I just found this post by Terry Montlick from the AVS forum..
There is no acoustical difference between using rigid fiberglass duct board and flexible fiberglass duct liner. There is some slight difference in going from lightest 2.25 pcf linacoustic to heaviest 6 pcf, but this is very minor and can be ignored in most situations.
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Make sure the foil is facing against the wall and not towards the theater room, and it will have absolutely no disadvantage.
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Regards,
Terry
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Terry Montlick Laboratories
Complete Home Theater Acoustical Services
Design, Evaluation, Alpha Certification
www.tmlaboratories.com
That pretty well confirms which side the foil should face...and the flexible duct liner will be easier for me to transport..