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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 - Reinstate Monica Nov 9 '17 at 19:15
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The US Dept of Energy also says 1/16" back drainage cavity for siding is appropriate even though capillary action occurs up to 1/4". Point being, the department of energy tends to lag significantly behind the latest in building technology. I did my undergraduate in environmental sciences at the University of Missouri and my masters in architecture at the University of Oregon (consistently #1 for sustainability in architecture). The belief that you shouldn't use a vapor barrier at all is terrible advice. You should include one and locate it as close to the center of your thermal barrier as possible (for this climate). This allows the moisture that condenses on the surface of the barrier from either side (interior in winter and exterior in summer) to dry. So, say you have a 2x6 wall with dense packed cellulose insulation, which is roughly an r-value of 20. You should place a vapor barrier to the exterior side of stud and then include 5" of eps (unfaced eps, and please use eps because xps is awful for the environment) to the exterior of the wall (equates to about r 19). This gives your wall assembly an R39 value and ensures that condensate can dry to the appropriate side. If you adjust your insulation type, thus adjusting r-values, you should adjust dimensions accordingly to ensure as close to a 50/50 split on either side of the vapor barrier as possible.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here. – Daniel Griscom Mar 14 at 23:46
  • When you say, “you should include one and locate it as close to the center of your thermal barrier as possible.” You don’t mean it should be in the middle of batt insulation or middle of rigid insulation, do you? – Lee Sam Mar 15 at 4:54

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