Take it to the bridge


Simon Hill of Schöck explains the complexities around balcony design detailing to avoid thermal bridges, and why it must incorporate efficient thermal insulation

Improving the thermal performance of a building envelope by minimising energy usage has become increasingly important in the drive for sustainability and energy efficiency – particularly with residential new build and renovation.

Critical to this process is the avoidance of thermal bridging. As such, designers need to be aware of how significantly thermal bridges can compromise the value of the installed insulation.

Thermal Bridging
A thermal bridge is a localised area of the building envelope with significantly higher thermal conductivity than surrounding areas, and typically occurs where a material with high thermal conductivity penetrates the insulation layer.

Cantilevered balconies are the most critical thermal bridges, and their presence results in a higher heat transfer through the building assembly and colder surface temperatures on the warm side of the assembly.

The main consequences will be higher energy consumption for heating, noncompliance with Building Regulations, and condensation. The latter leads not only to structural integrity problems, but the potentially serious occurrence of mould growth too.

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’s 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). There are of course also voluntary certification schemes such as BREEAM and Passivhaus, however, despite this background of increasingly stringent standards for envelope thermal performance and for heat losses, many designers are still not fully aware of how significantly some common thermal bridges compromise the value of the installed insulation.

Performance & 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 they require certain physical characteristics. These include 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, and steel bars transfer bending moment and shear forces. The stainless steel results in lower thermal conductivity and is also corrosion resistant.

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
  • Passivhaus.

Structural assessments should verify the thermal break specifics in accordance with UK Building regulations.

Condensation & Mould
One consequence of thermal bridging is that condensation forming on surfaces, 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 value for residential buildings is equal to or greater than 0.75.

Heavy Balconies
One area that demands a fine balance of design and technical optimisation is the design of particularly 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 be such that they can transfer the higher loads.

The likelihood here is that there will be more steel reinforcement required, in turn increasing 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.

It should also be mentioned that the number of facade penetrations is likely to have an impact on the rate of heat loss. By using an optimum number of the same type of thermal break connectors, there will be an acceptable level of heat loss. However, if too few are used, this results in the remaining connectors taking increased loads and the overall heat loss may marginally increase – but still possibly remain within acceptable whole-building levels. It is therefore necessary to check that the minimum localised fRsi value is achieved to ensure any risk of condensation and mould growth is eradicated.

Responsible Design
The UK has set in law a target to bring all its greenhouse gas emissions to net zero by 2050 – one of the world’s most ambitious carbon targets.

As part of that journey there is a commitment to introducing the Future Homes Standard in 2025, a key part of which 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 that by shaping tomorrow’s construction needs today, new homes will be future-proofed, avoiding any need for retrofitting in years to come.

Simon Hill is product and marketing manager for Schöck