FACADES, RAINSCREEN & LOUVRES
WHAT MAKES A GOOD RAINSCREEN INSULATION SPECIFICATION? The legislative landscape is changing at pace, not only bringing in stricter standards for how buildings must perform but also placing greater emphasis on accountability and product choice. So, to ensure your rainscreen projects perform in the real world, Kelly Westwood, Head of Construction Projects at Knauf Insulation, explains the factors that you need to consider to get your insulation specification right.
KELLY WESTWOOD IS THE HEAD OF CONSTRUCTION PROJECTS AT KNAUF INSULATION
KNAUF INSULATION
Failsafe fire safety When specifying rainscreen facade insulation, always start with fire safety. In December 2022, the ban in England and Wales on using combustible materials in the external walls of relevant residential buildings was widened to cover all residential buildings between 11 and 18m high. The only exception for rainscreen facade systems is where a full-scale fire test to BS 8414-1 or BS 8414-2 has been conducted. Such tests are prohibitively expensive, and the system must be identical to that used for the test, so combustible materials have effectively been banned in external walls of residential buildings with rainscreen facades. The changes also affect non-dwelling buildings. Materials used in the external walls of assembly and recreational buildings within 1m of a relevant boundary must have a reaction to fire classification of B-s3,d2 or better. For assembly and recreation buildings over 1m from boundary, classification B-s3,d2 or better is only required from 18m and above. FC& A – MAY – 2023
While designed to improve fire safety, the changes add complexity. So, the best way to mitigate risk is to use noncombustible mineral wool insulation with A1 or A2-s1,d0 reaction to fire classification regardless of height or building type.
Accurate thermal modelling Once you have fire safety covered, you can look at thermal performance. Approved Document L England and Wales was updated last year, bringing in much stricter thermal performance standards. The transition period ends on 15th June 2023. Stricter standards are essential, but buildings will only deliver that performance in the real world if designs are underpinned by accurate thermal modelling. Rainscreen facade systems feature numerous metal components that penetrate the insulation, compromising thermal performance. But the most common type of U-value calculation, the simplified methodology, cannot 28
accurately account for this, because a blanket correction factor is applied. This means insulation may not achieve the targeted U-value or may be overspecified, suggesting a thicker solution is required, increasing costs. Instead, architects and designers should insist on the numerical method or 3D U-value calculations to BS EN ISO 10211. These calculations account for the precise bar and bracket configuration, so you can be certain your insulation and wall build up will perform as it is designed to.
Ensure compliance In construction, there’s always the risk that one product can compromise the performance of another. Taking a system approach can help to mitigate this risk. For example, specifying a throughwall system gives reassurance that the different elements of the wall build up have been designed to work together and have been tested both individually and as a system.