Well too, car will ride quieter, eliminate NVH. Noise, vibration & harmonics as I recall and make carpeting plusher. Makes me think, I have a section of the under carpet padding for rugs I was going to put in my trunk- same idea, never got to it this year lol. And then the jacking rails I never installed from a year or so ago. Can use one jack to lift entire side of car is idea.
So I’m thinking for floor of car proper the shielded underlayment is a must, trunk not so much. Had just mat in rr but never thought of something under that. So the newer cars have that felt string/thread backing on carpet and yes that will catch fire, so...
Those road runners had huge trunks, not a problem putting all the groceries in them with space left even. Never noticed a heat buildup but never was looking either.