Everything has (at least) two faces: about walls and normals

At the walls, by S. Bakalovich
from wikipedia

 To explain how I am going to deal with constraints, I will use the simplest, and most common, type of them: walls. Although they often look just as think lines on a plan, walls cannot be represented as flat two dimensional areas. That would contradict our experience and wouldn’t make any sense geometrically, as walls are three dimensional objects with different depth. 

It is possible that at the very beginning of your 3D modelling career you are tempted, quite naively, to simply trace the shape of your walls and extrude them as a solid. In case, the puzzled expression of your supervisor and the question “what is that supposed to be? Minecraft?” will quickly tell you that you’re on the wrong track.

What you are supposed to do (or, at least, what I am going to do) is to build two separate surfaces, one for the interior and one for the exterior, without modelling what is in between (i.e. the actual thickness of the wall).

There are a few reasons why I agree it is a sensible approach.

First, walls (as well as every other constraint) may display different material or decorations on the inside or the outside. So, in the model, they will probably have different textures assigned. 

Also, as we were discussing in the other posts, there is a logical (or ontological, in the linked data meaning of the word!) benefit in it. A wall often delimits two different spaces. But, more specifically, one side of it delimits one space and the other side delimits another space (or spaces). For example, the wall that separates the ekklesisaterium from the sacrarium, is the internal southern wall of the ekklesiasterion on one side, but, on the other side, it is the northern internal wall of the  sacrarium.

It may sound confusing at the beginning, but, after you get used to that, it is a simple yet effective convention that has proven his usefulness in my previous models already. I think it will also suit well the RDF modelling and help dividing meaningfully my digital archive.

If there weren’t enough reason to justify a separate management of the two surfaces of a wall (or other constraint), game and VR engines give us another good one: normals. Surfaces are visible only from one view, either from the interior on the exterior. Even if we had a wall with the same identical texture (let’s say a generic masonry), to allow a 360° virtual view of it, we still would need two surfaces: one looking towards the interior and one looking towards the exterior. This is the only way a character (or a camera) can have a realistic view of a virtual building when it is walking into a space as well as when it is walking around it.