Characteristics of Sustainable Building Envelopes

Building envelopes in sustainable buildings are different from conventional buildings in that they aim to achieve a wider range of objectives and work in a different way.

Some characteristics of sustainable building envelopes are:

1. Responsive: Green building envelopes are designed to respond to their local context and work with external and internal conditions to achieve optimum environments within and around the building. Therefore, the building envelope may have additional acoustic treatment in areas which receive noise from external environments and have strong visual and physical connections (through balconies, windows and external doors) where external light and thermal conditions support human comfort.  
2. Dynamic: In order to achieve optimum conditions on an ongoing basis, green building envelopes are dynamic and adapt to changing conditions. Thus, more of the building envelope may have shading in summer than in winter to minimise unwanted heat gains. In winter, the envelope may allow more sunlight to enter the building than in summer to allow this to warm the building.  
3. Controllable: Providing users with greater control over local environments is a central strategy in most green buildings. Building envelopes, therefore, are likely to have large numbers of operable windows that can be easily opened and closed by occupants. They may also have controllable internal blinds and external solar shading which can be used to maximise internal daylight quality and avoid glare and solar gain. 
4. Ecological: Green building envelopes aim to support the development of ecosystems and plant and animal life around the building. Therefore, the envelope may be used to create habitat for animals such as birds and the roof and balconies may be planted. 
5. Breathable: Designers of green building often try and achieve the same performance qualities found in good outdoor clothing. The outer layer of the building envelope, like a raincoat and umbrella, provide protection against weather such as wind and rain. The middle layer, like shirts and jerseys, provide warmth and thermal insulation. The inner layer, like a vest, is comfortable to touch and wicks away excess moisture. 
6. Microclimatic: The building envelope is used to support the development of local microclimates. Thus, envelopes may be used to create sheltered, sunlit spaces around buildings as amenity areas for occupants.  They may also be used to create vegetated, shaded areas from which cool, fresh air can be drawn into the building. 
7. Energy generation: Building envelopes provide excellent opportunities to generate renewable energy for use in the building. This is done through photovoltaic and solar water heating panels and wind turbines. Ideally these are integrated in the design of the building envelope to improve the aesthetic quality of the building and minimise material requirements. 

A brief introduction to sustainable building envelopes can be accessed here.

Defining Sustainability

There are many definitions for sustainability. Probably one of the most well known is: 

“…development that meets the needs of current generations without compromising the ability of future generations to meet their needs and aspirations” (World Commission on the Environment and Development 1987).

However this definition, and similar ones, may have inadvertently been a stumbling block to the implementation of sustainability in built environment as the definition could not be readily translated into action. Button (2002), for instance, suggests that this definition has a biblical vagueness. He argues that this definition is very difficult to apply to urban areas as it only refers to temporal and generational effects of sustainability, without addressing key geographical aspects and the inherent dynamism of cities.

Newer definitions of sustainability are more relevant to the built environment. A number include resilience as a preferred, or essential, characteristic of sustainability. López-Ridaura et al (2005), for instance includes resilience as a key attribute of sustainable systems:

“..the degree to which a system is sustainable will depend on its capabilities to produce, in a state of stable equilibrium, a specific combination of goods and services that satisfies a set of goals (the system is productive), without degrading its resource base (the system is stable)
even when facing ‘normal’ (the systems is reliable), ‘extreme’ and ‘abrupt’ (the system is resilient) or ‘permanent’ (the system is adaptable) variations in its own functioning, its environment or co-existing systems”.

One of the most relevant definitions of sustainability for the built environment has been developed by the World Wildlife Fund (WWF). This describes sustainability as being the achievement of above 0.8 on the Human Development Index (HDI) and the achievement of an Ecological Footprint (EF) below 1.8 global hectares per person (World Wild Life Fund, 2006).

The Human Development Index was developed by the United Nations as an alternative to economic progress indicators and aimed to provide a broader measure that defined human development as a process of enlarging people’s choices and enhancing human capabilities (United Nations Development Programme, 2007). The measure is based on:  

• A long healthy life, measured by life expectancy at birth
• Knowledge, measured by the adult literacy rate and combined primary, secondary, and tertiary gross enrolment ratio
• A decent standard of living, as measure by the GDP per capital in purchasing power parity (PPP) in terms of US dollars

An Ecological Footprint is an estimate of the amount of biologically productive land and sea required to provide the resources a human population consumes and absorb the corresponding waste. These estimates are based on consumption of resources and production of waste and emissions in the following areas:

• Food, measured in type and amount of food consumed
• Shelter, measured in size, utilization and energy consumption
• Mobility, measured in type of transport used and distances travelled
• Goods, measured in type and quantity consumed
• Services, measured in type and quantity consumed
• Waste, measured in type and quantity produced

The area of biologically productive land and sea for each of these areas is calculated in global hectares (gha) and then added together to provide an overall ecological footprint(Wackernagel and Yount, 2000). This measure is particularly useful as it enables the impact of infrastructure and lifestyles to be measured in relation to the earth’s carrying capacity of 1.8 global hectares (gha) per person.