One more valuable link:
Although the subject of this thread was was voltage separation in the z-axis of a PCB,
I did find a very useful ONLINE CALCULATOR for Creepage and Clearance per IEC/UL
http://www.creepage.com/
It saves having to wade through the specifications and various tables, and if you are
calculating for a PCB construction, select the "IIIa/IIIb" Material Group
simple.
(big thanks to Dave Larusso!)
Jack