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3D psi value modelling must be used for compliant thermal assessment of junctions in reinforced concrete (RC) / steel frame system (SFS) construction.

Part L 2021’s stricter fabric and energy targets alongside photographic evidence has placed even greater emphasis on the accuracy of psi values and thermal bridging calculations.

BR497, Conventions for calculating linear thermal transmittance and temperature factors, is the standard used for the calculation. It requires 3D modelling to be performed to properly capture the influence of repeating thermal bridging elements when; “the plane elements of a structure being analysed are non-uniform in the third dimension.” This occurs when:

  • Repeating thermal bridging elements, parallel or perpendicular to a junction, in adjacent flanking elements must be included in any modelling of the junction and in the calculation of the modelling U-value of the flanking element.
  • Hybrid or cold steel-framed walls, where the primary insulation layer is bridged by sections of steel which have a significant effect on the inside surface temperatures.
  • Individual penetrations of the insulation layer – Chi values (point heat losses) associated with bolt-on balconies and metal brackets in rainscreen cladding etc.
  • Special-case junctions that connect to the ground floor:

There are a limited instances where 2D modelling is applicable but 3D modelling is always recommended.