Thermal bridges and energy calculations
It is possible to calculate thermal bridges using internal and external dimensions. The use of internal dimensions tends to underestimate heat loss. The use of external dimensions tends to overestimate heat loss. The two methods should not be combined.
The Passivhaus Standard uses external dimensions whilst UK conventions utilise internal dimensions. Calculations based upon UK conventions should not be used when calculating the heat loss from a building that is designed to meet the Passivhaus Standard. Likewise thermal bridging calculations developed for the Passivhaus Standard should not be used in SAP or iSBEM calculations.