Gary Robson, business development manager at EJOT UK, explains why the design and specification of the metal substructure is critical to maximising the thermal performance of rainscreen facade systems.
According to the Whole Life Carbon Roadmap published by the UK Green Building Council (UKGBC), around 75% to 80% of carbon emissions from the built environment result from the building’s operation and use – a large part of which is for heating and cooling. Improving the energy efficiency of buildings is, therefore, extremely important for the UK to meet its net zero goals.
Wrapping the walls of existing buildings and creating new buildings with a highly insulated rear ventilated façade is an effective and proven way to reduce heat loss and, provided the building’s overall ventilation and shading strategy is sufficient, lower the potential for summertime overheating. For the facade system’s insulating capacity to be maximised, careful consideration has to be given to the type of insulation used within the proposed system, of course. But that is not enough in isolation – it is also critically important to take into account the contribution that the metal substructure makes to thermal transfer.
Substructures or backing frames are formed of metal brackets, rails and fasteners which are attached to the building substrate, providing the frame onto which the outer cladding materials can be fixed. The framing allows a void to be created for the insulation material to sit as well as a ventilation gap – all positive from a thermal insulation perspective.
However, every bracket holding the frame and outer cladding to the structure breaks the layer of insulation, creating a thermal bridge. While this thermal bridging effect may seem relatively inconsequential, especially where a deep layer of insulation is specified, it can actually be so significant that it undermines the façade’s overall thermal performance.
Aluminium versus stainless steel
Aluminium is the most widely used metal to manufacture rear ventilated facade substructures with its popularity driven by it being lightweight, easy to work with and recyclable at the end of its service life. Stainless steel offers an alternative, with substructures such as EJOT CROSSFIX offering a solution with similar advantages whilst also enabling greater rigidity in the frame and – crucially – far lower thermal conductivity.
Aluminium brackets in facade substructures have a thermal conductivity of typically between 160 to 220 W/mK, compared to stainless steel’s range of between 15 to 20 W/mK. This means stainless steel has the benefit of being between ten and fifteen times less conductive than aluminium.
Thermal breaks or thermal stops, manufactured in materials such as polyamide, are integrated into some aluminium systems to help reduce conductivity. However, the impact they have is not enough to reduce the material performance gap – especially considering that most good stainless steel.
Leveraging thermal and structural gains
Stainless steel brackets substantially reduce heat flow through the façade assembly, improving the overall thermal performance of the rainscreen system and helping to lower heat losses, energy consumption and the risk of localised cold spots. That makes a significant difference in itself, but there is an additional benefit stemming from stainless steel’s greater rigidity versus aluminium.
In certain projects, stainless steel brackets may enable wider bracket centres and fewer anchors because of their higher load capacity and stiffness, subject to a full structural calculation. This means fewer stainless steel brackets may be needed across the facade without compromising the substructure’s ability to accommodate the calculated loads.
With a reduction in the number of brackets per square metre, fewer structural connections link between the wall and the outer cladding – lessening the opportunity for thermal bridging in the first place.
Scope for thinner walls or better performance
The cumulative effect of reducing thermal bridging in each bracket can be significant enough to improve the facade’s overall thermal performance, potentially allowing the required insulation thickness to be reduced while still achieving the target U-value.
This means choosing a stainless steel façade substructure can be commercially advantageous in a number of ways. Firstly, with thinner walls, the floor space within the building could be slightly increased without expanding the building’s overall footprint. The reduction in insulation material would also facilitate cost and carbon savings. Most importantly for developers and building operators, the increased square footage would attract a higher rental or sale value, increasing the return on investment over the building’s lifetime.
The alternative to reducing wall thickness would be to maintain the original design and use a stainless steel substructure to achieve a lower U-Value without having to up-spec the insulation. This would allow the rainscreen system’s thermal conductivity to be lowered further without increasing insulation depth.
Maximise the potential of insulation innovations
Every facade project has a different set of challenges and performance goals, but it is only as thermally efficient as its weakest link. As insulation materials continuously advance to offer ever higher thermal standards and improved energy efficiency across the built environment, the negative impact that the metal brackets and sub-frames can have is increasingly being recognised.
As the industry continues its drive towards net zero, reducing thermal bridging within the substructure should be considered a fundamental design principle. By addressing heat loss at every bracket and connection point, designers can create façades that not only meet today’s performance standards, but are better prepared for the demands of the future.