What is a Building Envelope?

Functions of a building envelope

The main function of the building envelope is to provide a controlled internal environment that protects from the variable and uncontrollable external climate. The nature of internal environment is dependant on the end use of the building and this in turn will determine the requirements for the building envelope.

Building Envelope
Photo: CileMontgomery

An office building will require control of internal temperature and humidity with appropriate levels of daylight to provide a pleasant working environment. Whereas with a storage barn the only requirement may be to keep the enclosed space dry. Principle building components

There are essentially three layers to the structure:

  • The primary steel frame
  • The secondary steelwork
  • The roof and wall cladding

The primary steel frame consists of columns, rafters and bracing.

The secondary steelwork consists of side-rails and purlins, these members serve 3 purposes:


1) To support the cladding
2) To transfer load from the cladding to the primary steel frame
3) To restrain the primary steel frame members Roof and wall cladding

It’s functions include some or all of the following:

  • Separating the enclosed space from the external environment
  • Transferring load to the secondary steelwork
  • Restraining the secondary steelwork
  • Providing thermal insulation
  • Providing acoustic insulation
  • Preventing fire spread
  • Providing an airtight envelope

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