Under my rubber mat??

I don't have factory carpet..."yet". I have the factory rubber mat.
On my 71 Valiant. I was running speaker wires the to the back the other day. Well, I removed my kick panel, and my rocker cover...etc.
The body Side behind kick panel nice and clean.
But under the factory rubber mat, is insulation. It was a bit damp.
And I see it made some surface rust. I will be removing the insulation this weekend, and wire wheeling and using rustoleum, so things don't get out of control.....Still nice and solid.
I don't want to replace with factory insulation, seems like a moisture holder.
I will re-use my factory rubber until I get carpet.
Any suggestions on what to do? I would like to put something under it, and/or under the carpet when I get it. What are you guys using??