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Providing a suitable external envelope for major projects has long been principal challenge aspects for designers, contractors and manufacturers. So much has been learned from the experiences of the 1970s and 1980s that today there is a marked improvement in choice, quality and performance. But problems can still arise.

The design of an external envelope has to strike a balance between visual intent, performance requirements, material selection, workmanship, testing and installation methodology, all within pre-set budgeting constraints.

The specifying of external envelopes has been made far easier by the Standard Guide to Good Practice for Curtain Walling, produced by the Centre for Windows and Cladding Technology at Bath University in the early 1990s. The centre's publications combine the expertise of many knowledgeable and experienced practitioners.

Consider the issues

When specifying external envelopes, specifiers should bear the following points in mind:

• Can an existing cladding system be used or adopted, or does the building need a bespoke cladding solution? The advantages of using an existing system are that engineering and testing have already been carried out and existing installations can be examined, thus reducing costs. Bespoke cladding systems are generally appropriate only on larger projects where the budget and programme allow for the cost and time of prototype production and testing prior to manufacture.

• Does the designer's appointment require him to deliver a fully detailed design? It is widely accepted that, in order to achieve the most appropriate cladding solution backed up by "real" warranties, the cladding manufacturer should be involved in the detailed design of the system. The level of this design input and its timing are critical to the way the specification is written.

• Can the prospective manufacturers be relied on to provide a fully detailed design within the parameters set by the specifiers? Proper and detailed pre-qualification of tenderers is an essential function. Different manufacturers specialise in different types of product, and it is important to compile a tender list confirming the firms that will be able to offer suitable systems, as well as adequate design input.

• Does the designer need help from a specialist consultant to fulfil his contractual obligations? The main factors to be considered are affordability and design responsibility. Cladding consultants can work equally well for designers and contractors but their role is different for each, as very often designers do not want to produce a fully engineered solution, whereas a manufacturer does.

  If the need for a specialist consultant is identified, the designer should ensure that the resulting design is not too detailed. This is especially important when performance-based documents are used, as the main objective is to get a technical response and solution from manufacturers based on their own preferred methods and expertise, and not to "tie their hands" in a single solution. Always remember that warranties are provided by contractors, not designers.

• What procurement provision and form of contract is being adopted for the project, and what is the timescale for the production of tender/contract design information?

  The specifier should check the level of drawn information being provided for tender, which should suit the performance and design-intent criteria stated in the specification. Too much detail tends to restrict specialists; too little will result in loss of the design intent. It is important for the specifier to ensure that an achievable solution is specified, having checked with manufacturers during the pre-qualification period.

Address the requirements

Once the above issues have been established, the specifier should address the specific requirements for the building, the key elements of which are:

• Incorporating detailed descriptions of each type of cladding, along with its key interfaces and locations. These should relay to the tenderer the precise design intent and visual characteristics that the designer is looking for, as shown on the drawings.

• Defining the contractor's post-tender and post-contract design responsibilities.

• Stating the required quality control standards, including the need for testing, and the submission of samples, prototypes, site benchmarks and so on. These should be determined at tender stage as they have cost and programme implications.

• Clearly stating the required performance criteria, including structural, environmental, acoustic, fire, security and durability, for all cladding types.

• Stipulating appropriate levels of materials and workmanship required. Some flexibility in this area is often needed, and details should be resolved before the contract is awarded.

  Ideally, the earlier the designer can work with the specialist the better, but unfortunately this is all too often restricted by contractual barriers.