Last edited 09 Sep 2024

Metal composite material panel systems MCM and MCP

1 In brief
2 Background
3 Sandwich solution
4 Aluminium Composites and Grenfell
5 Perceived advantages and risks
6 MCM finish types
7 MCM form types
8 MCM detail design types
9 MCM core types
10 Related articles on Designing Buildings

[edit] In brief

Metal composite panels (or metal composite materials - MCM) are typically used in the external cladding of buildings as single system build-ups or as multiple layer build-ups such as rain-screens. They can be bent, curved and joined together in an almost unlimited range of configurations, with a variety of finishes, making them popular with architects and engineers of both simple and complex structures. The use of MCM panels and in particular Aluminium Composite materials (ACM) has however in recent years been seen to pose significant fire risks, particularly where manufactured with an untreated polyethylene core and as such have been banned for use in high rise buildings.

Metal composite materials (MCM) are defined in approved document B of the building regulations as: '...any panel or sheet, having a thickness of no more than 10mm which is composed of a number of layers two or more of which are made of metal, alloy or metal compound and one or more of which is a substantial layer made of a material having a gross calorific value of more than 35MJ/kg when tested in accordance with BS EN ISO 1716. A substantial layer is defined as a layer which is at least 1mm thick or has a mass per unit area of at least 1kg/m².'

[edit] Background

MCMs first emerged commercially in the 1960's and have continued to be used as a wall cladding, in cornices and canopies, and for joining areas between other building materials such as glass and precast panels. Initially many examples employed singular extrusions of bonded sealed cladding units, with a tongue and groove system, cut to length and clipped above one another, with the lips externally, protecting the exposed edges of insulation, reducing thermal bridges and sealing from the weather, these are often referred to as insulated metal panels. These types of systems are single layer systems and often used for buildings having lower thermal requirements such as earlier industrial buildings, becoming less popular as thermal performance requirement increased.

MCMs were later developed as a component of two or more part thin rain-screen systems, installed within a ventilated frame system, protecting a separate layer of insulation beneath. Rain-screens could improve thermal performance, over single layer build-ups through an increase in thermal resistance at the surface, preventing risks of damp penetration into the insulative layer, reducing thermal bridges and allowing a flexibility in the selection of component parts, the build-up depths and selected finishes.

[edit] Sandwich solution

In general in both single layer and rain-screen systems comprise of two metal skins, bonded to an insulating core, forming a composite ‘sandwich’ panel. The combination of the two can create a rigid, lightweight rain-screen cladding system, or a single modular cladding system (often referred to as Insulated metal panel). In the case of the rain-screen cladding the rigid core is considerably thinner, gaining rigidity from the combination of core and metal surfaces as a thin sandwich, with the thermal performance coming primarily from separated thermal insulation beneath. In most cases fire rating is achieved by the metal sheet protecting the insulation layer, though this can be improved through different insulants, fabrication and installation approach it can also be significantly compromised by the same factors.

[edit] Aluminium Composites and Grenfell

The more commonly referred to ACM (Aluminium Composite Materials) or ACP (Aluminium Composite Panels also referred to as Aluminium Composite Panel Cassettes) are one type of MCM which became popular for use throughout the 1990's up to 2020s. ACMs as a rain-screen cladding material, in particular became infamous because of their role in the Grenfell tragedy, a 2017 tower block fire in which 72 people died. For further information see articles; ACM cladding, ACM PE panels, ACM FR panels and other variations, and Grenfell Tower articles, and Grenfell Phase 2 final report for construction at a glance. In 2018 the UK government banned the use of combustible materials in and on the external walls of new blocks of flats, hospitals, residential care premises and student accommodation over 18 metres in height. see Approved Document B. On 1 June 2022, changes were introduced, including an outright ban on the use of Metal Composite Material (MCM) / Metal composite panels (MCP) with unmodified polyethylene (PE) core on all new buildings at any height.

The metal component, can be aluminium, zinc, stainless steel, titanium and so on, available in a wide variety of colours, finishes and profiles, the core may be manufactured from a blown insulant such as modified polyethylene or from a fire-retardant material such as filled PE, or mineral wool, with a range of thicknesses available depending on performance requirements and substrate.

[edit] Perceived advantages and risks

The composite panel has a number of advantages compared to single-layer metal sheeting, including:

Weather resistance.
Acoustic insulation.
Thermal insulation.
Simplicity.
A consistency of finish that requires little maintenance.
They do not wrinkle as the external skins are bonded to the core under tension.
They are lightweight.

With improvements in manufacturing technology and installation techniques, metal composite panels have become very affordable compared to other systems and thus a popular choice. They can also be more cost-effectively installed and faster than precast panels, granite or brick exteriors, and have reduced structural support requirements because of their lighter weight. They have also however come under heavy scrutiny and criticism on many levels since the completion of the Grenfell Tower Phase 2 report.

[edit] MCM finish types

[edit] MCM form types

Whilst the make up of the core is a significant element that can alter the resistance to fire of a system, the panels do primarily rely on the metallic skin to protect the inner core from weathering and from exposure to fire. However, incorrect or inappropriate cutting, installation and location of firebreaks between the panels can expose the inner core from the sides or corners, impacting fire resistance, and during a fire, it is possible for the panels to delaminate, exposing the core material. For example the significant difference between flat panel systems and cassette systems was highlighted in the Grenfell Phase 2 final report ( see Grenfell Phase 2 final report for construction at a glance.) These issues were also highlighted in Grenfell Tower inquiry: 9 things we now know about the cladding (23 March 2021) and Cladding boss denies contributing to Grenfell disaster (5, September 2024)

Flat Panel systems - as the name suggest are simply flat panels cut to size and fixed to a framework, generally only used in rainscreen applications, where edges are sealed through different detail types.
Cassette systems - used in both single layer and rainscreen applications ( different designs) with panels that are formed at the edges, creating improved protection against weathering and ingress. Though some evidence also shows that some of the forms of cassette may also compromise fire ratings. See article Grenfell Tower inquiry: 9 things we now know about the cladding (23 March 2021) and Cladding boss denies contributing to Grenfell disaster (5, September 2024)

[edit] MCM detail design types

The Technical Bulletin MCM Panel System from the Metal Construction Association highlights a number of different detail solutions, when it comes to MCM systems.

MCM Wet Seal Systems - also referred to as single line barrier wall or fully-sealed wall, where an exposed sealant joint between adjacent panels fully seals between the exterior environment and the interior cavity behind the panel.
MCM Dry Seal Systems - also creates a single line barrier wall or fully-sealed wall, but through the use of interlocking joints, extrusions, metal profiles or gaskets as part of the system, rather than using a separate wet sealant.
MCM Drained and Back Ventilated Rainscreen Systems (DBVR) - also uses interlocking joints, extrusions or profiles but where a minimal amount of air and water is accepted as entering the build-up. They are designed so as to be able the drain and dry through ventilation, with any water passing through dissipating through the ventilated cavity. These systems may aso employ a secondary air/water barrier just on the cavity side of the insulation layer.
MCM Spline Systems - is one method used in PER and DBVR systems, where a metal or metal composite strip is inserted into the panels perimeter extrusions to close off the joint and provide a seal to water infiltration. The spline element does not have a transfer load function, justr as a seal and conceal any exposed fasteners.
MCM Hook and Pin systems - describes a fixing system employing a clip, rail or pin system installed on the building before the panels. The hook is then an integral part of the panel attaching to the already installed frame. The name does not describe weathering performance function, merely the structural approach.
MCM Pressure Equalized Rainscreen Systems (PER) - describes equal pressure on the exterior of the cladding and the interior air cavity, with no pressure gain/loss there is no method for the rain to enter into the system. Equalization is achieved by venting the interior airspace in precisely the correct locations and amounts. to allows air to quickly pass through the cladding joinery allowing the interior air space to quickly equalize with the exterior pressure while controlling water infiltration. However, this pressure equalization also means that the solid backing (such as gypsum board) must be properly designed and constructed behind the panel system. The air/water barrier becomes the primary air barrier for the building. In addition to the air/water barrier, the interior air space behind the panel system is typically divided into sealed and separate compartments. This prevents the air within the air space from migrating between higher and lower pressure zones.

[edit] MCM core types

There are many different possible core materials that can be used in the manufacture of MCMs. The core material used on Grenfell was ACM PE (Aluminium Composte Material Polyethylene) installed as cassettes, which caught fire and spread flames across the facade eventually igniting the insulation layer beneath. One year after the fire in 2018 the UK government banned the use of combustible materials in and on the external walls of new blocks of flats, hospitals, residential care premises and student accommodation over 18 metres in height. On 1 June 2022, further changes were introduced, which included an outright ban on the use of Metal Composite Material (MCM) / Metal composite panels (MCP) or ACMs with an unmodified polyethylene (PE) core on all new buildings at any height ie ACM PE.

However Polyethylene (PE) products can be manufactured with a number of different modifications to improve performance, such as the addition of fire retardents, vinyl acetate modification (PE-VA), ethyl vinyl acetate (EVA) co-polymers aswell as using a mineral filler added to the polyethylene to dramatically improve fire performance. Although the amount of mineral filler added to such products can vary from 5-90%, the higher percentages produce a product which is labelled as FR, for fire resistant, one that continues to be used in cladding systems.

In the case of the Grenfell fire the product Reynobond PE ACM cladding was used and reported by the press as being a ‘cheaper, more flammable’ option, when compared to the Reynobond FR, product which has a more fire-retardant core (through the use of a mineral filler and other additives) whilst the Reynobond A2 is quoted by the manufacturer as having a non-combustible core, 'containing over 90% inorganic mineral material'.

The University of Queensland, Australia have tested and studied a number of different cladding system and published their results in a library https://claddingmaterialslibrary.com. This library lists so 80 plus products that have been tested, many of which are ACM products, of the those products the library highlights slight variations in the make up of the differing products, even where the descriptive name such as ACM PE or ACM FR remains the same. Some of these are listed below, for further information and detailed results visit the library here.

Predominantly polyethylene core (e.g. ACM PE)
Predominantly polyethylene and a fire retardant (e.g. ACM PE)
Mineral (or in organic material) filled thermoplastic core (type of thermoplastic used is often undefined) (e.g. ACM FR) FR standing for fire resistant.
Predominantly polyethylene modified with vinyl acetate (e.g. PE-VA)
Predominantly polyethylene modified with vinyl acetate (e.g. PE-VA), a fire retardant, and / or an inorganic filler.
Predominantly polyethylene (PE) with ethyl vinyl acetate (EVA) copolymer, a fire retardant, and an inorganic filler.
Predominantly mineralwool.
Metal foam cores.