Specifying cladding and curtain walling means considering different materials and installation methods. Richard Teale of NBS helps out
1. Product and installation standards
As a specifier of cladding or curtain walling, you may wish to adopt innovative technology. However, there is one strong disincentive for this – the slow development of supporting product and installation standards. To overcome this, it is advisable to seek the advice of facade engineers, cladding consultants and – in the case of curtain walling, slope glazing and rainscreen cladding – the Centre for Window and Cladding Technology (CWCT). Its standard and guide documents provide a technical database for product, performance and testing requirements relevant to the UK climate that NBS have incorporated into relevant work section specifications. CWCT standard documents are presently being reviewed, to align where possible with recently issued European Standards, and are expected to be republished within the next 18 months.

2. Determination of materials
It is useful to ascertain whether the cladding makes up the whole wall or is merely a component within it. Curtain walling may comprise glazing units and solid panels that when erected will fulfil all performance requirements. Rainscreen and other sheet cladding on the other hand usually only provide the weather protection component to a backing wall. Increasingly complex forms of double- or multiple-leaf walling or cladding packages are emerging to optimise environmental conditions such as control of temperature, light and so on. The overcladding market is becoming equally sophisticated – recloaking with prefinished panels is being upstaged by solutions such as structural glass, brises-soleil, tensile fabric and ETFE foils.

3. Type of specification
Determine whether a single works package will encompass the external walling, cladding or recladding of a building. With many forms of composite metal panels and curtain walling systems, this will be the case – particularly where proprietary specification against stated characteristics is possible. However, with more complex, multi-layer systems, work packages will often be divided or sublet, and the specifier will have a difficult role in anticipating which components will contribute to which performance requirements. Bespoke performance specification requires project calculations, evaluation and testing, which can be carried out with the aid of a facade engineer and advanced modelling software.

4. Performance requirements
NBS guidance now offers specifiers the choice of more than 20 cladding or covering specification sections. Each section includes clauses relevant to the type of cladding covered, but there are performance requirements common to most sections. These are:

  • Integrity Secure attachment of cladding contributes to many of its performance requirements. Integrity calculations should address project specific wind and imposed loadings. In all but the simplest of packages, it is common for the cladding and fixing manufacturers to undertake or confirm the attachment design. The demands on fixings are becoming more severe in composite cladding systems as insulation layers increase in thickness and external components become further removed from the supporting structure.

  • Airtightness and air permeability Airtightness should be approached at whole-building and cladding element levels. All projects within the scope of Building Regulations Part L (England and Wales) now face testing to show that air infiltration through the whole building fabric does not exceed 10 m3/h/m2 at an applied pressure difference of 50 Pa. Most cladding systems will pass this test, providing installation workmanship and sealing of interfaces with other components is adequate. Specifiers of higher technology cladding systems such as curtain walling and structural glass should not be concerned with these air infiltration values, as these systems are often tested at element level at pressure differences of 300 or 600 Pa. Many cladding systems – drained and ventilated, pressure equalised, and so on – are designed to allow controlled air infiltration but will contain a very effective air barrier internal to the ventilated cavities.

  • Watertightness Cladding must exclude penetration of water to internal surfaces. Anticipate project exposure conditions by specifying an external to internal test pressure difference. Most cladding systems arrive on site as components for assembly into a watertight construction. Component joints are sealed, lapped, interlocked or framed to exclude water from interior surfaces.

  • Thermal insulation Buildings must comply with the thermal performance standards laid out in Building Regulations Parts L1 and L2 (England and Wales) and Scottish Technical Standards Part J. Cladding systems vary significantly in their ability to contribute to these objectives. The glass-based systems – patent glazing, structural glass and some curtain walling – usually offer low contributions to thermal insulation. Composite and built-up solid cladding systems can be selected or customised to meet more exacting thermal requirements.

  • Fire protection For cladding systems, specifications commonly cover fire resistance, flame spread and fire stopping within cavities. BS 476 in its many parts comprehensively covers requirements, but note the recent publication of Amendments 2002 to Building Regulations Part B. This introduces "recognition to the new European technical specifications and supporting fire test methods".