Hm ok i cant see anything on that pic (3 things for future pics: brighthen them up, downsize them, also plz post them in here). You probably mean that light is creeping through at the floor or the ceiling or in corners, right?
In that case Higgins is right about the way to build with brushes. That way to build prevents light leaking through in most cases.
It also reduces the amount of t-junctions you get from unclean brushwork (search for Ifuritas t-junction tutorial).
Only to prevent confusion: the light leaks are not caused by t-junctions, or the other way round. T-junctions really have nothing to do with the light leaks afaik, it just happens you can deal with both by using the same (clean) technique of brushwork.
T-junction means: there is the corner of a triangle/brush touching a side of a triangle/brush, and that causes the engine to split the touched triangle (read Iffy’s stuff, it even got pics in it). So by doing clean brushwork you get
less t-junctions = less tris = better FPS = cool = all the hot Wolfenstein-babes
