Specification

Specifications for building envelopes may be wholly bespoke or follow one of the recognised building envelope specifications.  However, all specifications have to be customised to take account of the external climate and the building use.

The CWCT ‘Standard for systemised building envelopes’ gives a framework for specifying building envelopes and provides a ‘Specifiers checklist’ showing information that will change from project to project.  This includes:

  • Internal and external environment
  • Air permeability
  • Thermal performance
  • Access and safety
  • Design life

Both the National Building Specification (NBS) and National House Building Council (NHBC) standards are based on the CWCT ‘Standard for systemised building envelopes’.  There are many British Standards relating to building envelopes but simply listing standards is no substitute for a comprehensive specification.

Building Envelopes

The new CWCT 'Standard for systemised building envelopes' is applicable to:

  • Curtain walling
  • Walls with ventilated rainscreens
  • Overhead glazing

Previously these forms of construction were covered by separate CWCT Standards.
  The Standard covers:

  • Weathertighness
  • Loading and movement
  • Thermal and moisture performance
  • Acoustics
  • Fire performance
  • Operable components
  • Safety
  • Service life

The Standard may be purchased here.

Glazing

The performance and quality of glass is described in nearly 100 British and European Standards. CWCT has not yet published detailed guidance on the use of glass in the building envelope’ although it has plans to do so.

However, we have published several technical notes on glass here

Meanwhile we can advise members on:

  • The relevant Standards for particular types of glass and usage.
  • Glass types
  • Appropriate use of glass
  • Mechanical strength of glass
  • Glazing for environmental control
  • Limitations on the use of glass
  • Glazing systems.
  • Stone

    When specifying stone it is necessary to cover three main aspects of performance;

    • Durability
    • Strength
    • Stone fixing

    CWCT has produced two guides to help the stone specifier.  These are:

    • Guide to the selection and testing of stone panels for external use
    • Performance and testing of fixings for thin stone cladding

    The documents may be purchased here.

    Stone walls have to meet the normal weathertightness and other performance criteria of the building envelope.  Stone panel walls are most frequently constructed as ventilated rainscreens and the specifier should also use the ‘CWCT standard ‘Standard for walls with ventilated rainscreens’.

    A new single document on the specification of stone panels and their fixing is under development.

    NBS

    The National Building Specification (NBS) is one of the most widely used frameworks for specifying construction in the UK.  It provides architects with specification clauses that match the architectural packages of work.  In particular:

    • NBS Section H11 – Curtain walling invokes the CWCT Standard for curtain walling
    • NBS Section H92 – Rainscreen cladding invokes the CWCT Standard for walls with ventilated rainscreens


    Further details of the National Building Specification are available here.

    NHBC

    The National House Building Council (NHBC) sets standards for all new dwellings that are covered by its warranty scheme.  This is around 85 percent of all new homes constructed in the UK.

    With the trend to medium- and high-rise blocks of apartments the NHBC has introduced its standard for building envelopes.  This invokes the CWCT ‘Standard for curtain walling’ and ‘Standard for walls with ventilated rainscreen’.

    More information about the National House Building Council is available here.

    History

    CWCT publishes standards for building envelopes. The table below summarises the current and earlier Standards along with their dates of currency:

    Curtain walling Ventilated rainscreen walls Slope glazing Guide to good practice for facades
    Standard for curtain walling, First edition, Published January 1993. Superseded August 1996. Standard for ventilated rainscreen walls, First edition, Published January 1999. Superseded March 2006. Standard for slope glazing systems, First edition, Published July 1998. Superseded March 2006. Guide to good practice for facades, First edition, Published January 1993. Superseded August 1996.
    Standard for curtain walling, Second edition, Published August 1996. Superseded March 2006.   Advice on selection of overhead glazing not superseded Guide to good practice for facades, Second edition, Published August 1996. Partially superseded April 2006.
    Standard for sytemised building envelopes, Published April 2006. Standard for sytemised building envelopes, Published April 2006#.

    #  Standard for systemised building envelopes covers access, workmanship, quality and maintenance. The remaining parts of the Guide to good practice may be used but note that they were published in 1996 and are no longer maintained. They may be viewed here.
    N.B.  The dates of publication given above are not the same as the copyright dates printed in the documents.

    The following Technical Updates have also been issued:

    TU1 # February 2000 Dynamic watertightness tests
    TU2 February 2000 Air and water testing with a site cabinet
    TU3 # June 2000 Condensation on glazing frames
    TU4 $ December 2000 Use of glass overhead
    TU5 % March 2002 Building Regulations Approved Document L2 (England and Wales) - showing compliance of curtain walls
    TU6 August 2002 Dynamic watertigthness testing of slope glazing
    TU7 August 2002 Buckling of curtain wall mullions
    TU8 # March 2003 CwCT Standards and European standards
    TU9 % February 2004 Specification and verification of thermal performance for Part L2 of the Building Regulations
    TU10 December 2003 Use of glass overhead
     
    TU11 March 2008 Acceptable water leakage
     
    TU12 April 2008 Redundancy of fixings
     
    TU13 March 2008 Measurement of displacement
     
    TU14 July 2009 Load combinations
     
    TU15 November 2010 Replacement of British Structural design codes by Eurocodes
     
    # Superseded by Standard for systemised building envelopes
    $ Superseded by TU10
    % Obsolete following publication of Part L 2006

    Technical updates are available in the Cladding Forum or from CWCT.

    Archive

    Standard and guide to good practice for facades
    This second edition of the 'Guide to good practice for facades' was published in August 1996 and is no longer maintained by the CWCT Technical Committee. Parts of the document have been superseded by the CWCT 'Standard for systemised building envelopes'. The guide may be viewed by using the links below:

    Section 4  Materials and components

    Section 5  Finishes

    Section 6  Quality

    Section 7  Fabrication

    Section 8  Handling and storage

    Section 9  Installation

    Section 10  Maintenance