EXTERNAL ENVELOPE 49
Bridge the thermal gap A
thermal bridge is a localised area of the building envelope with significantly higher thermal conductivity than surrounding areas, typically occurring where a material with high thermal conductivity penetrates the insulation layer. Cantilevered balconies are the most critical examples, resulting in higher heat transfer through the building assembly and colder surface temperatures on the warm side.
The main consequences will be higher energy consumption for heating, non- compliance with Regs, and condensation. The latter can lead not only to structural integrity problems, but potentially serious occurrence mould growth too. The latest version of Part L (2013, with 2016 amendments) and associated guidance for residential construction Approved Document L1A (ADL1A) require thermal bridging be included in fabric heat loss calculations. The SAP calculation on heat loss includes the term HTB (heat loss due to thermal bridging). However, despite this background of increasingly stringent standards for envelope thermal performance and losses, many designers are still not fully aware of how significantly some common thermal bridges compromise insulation.
Thermal performance & 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 manufactured 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 – thermal insulation of an optimum thickness for the application, load-bearing
ADF JUNE 2022
Improving the thermal performance of building envelopes has become increasingly important, and designers need to be aware of how significantly thermal bridges can compromise the value of insulation, says Simon Hill of Schöck
components, and a combination of reinforced and stainless steel. A wide variety of thermal break solutions are available for connectivity applications as diverse as concrete-to-concrete; concrete-to-steel; steel-to-steel; renovation projects – and even Passivhaus.
Condensation & mould growth One consequence of thermal bridging is that surfaces can form condensation, resulting in both 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
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