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 external thermal bridge surcharge is an allowance for additional heat losses where the building fabric meets the external environment e.g.
Window and door reveals
Roof-to-wall junctions
Balconies or cantilevered slabs
External wall corners
The standard value in autarc is 0.15 W/m² but if you have detailed Ψ-value calculations (e.g. from SAP or PHPP modelling), enter those instead for more accurate results.