you want to use foil first to reverse penatration of heat, then like heavy carpet pad,1/2 inch thickness. There is no-substitute
for mass, thickness. before foil seal all holes if not already done. I'm a insulation contractor
sprayed in place polyurethane foam,I have to sound deaden walls frequently. There is data to support mass.proven methods, will work on autos also. I am fimilar with Ice dam material, won't hurt but I'd take it a step further with pad.