I'm replacing some flex duct in my crawlspace and am wondering if there is a difference between flex duct with a "foil" wrapping as compared to a plastic wrapping. All the big box stores seem to carry the foil type, but the existing flex duct I have in the crawlspace has a black plastic covering. I presume the benefit of the foil wrapping is that it reflects sunlight, but this would be unnecessary in the crawlspace. Most of the big box stores only carry the foil wrapped type, but I'm just wondering if there is a reason why one is used over the other. Perhaps the plastic type serves as a vapor barrier as well?