The solar envelope: its meaning for energy and buildings
20 R.L. Knowles / Energy and Buildings 35 (2003) 15–25 side property lines as well as at front and back to allow the free-flow of summer breezes. Two European prototypes provide solar access and crossventilation to individual units in apartment buildings (Fig. 7). Higher densities in the US generally depend on ‘‘doubleloaded’’ corridors and mechanical systems. But in these European designs, hallways systematically skip some floors allowing units to pass freely both over and under for access to light and air in opposite directions. Units are deeper when facing E–W, shallower when facing N–S. Finally, the fourth project, located on a hillside close to downtown, achieves the highest densities of the 10-year study (Fig. 6, lower-right). The density range is 76–128 du/a with a matching V/S range of 7.9–10.5. Design requirements for unit size and parking are the same as for the third project, and the building sections diagrammed there are used here as well. The solar-envelope rules for cut-off times are the same as for the third project, but the space constraints have been significantly altered. The solar envelope does not drop at side property lines. Also, overshadowing is purposely allowed on a north-facing slope that has been left open as a park. Combined, these changes provide exceptional envelope height and additional space for construction. 3.3. Study findings A composite graph, representing all 150 student designs, falls short of the full range of Los Angeles zoning but for two different and opposing reasons (Fig. 8). The lowest density of the study (7 du/a) is deliberate, the result of an initial Fig. 7. Housing sections: the upper two sections, developed in Europe by Jacob Bakama and Le Corbusier, are best for E–W exposures. USC’s Solar Studio adapted the two lower sections to be both shallower and internally arranged for N–S exposures where the winter sun enters from only one side.
Fig. 8. Results of 10-year housing study: the graph shows round symbols representing all 150 student housing designs clustering in a density range of 7–128 du/a corresponding to a V/S range of 2.5–10.5 (square symbols represent the extent of Los Angeles zoning range); most round symbols cluster below V/S ¼ 10:0 corresponding to a maximum density of about 100 du/a; the few exceptions that rise a fraction above this plateau symbolize unusually tall buildings resulting from special site conditions such as the adjacent park in project 4. decision to exclude from investigation one-family dwellings on very big lots as inappropriate for urban housing. On the other hand, the high end of the study’s density range (128 du/ a) is the result of a step-by-step disclosure over the 10 years of testing. Between these values, the study finds a remarkable variety of ways to live in the city within a height range of 3–7 stories. The conclusion of the study is that ample opportunities do exist in this size range to provide both energy conservation and development options for urban growth. The clustering of symbols illustrates the most important finding of the study. The consistent effort to achieve both energy efficiency and life quality, while striving for higher densities, yields a critical cut-off value of V/S ¼ 10:0 corresponding with a maximum density of about 100 du/a. Special circumstances, as in project 4, produce the few exceptions on the graph. Otherwise, for good solar access and cross-ventilation in a compact and continuous urban fabric, the rule holds. Designers who break it lose the choice of architectural means to sustain building comfort and must depend on energy-intensive mechanical systems year-round. The cut-off value of V/S ¼ 10:0 provides a simple but powerful design tool. Architects do not have to wait until an advanced stage of planning to evaluate the passive-design potential of a project. A simple calculation, performed on initial massing schemes, provides an unequivocal basis for comparing the eventual character of their energy usage. If V/ S is