A summary of the basics of making a digital model of the geometrical layout of the building to be rendered:
Avoid digital building models with paper-thin walls that meet along a single edge or one where any building element such as floors and ceilings are simple planes, because such models can develop “light leaks” along these edges. To repair a model so these light leaks do not occur:

• Make sure floors and ceilings have thickness.
• Use the CAD program's Wall command to create walls. The Wall command is programmed to make sure corners are constructed of solid objects (objects with thickness) instead of leaving a single, thin edge adjoing the roof or wall.
• Ensure that floor and ceiling objects extend beyond walls. Floor objects need to extend under walls and ceilings need to extend over walls.

By building your 3D model using these guidelines, light leaks will be less likely to occur when you process Global Illumination after adding a Skylight to the scene.

Another issue here is the practice that used to be common when we used solid modeling to make buildings. Then we built walls that extruded through floors or vice versa. The crude scanline renderer dealt with providing an approximation for the join line where one surface protruded through another. Now, intersection of solids in this way is seen to create ambiguity for the rendering program.