Especially in densly populated city centers, it often occurs that neighboring buildings partly or fully touch each other. In this case, the building's energetic behavior significantly differs from a free-standing building. Unfortunately, CityGML completely lacks a concept for "shared walls/roofs", and existing CityGML building models therefore do not represent closed building structures correcty. A single CityGML Building object is always bounded by exterior BoundarySurfaces (WallSurface, RoofSurface, GroundSurface).

In the Energy ADE, a ThermalBoundary object can be specified as "sharedWall". In reality, often only a part of a wall or roof surface is shared with a neighboring building, and the remaining part is facing the outside air. Thus, for transforming CityGML BoundarySurface into Energy ADE ThermalBoundary objects, a separation of the BoundarySurface geometry is needed, which is a time consuming and error-prone process. In most cases, it is much faster and much more robust only to calculate the size of the shared area. It is therefore proposed to add a numeric attribute to ThermalBoundary to either specify the absolute sharedAreaSize (type Area), or the ratio of shared area to total BoundarySurface area (sharedAreaRatio, type Scale).