yup, rubber roofing membrane is basically the same thing... and its stickier... so make sure you want it there before you apply it.

www.parts-express.com has 'no name' dampening sheets for like $17 for a 3'x4.5' sheet.

BTW, where did gutter guard come in while talking about dynamat and fiberglass?? Yes, it will stand up to weather, but its kinda flimsy and it tarnishes over time... if you use it, give it a few coats of clear coat first.