Sorry about the delay but have only just noticed your question since the forum has been updated. You probably have it solved by now, but in case you haven't.......
The proper way to get a lintel over the opening is to do a full wall, (so you have a 'room's and hence the outside texture you require), is to use the Openings tool. This just cuts an opening like a door but with no actual door.
You could of course just do the 3 walls and set the exterior texture to both sides of the wall but that can cause photorealistic rendering issues. No sure which version you have but later versions also have an option to 'Force Exterior Surface' which made the 3 walls into exterior surfaces.