I'm in the process of insulating now, I used a productt from Speedway Motors
You have a choice of single or double sided
https://www.speedwaymotors.com/Alumi...ded,32193.html
https://www.speedwaymotors.com/Alumi...eets,2604.html
The tape helped with sealing the seams
https://www.speedwaymotors.com/Alumi...Ft-,94554.html
I undercoated the inside of the body before installing the insulation.
Last year the front of the car was completed, firewall, cowl sides, floor, doors and under the seat. I noticed a huge difference in sound and heat in the car, and the undercoating has rid the "tinny" sound when you close the doors. So seeing I have some extra time on my hands..... thought I would do the rear and complete the job.
I got the cans of undercoat at my local farm store.