Why is glass material (and custom ones) see through on BSP shapes?

Both my custom glass material and the default unreal glass material appear to be see through on the edges when applied to a windowpane BSP shape. In most games the windowpane glass material just ends on all sides however when applying the glass material, it makes the shape see through on all contact points with other BSP shapes (window frame). Anyone know why this is happening or if there is a solution to this?