Sound deadening Material on the Cheap.

Also, it ain't water absorbtion that's the problem. It's the INTRODUCTION & ENTRAPMENT of Water that are the issues.

Keep the water out as much as you can. But, because it will probably get in (it is an A-Body, ya know), you have to allow for it to escape. There are many ways to do that - they mostly deal w/ allowing water to drain (drain plugs can be modified to allow water to escape...but you've gotta be strategic about it - think of it like a roof: How do you bend, drill, cut, channel, etc to make the water go where you want it to.

Fact is, anything you put under your carpet, plus the carpet itself, will trap water. So whether it's some overpriced 'space-age thermal heat shield' (b/s, it's just foil on bubble wrap), or the 30 yr old carpet padding from your latest demolition job, you'll have to deal w/ keeping the water from getting in & getting stuck.

FWIW, I put cheap-O carpet padding under new carpet 5 yrs ago & have no issues w/ it. Plus, it is FAR superior to any thin space-aged stuff w/ regard to acoustics. Remember, a basic law of acoustics is 'mass'. And the cheap stuff has got lots of that. My ride is pretty pleasant & is reminiscent of a newer car because I've stuffed so much arpet pad under the carpet, around the shifter, at the firewall, inside the vertical spaces such as doors & rear seat sides, bechind the back seat & yes, above the headliner.

Hide it, secure it, seal it & do it right. Once you do that, you'll have the satisfaction of knowing that you paid squat for a top-notch job.

We're A-BODY Guys & we don't just pay top dollar for junk like a lot of other Body Types, daggumit! Remember, Necessity is the Mother of Invention. And finding ways to innovate makes building your car a lot more interesting.

Good luck w/ it!