sound/heat insulation - rear floor in convertible

am considering forgoing using this stuff I got with butyl rubber/foil /closed cell foam layers as I noticed the section I tested on the rear seat board behind the rear seat back really got drippy over summer. I imagine if I use this on the floor under the rear seat bottom and going up into the trunk area, the bottom rubber layer is going to cook and stay in a semi-liquid state all summer. Pretty obvious it is never ever coming out of there either. Anyone else dealt with this and said screw it or found better solution? BTW it's X-mat from Eastwood. Probably OK for doors etc..