I just put new foil-foam (1") throughout my engine compartment. The original stuff had crumbled and I had to scrape off the remnants about 15 years ago. Biggest reason for the original failure was that the factory had never sealed the edges where heat and fumes had attacked the exposed foam.
Now it's all new and all edges are sealed with silvery mylar tape ($$). I also used screws and fender washers at all corners upon the advice of a boat builder friend.
It is a hard job on most boats due to poor access --- I was able to do it relatively fast because we had removed the old engine in order to install a new Betamarine. I did scrape down the walls and clean 'em with solvent to get most of the original adhesive off.
I used spray adhesive for the new application. It all seems well-adhered, but we will only know for sure in another decade.
As for the double-layer foam with the lead in the middle... I have heard that it helps. The industrial supply place where I bought this product did not believe that it helped very much, even tho they sell it for more $. Plus, it is harder to apply. Maybe I made the correct decision; or maybe not. Anyhow, it's done.
(BTW, the new engine is so much quieter that the foam layer is a little less important than it once was !)
