I was in the same boat,what to use to fill the spaces.I now am useing dieletric silicone compound for ignition systems from napa auto parts, and fiberglass insultion.I coated the back of my mobo then covered it with fiberglass the same stuff in your atic, then i did the same with the space under the cpu then wraped my water block.the silicne and fiber glass is easy to clean off your mobo and hell i'm running @ 33F with no problems. "easy to find parts"!
