The lowdown on designing top-performing high-rise facades

With Building Regulation guidance tightening and standards rising, how can architects and specifiers ensure their rainscreen facade systems perform as designed?

What impact does product choice have on fire safety and thermal performance?

Fire safety

Approved Document B (England) requires non-combustible materials to be used in all relevant buildings above 18 metres.

As of December 2022, non-combustible materials must also be used in certain external wall system build-ups, including rainscreen facades, on all residential buildings between 11 and 18 metres. The only exception is if a full-scale fire test to BS 8414-1 or BS 8414-2 has been conducted. 

Meanwhile, the Building Safety Act is introducing ‘Gateways’ for high-risk buildings at the planning, design and post-construction stages to ensure specification integrity is maintained from design through to build.

As requirements become more complex, the simplest solution is to specify non-combustible insulation, regardless of the building height. Choosing non-combustible products provides additional peace of mind alongside other essential qualities, such as thermal and acoustic performance.

Thermal performance

Approved Document L (England) now requires all new homes to produce 31 per cent less operational carbon emissions (27 per cent for non-domestic) than under the previous regulations.

New-builds must also meet tighter limiting U-values, and a Building Regulations England Part L (BREL, or BRUKL for non-domestic) report must be produced, comparing the designed and as-built specifications.

Specifiers need confidence that the thermal performance of their design will translate into real world performance, but too often this relies on oversimplified U-value calculations.

The simplified calculation method isn’t recommended for rainscreen facade systems because it applies a blanket correction factor to account for things such as penetrations from the steel frame, which can result in inaccurate specification.

By contrast, 3D U-value calculations attempt to more accurately account for the impact of those penetrations, making them a better indication of as-built performance.

Installation can also impact a building’s thermal performance so it’s important to specify materials that are easier to install correctly.

For example, rigid board insulation won’t sit flush against a wall cavity unless it’s completely uniform and flat, which can lead to unintentional air gaps and wasted heat.

Rigid boards also need to be cut to size and taped with precision, creating further opportunity for human error.

Mineral wool insulation, on the other hand, is flexible and adapts to minor imperfections in the substrate. Where two edges meet, the fibres also ‘knit’ together, minimising air gaps and maximising performance.  

Current regulations address operational carbon, but a buildings’ embodied carbon measurement is just around the corner - that’s the emissions released in producing a building asset, from products and construction.

The use of insulation has clear operational carbon reduction benefits, but manufacturing it carries a carbon cost. Specifiers can create an optimal balance between the two, by choosing products that are lower in embodied carbon.

Details of the make-up and environmental impact of products can be found in Environmental Product Declarations, which are published by the manufacturer and verified by independent third parties.

Meeting the rising standards of Building Regulations guidance requires rainscreen facade systems that can match their designed performance in the real world.

The first step is for specifiers to choose products that are non-combustible and easier to install correctly, with minimal environmental impact.

Leading UK provider of high-performance glass and rock mineral wool insulation solutions, Knauf Insulation offers a CPD in High Performance Rainscreen Facade Systems