THE IMPACT OF THERMAL BRIDGES ON INSULATION VALUES

When changes to Part L of the Building Regulations came into force last year, CO2 emissions for dwellings had to be reduced by 31% and, for other buildings, 27% – this being an interim step towards the Future Homes Standard, which becomes mandatory from 2025. As a result, designers reall y need to be aware of how significantly thermal bridges can affect insulation values, says Simon Hill, Product a nd Marketing Manager at Schöck.

Insulation values can be seriously compromised by thermal bridges, which occur in a localised area of the building envelope with significantly higher thermal conductivity than surrounding areas. Typically, 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 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 (SAP) 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 an audit should be undertaken to confirm the designed details have been constructed –before different construction junctions are concealed by subsequent work. 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 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 betterperforming 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 0.20W/ (m 2.K) to discourage generic estimations – as this does not reveal the performance of thermal break junctions, including failure to meet (fRsi) figures.

Many designers though are still not fully aware of just how significantly thermal bridges can affect insulation performance – and the most effective way to minimise thermal bridging at cantilever balcony detailing is to incorporate a structural thermal break from Schöck. These are highly-efficient balcony connectors that minimise the flow of thermal energy between the interior and exterior of a building, providing both structural integrity and thermal isolation of the balcony. The Schöck range of thermal break solutions is for applications as diverse as concrete-to-concrete; concrete-to-steel; steel-tosteel; renovation projects, parapets and even Passivhaus. 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.

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.

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 a critical element in this is responsible design and the avoidance of thermal bridging.

One current project with a challenging variety of balconies is Hendon Waterside in north-west London, a mixed-tenure housing development. Situated alongside the Brent Reservoir, the scheme involves the regeneration of a 1960s housing estate being built in six phases and scheduled for completion in 2027. The masterplan will deliver around 2100 new homes consisting of social, affordable and private housing. These are being constructed in 23 blocks, varying in height from three to 23 storeys, along with an imposing tower building of 28 storeys – all of which offer waterside views.

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 13 different product variants. As a leading international supplier of structural thermal breaks, Schöck can provide a solution to practically every connectivity detail with the hundreds of variants available in its main Isokorb range; offering planners enormous freedom of design. The Isokorb products used here are for concrete-toconcrete applications and provide optimum solutions through the use of varying tension bar arrangements. For example, one type provides cantilever balcony connections and transfers both negative moments and positive shear forces. Another is a load-bearing thermal break element for transferring shear forces on supported balconies, recessed balconies and for occasional peak shear forces.

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