CPD 08 2023: Rainscreen facade system regulations

This CPD, sponsored by Knauf Insulation, takes an in-depth look at the systems that lie behind the rainscreen facades of buildings and sets out the regulations and relevant guidance that should be considered to ensure the specification of safe, healthy and sustainable structures

The Marshall Building for the London School of Economics included the specification of rainscreen insulation

High-performance rainscreen facade systems contain a number of key elements that combined provide the thermal, fire safety and acoustic performance required. These performance needs are mainly driven by the regulatory landscape. With a greater responsibility being placed on all those involved in the design and installation process, it’s important to be aware of the key changes to UK regulations when specifying these systems.

Learning objectives:

  • A brief overview of what makes a high-performance rainscreen facade system
  • Knowledge of key changes to the regulatory landscape
  • Awareness of the principal considerations for successful insulation specification

Facade systems

A typical rainscreen facade system includes the elements pictured in the diagram overleaf.

The internal lining adds an aesthetic finish, aids reduction of sound transmission and contributes to the fire resistance of the building element, while the vapour control layer (VCL) provides the structure with resistance to airborne moisture ingress.

The studwork is part of the steel framing system (SFS) that supports the wall boards and outer leaf of the facade, creating space for an infill panel and insulation. The SFS insulation shown is noncombustible glass mineral wool. This aids the reduction of sound transmission and contributes to the fire safety of the building element. It is advised to fully fill the studs.

The sheathing board strengthens the assembly, provides a surface for other materials to be applied and gives some degree of weather resistance. The helping hand brackets and cladding support rail provide the metal framework to which the external cladding can be fixed.

The rainscreen insulation shown is non-combustible rock mineral wool, which can provide thermal and acoustic performance while meeting fire safety requirements. The insulation fixings pass through the insulation and into the superstructure, to ensure that the insulation is held firmly against the sheathing board.

The ventilated cavity between the insulation and the outer cladding means that there is no direct path for moisture driven from the external environment to reach the inner leaf. It also allows any moisture within the structure to find its way out and is part of fire safety requirements.

The external cladding gives the building its aesthetic appearance, but it is also the first line of protection against weather and sources of external fire. The choice of materials suitable for consideration varies as external cladding will be driven by factors such as the height, use and proximity of the building to its boundary, which determine the exact fire performance characteristics required.

Material options include steel, timber, masonry and concrete. However, timber frames cannot be used in relevant buildings with a storey 18m above the ground. In addition, fire regulations in England and Scotland restrict the use of timber frames in buildings above 11m in height (depending on type and usage).

Regulatory landscape

There are slightly different approaches taken to the conservation of fuel and power between England, Scotland, Wales and Northern Ireland, as well as differing thermal performance requirements. However, the overall direction of travel is the same across the UK.

Large reductions in carbon emissions are required compared with previous regulations, ranging from 40% in Technical Booklet F in Northern Ireland through to 31% in Approved Document L (Part L) for England. We are moving away from fossil fuels towards renewables and using technologies that work more efficiently within a building fabric with high thermal performance.

In England, new metrics such as the primary energy target are being introduced to encourage the measurement and reduction in energy demand for buildings. As seen in the diagram, the net impact of these changes is to push the required U-values lower as we head towards the UK’s target of being net zero by 2050.

Another important emerging theme is the attempt to close the performance gap – when the as-built thermal performance of a building does not match the designed performance. The British Regulations England Part L (BREL) reports will be produced by SAP assessments, in an attempt to help address the gap. This requires photographic evidence of correct product installation to prove thermal continuity and quality of insulation in floors, walls, roofs and around openings.

The extra focus on as-built performance will encourage the specification of proven construction products that are easy to install correctly and are therefore less likely to be the subject of investigations, and possible costly reworking, should performance concerns be raised.

The key changes to Approved Document B in England and Wales, and to Section 2 covering fire safety in Scotland, are designed to ensure that the external walls of buildings do not consist of materials that could promote the spread of fire across the external surface of the building itself, or to adjacent buildings.

The regulations set out the classification of materials that can be used, depending on the height and use of the buildings. Relevant buildings are defined within the regulations as any building with a storey over 18m in height and which contains one or more dwellings, an institution or a room for residential purposes. This includes student accommodation, care homes, sheltered housing, hospitals, dormitories in boarding schools, hotels, hostels and boarding houses.

Other residential purpose groups such as flats and dwelling houses are defined within the regulations, with the permitted materials depending on whether the building is more than or less than 11m tall.

Building safety

The Building Safety Act (BSA) places new requirements on the industry with the goal of ensuring that high-rise buildings are constructed correctly, by competent professionals, while meeting all regulatory requirements to help ensure the safety of occupants.

There are sections of the BSA that apply to all construction projects, regardless of their height, and the emphasis for stakeholders will be on using tried and tested systems that have proven performance in order to fulfil their legal obligations.

The BSA has established a more stringent regime for higher-risk buildings, which are at least 18m high or have at least seven storeys and two residences. There are two parts to the regime, with the first establishing a regulatory framework for the design and construction of new higher-risk buildings and work carried out on existing higher-risk buildings. This places extra responsibilities on dutyholders — the principal contractor and the principal designer — who must ensure that all building and design work meets the functional requirements of the Building Regulations 2010.

The second part of the regime establishes a new regulatory framework when higher-risk buildings are occupied. The act requires evidence of compliance at every stage.

Gateways

The BSA features hold points known as gateways. This approach ensures that all potential safety risks are considered and captured throughout the construction process. Gateway 1 is at the planning stage, and designers will need to provide a fire statement as part of the planning application to show that all the fire safety requirements have been considered and incorporated into the proposals.

Gateway 2 makes it mandatory to seek Building Regulations approval for the project before it is allowed to start. The information required at Gateway 2 includes competency statements for the principal designer and contractor.

The Building Safety Regulator, run by the Health and Safety Executive (HSE), has 12 weeks to respond to the application, which does not include time for design development in response to any areas of concern raised. This often makes tried and tested solutions more appealing, reducing the risk of delays in getting the project started. If changes are required after the plans have been submitted and approved, the HSE must be notified.

Gateway 3 begins when construction is completed and the Building Control Body assesses whether the work has been carried out in accordance with the Building Regulations. Until this has been signed off, the accountable person cannot register the building and it cannot be legally occupied.

The “golden thread of information” refers to the requirement to digitally store vital safety information, gathered throughout the three gateways, detailing how the building has been designed, built and managed.

The safest option for any structure is to use non-combustible insulation with non-combustible outer cladding

Key considerations

There are a number of considerations to achieve a successful insulation specification for your rainscreen facade. This includes thermal performance, fire safety, acoustic performance, durability and sustainability.

Fire safety

There are three elements to consider when choosing fire safety and rainscreen facade systems:

  • Reaction to fire of the insulation materials and the materials around it
  • Fire resistance of the element as a whole
  • Provision of cavity barriers to hidden cavities.

Reaction to fire tests work at the product level, rather than for full systems or wall build-ups. They are dependent on the amount of organic material in the product, and this is measured in two ways: by mass loss and by quantifying calorific value.

There are seven groups within the Euroclass classification system, ranging from F (will burn) to A1 (non-combustible). The performance levels include properties such as the ease of ignition and the rate at which the product gives off heat when burning.

The proximity to the boundary of the external wall is an important determining factor in the materials that can be used, with the critical distance being one metre. The safest option is to use non-combustible insulation with non-combustible outer cladding.

In England’s Approved Document B, a building height of 11m has additionally been introduced to cover “all residential purpose groups”. For all relevant buildings above 18m, and for residential buildings above 11m in height, the requirement for insulation is A2-s1,d0 or better. The only exception for residential buildings between 11m and 18m is if a full-scale fire test has been undertaken, but such tests are expensive.

Some stakeholders go further than the regulations stipulate. For example, the Greater London Fire Safety Plan Guidance says: “The fire statement for major developments must include a commitment that the development will not incorporate combustible materials in its external walls. […] That is, only materials that are class A2-s1 rated and above under the European classification system, as set out in the standard BS EN 13501-1 and as reflected in UK Building Regulations will be used except for the exempt elements as set out under regulation 7(3) of Approved Document B.”

It is important to note that fire resistance is not the same as reaction to fire. They are two different considerations, both being crucially important. Reaction to fire indicates how ignitable the material is, and how much the material supplies fuel to the fire; this gives an overall indication of how the product will contribute during the development stages of a fire. Fire resistance indicates how long a particular construction element can withstand fire once it is developed.

Cavity barriers

Wherever there are small concealed spaces, there must be the provision of a cavity barrier. In ventilated rainscreen facade systems, horizontal open-state cavity barriers are required. When in place they leave a gap behind the outer cladding for ventilation. However, in the event of fire, the intumescent strip on the front of the open state cavity barrier expands and fills the gap to help prevent the passage of heat and smoke.

When conserving fire resistance, it’s important to remember that:

  • The element concerned may need fire resistance from inside or from both sides.
  • Fires in “relevant buildings”, “residential” and “assembly and recreation” buildings represent greater risk to life.
  • Periods of fire resistance required by the regulations in this context are typically 30, 60, 90 or 120 minutes. These ratings typically mean the period during which the wall construction can withstand a fire resistance test.

Approved Document B (England) requires cavity barriers to have a minimum fire resistance of 30 minutes’ integrity (E 30) and 15 minutes’ insulation (I 15). However, in many cases higher performance can be stipulated by building owners, financiers, or insurers of up to 120 minutes (EI) to match the performance of walls and floors.

Approved Document B states the limit on the maximum cavity dimension before cavity barriers are required in buildings other than dwellings is 20m in most instances. However, many building insurance providers will have stricter requirements.

Thermal performance

Updated building regulations will result in the construction of buildings with improved thermal performance and call for lower U-values in walls, floors and roofs. These regulations are concerned with operational carbon — the total carbon emissions associated with all energy sources used to keep our buildings warm, cool, ventilated, illuminated and powered.

The UK’s ambition to deliver net zero buildings cannot be achieved unless robust solutions are employed. Thermal bridging of the insulation, especially through the use of metal components, is a solution that can have a significant effect on the thermal performance of a wall.

Materials with a high thermal conductivity can transfer a lot of heat energy. Low thermal conductivity materials (such as insulation) transfer a much smaller amount of heat. More metal means more heat passing through components, requiring an appropriate U-value calculation method for the application.

3D U-value calculations provide a comprehensive and accurate reflection of the thermal performance of your specified rainscreen system. Having an accurate U-value calculation means that there is more chance of the as-built performance being in line with that of the original design.

When designing the thermal performance of a building, a key design element is considering thermal bypass (air movement through or around the insulation). While a small gap on either side of the insulation may not have a huge effect, research by Lecompte in 1990 would suggest that gaps of 10mm or more could seriously impact the U-value.

Sustainability

To confirm the sustainability of products, look for BRE’s BES 6001 framework standard for responsible sourcing. The overall environmental impact of products should be shared in independently verified environmental product declarations (EPD) that comply with the European standard EN 15804.

The indoor air quality of a building can be affected by products used in its construction if they emit high levels of volatile organic compounds (VOCs) into the air. Eurofins, a laboratory testing service, assesses construction products for their effect on indoor air quality. Products that achieve Eurofins’ Indoor Air Comfort Gold certification are the best in class for low emissions.

Construction products can contain chemicals that have been found to pollute the environment, with the worst of these chemicals appearing on the Declare “Red List”. The Declare label provides transparency about the ingredients used in building materials and reassures customers seeking products without harmful chemicals present.

Insulation products can contribute to sustainability scheme credits, including BREEAM. Specifying products that are thermally efficient, sound absorbent and have low embodied carbon can help maximise the number of credits available.

Final thoughts

The current regulatory landscape represents a step change in the way that buildings of all kinds, including high rise, will be scrutinised. Higherrisk buildings in particular will be examined at every stage of design, planning, construction and handover to ensure that they deliver compliant, safe and high-performance buildings.

This will increase the desire to choose products and systems that are easy to install, come with robust third-party testing and performance data and are fully supported by the manufacturer’s technical teams.

The early engagement of manufacturers in the design stage of projects to provide expert technical support and proven product solutions will make it easier to generate robust and compliant specifications.

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