I'm concerned about that as well, I'm curious if if the non Butyl compound gasses at all. I'd be concerned that the inside of my car would smell like roofing tar, so I thought I'd ask to see if any fellow members had any experiences to share.
Had the fatmat in for two desert summers and no roofing tar smell. The fatmat site claims a "rubber compound". As far as moisture goes, I figure that if you car is sealed and as sticky as this stuff is (once it sticks to itself then it's stuck forever) then you should have no issues. Do what you want, I am just letting you know that I am very happy with it. I just don't see the point in paying three times as much for dyno mat or hush mat.