I guess I'll bring this one up, as it had intrigued me for Years-The W.R Grace Company makes a Roofing Underlayment that is made from butyl Rubber with an adhesive backing-it's called Grace Ultra, and I wonder if it would work well in our application-it's 30 mils thick (Dynamat Extreme is 1.7 mil)-
Grace Ultra is designed to protect sloped roofs from the effects of wind driven rain and ice dams in applications where the membrane must withstand high, in-service temperatures for extended periods of time. This membrane offers excellent performance at elevated temperatures, in hot desert southwest climates or any application where superior heat resistance is a requirement.
(hope those Links work)-
What do youi guys think?