copyright: Ralph L. Knowles, 1999


The Solar Envelope


Ralph L. Knowles

Professor Emeritus of Architecture

University of Southern California


"Nothing is experienced by itself, but always in relation to its surroundings, the sequence of events leading up to it, the memory of past experiences."

Kevin Lynch, The Image of the City


Key Words: mama-plane; solar access; solar architecture; solar envelope; solar zoning; urban design.


The sun is fundamental to all life. It is the source of our vision, our warmth, our energy, and the rhythm of our lives. Its movements inform our perceptions of time and space and our scale in the universe.

Assured access to the sun is thus important to the quality of our lives. Without access to the sun, our perceptions of the world and of ourselves are altered. Without the assurance of solar access, we face uncertainty and disorientation. We may lose our sense of who and where we are.

The concept of solar access is an abstraction generalized from particular observations. The natural world appears to abound with examples of arrangements based in some measure on exposure to the sun.

More to the point, observations of the modern built world reveal that we have not usually followed nature's example in this regard. Our cities are non-directional. Our buildings are undifferentiated by orientation to the sun. They stand static, unresponsive to the rhythms of their surroundings.

Solar access has, over the past twenty-five years, come into focus as a topic of discussion in the USA. Beginning in the 1970's, we looked at the sun primarily as a source of energy, a replacement for uncertain supplies of fossil fuel. More recently, with deterioration of the urban environment, emphasis has shifted more to life quality. Whether for energy or for life quality, solar access remains a legitimate area of public policy in which the aim is to regulate how and when neighbors may shadow one another.

The solar envelope is a way to assure urban solar access for both energy and life quality (Knowles and Villecco, 1980). First conceived and tested by the author, working with Prof. Richard D. Berry at the University of Southern California (USC), the solar envelope regulates development within imaginary boundaries derived from the sun's relative motion. Buildings within this container will not overshadow their surroundings during critical periods of the day and year. Twenty-five years of design research in the USC Solar Studio have shown that, if generally applied as an instrument of zoning, the solar envelope will not only allow potential growth but will open new aesthetic possibilities for architecture and urban design.



The idea of solar access is ancient. It was practiced in the colonial cities of Greece and in the United States it is evident in the patterns of such early settlements as Acoma Pueblo (Fig.1). Located on a plateau about 50 miles west of modern Albuquerque, New Mexico, Acoma appears to have been continuously occupied for over a thousand years. Its rows of houses are stepped down to the south. Walls are of thick masonry. Roofs and terraces are of timber and reeds, overlaid with a mixture of clay and grass (Knowles, 1974).

Fig. 1. Acoma Pueblo, New Mexico. (Click image to enlarge)

The houses of Acoma are well suited to a high-desert climate (Fig.2). The sun's low winter rays strike most directly their thick masonry walls where energy is stored during the day, then released to warm inside spaces throughout the cold night. In summer, the sun passes high overhead and strikes most directly the roofs and terraces where the sun's energy is less effectively stored and transferred.

Fig. 2.


Fig. 3.

Houses do not shadow one another during the cold winter months and, by sharing side walls, they offer protection to each other in summer (Fig. 3). Their major exposure is southward toward the winter sun, not east and west where the summer sun would have a major impact. And the rows are spaced to avoid winter shadowing of terraces and heat-storing walls. It is this critical relationship of building-height to shadow-area that, in 1976, gave rise to the solar-envelope concept. But why the solar envelope rather than some other legal device?



Some legal experts in the United States have called for a clarification or change of laws or even the formulation of new laws if solar access is to be guaranteed. This raises an interesting question of legal precedents.

The most commonly cited law outside the United States is the English Doctrine of Ancient Lights but there are problems with its application (Thomas, 1976). Roughly, the doctrine states that if in living memory no one has overshadowed your property, they cannot now do so. However, this doctrine has been repeatedly disavowed in U.S. courts.

Some legal experts have suggested that American water law, especially the doctrine of prior appropriation, may offer a more useful precedent for sun rights (White, 1976). They point out that both sunlight and water are used rather than captured and sold; both may be consumed, but both are renewable. In addition, there is an equivalence between upstream and downstream in water law and the geometry of solar shadowing. But, like the Doctrine of Ancient Lights, there are problems with the application of water law.

The doctrine of prior appropriation is a formalization of the general practice among early Western settlers of appropriating available water according to who first put it to beneficial use. Simply put, "He who gets there first, gets the most." It was the American frontier's answer to the exigencies of pioneer settlement.

Prior appropriation is not likely to be applied to solar allocation in any simple way. Future access would not be assured for structures without present energy-conversion systems. Several permits acting on different, adjacent properties (as well as those on distant sites) may conceivably act to stop development completely on one of them. This point has been made abundantly clear in the writings of legal experts who point out serious weaknesses in any attempt to move directly from water law to solar law.

The difficulties in applying water law have led to arguments for straightforward zoning as a more appropriate approach to the problem (Hayes, 1979). First, it offers the possibility of more local administration of rules affecting the allocation of sunlight. Second, zoning is traditionally applied to all properties in a district thus assuring future access and bypassing the problems of preference based on prior use. Finally, existing zoning limiting heights and setbacks is already based on the concept of an envelope of buildable volume. These reasons have been found compelling and have led to development of the solar envelope.



The solar envelope is conditioned in space and time. First, it assures solar access to the properties surrounding a given site. The envelope accomplishes this by limiting the size of on-site buildings, thus avoiding unacceptable shadows above a boundary along neighboring property lines; these boundaries are called shadow fences (Kensek and Knowles, 1995).

The second condition is that the envelope affords the greatest potential volume within time constraints, called cut-off times (Knowles 1981). The envelope accomplishes this by defining the biggest container of space that would not cast shadows off-site between specified times of the day. Clearly, greater periods of assured solar access will reduce the solar envelope's size. Cut-off times that are specified very early in the morning and late in the afternoon will result in smaller volumes than would result from later times in the morning and earlier times in the afternoon.

Fig. 4. Space-Time Constraints on Solar Envelope.

An example of how shadow fences affect the envelope can be seen in 1994 guidelines prepared for consideration by the L.A. Community Redevelopment Agency (CRA) (Fig. 4). In the example, shadow fences vary according to the street character as set forth in a proposal by the CRA for the downtown plan. Shadowing is allowed up to 10' along alleys, up to 20' along paseaos and avenidas, and up to 45' along boulevards. Such differentiation anticipates varied land uses and street qualities.

The same example shows how cut-off times influence the solar envelope. Winter, because sun angles are so much lower at that season, has the greater impact on volume; the cut-off times are 10AM and 2PM thus providing four hours of direct access to sunshine, the minimum generally considered useful for good passive solar design in Los Angeles. Summer, because sun angles are so much higher, has less impact on the envelope; the cut-off times are 8AM and 4PM, a longer period than winter but considered desirable in a mild, Mediterranean climate where people enjoy gardening and outdoor recreation.



The solar envelope's size, shape, and orientation are greatly influenced by the street patterns of urban settlement (Knowles, 1981). In the United States, those patterns are usually comprised of orderly subdivisions of the U.S. Land Ordinance of 1785. Typically, throughout the midwest and the west, streets run with the cardinal points so that rectangular blocks extend only in the east-west and north-south directions. But in Los Angeles, where most of the solar-envelope research has been done, there are two street grids (Fig.5).

Fig. 5. Two Street Grids in Los Angeles.

Most of Los Angeles follows the U.S. Land Ordinance but some streets run 26 degrees off the cardinal points, following the older Spanish grid . This diagonal orientation, a 16th century adaptation to sea breezes, was ordered by the King of Spain. It now extends from the original pueblo over the land that is modern downtown Los Angeles.


The Importance of Orientation

Before discussing the street grid's influence on the solar envelope, there should be some mention of the important seasonal differences of streets themselves resulting from orientation. A comparison of the United States and the Spanish grids serves to demonstrate qualities related to shadows.

Fig. 6. Shadow Patterns Within Streets on the U.S. Land Ordinance.

During the winter, streets that run east-west in a built-up urban area are shadowed (Fig. 6). In Los Angeles at 34 N, they remain dark and cold. By contrast, streets that run north-south are lighted and warmed during the midday and are more pleasant during the busy noontime shopping period.

Summer presents an entirely different picture. Unlike winter, when the sun's rays come from the southern sky, the summer sun comes more directly from the east in the morning and the west in the afternoon. At midday, it is nearly overhead. Streets that run east-west receive a little shadow at midday, much less in the morning and afternoon --- a critical factor on a hot afternoon. Streets that run north-south will be shadowed in the morning and afternoon, but will receive full sun for a brief period at midday.

From the viewpoint of urban street quality, the United States grid leaves something to be desired. Its east-west streets are too dark and cold in winter, too bright and hot in summer. Its north-south streets, while pleasant in winter, lack any protective summer shadow at midday.

Fig. 7. Shadow Patterns Within Streets on the Spanish Grid.

In Los Angeles, the older Spanish grid has advantages regarding street qualities of light and heat (Fig. 7). During the winter, every street on the Spanish grid receives direct light and heat sometime between 9AM and 3PM, the six hours of greatest insolation. It is true that at midday, all streets have shadows; but because of their diagonal orientation, more sunlight enters than if they ran due east-west.

There is also a summer-time advantage to the Spanish grid. Shadows are cast into every street all day long, with the exception of a short period during mid- morning and mid-afternoon when the sun passes quickly over first one diagonal street and then the other.

These differences in street quality are felt, if only subconsciously, by people. They are even acknowledged by real-estate experts who are vitally concerned with commercial land values that vary with the favored pathways of shoppers. Unfortunately, street orientation is almost never considered as a basis for land-use planning decisions.


The Influence of Street Orientation on the Solar Envelope.

Street orientation affects the solar envelope in two ways. The first of these has important consequences for development while the second relates more to issues of urban design.

The size of the solar envelope, and hence development potential, varies with street orientation. Generally, more envelope height is attainable at either of the two possible block orientations within the U.S. grid while somewhat less volume is possible within the Spanish grid. The street's gain in quality thus appears to be the developer's loss in bulk. This has made downtown Los Angles an especially challenging problem from the viewpoint of solar zoning.

The urban design consequences of street orientation are important because they relate to what Kevin Lynch has called "the image of the city." Pathways, districts, and directions take on clear perceptual meaning when the solar envelope becomes a framework for urban development.

Fig. 8. Solar Envelopes Over Blocks At Three Different Orientations.

Cues to orientation come more readily if solar access is included as a development criterion (Fig.8). This assertion can be shown to be true by comparing three different block orientations. Immediately evident are not only variations of envelope size but shape as well. These differences will result in street asymmetries, district variety, and clear directionality along streets. Such differences tend to occur systematically, not randomly. They can thus serve as the sorts of definite sensory cues that Lynch says should come from the external environment (Lynch, 1960).



Most design studies of the solar envelope have been made in Los Angeles. A few have been undertaken in such far-flung settings as Hawaii and the Slovak Republic but the greater number have focused on a variety of urban sites closer to home. Often these studies have been made at the request of L.A. government officials, planning agencies, and even developers interested in trying out the concept. And a few, as for example the Park Mile of Wilshire Blvd, have been taken by planners as guides to actual development.

Since nearly all of the work has been done at USC in a studio setting, its primary aim has been educational, but always there has been a strong research motive as well. The research purpose of the work is to test the belief that a public policy of solar-envelope zoning will not inhibit, but will enhance development and design opportunities in urban settings. There are two assumptions underlying the work: first, that a public policy guaranteeing access to sunshine is essential to a future that is both high-quality and sustainable; and second, while there is presently no general requirement for solar access in Los Angeles, the assumption is that a zoning policy assuring sunshine to all properties has been enacted in the test area.

Within the primary research objective, design studies have shifted emphasis depending on site and building type. For example, some tests have emphasized development issues: trying for the highest densities possible while maintaining such energy-saving strategies as cross-ventilation, daylighting, and solar heating, especially for housing. Other tests have emphasized design aesthetics: the ability of sunlight to influence the rhythms and rituals of daily life, especially in public places. Consider first housing, an urgent problem in Los Angeles where densities and land values keep rising.


Development Possibilities

Nearly all housing studies have involved 16-18 separate but contiguous land parcels, one for each member of a design studio. This has the pedagogical advantage of helping students to see and to deal with the real complexities of urban housing where actual buildings are never executed in a vacuum. Moreover, it advances research objectives by providing greater statistical reliability for such measures as density and floor-area ratios, both important for understanding whether the project is economically viable under given conditions of land-use and community values.

Two projects are selected to show housing capacities for the solar envelope. The first presses the envelope for the highest densities in an urban setting. The second looks at lower densities, but greater design choice in the suburbs. Both hint at limits that subsequent studies have verified.


Urban Prospects

This first project, with resulting densities of 80-100 du/a, tests the solar envelope within the Spanish grid (Fig.9). Viewed from the east, the solar envelopes are crystal-like while existing buildings appear as rectilinear blocks. The envelopes are generated to provide four hours of sunshine in winter and eight hours in summer; they slope downward to a 20' shadow fence at property lines thus accommodating a base of street-front shops under housing. Tower-like shapes appear at some corners where shadows are allowed to extend into streets, but not onto surrounding properties.

Figs. 9 & 10. Southpark: Envelopes and Housing.

When buildings replace the envelopes, design elements appear that typify many subsequent urban-housing studies (Fig.10). For example, terraces occur where the rectangular geometry of construction meets the sloping envelope. Courtyards center many designs to achieve a proper surface exposure for light and air. Facades are elaborate, enriched by the porches, screens, clerestories, and other devices of solar architecture.

Buildings in the study meet each other gently, across sloping spaces, not abruptly across side-lines and alleys. The resulting spaces, not confining and dark but rather liberating and filled with light, allow view and the free flow of air through the city.

Although not central to this work, solar zoning turns out to be compatible with earthquake safety, an important design consideration in Los Angeles. The pyramidal shapes that often result from applying the solar envelop have the effect of shifting the building's center of gravity downward thus making it more stable under horizontal acceleration.


Suburban Prospects

A second project replaces the typical suburban densities of 5-7 du/a with densities of 25-45 du/a under the solar envelope (Fig.11). Viewed from the south, the solar envelopes rise and fall with changes in street orientation and lot size. The envelope rules provide longer periods of sunshine than the first project: six hours on a winter day; ten hours in summer. They are generated to a 6' shadow fence across streets at neighboring front yards and at rear property lines, but they do not fall at property side-lines as in the first project.

Figs. 11 & 12. Subdivision: Envelopes and Housing.

When buildings replace the envelopes, the result is remarkable innovation within harmony (Fig.12). The continuous envelopes result in a smooth flow of street facades. At the same time, building types range from town houses and courtyard clusters to apartments. Individual designers are clearly exploring separate formal ideas from one parcel to another. The consequence, if built, would be an enormous range of diversity and choice within a neighborhood.

The two projects shown are typical of completed housing studies in a variety of settings resulting in a range of 7-128 du/a. The highest density was achieved in a test using a solar-envelope protocol that allowed some overshadowing of a public park. Otherwise, for good solar access and cross-ventilation in a compact and continuous urban fabric, the more normal high figure turns out to be 80-100 du/a . This range can be accommodated in buildings of 3-7 stories, a scale familiar in some of the most admired cities in the world.


Design Possibilities

Most studies of the solar envelope have involved housing, but a few have included other building types that raise new possibilities for design. The foregoing exemplars have demonstrated that shadowing of neighboring properties is forestalled by the solar envelope, but the solar envelope is also an expression of the freedom to choose how and when to use the guaranteed sunshine. The designer, working within the envelope, must settle important issues: whether or not to shadow one's own buildings; how to develop the aesthetic potential of sunshine. Two projects are shown to demonstrate the design potential of working under the solar envelope.


Calling Forth Rhythm

The first project, Bunker Hill, is a 9-acre site in a key downtown location. A preliminary program was made available to the USC Solar Studio by the CRA in the fall of 1979. A similar program became the basis for a competition among five invited developer-architect teams. A final selection from among the entries was made by the CRA based partly on their review of the solar-envelope projects at USC.

The properties immediately surrounding the site are extremely varied. Commercial buildings, 15-50 stories high, are concentrated at the southwestern end of the site. Housing is generally grouped on the site's long sides. The least certain part of the surroundings takes up the site's northeastern end. The presumed uses here are institutional, either a cultural facility or a county municipal building about 12 stories high.

The resulting solar envelope rises and falls in concert with the surrounding skyline (Fig.13.) Rules for the solar envelope make general distinctions in the magnitude of acceptable shadowing of surrounding properties: least shadow on housing and most on commercial office buildings that may be shadowed to 33% of their window-wall areas. The final envelope that emerges from these rules is of variable height from 100' to 500'.

Figs. 13 & 14. Bunker Hill Envelope and Mixed-Use.

A typical design does not fully use up the floor-area potential of the solar envelope; instead, the designer has made a trade-off (Fig.14). The solar envelope has a potential FAR in excess of 20 based on a floor height of 15'. With such a huge volume, the CRA's programmed FAR of 8-10 could easily be met by following standard development procedures. But the designer has chosen instead to split the project open along a favorable N-S axis. This maneuver combines the best qualities of both the U.S. and Spanish grids, thus providing ample sunshine to housing and to public open space within a mixed-use project. There is another quality that the designer has achieved by slicing through the design.

When viewed at an instant in time, the designer's model reveals a picture of compositional richness; but with observations over time, made with a heliodon or computer animation, the continuity and rhythm of urban events can be imagined . As the day passes, impressions of the whole site swing to and fro --- buildings and spaces go from light to dark, from warm to cool. If the designer's intentions had been carried through into construction, the pulse of human activity within the spaces would be reinforced by these light and heat cycles of buildings and spaces.


Rhythm As A Design Strategy

The second project is a library. The actual program of required spaces was obtained from the Los Angeles library planners, but the spirit of the program was taken from observing the moving shadows of a wall.

Modern architects have generally defined space in opposition to change. They acknowledge that buildings may be transformed or deteriorate over time. Yet on the whole, their artistic idea of space is complete and static, the final product of many imaginative decisions.

By contrast, the Solar Studio advances the proposition that sunlight adds a dimension of time to our perceptions of architectural space. In other words, space is generated by flux itself. The notion of completion is antithetical to all our activities. Any execution is calculated as a measure of time, the whole as a consequence of daily and seasonal rhythms of sunlight. The proposition reached its greatest amplification and expression in the library project.

Fig. 15. Shadow Wall.

Design students were first asked to picture a free-standing wall with a gateway through it (Fig. 15). If the wall faces east and west, it will accentuate a daily rhythm; shadows will appear early on the west and later on the east, regardless of season. But if the wall faces north and south, moving shadows will emphasize a seasonal rhythm; they extend much farther northward in winter than in summer throughout most of the day. Finally, if the wall faces diagonally to the cardinal points, the accents will be complex and contrapuntal. These images of a wall were then extended directly into the design of the library.

The students were then asked to imagine a circumscribed wall that reaches to the upper boundaries of the solar envelope and that, when standing alone, can fill out the envelope's imaginary boundaries with shadow over time and seasons. The wall in sunlight can thus act as a generator, an allusion to invisible form. When the form is brought into concrete existence, sunlight replays a series of connections in space and time. The generating wall has been dubbed the mama plane . Its applications in the Solar Studio have sometimes been quite direct.

Fig. 16. Library. Designer, Anthony Reiter.

One design for a library develops a literal interpretation of the wall and gateway (Fig.16). Beginning with the mama plane, a gateway opening through it allows sunshine to penetrate to a second plane where the lighted area is removed. After a prescribed interval of time, sunshine passes through both gateways to a third plane where the lighted area is again removed. The designer continues this process throughout the course of a simulated day, interval by interval, until the final design embraces the entire set of planes. The designer's intention is that this arrangement of gateways will not only generate a system of transverse spaces but, on all future days, will act in sunlight to recite the original sequence of connections with seasonal variations.

Figs. 17 & 18. Library. Designer, Gustavo Koo.

A second design for a library uses shadows cast from edges of the mama plane to create both the building and a garden (Figs. 17 & 18). The mama plane, with a diagonal orientation to the cardinal points, has been given thickness by the designer and serves as book storage, service, and circulation. To the south-west of the plane, morning shadows moving at intervals across the ground have been used to outline the terraces of a garden with parking below. To the north-east, afternoon shadows are used to define the shapes of floor plates: one set is generated on a winter day, alternate plates on a summer day. And as with the previous design, the creative process will be recited over time.

The intention of such studio projects is not simply to describe solar phenomena by architectural means; the purpose has more to do with rhythm as a mysterious fact of aesthetic experience. Rhythm as a design strategy, as a medium of the designer, can express our most delicate feelings and moods. It is toward this end that the USC Solar Studio has so exhaustively investigated the solar envelope.



Without access to the sun, we cannot use it. The solar envelope is proposed as a zoning device to achieve solar access by regulating development within limits derived from the sun's relative motion. Buildings within its boundaries will not shadow surrounding properties during critical periods of the day and year. Assured solar access thus offers the chance to replace unreliable energy sources and to enhance the quality of urban life. Assured sunlight also suggests rhythm as a novel design strategy, as a way to bring us closer to a sense of nature in our buildings and urban spaces.


Hayes, Gail Boyer. Solar Access Law . Cambridge, Massachusetts: Ballinger Press, 1979.

Kensek, Karen M. and Ralph Knowles. "Work in Progress: Solar Zoning and Solar Envelopes," ACADIA Quarterly, 14.2 (Spring 1995): 11-17.

Knowles, Ralph and Marguerite N. Villecco. "Solar Access and Urban Form," AIA Journal, February 1980, 42-49 and 70.

Knowles, Ralph. Energy and Form. Cambridge, Massachusetts and London, England, MIT Press, 1974.

-----. Sun Rhythm Form. Cambridge, Massachusetts and London, England, MIT Press, 1981.

Lynch, Kevin. The Image of the City . Cambridge, Massachusetts and London, England, MIT Press, 1960.

Thomas, William. "Access to Sunlight," Solar Radiation Considerations in Building Planning and Design: Proceedings of a Working Conference (National Academy of Science, Washington, D.C., 1976): 14-18.

White, Mary R. "The Allocation of Sunlight: Solar Rights and the Prior Appropriation Doctrine," Colorado Law Review, 47 (1976): 421-427.


Return to Home Page