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I'm constructing a small house in southern Missouri, climate zone 4. My question about vapor barriers is multifold:

  • Should I install a vapor barrier at all?
  • Should it be on the interior (warm-in-winter) or exterior (warm-in summer) side?
  • What permeability/material is advisable?

As highlighted in existing answers on this site, this depends on where I am - however, having researched the recommended approach in my climate area, authoritative sources are giving me directly contradictory information, and I'm not sure which one to trust - the MO DoE, US DoE or the University of Missouri?

The University of Missouri extension and the MO Department of Energy say I should install one, and that it should be on the warm-in-winter side:

Vapor barriers are installed over the face of the studs or joists on the side closest to the inside surface of the home.

http://extension.missouri.edu/p/GH4881

A vapor barrier should be placed on the "warm-in-winter" side of the insulation.

https://energy.mo.gov/resources/insulation

The Craftsman blog says I should install one, and that it should be on the exterior, warm-in-summer side:

if you live in a hot climate like I do here in Florida the vapor barrier should be on the outside of the wall assembly

http://thecraftsmanblog.com/vapor-barriers-101/

And, finally, the US Dept. of Energy says I should not install any vapor barrier at all when building in Missouri:

Building scientists generally do not recommend putting a vapor retarder in walls in the mixed-humid climate. In the mixed-humid climate, walls should be able to dry to both the interior and exterior.

https://energy.gov/sites/prod/files/2013/11/f5/40percent_mixed_humid.pdf

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    I would say no. Let the wall breathe. Or at most install fiberglass bats with kraft facing. buildingscience.com/documents/digests/… – ArchonOSX Nov 8 '17 at 19:53
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    I think I agree with this - it's also the recommendation of the US DoE document, which gives extensive reasons for why, unlike any of the other sources. It also happens to be cheaper, which is nice :) – Jacob Davis-Hansson Nov 8 '17 at 21:14
  • The newer residential energy codes for various states say that you need to limit conditioned air leakage to the outside. If you don't have a vapor barrier, the air blower test might be difficult to pass. I'm familiar with Minnesota's energy code, so everything else I know doesn't really apply to hot/humid climates. – Dotes Nov 8 '17 at 22:20
  • @Jeff -- air != vapor. You can limit air leakage with taped drywall or OSB even though those materials are vapor permeable. – ThreePhaseEel Nov 8 '17 at 23:26
  • Yes, you can always install a vent through a vapor barriered wall, but retrofitting a vapor barrier to a wall not built with one is near impossible. – Harper Nov 9 '17 at 19:15

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