Hi,
In your GI Settings , I hope you are using mental ray, Turn maximum sampling radius, and set it lower. The radius it's sampling is to large and so the light is bleeding through, if you set it smaller it shoudlnt' go through the edges.
Or
You could also attach the floor, the walls and the roof to one poly mesh and weld the vertices's, so you kinda make those edges or intersections "water proof"... that way light should not go through.
Cheers!