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I'm adding insulation into a room I'm building here in the PNW. The insulation is unfaced R-30 fiberglass roll. The room is in an unfinished attic space with the ceiling, and two of the walls are external while two are internal.

What kind of material can I use as a vapor barrier for the insulation? Can it just be thin clear plastic painter's tarp? Can it be any kind of 6 mil plastic sheeting? Does it have to be specifically for "vapor barrier" purposes?

I've already gone through and purchased (and applied) some vapor barrier already. However, I've realized that the amount I would need to complete the remaining space would be cost prohibitive if other cheaper options exist.

Thank you in advance!

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  • The type of insulation is important. Faced batts do not need vapour barrier, some foam sheets are the same. Most vapour barrier is 4mil or 6 mil poly rolls.
    – crip659
    Commented Jun 10 at 15:46
  • Hey Crip - I added a response - it's R30 unfaced fiberglass roll insulation - homedepot.com/p/…
    – Helixdnb
    Commented Jun 10 at 16:08

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Typical vapor barrier is 4 mil polyethylene sheeting. It's about as cheap as it gets. It's nothing special, but it's impermeable (at least until you put a million staple and screw holes in it), and it's available sized for walls (a bit over 8 feet). In theory you could use about anything similar that's safe for humans. 6 mil sheeting is only used for exteriors and elsewhere that more durability is important, or in cases where fire-rated sheeting is needed.

Anything lighter than 4 mil (such as thin painter's plastic) is likely to be such a hassle to avoid tearing that it's not worth any cost savings.

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