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We are removing a corner prefab wood burning fireplace. One wall behind the fireplace and part of the ceiling are exterior structures. When we took the drywall down, we found that the fiberglass at the vent area is open and the fiberglass in the ceiling does not have a moisture barrier.

We know exterior walls should have a hermetic moisture barrier on the fiberglass. We wonder if we should cover the fiberglass that is uncovered so it doesn't get any moisture.

Context: This fireplace was installed when the house was built in 1986. The fiberglass doesn't have any mold. The new fireplace is coming in 2 weeks but we won't finish the remodel for several weeks (until our contractor can schedule us). Should we cover it somehow temporarily/permanently? Will it be fine without coverage? Summers here are quite humid and we don't want to have to change the whole fiberglass later on. We are kind of baffled here by it not having any moisture barrier. Thank you for any advice!

ceiling fiberglass

vent fiberglass

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  • Where are you on this planet? That interior poly looks awful concerning... Commented Apr 3 at 1:55
  • I'm in the Midwest.
    – Daltoniana
    Commented Apr 3 at 11:48
  • Yeah, that interior poly is going to be serious trouble as soon as you flick the air conditioning on... :/ Commented Apr 4 at 1:26

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Yes, you should "install an air barrier behind fireplaces installed on exterior walls." (reference: pnnl.gov).

However, you aren't required to use vapor barriers if you live in Climate Zones 1, 2 or 3. (Find your climate zone here.)

If you do decide to use a vapor barrier, be sure to use a fire-proof rigid air barrier.

Should we cover it somehow temporarily/permanently?

Vapor barriers help to control moisture, but they also help to control air movement (which affects energy costs). So, cover the area temporarily if you want to minimize energy costs.

More info on exceptions: the International Building Code requires a vapor barrier for homes, with the following exceptions noted in Section R702.7:

  1. Basement walls.
  2. Below-grade portion of any wall.
  3. Construction where accumulation, condensation, or freezing of moisture will not damage materials.
  4. A vapor barrier shall not be required in Climate Zones 1, 2 and 3.
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  • Note that more current advice may not call for a vapor barrier (look into vapor open assemblies) even if it has an air barrier/WRB layer as part of the wall, and that interior poly is generally strongly discouraged as a barrier wherever air conditioning is in play Commented Apr 3 at 1:56

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