How to properly use Polyisocyanurate (PIR) foam with the heated bed? From what I understand, it takes a really long time for the heated bed to heat up using an MK2a heated bed.  I've heard some people suggest that using Polyisocyanurate (PIR) foam (insulation that takes quite a bit of heat to catch on fire) can be used under the headed bed to make it heat up faster.   

Now of course there are other methods for doing this too, for instance using a larger power supply, but at this point I'd rather just use the parts that I have without re-soldering many of the components on the RAMPS board. 

I was wondering what a proper way to attach this to the bed would be for instance, should the springs go on the bottom or on the top of the bed?  Do I need some extra parts?  Are there any other considerations for doing this?
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.