I use this stuff alot when insulating telecomm equipment. It is not a very good insulator for heat but it yields good insulation properties for its thickness. It will be good to glue it down to force it to mold shape to your area. It is a pain to keep it in place unless it is glued down. What I would suggest rather than undercoat for sound deadening is just buy regular carpet pad from lowes or that mernerds store and put that down to your metal first (inside the car), then over that run the thermal barrier, glue thermal barrier to the carpet pad. Several advantages. You can remove the thermal and sound dampening assembly to work on your floor board. The heavier and more absorbent the material which contacts the metal, the less sound will come thru. Plus with that pad between the thermal barrier will be a big thermal barrier enhancement.
I am debating just gettting some good quality water proof carpet pad for my floor and using the alum bubble wrap stuff too! Mostly because the combo would be an excellent sound and thermal barrier. Until it caught on fire. Then it would create noxious gasses.
Oh, I made a headliner out of it for my ranger, glued it to the existing backboard. Painted it black. Looks like the inside of the space shuttle.
