Thanks guys,
RJ it pleases me that you are being safe and we are thinking alike.
If you have a friend or know anyone in the AC trade see if they have some spare 3M 'firesafe' duct wrap. If you are doing a dual skin application putting that inbetween will not only increase the frie resistance but it will deaden the sound.
If not you can PM me and I can send you some for the cost of shipping, I have a full roll.
Andrew