You should have a layer of heat reflector, then underfelt, then carpet.
Thats from the "build book" for my 78. Alas Convertors seem hell bent to rip all this out, and your lucky if you get the old carpet plonked back up against the inside of the fire wall.
I purchased a roll of heat reflective sound deadner - looks like under felt with Silver Alli coating.
Cant remember where from, I can see the leftover roll but I cant physically get to it to see if it has any branding.
Its about 1/2" thick (10~13mm) Foil on one surface, approx 900/1000mm wide, I tried a few via samples - I'm pretty sure one even had a side which was preglued - ie peel and stick with a pressure senstive surface.
I would be carefull with polyethelene or similar plastic products, make sure first that all holes in the fire wall are sealed - I had 32 in the 78.
Even a 1/8" hole is like a thermal lance once the donk and manifold reaches temperature
Once the car has forward motion, the lower pressure in the cab (even with windows open) with literally SUCK the heated air into the cab.
Once fire wall sealed, new heat/sound deadner, new carpet - it was like another car.
Another thing to watch if converted is manifold air escaping over the fire wall and into the plenum, then the heat/AC system will draw it in.
There should be a rubber seal which runs across the top of the fire wall thence forward down each side of the guards.
One of our club members drives his C3 with the hood up on the safety catch - this allowes hot air directly into the Plenum and also has the effect of reducing the "Ground Effect", this is a partial low pressure created under the vehicle once forward motion of ~>50kph is reached, loss of this also destroys the vehicles road hugging and therefore handling. But each to their own.
