The appearance, quality and performance of a building envelope are totally dependent on good construction.  The use of qualified installers and trained supervisors is the single most effective way to achieve a good building envelope.

The building envelope may be installed by specialist contractor direct labour or by a labour only subcontractor.  In both cases it is recommended that installers are trained by the company that designed the system and supplied the materials.  CWCT has implemented a system of installer training leading to a CWCT ‘Installers card’.

It is equally important that architects, engineers, main contractors and others with a supervisory role on site are trained and capable of inspecting the work, again CWCT have implemented suitable training programmes.

As a check on workmanship site testing may be undertaken during installation and on completion.  This may include:

  • Whole building air leakage testing
  • Hose testing for watertightness
  • Thermal imaging
Specialist Contractors

Specialist Contractors are responsible for the construction of the building envelope. They may undertake the installation themselves or may use an installation sub-contractor.

The Specialist Contractor is also responsible for detaiiled design and should be able to design for buildability taking account of tolerances, component sizes and configuration.

The employment of a Specialist Contractor with appropriate skills and experience is essential if the building envelope is to be erected on time to the required standard. Advice on procurement is available here.


Installation is generally the responsibility of the specialist contractor.  It may be undertaken by directly employed labour or by a labour only installation subcontractor.  When using installation sub-contractors it is important that the installers are competent to install the wall to the required standard and quality.

Competent installers have both generic skills and knowledge and also specific knowledge of the cladding system being installed.

The knowledge required comprises:

  • General knowledge of glass and glazing, gaskets, sealants, hardware
  • Particular knowledge of how the system being used is assembled including, joint assembly, drainage and bracketry.

CWCT has established a training scheme where installers may be trained by the system designers.


Supervision of installation

It is essential that all those who purport to supervise building envelope installatin have adequate knowledge.  CWCT run a course ‘Managing the cladding package’ that is attended by architects, main contractors and specialist contractors to teach an understanding of the ‘nuts and bolts’ of cladding and glazing.


Practical guidance

  • CWCT have produced a guide to installation that may be downloaded here.  It should be noted that this guidance is no substitute for proper training.
  • The better system suppliers and specialist contractors are able to supply simplified site drawings and/or exploded views of mullion-transom joints and the like.  These facilitate both installation and checking.

The quality and performance of a bulding envelope are totally dependent on the standard of workmanship and knowledge available at site. Workmanship is a dominant cause of premature failures, delayed completion and additional costs arising from rework.

It follows that adequate and knowledgeable supervision is equally important.


Qualified installers

CWCT has developed a training scheme for curtain wall installers and over 1500 installers have been trained by all of the major curtain wall system suppliers. Details of the scheme are available here.

To ensure that competent installers are used on a project it is possible to specify that at least a proportion of installers should be qualified and hold a CWCT installers card. This implies knowledge of the components being used. Other qualifications such as CSCS cards are based only on generic knowledge.


Trained site supervisors

CWCT provides training for those with a supervisory role at site. This includes the 'Managing the cladding package' course and similar in-house courses.

Site Testing

Site testing may be used to check workmanship, occasionally it is used to check design performance. As a check on design performance it is no substitute for controlled testing of a specimen in a laboratory. Lessons learnt from site testing will not be fed in to the design but will only be used to assess options for rectification work.

Site testing to assess wokmanship should be timely. It should be undertaken early in a project if the desire is to improve workmanship, and at the end of construction if it is to prove compliance.


Air leakage testing

Whole building air leakage tests are required by Part L of the Building Regulations (England and Wales). Air leakage arises principally from poorly designed interfaces between cladding packages. A whole building air leakage test checks that interfaces and other sources of air leakage are correctly sealed. Guidance on whole building air leakage testing is given in CWCT Technical Note TN44


Water testing

Water penetration resistance may be tested to check workmanship or design. Workmanship is checked using the hose test whilst design is tested using an improvised cabinet test or in the case of patent glazing and rainscreens a spray bar test. All of these tests are described in the CWCT Technical Note TN41.



Thermal imaging can be a useful way to identify thermal bridges, the location of air leakage and other factors that affect surface temperature of the building envelope. Thermal images have to be interpreted with care to separate the different factors that may lead to variations in surface temperature. The CWCT has published Technical Note TN45 which describes thermography and its use as a diagnostic tool for building envelopes.