Thermal bridges are junctions in the building where insulation is less effective and heat escapes more quickly (for example, at corners, window frames, or where floors meet walls). In MCS-compliant heat loss calculations, thermal bridge surcharges are added on top of standard U-values to make sure these “hidden” losses are included.
The internal thermal bridge surcharge is an allowance for minor additional heat losses at internal junctions within the building structure, e.g:
Wall-to-floor junctions (within heated zones)
Internal corners and connections
Party walls
The standard value in autarc is 0.10 W/m² but if you have detailed Ψ-value calculations (e.g. from SAP or PHPP modelling), enter those instead for more accurate results.