According to sherwindu :
I am in the process of wrapping the forced air heating ducts in my crawl
space with
a material that looks like bubblewrap with foil on both sides. Does
anyone have any
idea of how much savings this will be on my gas bill? The insullation
is rated at R6, but I cannot relate this to actual savings in energy.

It's very difficult to even try to estimate without having any knowledge
of your installation/climate.

That said, chances are that it will be noticable savings if you
compare equivalent periods.

In an externally vented/drafty crawlspace, you could be losing a _lot_ of
heat from the bare ducts.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.