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Is an exterior vapor barrier ok on the exterior in a mixed climate?

Danny Kelly | Posted in Green Building Techniques on

We are building in Charlotte, NC – mixed/humid climate. We learned long ago to stop installing a poly vapor barrier on the inside of the wall over our insulation – we have moved to kraft faced batts. Have also read to not have any vapor barrier on the wall in our climate whatsoever(interior nor exterior) so we have always used 15# felt on our exterior sheathing (typically OSB).

We are building a LEED-H platinum house – exterior wall is a 2×6 with 1″ DOW SIS – we are using climate pro blown in fiberglass (R 4.2/in.) for a total R28 wall. While reading the installation manual for the DOW product, we learned that it is indeed a vapor barrier so was mentally scrolling through all I have read about not installing a vapor barrier in our climate. I am thinking with our upgraded wall assembly we are going to be ok but want to double check myself.

During the summer we would be concerned with vapor diffusion through the brick veneer – with the increased insulation in the wall and the vapor barrier – i do not think any condensation can occur and the vapor should not pass to the interior – so we should be ok in the summer. My next thought would be – what about the winter – warm air trying to get to the exterior – I am assuming the 1″ SIS will have enough R value to avoid condensation from vapor diffusion coming out from the interior. Only time we should be getting any moisture in the wall will be if we somehow have a positive pressure inside the house but the wall should always be able to dry to the inside.

Am I missing anything? Always worried about trying something new in our climate. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    Your design is fine, for exactly the reasons you have provided. One-inch SIS sheathing provides R-5.5, which is enough in your climate zone to prevent condensation problems during the winter.

    Dow SIS is an excellent choice under brick veneer, for the reason you state — to limit inward solar vapor drive during the summer.

    You're good to go.

  2. Danny Kelly | | #2

    Thanks Martin - while I have you one more quick related question. We have upgraded to using high performance felt on all of our roofs as underlayment in lieu of 30# felt. As the houses we have been building have gotten larger the felted roof remains exposed to the elements longer before the permanant roof is getting installed and the high perf. felts seem to hold up better - they also seem to be a greener product, another reason we have gone that route. I've noticed these new high performance felts mostly are vapor barriers as well. We have used this product on both conditioned attics (foam in the rafter bays tight to the roof sheathing) and in a traditional attic - batts at ceiling with soffit/ridge vent. Is there any situation that this could result in a problem? Thanks again.

  3. Riversong | | #3

    To your second question, while low-perm roofing underlayments won't make a difference in a vented roof assembly, in a hot roof insulated with closed-cell foam it creates an assembly that cannot dry in either direction should water accumulate in the sheathing from exterior leakage or interior air movement or accumulated diffusion through framing.

    To your first question, while Martin's answer is consistent with current building science "rules of thumb", it is dependent on proper indoor moisture management and the vagaries of local seasonal climate.

    Building science has wisely expanded the categorization of vapor retarders to four classes: impermeable, semi-impermeable, semi-permeable, and permeable. But even this broader classification over-simplifies the process of moisture movement and containment.

    We don't speak of thermal "barriers" as impermeable or semi-permeable, but rather define them by a spectrum of conductivity or resistance (R-value). In the same way that insulation material only slows the movement of heat, all vapor diffusion retarders merely slow the diffusion of water vapor.

    With thermal insulation, heat movement is as dependent upon the delta-T (driving force) as it is upon resistivity (R-value). Likewise, with vapor diffusion retarders, moisture movement is as dependent on the vapor pressure differential as on material characteristics (permeance). And condensation potential is dependent both upon surface temperature and relative humidity at that surface (dew point).

    Hygro-thermal design is far more complex than rules of thumb can capture, and anyone designing or building high-performance homes should get at least a "primer" in moisture management before specifying materials.

    Brick cladding requires an air space, ideally vented and drained, as a capillary break. A "solution" to summer radiant moisture drive may become a problem for winter condensation. While drying in one direction is better than none, drying potential in two directions is (generally) twice as safe, particularly in a mixed climate.

  4. Danny Kelly | | #4

    Thanks Robert - very helpful. We use an open cell down here so they say vapor can pass through there - so hopefully will take care of the roof issue.

    As for the wall assembly - luckily due to the LEED-H requirements, we have a very high efficient heat pump and air conditioner with a variable speed blower to control humidity. That will hopefully keep the vapor pressure differential down if I am understanding you correctly.

    We do have about a 1.5"-2" air space between the brick veneer and the SIS.

    Thanks for the response.

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