Quote Originally Posted by JJK View Post
I spent some time researching this we well for my build. Here is a quick summary. Various materials can be used for insulation, for both thermal and acoustic. Closed cell foam and butyl rubber each insulate both, and thicker variants insulate more. For acoustics, closed cell is better at sound blocking while butyl is better at sound deadening (removes hollow sound from panels). For thermal insulation, closed cell is better but butyl still has thermal insulating properties. Butyl takes the edge in filling in small gaps in panels that could let heat pass through. The foil layer is predominantly there for radiant heat transfer, but you lose all those benefits once you put carpet on top. Those benefits only apply if the shiny surface faces the environment. The foil also helps with vibration damping on the acoustic insulation though, so it is not totally useless under carpet.
Like egchewy noted, I also used a butyl rubber foil faced mat for my build. I used a thicker mat for the footboxes and transmission tunnel, and a thinner mat everywhere else to remove the hollowness and make the overall car feel more solid.
This is exactly right! In my experience, the issue with heat in the footbox comes more from gaps in the panels and where the panels contact the body more than from radiant heat. The best advice I can offer is to eliminate any source of air flow from the engine bay into the passenger areas of the car and you'll be good to go.