The solar envelope evokes a rhythmic motive for architecture in which buildings may change, decay, move or disassemble with the seasons. First offered in 1980 as a way to zone cities for solar access, recent studies now show that, while originally conceived as a fixed volume, the solar envelope's boundaries can cycle between seasonal extremes without denying year-round solar access to surrounding properties. Between the winter envelope and the generally higher summer envelope lies a region of space that adjusts for modern programming and for low-energy architecture. Analogies drawn from nature and vernacular architecture have provided a name for this region: Interstitium.
Keywords: courtyard, interstitium, solar access, solar envelope, solar zoning, toldo.
SEASONAL SOLAR ENVELOPES
Our need for solar access is not constant but varies according to season. In winter, we value direct access to sunshine for passive heating. In summer, we may actually prefer shade or filtered light to prevent overheating of spaces and building mass. Only for active energy systems, flat-plate collectors for heating water and air or photovoltaics to produce electricity, do we need year-round access to sunshine.
Conventional references to solar-access zoning emphasize fixed time and space constraints on buildings but, in actuality, the solar envelope is kinetic. Its form cycles with the seasons. This recognition, supported by recent computer studies at USC led by Karen M. Kensek, has suggested a different interpretation of space that includes the Interstitium, an interval between two quite different worlds (Knowles and Kensek 2000).
The term "interstitium" is borrowed from human anatomy. The interstitial space of the lung is that area of tissue between the alveoli (tiny air sacs) and the capillaries that carry the blood. During inspiration, the alveoli expand with air, and the interstitium stretches into a very thin layer. In this way, alveoli and capillaries are brought into close proximity so the oxygen has less distance to travel in its diffusion from outer world (aveolus) to inner world (capillary). The dynamic of this process offers a useful model for seasonally adaptive zoning and design.
The Interstitium of the solar envelope responds to changing requirements for sunshine by changing the proportions of available space. Its low winter boundary may confine buildings while its higher summer boundary limits deciduous trees that lose their leaves in winter to provide passive solar gain. Another example is a building that transforms from a tighter, compact winter mode to a looser, expanded arrangement in summer. Consider the example of the Spanish toldo
Reynolds has recently expanded this theme (Reynolds 2002) He describes one of the most appealing characteristics of Carrascos' courtyard: the sound of water. Several small fountains echo softly in the resounding space. Additionally, the patio floor is made of absorbent brick set in porous mortar. This floor is capable of absorbing water, splashed on it during the watering of the plants and deliberately sprayed for cooling several times daily.
Shading for the patio depends on a movable horizontal white transparent canvas cover or toldo. Reynolds says, "Like a large tree, the toldo casts shade over the whole patio; unlike a tree, it is swept away in the early evening to facilitate both ventilation and cold-sky radiation at night." He describes another advantage shared by deciduous trees. "As falling leaves permit warming rays of the sun to pass through a mantilla of bare branches, so the toldo is folded back in winter to let sunlight flood the patio."
Typical Courtyard House in Bornos, Spain: (Left) Toldo in place, shading courtyard but also interfering with ventilation; (Right) Toldo withdrawn, exposing courtyard. (Photos: Reynolds, 1996.)
The toldo in Carrasco's house rises several meters above the courtyard to allow
the escape of hot air but Reynolds says this is not the usual case. He points
out that as typically applied, the toldo nearly fills the sky opening, interfering
with ventilation. It might thus be seen as disadvantageous despite its obvious
advantage for shade. It is this contradiction of shade and ventilation that
led to a recent study of dynamic adaptations for comfort in modern courtyard
buildings.
A TEST OF THE INTERSTITIUM
Working with USC architecture students, the authors recently tested the idea that, within the interstitium, flexible structures may expand and contract to provide year-round comfort in courtyard buildings without denying solar access to adjacent properties (Knowles and Koenig 2002). The study assumed the existence of urban solar access using the interstitial space of the solar envelope as an instrument of zoning policy.
The circumstances for comfort vary seasonally. Prevailing winds from the west were considered desirable for summer ventilation and cooling. (Winds do sporadically blow from other directions but were not tested because of their infrequency and unpredictability.) Winter sun in the courtyard was considered desirable while summer sun was not. All cases were appraised with the courtyard open before alternative cover designs were evaluated.
Wind tests were done by visual inspection on scale models in a low-velocity tunnel. The tunnel floor was roughened upwind of the test section to simulate a mid-rise urban surround. Tests were conducted at two speeds: Low (4 mph) and high (26 mph). Two means of measure were used. Trash tests (small bits of scattered paper) were used to establish whether or not there was any air movement in the courtyard. Additionally, tufts were used to establish more precisely where surface flow occurred and its direction. (Pressure tests were not employed.)
Sun studies were conducted by using
either scale models on a sun machine or computer simulation. The sun machine
was generally used first to get a general idea of how and when the sun entered
the courtyard. Then as the study advanced and alternative cover designs were
tried, the computer proved more valuable. As with the wind studies, the sun
tests were also done by visual inspection: Dark-light patterns only. Precise
thermal measures were not used.
The test site is actual, a corner lot surrounded on two sides by city streets
and on the opposing two sides by adjacent building sites. The lot measures 141'
x 216', oriented on the cardinal points following the US Land Ordinance of 1785.
The test building has a courtyard measuring 47" x 122', placed symmetrically
on the site. Depending on overall building dimensions, this leaves a perimeter
of usable office space approximately 50' deep.
The site is rotated to provide a variety of test conditions. All cases match
cardinal compass points to simulate typical conditions in much of Los Angeles.
Except for different orientations, the outline of the site and courtyard remain
constant for all tests cases.
The study of mid-rise commercial buildings took several local conditions into account. For example, prevailing west winds are considered desirable for summer ventilation and cooling but not necessarily desirable in winter. Winter sun is considered desirable for passive courtyard activity while summer sun is not. Taken together, these conditions generally require an open courtyard in winter, a covered one in summer. (Only seasonal variations, not daily, were investigated in this study.)
Orientation profoundly affects how and when sunlight enters courtyards. The solar envelope generates a separate building mass for each site. Some cases are generally taller than others; there is also considerable variation in the heights of walls around any given courtyard. So in spite of the pervasive earth-sun geometry, the effect inside different courtyards varies in important ways with implications for comfort.
Depending on the seasons, a direction that aids the sun can hinder the wind and vice-versa. For example, courtyards that are elongated north-south receive a fair portion of the midday winter sun, an advantage that is contradicted by poor summer ventilation from prevailing west winds that barely enter the narrow dimension. Contrariwise, courtyards that are elongated east-west are filled with winter shadows but the west wind more easily enters the long dimension for summer cooling. The table below compares two cases that typify such contradictions.
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summer
midday
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winter midday
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comparison
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case a |
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Courtyard Elongated North-South: At midday in summer, the patio is nearly all in direct sunshine. At midday in winter, the patio is still half in sunshine. Prevailing west winds ride up and over the narrow dimension of the courtyard. |
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case d |
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Courtyard Elongated East-West: At midday in summer, the patio is nearly all in sunshine. At midday in winter, the patio is completely shadowed. Prevailing west winds enter more easily into the long dimension of the courtyard. |
The objective of the study was to design courtyard covers or toldos that simultaneously exclude summer sun and enhance the low-velocity flow of air at the patio level. This combined function means that a given amount of covering material ideally works at the same time in two ways. This multiple use of material was, with only a few exceptions, achieved in the study.
The study produced four unique building shapes and corresponding toldos based on separate cardinal orientations. The requirements for sun control were, in all cases, completely met by the courtyard covers. The requirements for ventilation were met with varying degrees of success but, with only one exception, there was always significant improvement over the open courtyard.
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1. solar
envelope
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2. building
mass
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3. interstitium
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4. toldo
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5. wind
patterns
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case a
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case b
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case c
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case d
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Column 1: Solar Envelope.
The solar envelope is a space-time construct. Spatially it is restricted by shadow fences of 20' on commercial properties across streets and by 10' shadow fences on adjacent multiple-housing lots. Time constraints provide 6 hours of solar access, 0900 - 1500 Standard Time; this time limit applies for all seasons but is naturally most constraining on height during winter when sun angles are low.
Column 2: Building Mass.
Several study conditions pertain to the building itself. The mass must completely fill the solar envelope at a floor-to-floor height of 14', even when changes of shape might benefit the courtyard.. Walls are presumed to have no wind openings. They are closed at the street level to increase shop frontage and to avoid street dust and noise. Windows at the upper levels are understood to be a hit-or-miss proposition thus not considered reliable for ventilation of the courtyard. In other words, for the study, all air flow occurs only through the courtyard opening. (All buldng-mass images are shown in midday winter light.)
Column 3: Interstitium.
The interstitium lies between the building mass and the ( transparent) summer envelope . While the primary building mass is fixed by a synthesis of all seasons, but especially the winter, flexible structures can expand and contract seasonally within the interstitial space without denying sunshine to neighbors.
Column 4: Toldo.
Courtyard covers expand upward into the interstitium during hot summer months, thus catching ocean breezes from the west and simultaneously shading the courtyard from 0900 - 1500 Standard Time. Then during the winter, when the sun is lower and there is less need to direct cooling breezes downward into the courtyard, the structure withdraws, opening the courtyard to the sun's heat and light. (All toldo images are shown in midday summer light.)
Column 5:Wind-Flow Patterns
The study first tested each case with an open courtyard as a reference for the subsequent design of covers. The objective was to ventilate the patio level at a comfortable wind velocity of about 4 - 5 mph, the prevailing summer wind speed in Los Angeles.
A NEW IDENTITY
If generally applied, the interstitium changes the way people identify with their environments. Instead of a fixed image of the city, people see a transforming picture that, as in the Spanish courtyard, corresponds with the habits and rituals of life. The consequence can be a connection to nature that has been sorely missing in our mechanized era. Space and form are no longer static concepts. Rhythm and ritual become motives for design.
Traditional architecture, as the Spanish courtyard, has often responded to the rhythms of nature by dynamic means that conserve energy and enhance life. The interstitium of the solar envelope provides a way to accommodate modern flexible structures that can advance and decline, grow and decay with the seasons. While solar-access zoning typically provides only a fixed image of the city, the interstitium of the solar envelope allows architecture to explore a rhythmic design strategy with implications for energy and life quality.
Knowles, Ralph L. and Karen M. Kensek. 2000. "The Interstitium: A Zoning Strategy for Seasonally Adaptive Architecture" Proceedings of PLEA. Cambridge, UK
Knowles, Ralph L. and Pierre F. Koenig. 2002. "Dynamic Adaptations for Courtyard Buildings" Proceedings of 27th National Passive Solar Conference. Reno, Nevada.
Reynolds, John S. 2002. Courtyards: Aesthetic, Social, and Thermal Delight. New York: John Wiley & Sons, Inc.
