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Linear Thermal Bridging - BBA launch new details with Aircrete products Association
Changes to the way heat losses along the length of junctions of constructions (linear thermal bridges) are calculated in the 2010 revision to Part L has had a significant impact on the specification for external walls, with aircrete benefiting from its innately low thermal conductivity.
The issue of linear thermal bridging is hugely significant. As U-values get even lower heat loss from junctions can now account for as much as 40% of the heat lost through the fabric.
The spotlight was turned on this issue in the 2010 revision as designers are required to include individual heat loss calculations for all junctions including those at the party wall edges. The resulting additional heat loss value has a significant impact on the overall energy conservation and carbon emissions.
Under the previous, 2006 regulations, heat losses at thermal bridges were calculated by applying a global “y” value, which was taken as 0.08 where Accredited Construction Details (ACDs) were adopted or 0.04 if Enhanced Construction Details (with enhanced insulation at junctions) were used.
SAP does provide a list of default linear thermal bridge (psi value) for each junction of construction, but since they are for generic constructions they tend to be on the conservative side.
The Aircrete Products Association (APA) had previously developed a set
of typical Construction Details which showed that by using aircrete blocks
within the standard ACDs, the enhanced y- value of 0.04 could be adopted
without the need for further enhancing the details or adding insulation. To
build on this work, more recently the APA has been working with the British
Board of Agrement and Robust Details Ltd as part of their ‘Constructive Details
Limited’ scheme to provide the first set of standardised and independently
produced construction details that are aircrete masonry specific. The details
have been made as flexible as possible for the designer to achieve optimum
energy savings when using SAP. Based on a cavity construction using partial
fill insulation, each detail gives psi-values (a measure of thermal bridge
performance) and covers a range of aircrete block types and U values as well as
highlighting the key areas to check on site. The format closely mirrors the
Part E ‘Robust Details’ with specific drawings of the details, the psi values
obtained for the variants of the details and a site check list.
The benefit in using these simple constructions, which have been based on common build methods and follow closely the Government’s ACDs is that the design heat loss at junctions should be reduced by some 50%, providing significant cost savings in the build process.
“Sensitivity checks carried out on numerous variables during the developing stage highlighted that influencing factors are different for each junction type. For example the thermal conductivity and thickness of insulation may be significant for some details but for others it is more important to limit certain dimensions. Constructive Details have adopted a user friendly approach which gives limiting parameters relevant to each junction, thus enabling most accurate results to be adopted whilst maintaining maximum flexibility for the designer. Agreed, Alan Thomas
Although the number of publically available sets of construction details and psi-values has recently increased, the user has to carefully choose the correct detail and, more importantly, ensure that it will be correctly re-produced on site as any deviation can significantly affect the thermal bridge performance. In the absence of an Accredited Scheme to monitor the standard or quality of such details, Constructive Details Limited have set the Standard for others to aspire to.”
The details can be found at www.constructivedetails.co.uk and are available for free, via a straightforward registration process. On the same site you can also register for updates and follow any CDL developments via our Twitter feed @ConstructiveDtl.