Here are some discussions dealing wit various aspects of voids:
http://thebuildingcoder.typepad.com/blog/2011/06/boolean-operations-and-instancevoidcututils.html
http://thebuildingcoder.typepad.com/blog/2010/07/beam-maker-using-a-void-extrusion-to-cut.html
http://thebuildingcoder.typepad.com/blog/2014/04/instancevoidcututils-and-need-for-regeneration.html
http://thebuildingcoder.typepad.com/blog/2017/03/wta-mech-and-ttt-for-provision-for-voids.html
Cheers,
Jeremy