Heat loss areas
Heat loss areas in Passivhaus are measured to the outside of the thermal envelope. This can be less straightforward than the SAP internal measurement methodology, as what constitutes the outside of the thermal envelope may not be immediately clear.
Put simply, the thermal envelope ends at the last element used in your U-value calculation. So for a masonry wall with fully filled cavity the last element is the exterior brick or block; for a timber frame wall with a ventilated cavity and rain-screen, the thermal envelope will end on the outside of the timber frame wall, because the thermal performance of the rain-screen and cavity are unpredictable and are therefore ignored.
Using dimensions which match the element build-ups you use for U-value calculations will help ensure an accurate PHPP model.