Bonded Logic Heat/Sound Insulation - JCWhitney
This is what we paced under the carpet of out 1999 Class C. You have to pull the seats and carpet. Also lined the engine cover on the back side. I used a high temp spray adhesive to secure it and sealed the edges with the foil tape.
Keep the edges cut back about an inch, so that you can get the door sills and trim back down.
Made a world of difference in the cab.