How to properly use Polyisocyanurate (PIR) foam with the heated bed?
An alternative solution that is used with some Kossel Mini models, is a simple cardboard covered with aluminum tape (docs).

This solution has the added benefit of actually improving heat-up time, since the reflective surface of the aluminum tape reflects the heat back into the bed, instead of getting absorbed. 

It is also a very simple solution, without costly materials. I guess mounting it properly could be an issue depending on your printer, but this is a fine solution for the Kossel models, at least.