The foil-wrapped bubble plastic should work fine where it is not compressed over time. That compression, such as on the floor of the cockpit, will eventually release the air cushion and you will be left with only two sheets of thim aluminum to do the job. Also, your insulation footwell liner needs to be strong enough to hold the carpet, and this bubble stuff may not be up to that job over time. The materials savings may not offset the difficulties of dealing with the problems that might crop up later. Insulating a car like the FFR presents unique problems, not the least of which are related to the tight quarters.