SpecificationSpecifications 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.
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:
- Loading and movement
- Thermal and moisture performance
- Fire performance
- Operable components
- Service life
The Standard may be purchased here.
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:
When specifying stone it is necessary to cover three main aspects of performance;
- 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.
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.
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.
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 $||December 2000||Use of glass overhead|
|TU5 %||March 2002||Building Regulations Approved Document L2 (England and Wales) - showing compliance of curtain walls|
|TU6||August 2002||Dynamic watertigthness testing of slope glazing|
|TU7||August 2002||Buckling of curtain wall mullions|
|TU8 #||March 2003||CwCT Standards and European standards|
|TU9 %||February 2004||Specification and verification of thermal performance for Part L2 of the Building Regulations|
|TU10||December 2003||Use of glass overhead|
|TU11||March 2008||Acceptable water leakage|
|TU12||April 2008||Redundancy of fixings|
|TU13||March 2008||Measurement of displacement|
|TU14||July 2009||Load combinations|
|TU16||July 2020||Site Hose Testing|
|TU15||November 2010||Replacement of British Structural design codes by Eurocodes|
|TU18||August 2015||Displacement at supports|
|TU19||April 2015||Dynamic watertightness test|
|TU20||March 2015||Structural calculations – CE marking of curtain walling|
|TU21||August 2014||Safety and fragility of roof glazing|
|TU22||March 2014||Allowable deflections - distributed with Standard for systemised building envelopes, 2006|
|TU23||October 2011||Calibration of test chamber for air leakage - from November 2011, distributed with Standard for systemised buildings envelopes, 2006|
|TU24||January 2011||Response to Approved Document L, 2010|
|#||Superseded by Standard for systemised building envelopes|
|$||Superseded by TU10|
|%||Obsolete following pu|
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: