An external door is any operable door that forms part of the external envelope of a heated building and provides access to the outside (or to an unheated space).
It’s treated as a separate building element in heat loss calculations, with its own U-value and area, just like walls, roofs, floors, and windows.
The classification is based on function (entry/exit point), not the glazing percentage.
This includes:
Solid entrance doors (timber, composite, metal, insulated).
Partially glazed doors (e.g. front doors with small glass panels).
Fully glazed doors (e.g. French doors, patio doors, balcony doors).
Sliding or bi-fold doors leading to outside or to an unheated space.
Garage doors only if they directly connect a heated space to outside.