External wall issues

What are ACM panels and the risk factors?

This section will help you understand the difficulties in assessing the fire resistance adequacy of ACM external panelling systems in view of the numerous alternative ‘physical solutions’ and plethora of ‘technical guidance’ available.

ACM panels are an external wall panelling system comprising of 2 aluminium sheets (the skin) encasing an insulation or filler core material. These filler materials are typically polyethylene (PE), polyurethane (PUR), profiled metal or a mineral wool. The aluminium sheets are normally between 3 and 7mm thick, with the whole panels varying in depth, width and height depending on their performance requirements.

ACM panels typically have a powder coated or metallic finish. They are traditionally used to form either a single skin external wall or as over cladding to enhance an older building's appearance or improve thermal/sound insulation properties.

The diagram below illustrates a typical ACM ‘over cladding’ arrangement including a layer of insulation board within the ‘sandwich’.

Figure 1: Typical ACM cladding arrangement including insulation panel

As seen in figure 1, the ACM panel is only one element of the external wall ‘sandwich’ and provides the outer face of the overall cladding ‘system’. The constituent parts of the ‘sandwich’ in more detail are:

The design, specification, installation and performance of ACM Panels during a fire - the jigsaw

There are several key documents relating to the design, specification, installation and performance of ACM panels to ensure that they meet reduce the risk/do not contribute to the risk of fire spread including:

1. The Building Regulations Approved Document B – Volume 1: Dwellinghouses.
2. BR 135 - Fire performance of external thermal insulation for walls of multistorey buildings, 3rd edition.
3. BS 8414-1:2015+A1:2017 - Fire performance of external cladding systems.
4. BS EN 13501-1:2007+A1:2009 - Fire classification of construction products and building elements.

1. The Building Regulations Approved Document B – Volume 1: Dwellinghouses

This document provides guidance only on how to 'ensure external walls and roofs　have adequate resistance to the spread of fire over the external envelope'. There is 'no obligation to adopt any particular solution ... if you prefer to meet the requirement in some other way'.

2. BR 135 - Fire performance of external thermal insulation for walls of multistorey buildings, 3rd edition.　

This BRE-produced document focuses on the fact that external cladding systems offer a potential route for fire spread through multi-storey buildings. The third edition of BR 135 sets out the principles and design methodologies related to the fire spread performance characteristics of non-loadbearing cladding systems. It also provides revised guidance in the light of the growing market for cladding systems and increasing thermal performance requirements.

It further updates the guidance on fire performance of currently used materials and technologies. The illustrations and scenarios given are based on typical examples of current practice, but in view of the rapid changes in designs and materials, the guidance aims to enable designers to understand the parameters relating to fire safety design and construction of external cladding. It also consolidates the fire performance classification systems for full-scale fire tests.

3. BS 8414-1:2015+A1:2017 - Fire performance of external cladding systems.

This British Standard document covers the fire performance of external cladding systems. It defines test methods for non-loadbearing external cladding systems such as rainscreen cladding and external wall insulation systems fixed to masonry faces of buildings. The document also evaluates whether a cladding system subject to fire breaking out of an opening (such as a window) in an external wall, will result in excessive fire spread up the outside of the building and the potential for fire to re-enter at a higher level.

The tests are carried out in specialist laboratories, and are performed on full-scale systems (rather than small-scale samples) incorporating; joints and corner details, fixings, insulation, firebreaks, cavities and other elements of the system construction.

4. BS EN 13501-1:2007+A1:2009 - Fire classification of construction products and building elements.

This British Standard provides the reaction to fire classification procedure for all construction products, including products incorporated within building elements. It covers three categories:

1. construction products;
2. floorings; and
3. linear pipe thermal insulation products.　

The two key elements of ACMs

There are effectively 2 key elements to the composition of ACM panels specifically, which are critical to their performance in terms of resistance to combustion and spread of flame during a fire. This applies particularly when used at a height above 18 metres as this represents a greater problem for fire fighting services. The 2 key elements are as follows:

1. Combustibility of the 'filler' material within the panel leading to fire spread.　
2. Performance of the aluminium 'skin' during a fire, i.e. will it disintegrate/delaminate, exposing the inner core 'filler’ material to the fire.

The Building Regulations state that the 'filler' material (normally providing insulation) must have 'limited combustibility' (AKA Class 2) and also highlights products and materials that present a risk to the objective of preventing fire spread within Part B, Volume 2 (table A7).

Schedule 1 Section B4(1) of the Building Regulations 2010 requires that:　

‘The external walls of the building shall adequately resist the spread of fire over the walls and from one building to another, having regard to the height, use and position of the building.’

The Building Research Establishment further state in BR 135 that this can be achieved by either:

1. each individual component of the wall (insulation, filler, etc) to be of limited combustibility as　set out in the Part B paragraphs 12.5 to 12.9; or
2. ensure that all the combined elements of a wall (ie. skin and filler), when tested as whole system, have sufficient fire spread resistance meeting the　acceptance criteria in BR 135, in accordance with BS 8414.

Summary

It is clear there are a plethora of documents relating to the design, specification and installation of ACM panels and associated insulation products aimed at reducing the risk of spread of flame during a fire.

It is also clear there are many alternative ways of achieving the overall aim of limiting/eliminating external fire spread of retrospectively over cladded high-rise buildings using ACMs.

In theory, ACM panels that have a core material which is not of limited combustibility, may be compliant if installed as part of a ‘whole wall’ system that meets the test described in option b above.

It must be remembered that a ‘whole wall system’ includes the following items:

• The ACM panel.
• The ACM panel fixing system.
• The internal cavity.
• The ‘inner’ thermal/sound insulation product.
• Fire breaks/cavity barriers behind the external aluminium panel.

The fact that there is such a vast quantity of guidance/standards and alternative systems and products available to designers makes the design, specification and installation of a ‘compliant’ solution a less than simple process.

The various permutations/combinations of products available to provide an external cladding system that meets all the relevant standards and legislation, almost allows too much choice without giving definitive guidance in some instances.

Another major difficulty with this particular issue is the fact that while the quality of the products used and workmanship can be inspected during installation, the only accurate performance testing of a newly installed ACM system is to assess after a fire.

Along with the key issue of combustibility of the materials used, the integrity of the ACM panels themselves during a fire and the adherence to the specification by the installation contractor are equally critical factors.