Luke Davies, Rock Mineral Wool Product Manager for Knauf Insulation, looks at how architects can specify rainscreens that deliver on their designed performance.
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The last few years have seen Building Regulation guidance tighten significantly. Requirements for energy efficiency and safety are becoming far stricter and more complex. As a result, the onus is on architects to deliver rainscreen designs that translate into real world, ‘as-built’ performance. Closing that gap between designed and built performance can be a challenge, but there are steps architects can take to set their rainscreens up for success.
Detailed calculations
Last year’s updates to Approved Document L (Energy Efficiency) require buildings to produce less operational carbon emissions than those built under the previous regulations. Next year the Future Homes and Buildings Standard is due to tighten this even further.
For any building to live up to its designed thermal performance, accurate U-value calculations are essential. However, rainscreen façade systems can be prone to calculation errors because of the metal components that penetrate the insulation, compromising thermal performance. Simplified U-value calculations account for this by applying a blanket correction factor. The lack of detail in this approach can mean target U-values are not met. To avoid this, architects should insist on 3D U-value calculations, which incorporate a detailed analysis of the interaction between the bar and bracket configuration and the insulation, resulting in far greater accuracy.
It’s also important to request a detailed report that includes the components used in the calculations. This can then be passed to the supply chain, showing how changes to the specification could affect the U-values.
Improved fire safety
In 2022, Approved Document B (Fire Safety) banned the use of combustible materials in certain external wall system build-ups, such as rainscreen façades, on all residential buildings between 11m and 18m. The only exception is if a full-scale fire test to BS 8414 has been conducted. Requirements also changed for other types of building, adding further complexity to the design process. In response, some planning policies and funding requirements have gone beyond these minimum standards. For example, the Greater London Authority’s Homes for Londoners programme states that ‘no combustible materials may be used in the external walls of all homes and buildings, regardless of their height’. By using non-combustible insulation, regardless of the height or use of a building, architects can simplify the specification process and help mitigate fire risk.
Resilience to real world challenges
Having designed with real world performance in mind, it’s also important to consider what happens on site. This might seem outside of the architect’s remit, but material choice can have an impact on the success of an installation. For example, rigid board insulation won’t sit flush against a cavity unless it is perfectly uniform and flat. Boards also need to be precisely cut and taped, all of which can introduce air gaps that compromise thermal and fire safety performance. By contrast, mineral wool insulation is flexible and maintains close contact with the substrate, fully filling the cavity. Where slabs meet the ends also ‘knit’ together, making it easier to install correctly. This minimises air gaps and maximises thermal performance.
Another common challenge with rainscreen façade installations is exposure to the elements during construction. Best practice is to install insulation on a ‘rolling front’, meaning each section is immediately covered by panels to protect it. But a recent survey suggests that this is only achieved in approximately 50 per cent of projects. In 70 per cent of cases, the sheathing insulation is left exposed for over a week¹ – enough time for weather conditions to cause damage and compromise performance.
Fortunately, architects can respond to this challenge by specifying insulation that has been engineered with the realities of construction in mind. Knauf Insulation’s new Rocksilk® RainScreen Slab EE rainscreen sheathing insulation has an enhanced water-repellent facing that reduces the risk of water damage when the insulation is left exposed. This helps architects to preserve the intended thermal, fire and acoustic performance of the building.
By insisting on accurate thermal calculations and specifying insulation that is non-combustible, buildable and more resilient to weathering, architects can design rainscreens that deliver real world performance.
Contact Details
To learn more about Rocksilk® RainScreen Slab EE, please click here.
¹ Survey of rainscreen installers by RCI magazine, conducted 2023. Sample size 10.
Source: Architecture Today