The design of a building envelope will determine:
  • Building appearance
  • Internal building environment
  • Whole life costs of energy and envelope maintenance
  • Life to refurbishment
These pages give some initial guidance on who designs, what influences design, design process and design outcomes.
    Who Designs?

    Many parties may be involved in the design of a building envelope including:

    • Client
    • Architect
    • Engineer
    • Main contractor
    • Specialist contractor

    Clients and architects are predominantly involved in scheme design.  Traditionally it has been assumed that most scheme designs could be ‘made to work’ at the detailing stage.  This is no longer the case and seemingly plausible schemes may be impossible to construct for reasons of building physics or structure.

    Increasingly building services engineers are being drawn into the building envelope design process as the energy performance of buildings is dominated by the building envelope performance. Structural engineers are required to consider building and façade movements and cladding loads.  It is important to involve engineers at the scheme design stage to at least ensure that what is proposed may be built in compliance with the Building Regulations and the laws of physics.

    Main contractors and specialist contractors can make a valuable contribution to design, as they are aware of issues affecting the ease of construction that will impact on cost and construction time.  Contractors are also likely to request design changes where they are necessary to ensure safety during or after construction.  These late design changes can be avoided by involving the constructors at an early stage in the design.

      Design Options

      With modern non-traditional facades the link between appearance and form of construction is more flexible.  For instance, stone may be placed using traditional methods or may comprise thinner slabs bolted to a carrier frame.  Conversely metal panels may give a modern appearance to a traditional blockwork wall when they are used as overcladding.

      It is common practice to think of design options in terms of materials, appearance and image, and indeed the client may well express preferences in terms of precedent buildings.  However, the constructor will be thinking in terms of function, buildability, safety, time and cost.

      These latter aspects of the façade depend on the following primary decisions:

      • Should the façade be a heavyweight or lightweight construction?
      • What areas of glass should be used and in what orientation?
      • Should the façade be prefabricated or site-assembled?
      • Should the façade be a bespoke design or a standard system?
      • What are appropriate combinations of primary structure and façade?


      Design guidance

      CWCT has written a number of design guides as follows:


      Bespoke or standard design:

      Although two building envelopes are seldom the same they are equally unlikely to be entirely bespoke. Most building envelopes are constructed from systems of components. These offer benefits over bespoke design but are unable to deliver a completely unique construction. Some of the benefits to consider are:

      Benefits of standard systems Benefits of bespoke design
    • Cost
    • Shorter design and manufacturing period
    • Familiarity of construction
    • Use of existing test results
    • Appearance
    • Greater structural capapabilities
    • More advanced shading devices
    • Standard system walls can be given a variety of styles by selecting appropriate infill panels and additional items such as brise soleil. It has to be remembered that even the most standard of systems will have bespoke components such as brackets, flashings, and interfaces with adjacent cladding. Modified standard systems are sometimes used. In these one or more of the framing profiles, such as a pressure cap, is changed but the structural and sealing components remain unchanged.

      Bespoke systems comprise framing components that are bespoke to a project. The profiles are designed and dies cut to extrude the required cross-sections. Note that bespoke wall may also comprise standard systems components were appropriate.


      Envelope types:

      Walls are constructed using on of the basic forms of construction shown below. Further information is available in CWCT Technical Notes:

    • TN14 Curtain walling types
    • TN15 Cladding types
    • Stick construction
      Unitised construction
      This is the most common method of constructing curtain walls in the U.K. although it is less popular elsewhere.  Simiar techniques are used to assemble roof glazing.

      Stick construction consists of a grillage of mullions supported from the floor slabs and transoms.  These are assembled at site and the resulting openings are infilled with glazing or opaque panels.

      It has the advantage that it is economical and the construction sequence is flexible.  Disadvantages are the lengthy intallation time and high dependence on site workmanship to ensure acceptable quality.

      Unitised construction consists of pre-fabricated units normally the width of a glazing bay and one or two storeys in height.

      Units are supported from the edge of the floor slab.  Units are delivered to site glazed and infilled and only the unit perimeter seals are made at site.

      Unitised construction is marginally more expensive than stick construction but normally leads to a higher quality wall.  It should be noted that the choice of materials has a greater effect on cost.

      Unitised construction is quicker to install but requires better planning and larger lead times are required.

      Panellised construction
      Panellised construction comprises pre-fabricated panels that span from column to column. The panels are supported from the columns and not influenced by movement of the floor slabs.

      The panels require structural integrity to transfer the loads on them back to the columns. Either a precast concrete panel or a steel truss is used. Hence the North American term 'Truss wall'.

      Panellised wall can be installed more quickly than other forms of wall.

      Rainscreen is a plane of panels designed to protect the wall from rain. It may be constructed as overcladding supported on a brick or block wall or as an integral wall supported from mullions or studs spanning from floor to floor.

      Rainscreen overcladding is often used on refurbishment projects but may be used to advantage when constructing new buildings.

      Integral rainscreen walls supported from framing members are dependent on an inner air barrier to carry wind load and give the required air permeability.

      Structural glazing
      Structural silicone glazing


      Structural glazing comprises frameless glazing in which each glazing unit is supported by point fixings, normally bolted connections.

      The glazing units may be supported individually from a separate framed structure similar to that shown or a stick construction curtain wall. Walls of this type are frequently called 'Planar' walls although 'Planar' is a brand name and other products exist.

      In true structural glazing the glass components transfer load. It may comprise glazing units suspended one from another or glazing units supported from glass fins or portals.

      Structural sealant glazing is a method of bonding the glazing units to a frame. This has the advantage that the glazing appears from the outside to be frameless.

      In practice the glazing is bonded to a carrier frame that is bolted to a framing system such as that for a stick construction curtain wall.

      The glazing may be bonded on two edges (and framed on the other two) or it may be bonded on four edges. In the latter case small clips may be provided to provide mechanical retention.

      Insulated panels
      Built-up cladding


      Insulated panels comprise a a layer of insulation faced with metal or plastic sheets. The perimeter comprises a profile that enables them to interlock to provide a sealed structural connection.

      Insulated panels may be manufactured with windows installed but more normally windows are installed at site as the wall is assembled.

      Insulated panels have to be supported from a framework of columns and/or purlins. Profiles are often available to allow the use of different window systems as part of an insulated panel system.

      Built-up walls are constructed at site and comprise inner and outer skins with insulation and a structural frame between them. Windows and doors may be included in the construction.

      Profiled steel walls for industrial buildings are constructed in this way. Recently these construction techniques have been adopted for residential buildings and other materials have been used as the inner and outer skins.

      This method of construction may be used with a rainscreen outerlayer provided a cavity is included for drainage.

      Scheme Design

      Scheme design for a building should include a comprehensive review of the intended building envelope. In addition to providing the required apearance the building envelope design:

      • Has a dominant effect on the energy use and hence carbon dioxide emissions from a building
      • Determines the method and speed of construction
      • Affects the safety of construction, operation and maintenance

      The building envelope also has a major influence on the cost of a building, directly as a capital cost and indirectly in operating costs. The building envelope is the weathering surface of the building and its durability will determine the future value of the building.


      Construction and maintenance

      The method of construction and construction timescale will be heavily influenced by the scheme design.

      The lead time from entering a contract to commencing work on site will depend on the originality and complexity of the design. A totally bespoke wall will take 40 to 50 weeks to detail design, gain approvals, procure materials and manufacture. By comparison the lead time for a totally standard system wall to be arranged as a single plane of the envelope will vary from 10 to 16 weeks depending on the procurement time for the glazing.

      Ease of access and requirements for safe access are all dictated at the scheme design stage where complex geometries have to be matched with suitable access arrangements. The CWCT 'Guide to safe access' gives guidance on this.


      Energy issues

      The area of glass and its orienation are set at an early stage of the scheme design and yet they totally dominate the service requirements for the building and its energy use. As limitations on energy use have been tightened it is possible to design buildings based on precedent that are now impossible to build. CWCT 'Guide to the thermal assessment of walls' gives guidance from scheme design through to detailed calculations.



      The scheme design may suit a particular construction technique or may make the building almost impossible to construct by any means.

      The building envelope has to be co-ordinated with the building structure from the earliest design stages. Decisions on frame centres and panel sizes affect cost, construction method and ease of construction. The juxtaposition of components determines allowable tolerances and ability to construct and repair.



      The choice of materials to be used in the building envelope governs the service life of the envelope and therefore the refurbishment cycle of the whole building.

      Some commonly used materials have service lives that may be greatly reduced by inappropriate use. Conversely the service life of a component can be much extended by suitable choice of materials. The CWCT publishes a 'Guide to durability' for use by its members.

      Detail Design

      Specialist contractors are defined as those subcontractors who provide a design input as well as undertaking manufacture and construction.  With the exception of some simple window contracts all building envelope contracts have a design element within them.

      Even a plain wall of stick construction will have bespoke components designed by the envelope contractor.  These include:

      • Bracketry
      • Perimeter interfaces
      • Penetrations through the envelope


      Engineering design

      Most building envelopes are specified using a performance specification. This transfers certain design resonsibility to the specialist contractor. The specialist contractor will exercise engineering judgements when interpreting a performance specification; in particular how performance requirements are to be met. There are many design solutions that may be used to meet a project specification.


      Selection of components

      The specialist contractor will select the components and materials to be used in the building envelope unless they are specified prescriptively, for example, the specialist contractor will select the glass type, operating hardware and so on. They may even select the framing system to be used.


      Design detailling

      The specialist contractor will produce workshop drawings and site drawings. They will always decide on joint locations, bracket design and other bespoke elements of the building envelope. They will detail additonal elements of the building envelope such as brise soleil. It is important that a specialist contractor with appropriate design skills is employed.


      When designing a building envelope it is necessary to be mindful of how it will be procured and who from?  The envelope will eventually have to be design detailed, fabricated (manufactured) and constructed.

      The good designer will always have in mind a specialist contractor who has the required mix of skills and capacity at the right price.  In that way he will not design something that cannot be built, other than at exceptional expense, and will normally be able to find at least a second company to bid competitively for the work.

      The larger custom curtain wall contractors are used to doing most things, at a price. Small projects of completely standard solutions using a single system may be readily procured from a number of specialist contractors (the system company’s dealer network).  Greater difficulty is experienced if a system is to be partially customised, of complex geometry or include features such as automatic blinds and windows.


      Identifying appropriate specialist contractors

      The success of any building envelope contract depends on the employment of a specialist contractor of appropriate size and with relevant skills and knowledge. CWCT manages a directory of specialist contractors showing the size of contract they typically undertake and the scope of their work.  The directory of specialist contractors can be accessed here.


      Better procurement

      Procurement practice continues to evolve and CWCT runs a course on best practice for building envelope procurement.  Speakers from industry and CWCT cover: design options, specification, design procurement, shortlisting of specialist contractors and the tendering process,  Details.


      Types of specialist contractor

      Specialist envelope contractors take several forms.  The different business models are based on the following activities:

      System design and manufacture is the design of framing members and other components.  The system may be developed for a single construction project or may be placed in the market place by a system company for use by more than one specialist contractor and on many buildings.

      Project design is the arrangement of system components to match the architect's scheme design and realise the building envelope.  This may be undertaken by the specialist contractor or may be subcontracted to a design house.  Where the project is to use a system company's products the system company may advise the project designer but will not generally be responsible for design.  Note that the input from the system company will vary from system to system and the specialist contractor may not need, or wish, to take advice.  The design team will produce workshop and site drawings.

      Fabrication is the machining of framing members and panels, the forming of flashings and assembly including fitting of hardware and so on.  Fabrication includes all factory based activities involved in the manufacture of the wall.

      Installation is the site operation of fixing and sealing the envelope on the building.  It includes all activities of fixing, assembly, glazing, cleaning down and commissioning the envelope.
      Basic business models exist as described below. However, there are many variations on these four models depending on whether design is subcontracted, manufacturing is factored or components and assemblies are bought in.

      a) b)
      System design
      System design
      Project design
      Project design

      In this model a single company, the specialist contractor, undertakes all of the activities required to realise the building envelope. Once common, this method of working is now restricted to the largest companies and some smaller companies undertaking small but highly bespoke contracts.


      Companies following this model subcontract the installation to an independent installation company. This enables them to work economically over a wider geographical area. These companies may undertake envelopes of the greatest complexity but are generally smaller companies than those following model (a) and may not undertake the largest contracts. Good installation depends on good inter-company communication and adequate on-site supervision.


      c) d)
      System design
      System design
      Project design
      Project design

      In this model the specialist contractor is independent from the system company that supplies components. This has become the most common model for realising all but the largest envelopes in the U.K. Although specialist contractors rely on a system of components many are able to modify the system design if required. Companies of this form do not generally undertake contracts in excess of £5M but this limit is due to commercial constraints other than the business model. Good design depends on the provision, and acceptance, of appropriate support and advice from the system company.


      In this model the specialist contractor relies on separate companies for both system supply and for installation. This is a common variation on model (c) for no other reason than that specialist contractors do not have to keep installers fully employed. Again good design depends on the provision, and acceptance, of appropriate support and advice from the system company. Good installation depends on good inter-company communication and adequate on-site supervision.