Maybe you should add a solid frame around your window, like I did for the double-hung window (see http://sweethome3d.cvs.sourceforge.net/sweethome3d/SweetHome3D/src/com/eteks/sweethome3d/io/resources/doubleHungWindow80x122.obj?revision=1.3&content-type=text%2Fplain for its OBJ file), and choose a depth for this frame equal to default wall thickness.