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Q&A Spotlight

Do These Walls Need a Poly Vapor Barrier?

The owner-builder's uncle suggests poly in both the walls and ceiling. Is he right?

At work on a new house in New Mexico, the owner-builder wrestles with questions about air and vapor barriers in the exterior walls. (Photo: Zane Bridgers)

Zane Bridgers is building a single-story house in northern New Mexico and has nearly completed the framed exterior walls. As his mind turns to air sealing, he’s considering whether to install an interior vapor barrier — and whether his uncle, a builder of 40 years, is giving him good advice on how to proceed.

Walls on the slab-on-grade structure will include R-19 fiberglass cavity insulation, 5/8-inch OSB sheathing, a 2-inch layer of polyiso rigid insulation, a drainage layer and, finally, three-coat stucco. In the roof, Bridgers plans 6-mil poly under the drywall, followed by 2 inches of polyiso, R-38 fiberglass batts, a 2-inch ventilation channel, OSB sheathing and metal roofing.

“This is a predominately cold and dry heating climate with big temperature swings,” Bridgers writes in a Q&A post of the Climate Zone 5 locale. “I was talking with my uncle who has been a builder here for 40 years. He was explaining the importance of dry heat for optimal performance of fiberglass insulation, hence his recommendation to put the 6 mil plastic on the ceiling … I was planning to tape the foil faced polyiso for this effect, but he thinks it’s a waste of time and effort vs. the 6 mil poly.”

Bridgers is equally concerned about another bit of advice he’s been given: Add a layer of poly to the walls.

“This caused a bit more concern as it seems it could potentially trap moisture in the wall cavity, especially since I had planned foil-faced polyiso under the stucco, also taped,” Bridgers adds. “He suggested leaving somewhere for the moisture to go.”

Bridgers has two other questions. First, is it a waste of time to seal the OSB and framing when the exterior foam and drywall seem like much easier ways of controlling air leaks? And second, with a continuous layer of rigid foam over the wall sheathing, where is the point in the assembly where condensation is likely to occur?

Those concerns will get us started on this Q&A Spotlight.

Water doesn’t need to escape

GBA editor Martin Holladay, referring Bridgers to an article he had written on the topic previously, notes that interior moisture doesn’t really need to go anywhere.

“Water doesn’t need to escape from your house,” Holladay writes. “Although it’s true that indoor air is warm and humid during the winter, while outdoor air is cold and dry, that doesn’t mean that indoor moisture needs to ‘escape’ from your house. It’s perfectly OK if the indoor moisture stays where it is without ‘escaping.’ ”

Holladay explains that walls with exterior rigid foam should never have interior polyethylene, since foam-sheathed walls need to be able to dry to the interior. Either polyiso or expanded polystyrene (EPS) insulation would make a good choice, and taping the seams would be time well spent.

“Air sealing efforts are almost never a waste of time,” he says. “Whether or not you need redundancy (basically, multiple air barriers) depends on your airtightness target and your budget.”

What about Bridgers’ concerns about condensation inside the walls?

“The idea is to specify enough rigid foam on the exterior of your wall sheathing so that condensation does not occur,” Holladay says. “In your climate zone, your rigid foam needs a minimum R-value of R-7.5 if your walls are framed with 2x6s.

“Skip the interior polyethylene,” he adds, “pay attention to airtightness, and everything will be fine.”

What about venting the roof?

If the walls are better off without the poly, should it still be used in the ceiling? Bridgers asks.

There’s no code requirement for including a vapor barrier on the interior side of a vented roof assembly, Holladay replies. The most important thing is airtightness, not preventing vapor diffusion. Drywall can be an air barrier, providing Bridgers pays close attention to sealing any penetrations, particularly the electrical boxes.

Bridgers sees a problem with adding foam or plastic ventilation baffles above the fiberglass in the roof because either would prevent moisture from being wicked out of the insulation.

“The purpose of the vent channel is to help keep the roof sheathing dry,” Holladay says. “You are not trying to wick moisture out of the fiberglass … You aren’t trying to help indoor moisture escape. It’s OK if indoor moisture stays where it is, all winter long.”

One air barrier is enough

Peter Engle writes that Bridgers has three potential air barriers in the ceiling: the foil facing on the foam insulation, the poly, and the drywall.

“You only need one, well detailed and airtight,” Engle says. “If you can make the drywall airtight, you can skip the poly and use any polyiso foam you want, or EPS foam. If you are worried about the drywall being airtight, you can tape the seams in the foil polyiso.”

Jon R, however, suggests there’s nothing wrong with multiple air barriers.

“More air barriers will generally outperform one,” he says. “If you can only have one, the best (for your climate) is the interior side.”

Addressing the risk of condensation

Jon R adds that the choice of exterior foam may make a difference to how well the wall performs: “With external foam, a wall that can dry a little to the exterior (say EPS) will outperform a similar wall that can’t (say same R-value of foil-faced foam).”

He takes issue with the idea that a wall must be designed so that there is a zero chance of condensation taking place.

“The idea is to reduce the amount of condensation to the point where it sometimes occurs but isn’t enough to cause a problem,” he writes. “Going beyond that, all the way to ‘no condensation’ is unnecessary expense. Some in-wall condensation *will occur* at the minimum recommended foam R values.”

Holladay replies: “When sheathing is cold in the winter, and in contact with warm humid air, what happens isn’t really condensation. It’s sorption. The moisture content of the cold OSB or plywood sheathing increases when the warm humid air is in contact with it.”

That said, the building science in this case is clear. When the exterior foam is thick enough, the sheathing stays dry. If the foam is too thin, the siding may get damp, which is risky.

With that in mind, Bridgers asks, would a 2-inch layer of polyiso with an R-value of 13 be a better bet than a 2-inch layer of EPS, with an R-value of 8?

“That’s exactly the right type of question,” Jon R replies. “WUFI might provide a useful answer. Forced to weigh various factors and guess in your case (never a good way to do design), I’d say the R-13. Better than either if it were unfaced/higher perms.”

Agreed, says Holladay: R-13 is preferable to R-8.

Our expert’s opinion

GBA technical director Peter Yost made the following points:

Cavity or interstitial condensation: There are two primary drivers of this phenomenon: the difference in temperature between the interior and exterior, and the interior relative humidity. Yes, it’s plenty dry in New Mexico during the winter, but occupants can generate quite a bit of moisture (for more, see this). So, make sure you manage interior sources of moisture and have humidity-sensing devices so that occupants know what the interior relative humidity is.

Location and nature of air control layer/barrier: If I only get to choose one location for an air barrier, I choose the exterior, for two reasons. One, it’s much easier to get continuity on the exterior (no intersecting interior partitions or floor assemblies to worry about). Two, exterior air barriers deal better with wind-washing at the corners of buildings. Continuity is key. It’s easy to designate elements of the air control layer, but more difficult to get them all connected. Pick one plane for the air barrier, and then make it continuous. If you can get more than one air barrier, great. But one continuous barrier is way more beneficial than two or more discontinuous ones.

Building assembly drying potential: It’s ideal to select every individual layer of an assembly (based on vapor permeance) so that there is drying in both directions, but our assemblies are complex enough these days that settling for drying potential in one direction is reasonable. Avoid selecting Class I — and if you can, Class II — vapor retarding materials on one side of your assembly or the other to get that single-direction drying potential.

Bottom line: There’s absolutely no need for polyethylene. Don’t put in a Class I vapor retarder/barrier unless you have to.



  1. Steve Grinwis | | #1

    So, what happens when you have no sheathing, but instead have the metal T's to prevent racking?

    Also, how cold does it have to be before you need interior poly? Houses in Ontario all have interior poly and there don't seem to be mass failures...

  2. Yupster | | #2

    This article by BSC should provide you with an answer on the houses in Ontario question.
    I'll add one other thing to the article though. While they often work here, they are not very forgiving when there is a leak at a window or in the siding. Lots and lots of rot. We just tear them open, fix it, put it back together the same way. So even though it works, it's not best practice and not very resilient.

    1. User avater
      Jon R | | #3

      Note that as explained in BSI-092, a reason that external R5 foam and interior side poly works is that the interior air sealing is better than the exterior air sealing. Tape the R5 foam (or sheathing) and this is no longer true. Not all partitions are safe with external air barriers!

      1. Steve Grinwis | | #4

        So, what happens if there is no sheathing, just R-10 XPS nailed to the studs, and the entire thing is really well air sealed?

        This is my new house. We hit .6 ACH, so she's pretty tight, but there's poly on the inside, and the outside is taped and sealed as well.

        1. User avater
          Jon R | | #5

          No sheathing to rot significantly decreases moisture risk. As does R10 external foam (depending on R ratio and temperature). Even .55 perms of outward drying helps. So I expect you have an excellent wall.

          But imagine some no-interior-side-air-barrier scenario where lots of moist air flows from the interior, to cold sheathing/foam where it condenses (depending on R ratio and temperature) and then returns to the interior (convection will cause this loop). You could get enough water accumulation to cause problems. Be careful with external only air barriers.

        2. Yupster | | #6

          2 inches of XPS would be vapour semi-impermeable, so while some drying will occur, it will be very slow. Like many people in Ontario, you have a somewhat risky assembly. Since it's already built there isn't much you can do. At this point, sit back, don't worry, and enjoy your new home. If you get a leak and water damage, you'll have to fix it. But I wouldn't worry too much. Your house has a much lower risk than many others in Ontario.
          Next time you build, you'll know better. As my neighbour used to say, by the time you are old, you'll know everything you need to and then you'll be dead. Cheery fellow...

  3. Azad | | #7

    I have a question about how a standard wall performs as far as condensation forming on the interior of the sheathing.
    Wall construction like this:
    Hardie Board, Tyvek, Sheathing, 2x4 studs filled with R13 and then Drywall?

    Does this wall not have the same effect that the sheathing will get cold and condensation will happen on the interior?

  4. User avater GBA Editor
    Martin Holladay | | #8

    Q. "Does this wall not have the same effect that the sheathing will get cold and condensation will happen on the interior?"

    A. Clearly, climate matters. Wall sheathing in Minnesota gets a lot colder than wall sheathing in Georgia. Even in a cold climate like Minnesota, what happens isn't really condensation -- it's sorption. The sheathing moisture content rises over the winter, generally peaking in February. Whether or not this annual wetting is problematic depends, in part, on how quickly the sheathing can dry in April and May. In your case, the siding and Tyvek allow some outward drying -- but the rate of drying won't be as fast as it would if you had included a rainscreen gap between the siding and the Tyvek.

    1. Azad | | #9

      that makes sense. So then if all of the walls are getting wet the difference between a wall with exterior rigid insulation is that it will be unable to dry at all?

  5. User avater GBA Editor
    Martin Holladay | | #10

    If a wall has an adequately thick layer of exterior rigid insulation, the sheathing stays warm and dry all winter long. It never gets cold enough for the moisture content to rise.

  6. Augustus Elliott, III | | #11

    Great article. I have some moisture management/building envelope questions on a few aspects of a current remodel. Is there anyone at GBA I can consult with for advice via email or phone. I need and would very much appreciate the expertise. Thanks!

  7. Thomas Peterson | | #12

    I am a double-wall super-insulation fanatic with 40 years experience. I will keep things simple & short, but obviously more detail would be required for actual construction.

    For the last two decades I have settled on using an exterior 2x6 structural wall with 6" of fiberglass (ideally rockwool). Working towards the interior, I use 2" of XPS or EPS, then a 6 mil vapor retarder, which a;so serves as my only "official" air barrier. Wall air barriers are attached to a ceiling vapor retarder / air barrier. This is then followed by a 2x4 wall with studs offset from exterior wall, another 3-1/2" fiberglass or rockwool then sheetrock. Window & door openings connect both walls with 1/2" underlayment plywood and are sealed to the vapor retarder / air barrier. Such a wall system can be built with multiple thermal breaks with the weakest areas having an R-factor of around 30.

    Such a system is easy to explain to the average builder, easy to inspect all of the components in stages (with easy corrections), and has many more construction advantages. Drys to both inside and outside and I have never had a failure. Additional costs for such a wall system can usually be easily be covered by energy dollar saving as the system uses "at least" 75% less energy for both heating and cooling.

    1. Malcolm Taylor | | #13


      I bet that wall does perform well, but it would be a lot more robust if it included a rain-screen cavity behind the siding.

  8. User avater GBA Editor
    Martin Holladay | | #14

    Response to Augustus Elliott III (Comment #11),
    I'm an editor, not a consultant. That said, GBA is ready and willing to provide free advice -- worth every penny you pay for it. I suggest that you start a new Q&A thread describing your dilemma. In most cases, readers who take that approach get sound advice. And the price is right.

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