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Vital Signs
Project: Daylighting in Three Libraries
(Overview) (Method) (Survey)
(Mt. Airy) (Michigan) (Exeter) (Syllabus) (Mt. Airy Intro) (Mt.
Airy Model Study) (Mt. Airy Survey) (Mt. Airy Student Evaluation) Mt. Airy Public Library (1984 Edward Mazria, Architect) was designed with assistance from a U.S. Department of Energy Program to demonstrate passive solar and daylighting design. The intention to provide maximum daylighting is an evident progenitor of the cross-section of the design, through a combination of clerestory lighting, light shelves and roof and ceiling reflection. Model and field testing proved the efficacy of its original goals. Mt. Airy Library is a well known "daylighting" exemplar. Light modeling was used by the architects as part of its design development. A goal was to provide daylighting throughout the library, sufficient to preclude the need for electric lighting during daylight hours. The findings of the Mt. Airy Library field visit were close to the results anticipated from the lighting model study, e.g. daylighting is more than ample, with the clerestory giving the effect of a very powerful illuminaire. A feeling of clarity and brightness pervades the entire library. Due to evident care in calculation of sunshading at south facing windows and clerestories, there is no observable bright sun or contrast in the reading room. Field tests and occupant surveys confirmed that the daylighting is very well designed (requiring little or no electric lighting during daylight hours) and appreciated by a majority of users. Because the library is relatively small (compared to the other examples), lighting switches at the main desk allow the librarian to turn on lights when required (reportedly only in relatively cloudy conditions and at twilight).
The intent of the "building work up" assignment was to understand how well the goal of designing a library predominantly lit by daylight was accomplished. Based upon the model testing (prior to the field visit), it was determined that daylighting appeared to be very effective, with very high light levels measured especially due to reflections from adjacent roofs. It was seen by modeling that direct sun lighting is carefully controlled, and minimized if not eliminated by careful sizing of the light-shelves / sun shades. The resulting hypothesis then was that daylighting design was effective, to be verified by field measurement and observation. The daylight factor under "artificial sky" sky (simulating cloudy day illuminance) is high, in the range of 9%, with the most significant variable being the actual roof reflectance, to be verified by on-site inspection. At the field visit, the roof was determined to be black EPDM, a bit darker but comparable to that used in the model. Comparable measurement at the field visit (cloudy sky in the range 1500fc) indicated that this was high by several percentages but accountable due to furniture (book stack) arrangements which blocked and absorbed light reflection. The effectiveness of sun reflecting on inside surfaces, especially the north-facing side of the interior clerestory was confirmed to be very effective in creating bright lighting without direct sun (onto stack or reading areas).. Methods The procedure followed the methodology outlined above. Two students, Tara Borodin and Greg Malone, traveled by car to the building, taking the lighting model for on-site comparison. The variation was explainable due to several surface material assumptions in the scale model. The students spent approximately four hours in the building taking measurements and administering the survey. The limiting factor of the single-day site visit (a ten hour car trip from campus, fit in between classes) proved to be the weather which was only partly sunny to cloudy. This provided light readings in the "low-end" of what was measured in the scale model (under brightly sunny conditions) but nonetheless provided a "close to worse case" and thus an adequate evidence of the effectiveness of the design. The building was revisited a second time to reconfirm lighting performance under brightly sunny conditions. Results and conclusions The results of the Mt. Airy Library field visit were comparable to those predicted, in a sense, confirming the original hypothesis. The daylighting design provides a high level of illumination for most daylight hours. It is the Library's practice to utilize electric lighting only as needed under dark and cloudy conditions. Model testing, in this case, was effective in revealing the design's intent and effect. Daylighting is ample, diffuse and reflected throughout the library and does not create any unacceptable hot spots in the reading room areas. Two concerns related to the daylighting design came from comments of the librarians. Firstly that the roof leaked and was an evident continuing concern (this is the case in all three examples! Is there a design principle here. . . "With light, comes water"?). Secondly, that there was some reported uncomfortable sun and glare into the librarian's reference desk area during late afternoons for some of the year (most apparent during equinox months). In all other respects, the Library daylighting is exemplary. Daylight is reflected off the ceiling and clerestory planes providing a more than satisfactory level of illumination throughout (requiring little or no electric lighting even under cloudy conditions). The occupant survey indicates that the lighting is appreciated by a majority of users (96% reported satisfaction with lighting comfort). In the next section we will present the results of daylight model studies conducted prior to field investigation. |
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author: vitalsigns@ All contents copyright (C) 1998. Vital Signs Project. All rights reserved. Created: 05/13/97 |
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The Mt. Airy, North Carolina Public Library was designed by
Edward Mazria. The building was completed in 1984.
