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concrete-to-steel and steel-to-steel. Some are designed specifically for renovation projects, parapets, balustrades and even Passivhaus.
Condensation & mould growth One consequence of thermal bridging is that surfaces can form condensation, resulting in visual deterioration and structural damage. However, an even bigger concern is mould growth. To identify areas where there is a risk of condensation and therefore mould growth, a ‘surface temperature factor’ (fRsi) should be used. It allows surveys under any thermal conditions and compares the temperature drop across the building fabric with the total temperature drop between the inside and outside air. Using the formula, the recommended (fRsi) value for offices and retail premises is equal to or greater than 0.5; and to ensure higher standards for occupants in residential buildings, equal to or greater than 0.75.
Passivhaus – a special case With Passivhaus being the highest building insulation standard, the use of energy- efficient building materials and the
prevention of thermal bridges are essential to the concept. Until recently, cantilevered building components – particularly balconies – posed a construction challenge for Passivhaus design. As the components penetrate the insulating outer shell of a thermally highly sensitive building, the performance demanded could not be met totally effectively by thermal break products on the market at the time. However, product development means that high specification ‘Passivhaus certified’ structural thermal breaks are now available for some applications.
A fine balance with heavy balconies
One consideration that demands a fine balance of design and technical optimisation involves heavy balconies. Where a balcony is heavier due to its method of construction and/or its unusual cantilever length, there will be a greater load transferred back to the structural thermal break connectors. The load capacity of those connectors will therefore need to transfer the higher loads. The likelihood here is that there will be more steel reinforcement required, which in turn
will increase heat loss. However, as long as the total heat losses remain within acceptable levels and the minimum temperature factor requirement (fRsi) is exceeded at a specific junction, then it is usually acceptable.
Responsible design for the future The UK has set in law a target to bring all its greenhouse gas emissions to net zero by 2050. As part of that journey, there is the commitment to introducing the Future Homes Standard in 2025. A key part of this involves uplifting the minimum standard of whole building energy performance and improving minimum insulation standards. The thermal performance of the building envelope is therefore becoming increasingly important – and critical to this process is the avoidance of thermal bridging. Mitigating this problem may result in a limited upfront cost, but represents a small investment when weighed against the long-term savings gained through energy savings and future maintenance issues.
Simon Hill is product and marketing manager for Schöck
WWW.ARCHITECTSDATAFILE.CO.UK
ADF NOVEMBER 2021
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