Formalised performance specifications for rainscreen cladding

CIMclad: Formalised performance specifications for rainscreen cladding
1.0 Introduction
The CIMclad project is investigating the feasibility of improving the efficiency and competitiveness of the
cladding sector through the development of a standardisation framework for computer-integrated design
and manufacture of cladding systems. This is the fourth report of the CIMclad series, which structures
relevant coherent rainscreen cladding specification identified in Report 3 - Review of Specifications on
Rainscreen Cladding.
It is recognised that there is no generally accepted standard specification for rainscreen cladding. Some
designers/specifiers have developed their own in-house specification in a generic format but tailored to
meet the requirements of a specific rainscreen cladding project. In-house standards have often been a
mixture of existing standard and specification clauses on rainscreen cladding such as Standard for Walls
with Ventilated Rainscreens (CWCT 1998) and Section H92-Rainscreen Cladding (NBS 2000)
respectively, and experience. The two documents make references to relevant sections of Building
Regulations, British Standards, and other guidance documents.
The CWCT standard and NBS specification are widely used as guidance documents for writing
performance specification and other requirements for rainscreen cladding in the UK, and both are publicly
available. The two documents are similar with respect to performance requirements. The CWCT standard
sets out a framework for the specification of a rainscreen and establishes specific requirements for its
performance. The NBS specification could be partly regarded as an interpretation of the performance
requirements in the CWCT standard in terms of specification clauses. It invokes clauses given in the
CWCT Standard for Walls with Ventilated Rainscreens and other CWCT documents such as Guide to
Good Practice for Facades [CWCT 1996]. Thus, this report largely focuses on the NBS specification
clauses.
Section 2 presents a working definition of rainscreen cladding in the context of this work including its key
features. The structure of Section 3 on entity-based rainscreen cladding specification is guided by the
rainscreen cladding product model requirements specification developed in work package (WP) 3.
Performance and other requirements of the product model entities are extracted from the NBS specification
and structured for computer implementation.
2.0 Rainscreen cladding
In the context of the CIMclad project, rainscreen cladding is defined as a layered cladding system
comprising of:
• A visible outer skin (or “rainscreen”) that also forms the primary rain barrier. This layer sheds
the majority of water down its surfaces, but does not prevent the passage of air into the air gap.
• An air gap that prevents water ingress into the building. This provides ventilation and,
depending on the design and dimensions of the rainscreen and air gap, may be intended to provide
pressure equalisation across the outer skin.
• A backing wall that forms an effective air barrier and frequently includes an insulating layer.
Typically the supporting system to the outer skin is fixed to the backing wall.
A more concise definition is:
“Rainscreen cladding is a layered cladding system typically comprising an outer skin which incorporates
air gaps but forms the primary rain barrier, a ventilated air gap that prevents water ingress, and an
effectively impermeable backing wall”.
Figure 1 shows the key features of a rainscreen cladding.
July 2001 Page 3 of 3