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

Critical 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.

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 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, 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