Following the laws of thermodynamics, a system can only be in equilibrium when it is a closed system. The envelope forms the limit to the building’s systems and establishes the processes of homeostasis between a space and the outside. Certain envelopes-such as fortresses-are designed to stabilize the regimes of occupation of space, while others are designed as a trigger of exchange, climatic or economic. The capacity of the envelope to regulate the exchange of temperature, of air, of light, of people’s flow may turn the buildings into closed systems (i.e. sealed envelopes are closed environmental systems that rely on mechanical ventilation, heating and humidification to preserve a stable climate inside of the building). Fences around gated communities or concentration camps are aimed at stabilizing the population inside the space they delimit.
The envelope is the element that opens or closes the building or the urban systems, and therefore the element that “stabilizes” the building population, climate, energy balance, etc. As the recent history of architecture has been driven by a constant tension between the positivist belief in progress, planning and determination, and the use of disorder and destabilization to induce change, the envelope becomes a crucial element in order to plan the built element.
One of the most important factors of performance of a building envelope is its capacity to react to changing climatic conditions such as daylight, solar exposure, ventilation, but also security controls and pedestrian flow and even iconography. Time may have an effect in the iconography of the building as a changing membrane capable to adjust to solar paths, seasonal changes, and different conditions of use, and permeability.
The importance of phasing in the very large complexes such as trade fairs and retail complexes, may also place crucial constraints on the determination and design of the building envelope.
The instability and mobility of the envelope, its capacity to mutate its permeability to adjust to changing conditions is one of the capacities that can be used to produce particular effects, social or environmental.