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I am insulating my attic which will soon be drywalled. Is it better to go with faced insulation or unfaced insulation with 6 mil plastic sheeting as a vapor barrier? I live in Michigan (zone 5). Is there a problem with using faced insulation with a plastic vapor barrier?

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    My opinion only is that 6 mil vapour barrier seals against air drafts better. One full width/length sheet instead of dozens of small part sheets.
    – crip659
    Commented Apr 20, 2023 at 19:02

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Poly sheeting provides an unbroken vapor barrier which can be sealed to adjacent sheets, framing, etc. using proper caulk sealant. Faced insulation leaves less effective joints between every cavity. Clearly (hee!) poly sheeting is better in terms of raw performance. However, there may be other factors, such as obstructions or access limitations, which make faced insulation a better choice.

4 mil is standard in new construction in my similar region.

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For your second question, you should only use one vapour barrier. Adding a second barrier can trap moisture between two surfaces which can lead to problems.

There seems to be some debate if a vapour barrier is really needed. My opinion is it helps, but 4 or 6 mil plastic that is well sealed will lead to less drafts in the home better than faced insulation. Faced insulation is better than nothing, but should not be used if another vapour(air) barrier is used.

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