Components and Types of Building Envelope

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Building Envelope
Building Envelope

Building envelope refers to an enclosure of a built environment, which comprises walls, doors, windows, roof, skylights, and other openings for light and ventilation. Building envelope as the totality of (building) elements made up of components which separate the indoor environment of the building from the outdoor environment. The envelope protects the building’s interior and occupants from the weather conditions and other external elements. Design features of an envelope strongly affect the visual and thermal comfort of the occupants, as well as energy consumption in the buildings. Emergence of a building envelope design depends upon the ‘Skin’ (material used), built form, building pattern and building scale or its proportions. A building envelope is usually designed with respect to various factors such as: environmental, technological, socio- cultural, functional and aesthetic.

Building envelope components can be divided into opaque and transparent components. The opaque components include walls, roofs, slabs, basement walls and opaque doors. Transparent components (fenestration system) of a building envelope includes windows, skylights, ventilators, doors that are more than one half glazed, and glass block walls. Common measures of the effectiveness of a building envelope components include physical protection from weather and climate (comfort), indoor air quality (hygiene and public health), durability and energy efficiency. It should also satisfy the user psychologically and environmentally. Psychologically, outside views are very important. Environmentally, the questions that need to be addressed are: how they respond to solar radiation (both from sun’s heat and light), how is ventilation made possible, how is heat loss minimized and how is noise controlled? Development of a building envelope or skin is likely to be rapid in the next decade or so. Technological innovation in glass will allow window systems to respond according to environmental conditions to achieve sustainability in built form.

Building Envelope Components

Building envelope’s component design must take the consideration of both the external and internal heat loads, as well as daylight benefits. Building envelope components are always design with an objective to achieve environmental, technological, socio cultural, functional and aesthetic design factors to achieve its highest workability, efficiency and sustainability. These design factors include study of aspects like maximization of the daylight entrance, controlling the direct sunlight, minimizing the heat gain during overheated period, providing glare control and view to the outdoor environment. To design a building envelope for a space, determination of component’s properties and parameters will help in understanding the physical process of heat, air, moisture and light through the building envelope. In the comfort domain, most important factor is thermal comfort and which is measured by variables like dry bulb temperature (DBT), wet bulb temperature (WBT), Relative Humidity (RH), globe temperature (GT), clo value, heat exchange, air velocity etc., on which various model & equations are already developed. Thermal comfort can be defined as the condition of mind which expresses satisfaction with the thermal environment. This depends upon the environmental parameters such as physiology, psychology and behavioral factors. As such comfort is a complex relationship between parameters such as metabolic rates, the level of clothing being worn, air temperature, relative humidity, mean radiant temperature, local air velocity and radiant asymmetry.

The use of daylight is more of an architectural than a building-systems challenge. Daylight helps in improving the functional determinant of built space and also helps in making social or user’s activities productive. Optimizing the distribution of daylight saves energy and reduces the heats generated by artificial lighting. First and foremost, however, natural lighting is a key factor in architectural design of a space and promotes the visual comfort of the user. Many buildings with sufficient daylight in the interior are nevertheless plagued by problems arising by glare, usually in connection with computer related work. What is needed in these situations is dispersed light or deflection of incident sunlight, which can also improve the distribution of light in the interior. Daylight intensity diminishes rapidly as the distance from the window increases. Given an average room height and fenestration, the maximum depth of natural light in a room is about 6m. Light distribution system can improve the light distribution in the room without, however, greatly expanding the effectively lit room depth. There are some design strategies that make natural lighting in deep interiors possible, for example, sky lights, light domes, light wells and atria.. Sun protection in summer (like shading devices, horizontal & vertical louvers, blinds and shade or wall projection) is vital in connection with these functional, environmental and aesthetic factors.

Environmental determinants plays a very important role and directly impacts the energy efficiency and sustainability of a built space. In the domain of environmental factors, solar radiation is the most important climate factor. As solar radiation is responsible for the heat gain in a built space or form and also lighten up the space. Energy exchange between building and its environment is characterized by a continual crossing of thermal boundaries (building envelope components) between interior and exterior. This unique interior – exterior relationship occurs in various ways, most of which are barely noticeable. Energy exchanges happen due to heat gain through solar radiation and convection processes, which also affect people and the building’s space environment. Externally, the building envelope is subject to solar gain, radiation exchange with its surroundings and convective heat loss or gain owing to the winds that almost continuously flow past it. To control and maintain an adequate environment in the built form, building envelope design takes the lead role. The design of the envelope includes shape of building (massing), fenestration (size, position and orientation), solar control by shading devices and surface finishes,  building facade fabric or material (insulation and thermal storage) and opening size and shape for light and ventilation. In simple terms, this design process and concept is also known as “solar architecture”. In this the main principle is to understand solar geometry, both in cold climates where its utilization can contribute greatly to heating, and in warm zones, where the focus is on avoiding solar incidence in summer. Dependent on cloud cover, global radiation is composed of a direct and a diffuse component. The diffused component of solar radiation is non-directional. Design measures for passive utilization of solar energy are chiefly based on direct solar radiation; which may be influenced by the orientation of the façade.

Determination of the building envelope alternatives can be done for transparent and opaque elements with an objective to achieve environmental, technological, socio cultural, functional and aesthetic design determinant’s parameters. Parameters which are observed to study properties and behavior of components may be separated into two groups i.e. parameters related to an external environment and parameters related to the built environment. Parameters related to the external environment are outdoor air temperature, solar radiation, outdoor humidity, outdoor wind velocity, outdoor illumination level, and outdoor sound level. These values for the local environmental conditions can be obtained from geographic, meteorological and topographical data. A building’s location and surroundings also plays a key role in regulating its temperature and illumination through the building envelope. For example, trees, landscaping, and hills can provide shade and block wind. In cold climate, designing buildings with south-facing windows increases the amount of sun (ultimately heat energy) entering the building, minimizing energy use, by maximizing passive solar heating. Tight building design, including energy-efficient windows, well-sealed doors, and additional thermal insulation of walls, basement slabs, and foundations can reduce heat loss by 25 to 50 percent. Parameters related to the built environment can be obtained separately for opaque components and for transparent components of building envelope with an influence on the control of heat, light and sound, as well as energy conservation as shown in Table 1.

Alternate design of building envelope components is possible by making different fenestration assembly and wall layers combinations. These alternative designs construct different building envelope design types. Building envelope types defines the degree of compactness and identity that can be achieved in different kinds of buildings like office or commercial buildings, educational campuses, housing, school buildings etc. Building envelope different designs would depend on the wall design with opening(s) as well as on the number of layers in wall structure in regard to compactness, natural lighting and solar heat gain.

Types of  Building Envelope

To understand the behavior of building envelope components, categorization of different types of building envelope needed to define suitable combinations for the site and climate. Normally building envelope is categorized into two parts:

  • Single skin façade building envelope
  • Double or multiple skin façade building envelope

As this has been discussed a building element has evolved from past to current architecture style and now it is being used in multiple layers of material or skin in contemporary building façade, which are termed as double skin façade. The benefits for using the concept of double skin building envelope in buildings to encourage sustainable building design and save energy to make built thermally and visually comfortable. In the contemporary period, designers and researchers from the building community have integrated sustainable design concepts that can improve the overall design factors results through enhancing indoor air quality, thermal and visual comfort while conserving energy in buildings. Double skin façade concept is a construction element that is integrated in buildings to achieve several properties that can increase the performance of a building.

Single skin envelope is a basic need for providing enclosure to the built environment. This type of building envelope simply consists of walls (can be of bricks, stones, prefabricated blocks) with opening for fenestration and roof with provision of skylight if required. Extra skin for a single layer envelope always offers improved thermal insulation, which can reduce both cooling demand in summer and heating demand in winter. To describe extra skin on a single building façade, a number of terms have been used like multiple skin envelope, double skin envelope, twin skin, airflow window, and ventilated façade. Concept of a double skin façade is to achieve several properties that can increase the performance of a building. The basics of this system are that an additional skin is applied to a building with a cavity between the external wall and outside façade. The operational and working system of the double skin façade could have the upper hand compared to conventional building façade systems. The basic concept behind is to optimize the properties of the cavity between the two facades, this could result in a decreased need for service installations and mechanical service systems in the building itself. There are several ways to describe and develop different types of a double skin façade. The most common approach of categorizing different types of the system was made by Oesterle. Variation can be done in this kind of system by doing different arrangements of air cavity section like as box window façade, shaft- box window façade, corridor façade, and multi-story double skin façade. These are the most common ways of categorizing different types of facade systems.

Another type of building envelope can be defined as a complete enclosure type envelope. It is an appropriate example of complete enclosure type building envelope which synthesizes many green strategies, as well as pioneers the ‘micro climatic envelope,’ whose vast interior shelters a microcosm of urban life. These are located in two rows of buildings flanking a tapering central street sheltered within the 123,000 square foot glass shed.

One of the major aesthetic and functional elements of this building is its high tech glass envelope which fulfills its technological and environmental design aspects. Motorized openings and timber structure whose tree-trunk columns are exposed along an open front porch serves several purposes. The roof and west elevation are covered with 100,000 square feet of photovoltaic cells, which generate two and a half times the energy consumed by the complex. Even without the photovoltaic, however, the strategy achieves considerable economies in energy use.

Box Window Façade: First typology of façade introduced in building industries. Façade divided horizontally along the building with vertical division. Such a façade is divided into independent window boxes. Good for high levels of thermal and sound insulation. This is the only form of construction that provides these functions in facades with conventional rectangular openings. This system is used for retrofitting buildings to improve performance and easy to apply double skin façade on older buildings.   
Shaft Box Type Façade: This system is based on the box type window. It consists of an independent horizontal box window element with provision of vertical shaft. They benefit the stack ventilation by harvesting solar radiation, high level sound insulation. Shaft can be used for air flow system (natural or mechanical). Since, in practice, the height of the stack is necessarily limited, this form of construction is best suited to lower-rise buildings.   
Corridor Façade: Cavity space or open space between external and internal façade, and divided horizontally by each floor. It is accessible and wide enough to be used as a service platform. Ventilation can be both natural and mechanical. Air-intake and extract openings in the external façade layer situated near the floor and the ceiling. Advantages for corridor facades are that they do not limit the height of buildings.               
Multi Storey Double Skin Façade: There is no horizontal or vertical partitioning between the two skins; instead, the air cavity is ventilated via large openings near the base and roof of the building. It combines the typology of both the corridor façade and the shaft– box. It is used as a supply air facade in winter and as an exhaust air façade in summer. This is suitable where external noise levels are very high and does not necessarily require openings distributed over its height. Multistory façade can be used as a joint air duct. Example: Torrent research center, Ahmedabad, India   

Conclusion

In summary, this could be stated that the building envelope and its components always play an important role in providing a comfortable enclosure for users. Designing a building envelope with consideration of environmental, technological, socio-cultural, functional and aesthetic factors helps in achieving energy efficient and sustainable built forms. Further study of building envelope and its components’ evolution from past technology to contemporary could make the environment of built space user friendly in reference to social culture factors. Study and examine the properties of transparent and opaque parameters would guide in the selection of facades typology for understanding building envelope behavior as per site condition and requirements.

It could be recommended to use an innovative combination of different façade types with application of basic design principle and strategies as well as using renewable energy technology like integration of PV cells into façade components. This would increase overall energy efficiency and environmental quality or determinant of built space like in Mont Cenis Training Center. For moving towards advancement in technology, further research is needed to be done on different components of building envelope with integration of renewable energy considering local site context to achieve sustainable built form. This would be done by determining the peculiarities of design factors in accordance with the physical environment needs of space, technology, materials selection for comfort and aesthetic, climate study and user’s inputs. These strategies would minimize the environmental load as well as fulfill all the requirements of design factors for overall comfort, and be filled with graceful techniques for making maximum use of natural energy.

Authored By:- Animesh Nag, Structural consultant. PD. Management Consultant, Kolkata