By Kelly Westwood, head of construction projects, Knauf Insulation
Rainscreen facade systems are a popular choice for mid and high-rise developments. But the specification of insulation for these systems can be complex, especially when you factor in changing legislation and an increased scrutiny on product choice.
The Building Safety Act applies to all residential buildings, although there’s a focus on higher-risk buildings that have two residential units and are over seven storeys or 18 metres tall. It introduces three new gateways that act as ‘checkpoints’ to assess Building Regulation compliance and requires the creation of the ‘golden thread of information’ to show how higher-risk residential buildings were designed and built.
Parts B and L in England and Wales have also been updated, bringing in much stricter requirements for fire safety and thermal performance. As well as improved energy efficiency, every new home must also have a Building Regulations England Part L (BREL) Report recording the original plans, specification changes and photographs of the insulation after installation.
The most important takeaway from these legislative updates is that the construction and performance of the finished building must not only meet the Building Regulations but must also mirror its design.
Focus on fire safety
When specifying insulation for rainscreen facade systems, always start with fire safety. That’s because the materials you use must comply with the strict non-combustibility requirements of Part B in England and Wales.
This was updated in December 2022 when the definition of relevant residential buildings was expanded. The ban on using combustible materials in external walls was also extended to include certain build-ups, such as rainscreen facade systems, of residential buildings between 11 and 18 metres. This means that materials with Euroclass A1 or A2,s1-d0 reaction to fire classification must be used. The only exception is where a full-scale fire test to BS 8414-1 and -2 has been undertaken.
So, to minimise risk, the simplest solution is to choose non-combustible mineral wool sheathing and SFS insulation, such as our Rocksilk® RainScreen Slab and OmniFit® Slab, for every project – regardless of building height or use.
Ask for 3D U-value calculations
The next consideration is the thermal performance of the build-up. Updates to Part L in England mean new residential buildings must produce 31 per cent less carbon emissions compared with the 2013 standards.
But meeting the U-value on paper isn’t the only consideration. They must also deliver it in practice, which can be affected by the type of calculation used.
The most common method, simplified or 2D calculations, is best suited to straightforward build-ups with few variables. They are not recommended for rainscreen facade systems because they apply a blanket correction factor that fails to accurately account for the metal bracketry.
Instead, contractors should insist on numerical modelling or 3D U-value calculations. These calculations provide the most accurate representation of the thermal performance of the finished building because the methodology considers the complex interaction between the insulation, the facade, and the rail and bracket system.
Specifiers should also ask for an in-depth report so the whole supply chain can see what components were included in the calculation. This means they will understand how changes to the specification may affect it, helping to safeguard the design and giving specifiers reassurance that the finished building will deliver the required performance.
The move to real performance is being driven by legislation and brings with it greater scrutiny on the specification and use of construction products. By focusing on fire safety and insisting on 3D U-value calculations, specifiers can help ensure their choice of insulation is compliant and that the performance of the finished building matches its design.
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