Building envelope systems separate the indoors from the outdoors—they provide the “skin” of architecture. For this reason, they become the fundamental interpretation between the interior and the site condition. This interpretation sets in motion a large number of modulating functions of the envelope system — thermal, solar, acoustic, aerodynamic, and other forces largely invisible to direct observation but highly significant to human occupation.
The envelope is also the most visible element of the building and must respond to our appreciation of image, form, and orientation to the building. These dual roles of invisible forces and visible mandates are repeated to a lesser degree in other major building systems, but the envelope is the most obvious point of their resolution.
• Walls
• Fenestration
• Roofs
Separation/Connection. The distinction between indoors and outdoors is not always an exclusive one. There are often ambient conditions that benefit the interior environment and can be employed passively to condition occupied space. The dynamic relation of these indoor and outdoor conditions leads to an interactive definition of the envelope system. As discussed in the previous chapter, the ability of designers to respond to this interaction is one of the important trends in the systems view of architecture. Reynolds and Stein (2000) (after Christian Norberg-Schultz, 1965) describe this concept as “filters, barriers and switches” or as Stephen Groak says in discussing flows of matter and energy, “The building forms a system of barriers, filters, containers—sometimes condensers— for an enormous collection of materials and energies.”
Control over levels of connection and separation to the outdoors plays a direct role in the comfort of occupants. Weather has to be excluded so that the indoor environment can be controlled. But rooms allowing visual contact with the natural environment are more restful than windowless rooms. Workers with a view, for example, are more productive, and hospital patients in windowed wards heal faster. On the other hand, people require various degrees of separation from the outdoors for privacy, security, and control of indoor conditions.
Because the envelope must satisfy both the barrier and the filter roles, switches have to be provided. Window blinds are a good example of switches; they allow the window glazing to be effectively turned off, along with view, light, and sun. Operable shading devices such as movable awnings, movable insulation like window shutters, and operable windows for ventilation are further examples.
Weathering. Upon the completion of a finished exterior envelope surface, there begins a continuous relationship with the elements of weather. Sun and wind, moisture and ice, dust and decay all tend to age the building, diminishing its pristine newness. Erosive elements like acid rain, ultraviolet radiation, and chemical interactions with building components accelerate this process. Either the building will require continual renovation to maintain its newness, or it will be, to some extent, reclaimed by the site. The former choice results in a building that has been “stained” by time and the elements; the later can produce a building that acquires a graceful “patina” (see Mostafavi and Leatherbarrow, 1993).
Structural Form. The structural possibilities of envelope systems range from Louis Kahn’s cycloidal vaults of concrete at the Kimbel Art Museum in Fort Worth, Texas, to Philip Johnson’s transparent space frame structure at the Crystal Cathedral in Garden Grove, California.
Thermal Form. The thermal form of a building refers to its level of exposure between inside and out. For heat transfer by temperature difference, this exposure is a product of the area of exterior skin and its thermal conductance. From a purely geometric view, an efficient form has a low surface-to-volume ratio, enclosing the maximum amount of interior space with the minimum of skin exposure, and will also have a low heat transfer conductance. An inefficient form, conversely, will has a high surface-to-volume ratio and high-conductance envelope assemblies.
In actual practice, of course, as compared with a purely geometric view, the envelope becomes more efficient if it is shaped to maximize good exposures. Although a sphere would be the most efficient thermal container of space, a long rectangle stretched from east to west offers better opportunity for collecting low winter sun and avoiding harsh summer solar impacts. For this reason, various ratios of length-to-width can be considered before determining an appropriate aspect ratio (see Olgyay, 1963).
Thermal form is a way of talking about the “leakiness” of the envelope. This encompasses not only heat transfer by temperature difference, it also includes the consideration of accidental air change by infiltration and exfiltration as well as the migration of moisture into and out of the building.
Solar Form. Solar impacts on a building are determined less by its overall shape as considered in thermal form than they are by the placement and distribution of glazing and the use of shading devices to protect them. So solar form results primarily from window size and orientation.
Mitigating factors can also play a large role: appropriate use of external shading devices, both fixed and operable; the selection of glazings with differing ratios of visible light transmission versus total solar transmission; the slope of window glazings toward the sky or the ground. Even the configuration of the building envelope into selfshading forms such as courtyards and setback stories makes a significant difference.
Unlike thermal form, solar form is bimodal. In small surface-load-dominated buildings in particular, the need for solar gain in colder months will be replaced by a need for defensive shade in comfortable and warm seasons. Because every latitude has its own schedule of solar geometry and every building a particular set of sun-to-shade requirements, the interplay of these factors is one of the enduring truths to which a designer must respond.
Luminous Form. In terms of daylighting, the luminous form of an envelope is bound up in the solar form. Because most building spaces are occupied for daytime activity, daylight through the envelope is generally a desirable resource. In fact, the abundance of available natural light is usually much less of a resource problem than the difficulty of distributing daylight any distance away from a glazed aperture.
Separating natural light from the heat it eventually becomes is possible, to a limited extent, by admitting light in measured portions and avoiding the Promethean overheating that usually accompanies daylight saturation. Because daylight is generally useful year-round, it is not a bimodal condition as found in solar form, which confronts sun versus shade. But because available daylight levels vary with sky condition, time of day, and season of the year, it is no less a dynamic condition than solar geometry.
The envelope glazing is the lens of a daylighting system and functions just as the cover lens of an artificial lighting fixture does. Additional envelope components, such as reflectors and baffles, can also serve as artificial lighting fixture equivalents. These components bounce the light deeper into a room or modulate its penetration in other ways. When total control of natural light is important, such as in theaters and auditoriums, the envelope must function as a switch by means of daylight-control curtains or blinds, to close out the sky. Switchable glazing that employs laminated glass with an electrically excited diode layer of mylar to change from opaque to clear will be an option when the technology matures. Phototropic and thermotropic glass that darkens in response to high light levels and warmer temperatures (like darkening sunglasses), are technically proven but have not yet demonstrated economic practicality.
Aerodynamic Form. Wind is both a structural and a comfort factor in envelope design. The magnitude of uplift and surface wind loads are the important structural considerations. Cross-ventilation and buffering of cold winds are prime environmental concerns. The environmental aspects qualify as bimodal dynamics because of the changing seasonal patterns of prevailing winds and the alternating needs of a building for opening to ventilation versus closing for heating or cooling.
Computational fluid dynamics (CFD) is the science of predicting airflow in closed spaces and around freestanding obstructions. Recent advances in computing power have made CFD an important tool in studying the complexities of convective air currents. Three-dimensional modeling is accomplished in carefully constructed wind tunnel tests. Two-dimensional modeling can be performed on fluid mapping tables by standing a scale profile section of the building in a moving horizontal film of water.
Acoustical Form. The acoustical form of an exterior envelope is primarily related to site noise control. Self-sheltering forms include courtyard and atrium plans. Because mass is the primary source of noise attenuation, the solid, heavy elements of the envelope can be directed to reflect unwanted sounds away from occupied spaces.
Hydrological Form. The rainscreen functions of an envelope mandate the routing of precipitation away from the structure to the storm drainage system. In many cases this integrates the form of the roof into the topography of the site. Other factors, such as the need to provide a dry cover for entry to the building, may also be considered.