I have done quite a lot of soundprrofing work to my westy. As a result, I am
very doubtful of the economic viability of the roofing stuff, unless you
have a very cheap source for it.
the roofing stuff loren linked from lowe's is about $1.30 sqft. And it does
not have foil backing which gives you better radiant heat retention.

Heres my experience: I did fatmat from fatmat.com for the rear of my van.
Fatmat is not really more than the roofing stuff in price.  while the stuff
is much thicker than roofing stuff, you still roll out two layers. it comes
on a roll, delivery takes about two months. Sound insulation is ok, but not
groundbreaking (subjective of course). it sticks, but only when the surface
is perfectly clean and dry. you have to use a lot of pressure to roll it on.
After I glued 100sqft in the van i bough a pile of second skin audios
damplifier, which i got from a listmember who bought too much. list price is
about $2 sqft. Might be a lot, but this stuff is more than twice as good as
fatmat. its thicker, so you could go with one layer, but of course two
layers help even more. its precut in 12x24 sheets, which makes installation
much faster. It sticks EXTREMELY well which makes installation even easier.
The foil backing is so thick you wouldnt even be able to roll this stuff. I
put this in the cabin area and now the noise levels are way down in front -
hardly any road noise left. of course, wind noise is still there since i did
not do the area behind the dashboard or the overhead parts yet.

bottom line - dont just look at the per sqft price. the stuff isnt all the
same.

behind the panels i did the following: one layer of any of the products
above, then two layers of closed cell camping pad (pads are as low as $6
each on amazon sometimes) (this is the same stuff that reimo sells as xtreme
isolator) then spray all the nooks and crannies with "great stuff door and
window" which is a sprayfoam that expands without bending metal.
its much warmer in the van now and it heats up less in summer. the doors
sound really solid when you close them.
florian