IsExternal definition

https://github.com/buildingSMART/IFC4.3.x-development/blob/master/docs/properties/i/IsExternal.md

The definition: 'Indication whether the element is designed for use in the exterior (TRUE) or not (FALSE). If (TRUE) it is an external element and faces the outside of the building.'

In most EU countries, there is the Energy Performance of Buildings Directive which obliges new buildings and renovations to submit a score on the energy performance of the asset. Therefore one needs to know what elements are part of the outer shell (between heated and non-heated space). It would be great to have a parameter (true or false) where you could indicated that. For a long time I thought this was the parameter, but a wall between the garages (non-heated) and the living room (heated) will have IsExternal 'false' because it is not designed for exterior use. So one needs to add a custom parameter to be able to extract all walls that are part of the outer shell. Since energy calculations, in light of the directive, are a hot topic, I wonder if there shouldn't be a parameter for that by default in the IFC schema. At the same time I wonder, maybe this parameter was meant for that, but maybe the definition is incomplete?

Start with docs/properties/i/IsExternal.md and the issue's examples about exterior elements and heated versus non-heated spaces. Determine whether the existing definition needs clarification or whether a separate schema parameter is required. Done means the specification direction and resulting definition or parameter are agreed and documented.

Written by the indexing model from the issue text.