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
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 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 than 0.75. In more extreme conditions of high humidity, such as swimming pools, 0.9 would be anticipated.
Passivhaus: a special case
The use of highly energy-efficient building materials and the prevention of thermal bridges are essential to Passivhaus. Until relatively recently, cantilevered building components – particularly balconies – posed a challenge. As they penetrate the insulating outer shell, the performance demanded could not initially be met effectively by thermal break products. However, product innovation has led to high spec ‘Passivhaus certified’ structural thermal breaks being available for such applications.
Heavy balconies
These demand a fine balance of design and technical optimisation, due to the greater load transferred back to the structural thermal break connectors. The load capacity of those connectors will therefore need to be such that they can 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 a commitment to introducing the Future Homes Standard in 2025. A key part of that involves uplifting the minimum standard of whole building energy performance and improving the 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 this represents a small investment when weighed against the long-term savings gained through energy savings and future maintenance issues. It is therefore crucial to shape tomorrow’s construction needs today, ensuring new builds will be future-proofed, and avoiding any need for retrofitting in years to come.
Simon Hill is product and marketing manager at Schöck