Building envelopes are expected to meet many performance criteria.  These cover:
  • Safety
  • Building physics
  • Integrity
  • Serviceability
  • Durability
A comprehensive guide to performance for specifiers is available in the CWCT Standards. More detailed information on specific aspects of performance may be accessed through this page.
    CE Marking

    With effect from 1 July 2013 curtain walling and windows will have to be CE marked in accordance with the Construction Products Regulation.

    The responsibility for CE marking of a curtain wall lies with the specialist contractor but system and component suppliers will need to test their products and provide supporting documentation to their dealers.

    Guidance for CWCT members is available in the Cladding forum here.

    CWCT gave a series of regional workshops on CE marking of curtain walling. The presentations made at those workshops by Stephen Ledbetter may be downloaded here

    The following documents may also be downloaded.

    • CWCT Briefing note - this gives guidance on the application of CE marking to curtain walling and explains the duties of the system supplier and the specialist contractor. It may be downloaded here
    • CPA Guidance note - published by the Construction Products Association and the Trading Standards Institute, this document gives more general advice on CE marking of construction products. It may be downloaded here
    • Powerpoint presentation - talk given at industry meetings and CWCT Members' meeting, this talk covers the scope and process of CE marking of construction products. It may be downloaded here

    It is fundamental to building envelope performance that envelopes:

    • Are resistant to water penetration
    • Control air leakage
    • Resist wind loads

    It is equally important that building envelopes remain weathertight throughout their service lives.


    Secondary defence

    The principle of secondary defence is that any water leaking past an outer defence in the envelope should be intercepted and drained to the outer face of the wall. This principle is always applied to ventilated rainscreen which should have a drained and ventilated cavity behind it. It is also used in most glazing and framing profiles that contain drainage holes for any water that leaks past the outer glazing seals. In these systems the outer face is the primary barrier to water penetration whilst an inner seal acts as an airseal and secondary defence against water penetration.

    Building envelopes that are not built as a drained construction have to be face sealed to prevent water penetration. The single outer seal is exposed to weathering and is prone to failure due to accelerated ageing or poor workmanship. Failure of an outer seal in a face sealed envelope always allows water to leak into the building.

    Envelopes built on the principle of secondary defence and properly drained are far more robust than face sealed constructions.


    Suitability for site

    The water penetration resistance performance required of the envelope will depend on the degree of exposure of the building being clad. The CWCT 'Standard for building envelopes' gives suitable performance categories for different levels of exposure determined by wind loading. They are as follows:

    Design wind pressure Water penetration test pressure
    Upto 800 Pa 300 Pa
    801 to 1200 Pa 300 Pa
    1201 to 1600 Pa 450 Pa
    1601 to 2000 Pa 600 Pa
    2001 to 2400 Pa 600 Pa
    Over 2400 Pa 0.25 x Design wind pressure Pa


    Appropriateness of all components

    It is important that all the components of the building envelope meet the required weathertightness performance requirements. Building envelopes often include components that are imported into a standard system. The most common examples are the inclusion of opening lights in a curtain wall or windows in a rainscreen wall. Many windows cannot achieve the same levels of performance as curtain walls and sometimes specifiers set lower performance standards to accommodate a particular style of window or mode of operation. Care should be taken when specifying non-standard components to ensure that the overall performance of the building envelope is adequate.

    Windows that work well when installed in a brickwork wall will be exposed to more water when mounted in a large impermeable zone of glass and metal. Brick and block walls are less flexible than framed walls and the perimeter seal around a window performs differently when it is mounted in a curtain wall. BS EN 12208 classifies windows accordingly rating them as fully exposed or partially shielded.

      Thermal Performance

      The building envelope is a major factor determining energy use in buildings along with factors associated with usage patterns. The building envelope contributes to thermal comfort and energy efficiency through its:

      • Insulation
      • Control of solar radiation
      • Control of air permeability
      • Control of long wave radiation
      • Thermal mass

      A further consideration is the avoidance of condensation resulting from thermal bridging.


      Part L

      Part L of the Building Regulations (England and Wales) covers the energy efficiency of buildings. The 2006 Regulations require the design and construction of the building envelope and building services to result in sufficiently low carbon dioxide emissions from the building. Compliance can only be shown by considering all aspects of the bulding use and servicing as well as the envelope performance.

      It is no longer be possible to state that a zone of the building envelope complies with the Building Regulations. In whole building energy models, specifiers must now state what U-values, g-values and air permeabilities have been assumed for a particular zone of the building envelope.


      Whole building air leakage

      Air leakage is a major contributor to energy loss in buildings. From 2002 the Building Regulations (England and Wales) have required whole building air leakage of larger buildings, from 2006 this will apply to most buildings other than dwelings. Whole building air leakage of the finished building has to be less than the air permeability assumed in energy calculations and less than 10 m3/hr/m2 at 50 Pa.

      High levels of air leakage are normally associated with the interfaces between elements, particularly between different cladding contract packages of work. CWCT gives guidance on whole building air leakage testing in Technical Note TN44.


      Environmental control glass and shading devices

      It is predicted that global warming will lead to an increase of several degrees in summertime temperatures over the next few decades. This has the potential to increase the demand for energy for cooling. To reduce the need for cooling energy the amount of summertime solar gain has to be limited.

      Solar transmission into buildings can be controlled by shading devices such as blinds and brise soleil or with environmental control glasses. These can reduce solar transmission whilst giving a smaller reduction in transmission of daylight. It is also possible to reduce the area of window but this wil reduce the incoming daylight and increase any need for artificial electric lighting.


      Thermal performance guidance

      CWCT, jointly with the Council for Aluminium in Building, has developed 'The thermal assessment of window assemblies, curtain walling and non-traditional building envelopes' to support the 2006 Building Regulations. This gives advice on how to specify, design and analyse building envelopes that will, when considered at a whole buiding level, allow compliance with Parts L-2A and L-2B of the Regulations.

      CWCT also publishes guidance on U-values and condensation.

      Glazing Performance

      The variety of glasses available for architectural use continues to expand to give performance of:

      • Strength
      • Appearance
      • Environmental control
      • Post-failure integrity
      • Fire resistance
      • Resistance to attack

      The combinations of products in laminated glass and glazing units leads to a large number of glazing options. CWCT is publishing 'The use of glass in the building envelope'.


      Environmental control glasses

      Environmental control glasses take many forms including; insulated, tinted, filmed and switchable glass. CWCT has published 'guide to advanced glazings'.


      Advice on glass usage

      CWCT has published the following Technical Notes on the use of glass:

      • TN35 Assessing the appearance of glass
      • TN42 Safety and fragility of glazed roofing
      • TN43 Film backed glass
      • TN61 Glass types (supersedes TN 11)
      • TN62 Specification of insulating glass units (supersedes TN 12)
      • TN63 Glass breakage (supersedes TN 13)
      • TN65 Thermal fracture of glass
      • TN66 Safety and fragility of glazed roofing: guidance on specification
      • TN67 Safety and fragility of glazed roofing: testing and assessment
      • TN68 Overhead glazing
      • TN69 Selection of glass to prevent falls from height

      Technical Notes are available here.