Update – IMPORTANT: On 23 June 2017, in view of the focus on components of rainscreen cladding systems, Celotex stopped the supply of Celotex RS5000, pending further clarity. RS5000 remains suspended. Celotex do not currently supply a solution for buildings over 18 metres.
In December 2017 we identified a compliance issue relating to our calculation and testing of the lambda value of products in our 4000 and 5000 range and the Crown-Bond and Crown-Fix products within Crown Flat Roofing range. Material such as product downloads & specifications is for information only.
On 30 January 2018, Celotex made a further public announcement in relation to the full system testing of RS5000 pursuant to BS8414:2. The announcement can be read here
On September 19th 1928, construction started in New York on the Chrysler Building. Standing 319m high to the top of the spire the Chrysler building held the record for the world’s tallest building for less than a single year, when it was surpassed by the newly completed Empire State Building.
Designed in a distinctive Art Deco style, the façade of the Chrysler Building is constructed entirely of bricks over a load bearing internal steel frame.
Approximately 3,826,000 bricks were used in the construction of the outer walls – each laid by hand. The building was constructed over the course of 18 months and completed on 20th May 1930.
The Chrysler building stands today as an American cultural icon and still remains the world’s tallest brick structure. Whilst this building stands as a unique example of the versatility of bricks, the imperatives of modern building design make them a challenging choice for today’s high rise buildings.
Ventilated facade systems offer a number of benefits to the building designer.
Lightweight, durable and available in a huge range of materials, rainscreen systems allow buildings to achieve their aesthetic goals whilst utilising practical construction methods.
A typical rainscreen construction consists of non-structural cladding panels supported by aluminium brackets and rails. A drained and ventilated cavity is maintained behind the cladding and the system is fixed back to a lightweight metal frame or masonry substrate. Insulation material can be provided behind the cladding and optionally between the metal frame where present.
Modern Methods of Construction
Buildings designed using modern methods of construction (MMC) can be built quickly and easily. Metal framed and cladded buildings are much lighter than those built using traditional materials. The Chrysler Building for example clocks in at an impressive 47,000 tons.
Modern building methods present their own challenges. The Building Regulations place great emphasis on areas such as energy efficiency and fire performance and these must be considered carefully.
Well insulated buildings reduce running costs, reduce CO2 emissions and help to maintain comfortable internal conditions. However, achieving high levels of thermal performance can be difficult where metal components interact with the thermal insulation.
All metals conduct heat extremely efficiently. Where metal components such as rainscreen support brackets penetrate insulation layers, the overall U value of the wall can be significantly compromised. The thickness of insulation must be increased to compensate for this phenomenon so that the thermal efficiency target is maintained.
Aluminium has a thermal conductivity of approximately 205 W/mK at 25 degrees Celsius. This means that it conducts heat over 1,600 times more efficiently than softwood timber. To put this in perspective, you would need 488 metres of aluminium to provide the same thermal resistance as 50mm of Celotex insulation – twice the height of 1 Canada Square (Canary Wharf).
External Fire Resistance
External fire resistance is another critical area that must be considered.
If a fire develops within the building it can break out of windows and attack the exterior facade. If the cladding system provides a medium for vertical fire spread, then this can cause secondary fires to quickly break out on storeys above the initial fire.
Rainscreen systems often incorporate a drained and ventilated cavity behind the façade and this can provide a potential avenue for fire spread.
Fire requires three primary elements; heat, fuel and oxygen in order to propagate. If fire breaks in to the cavity behind the cladding panels then the supply of oxygen is effectively limited by the enclosed space.
The fire will naturally expand to the fresh sources of oxygen in the cavity above, causing flames to extend vertically very quickly. This is known as the chimney effect.
Cavity Fire Barriers
A well designed system will consider such factors as the provision of cavity fire barriers and the fire resistance of the various components; including the cladding panels, support brackets and insulation material.
Cavity fire barriers are materials designed to restrict the spread of fire within enclosed spaces. The nature of rainscreen systems mean that the provision of fire barriers is more challenging than in other constructions. This is because under normal operation the drained and ventilated cavity must be maintained behind the cladding panels. This cavity provides a plane for water drainage and permits air circulation to allow convective drying.
Innovative products allow cavity fire barriers to maintain this air path, yet effectively seal the cavity in case of fire.
A common cavity fire barrier solution consists of a non-combustible mineral wool core which incorporates a facing of intumescent material. Intumescent materials expand as a result of heat exposure.
Cavity fire barriers are typically positioned horizontally at each separating floor within the building and vertically at centres determined by the overall building design. Additional fire barriers may be required around openings and other penetrations in the façade.
Horizontal cavity fire barriers are secured to the supporting structure and extend to a point just behind the external cladding – maintaining the drained and ventilated cavity. In the case of fire, rising heat levels cause the intumescent material to react and expand to seal this airspace – effectively blocking the passage of further vertical fire spread within the cavity.
Building above 18 metres?
Buildings with a storey height greater than 18 metres have additional requirements under the national Building Regulations.
Section 12.5 (External Wall Construction) of Approved Document B2 states the following routes to compliance:
“External walls should meet the guidance given in paragraphs 12.6 to 12.9 or meet the performance criteria given in the BRE report Fire performance of external thermal insulation for walls of multi storey buildings (BR 135) for cladding systems using full scale data from BS 8414-1:2002 or BS 8414-2:2005”
In summary, the use of insulation material in the building fabric of multi-storey buildings is restricted unless it meets stringent testing requirements. This can make it difficult for the overall design to meet its goals of thermal efficiency and fire safety.
Celotex RS5000 is the first PIR board to successfully test to BS 8414-2, making it acceptable for use in buildings above 18 metres in height, offering class leading thermal performance. Additionally to its BS8414-2 classification, Class O fire performance and LABC approval, RS5000 allows thinner solutions, lower U values and is suitable for use in buildings of any height.
A number of resources are available to help you specify RS5000 in your project.
- The Celotex Technical Centre can provide U values and condensation risk analyses for your through wall design.
- Our Rainscreen Cladding Compliance Guide details the test details and performance criteria to aid with your specification.