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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, 2017 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, 2017 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, 2017 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, 2017 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, 2017 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.

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  • 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, 2020 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, 2020 at 4:54
  • Not within the material themselves, but between the assemblies. So, per the example provided above. The wall assembly may look like this : (from exterior to interior) 1) Cladding (brick, siding, etc.) 2) 3/8" min. air cavity for back drainage (vertical furring) 3) Water resistive barrier (such as Tyvek) 4) 5" unfaced EPS insulation (seams staggered and taped) 5) Self-adhered member (Vapor/air barrier) 6) 5/8" OSB substrate 7) 2x6 stud wall (Filled with dense packed cellulose) 8) 1/2" Gypsum Wall Board 9) Vapor open paint finish
    – Joel
    Apr 10, 2020 at 0:35
  • @Joel, can you please explain the concept of a symmetrical vapour barrier? What, for example, happens in winter? As I understand, the inside vapor has a higher partial pressure than the ouside, and so is "pushed" against the internal face of the vapor barrier, where it condenses if the dew point of the inner air is greater than the average between the insde and outside temperatures (for we are in the middle of thermoinsulation). Does that mean that the inner face of the vapor barrier will be moist through most of the winter?
    – Anton Shepelev
    Jun 15, 2020 at 12:57
  • @Joel, you say that the recommendatioon not to use any vapor barrier at all is terrible and obsolete. What, then, can you say of Allison Bailes's blog entry titled You Don't Need a Vapor Barrier (Probably), where he provides a sound-sounding rationale: it is only a negligible amount of moisture that can enter a house by diffusion even it is not protected by a vapor barrier.
    – Anton Shepelev
    Jun 15, 2020 at 13:13

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