space the IESNA daylight autonomy metric and the window-head-height rule of thumb both correlate well with the 60 student assessments. This result is encouraging for the IESNA Daylighting Metrics Committee. The close agreement between the rule of thumb and the daylight autonomy calculation are not surprising given that the rule was previously validated based on daylight autonomy simulations [2]. For the daylight factor,
Conclusion
Regardless of a computer algorithm’s sophistication, at the end of the day, recommended practices and digital models exist to test conditions in reality, and provide feedback for design. It is still unclear how such metrics stand up to the realities and nuances of human perception. Yet, models must be correlated to how humans perceive the world. This study proposes a simple method for such validation. Secondly, in this world of perpetuating standards and ever-more sophisticated and easy to use
Acknowledgments
The authors are indebted to the Le Corbusier Foundation and the Harvard University Department of Visual and Environmental Studies for letting us use the Carpenter Center for the student assignment. We also thank Ms. Shelby Doyle for building a detailed Rhinoceros model of the Carpenter model. This work has been supported by the Office for Executive Education at Harvard University Graduate School of Design as well as a Dean’s Grant.
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