23 Bridging the gap in design
Improving the thermal performance of the building envelope is central to the drive for sustainability, energy efficiency and meeting the criteria for the impending Future Homes Standard. Simon Hill of Schöck considers the issues involved
ritical to improving the thermal performance, is the avoidance of thermal bridging. Understanding how significantly thermal bridges can compromise the value of the installed insulation is essential.
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A thermal bridge is a localised area of the building envelope where the heat flow is usually increased in comparison with adjacent areas. In other words, thermal bridges act as a ‘heat highway’ headed straight out of the building. As a result, the temperature of the interior surface near the thermal bridge is lower and if this area becomes too cold it can lead to condensation. Over time, mould is likely to form, potentially damaging the building structure and even posing a health risk to the inhabitants. Cantilevered balconies are the most critical thermal bridges. Poor design detailing at these connectivity points will, in addition to higher energy consumption and condensation problems, result in non-compliance with the Building Regulations.
Meeting required standards The latest version of the Building Regulations Part L (2013, with 2016 amendments) and associated guidance document for residential construction Approved Document L1A (ADL1A) require that thermal bridging be included in the fabric heat loss calculations. The Government Standard Assessment Procedure (SAP 2012) is the model used to provide evidence that the carbon emissions target has been achieved. Also the SAP calculation includes the term HTB (heat loss due to thermal bridging). The Future Homes Standard will come into effect in 2025, with the aim of ensuring that new homes in England are futureproofed – and there are of course also voluntary certification schemes such as BREEAM and Passivhaus. However, despite this background of increasingly stringent standards for
ADF NOVEMBER 2021
envelope thermal performance and heat losses, many designers are still not fully aware of how significantly some common thermal bridges compromise the value of the installed insulation.
Thermal performance and structural integrity
The most effective way to minimise thermal bridging at cantilever balcony detailing is to incorporate a load-bearing structural thermal break. This is a highly efficient balcony connector that minimises the flow of thermal energy between the interior and exterior of a building, providing both structural integrity and ensuring that the balcony is thermally isolated. The units have a very specific purpose and to work effectively over a long period require certain physical characteristics. Namely, thermal insulation with an optimum thickness for the particular application, load-bearing components and a combination of reinforced steel and stainless steel. The bearings in the compression module transfer the compression forces, steel bars transfer bending moment and shear forces; whilst the stainless steel results in lower thermal conductivity and is corrosion resistant too. A wide variety of structural thermal break solutions are available for connectivity applications such as concrete-to-concrete,
Understanding how significantly thermal bridges can compromise the value of the installed insulation is essential
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