Agree that is best for heat reduction, but I am not pulling the engine. The next owner can 
I do have some concern about moisture getting trapped between the material and the body when insulating on the exterior. In building homes you never allow a space between two impermeable materials if it can get wet
Under the carpet, front and rear and in the front as far up on the inside of the firewall and front wheel well I could reach to apply from inside the car. .
Under the rear seat. I also laid a layer of E-type under carpet underlayment over the Noico mat and under the rear seat.
Where I could reach in the space between the inside structural and outside panels on the rear quarters.
The quarter panels I could reach from inside the trunk.
The trunk floor and spare tire well.
The doors.
It took most of a day. The difference is quite noticeable.
I had the same concern and sealed all edges around the belly pan (sealant applied subsequent to the photos.) Further, the entire bottom of the car was stripped and then sealed with multiple coats of epoxy paint. But if I ever moved to a northern or wet environment, I’d pull the bottom pan once in a while to inspect. That said…the factory installed heat shields…and nasty undercoating…