There's no method in the program to detect outside walls.
Maybe you could reuse [font=courier new]wallsAreaCache[/font] handled in [font=courier new]PlanController[/font] to get its exterior path(s) using [font=courier new]getAreaPoints[/font] in [font=courier new]Object3DBranch[/font]. The exterior walls should be the ones that intersect with this exterior path, but that won't work if an interior wall touches the border of an exterior wall.