Following changes to Part L in 2021, carbon emissions in new builds have to be significantly lowered. Simon Hill of Schöck says specifiers need to be aware how greatly thermal bridges can affect insulation values

A localised area of the building envelope with significantly higher thermal conductivity than surrounding areas is known as a thermal bridge. Typically occurring where a material with high thermal conductivity penetrates the insulation layer. Cantilevered balconies are critical examples, resulting in higher heat transfer through the building assembly and colder surface temperatures on the warm side. The main consequences will be non-compliance with Building Regulations, higher energy consumption for heating, condensation and the formation of mould growth. The 2021 edition of Part L (which replaces L1A 2013) sets out the minimum thermal requirements for avoiding such issues. 

Thermal bridging must now be included in fabric heat loss calculations and the Standard Assessment Procedure calculation (now updated to SAP 10.2) includes the term HTB (heat loss due to thermal bridging). A further change involves an onsite audit for new dwellings. It is now required that before different construction junctions are concealed by subsequent work, an audit should be undertaken to confirm the designed details have been constructed. Photographs should be taken to verify that the products used are those shown in the original design. If there are substitutions, the revised specification should be reflected in the SAP calculation and report in the Building Regulations England Part L compliance report (BREL report). This information needs to be signed off by the SAP assessor.

General tightening of U-values – requiring greater insulation requirements and the construction of better performing thermal break details – are also included in the revised Part L. Additionally, it encourages HTB assessment to be carried out through thermal calculation for a more realistic evaluation of existing thermal junctions. The default Y-value has been increased to 0.20 W/m².K in the new update to discourage generic estimations – as this does not reveal the performance of thermal break junctions, including failure to meet surface temperature performance factor (fRsi) figures. 

THERMAL PERFORMANCE & STRUCTURAL INTEGRITY

Many designers are still not fully aware of just how significantly thermal bridges can affect insulation performance – and that 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 thermally isolating the balcony. 

A structural thermal break has a very specific purpose and for long-term effectiveness requires certain physical characteristics – optimum thermal insulation thickness for the application
in question, load-bearing components and a combination of reinforced and stainless steel. Solutions are available for applications as diverse as concrete-to-concrete, concrete-to-steel, steel-to-steel, renovations, even Passivhaus.

CONDENSATION & MOULD GROWTH

One consequence of thermal bridging is that cold 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. 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. 

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 – and the higher performance targets of CO₂ emissions being reduced by 31% for dwellings and 27% for other buildings – is an interim step towards that standard. The thermal performance of the building envelope is therefore of increasing importance – and critical to this is the avoidance of  thermal bridging. 

THIRTEEN PRODUCT VARIANTS
FOR HENDON 

One ongoing current project with a demanding variety of cantilever balconies – and therefore thermal bridging challenges – is Hendon Waterside in North West London. Situated alongside the Brent Reservoir, the scheme involves a six-phase regeneration of a 1960s housing estate. The masterplan will deliver around 2100 new homes being constructed in twenty-three blocks. The construction of the many balconies involved vary in their design demands and to ensure the risk of thermal bridging is minimised, Schöck has supplied over thirteen different structural thermal break variants.

Simon Hill is product and marketing manager at Schöck