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Q&A Spotlight

Choosing an Air Barrier and a Housewrap

If Zip System sheathing is out of the running, what's the best way to air seal the exterior of walls?

Builders have many options when choosing a water-resistive barrier. Do more expensive options make better air barriers? Photo courtesy of DuPont.

In the midst of framing his new house, Joe Norm has switched gears and opted out of Zip System sheathing in favor of CDX plywood. The question he faces now is how he should air seal the exterior side of the walls—tape the seams between sheets of plywood, or tape the seams of the water-resistive barrier (WRB) he installs over the sheathing? More to the point, will a common WRB like DuPont’s Tyvek be good enough, or should he be prepared to spend more on an “exotic” product?

“I priced out VaproShield IT and it looks to be 3x the cost of Tyvek,” Joe writes in a Q&A post. “Solitex Mento looks to be about 2x the cost. Why are these products so much more and how are they justified over Tyvek?”

That’s where we start this Q&A Spotlight.

Staple-up WRBs are not the best air barriers

GBA Editor Brian Pontolilo has just finished a series of blogs on WRBs and has come to at least one conclusion: “There is no easy answer when it comes to the performance of individual products.”

In order to win code approval, a WRB must pass certain criteria of the International Code Council’s Evaluation Service. Reports on test results offer only limited information, but Pontolilo adds that there are better choices for an air barrier than a staple-up housewrap.

“It is well known that staple-up housewraps like Tyvek don’t make the best air barriers,” he says. “At least they are challenging, at best, to detail as an air barrier. If you choose this type of product, taping the plywood seams is a more straightforward approach to air sealing.”

If Norm wants his WRB to double duty as an air barrier, Pontolilo would recommend the Zip System sheathing, peel-and-stick membranes, or liquid-applied products instead.

Beyond the question of which WRB to use, Pontolilo adds, is the importance of including a ventilated rainscreen in the design.

Tighter houses need better WRBs

Houses with drafty exteriors can tolerate water more readily than tight houses, AlexPoi writes. If Norm is building a tight house, he’ll be better off with a high-quality WRB.

“A little bit of water in a leaky house is not a problem because air movement will dry the sheathing pretty quickly if it gets wet,” AlexPoi says. But when the house is tight, and air movement through the walls is limited, is takes longer for moisture to dissipate.

“So, if you are building a tight house with OSB sheathing in a wet climate, I would personally not take the risk to cheap out on the WRB just for the peace of mind,” AlexPoi continues. “After all, a good WRB will cost you just a couple of thousand more but could save you big time.”

AlexPoi suggests Norm pay particular attention to flashing details to keep bulk water out.

Sheathing makes a robust air barrier

Andrew C believes that taping the seams of the plywood sheathing is Norm’s best bet for a durable air barrier.

“Properly detailed, the WRB can be a belt-and-suspenders backup air barrier to the taped sheathing, but the sheathing remains primary,” Andrew C writes. “As others have pointed out, flashing design details and attention to these details, along with avoiding dumb designs like dead-ending a roof valley into a wall, are critical to durability.”

To Andrew C, a wall design with taped plywood and Tyvek CommercialWrap (a suggestion from Malcolm Taylor as a more durable alternative than Tyvek) makes the most sense.

Why opt out of Zip System sheathing?

Norm opened his original question by noting that he had decided against using Zip System sheathing, an OSB product with an integral WRB from Huber Engineered Woods that has become increasingly common on both walls and roofs.

How come? Patrick O’Sullivan wants to know.

“No good reason really,” Norm replies. “I’m not fond of OSB in general and I couldn’t totally get past the idea of hundreds of feet of reverse laps for water to get caught up in. I”m sure Zip is great and in the end I may regret not trying it out.”

Even though Taylor thinks taped plywood sheathing and a WRB in sheet form would be a good route, he also thinks Zip Sheathing is worth considering. “Zip-R [a version of the sheathing bonded to a layer of foam insulation] makes some sense to me, and I’m pretty sure Zip will turn out to be a good long-term sheathing/WRB,” he says.

But when it comes to choosing a particular WRB, Taylor says that as long as it is installed “diligently,” he hasn’t seen anything suggesting one WRB is significantly better than another.

As to the long-term durability of plywood vs. OSB, a question that Zerphyr7 raises, Alex P refers him to an Instagram photo posted by architect Steve Baczek showing two small pieces of Zip sheathing taped together with Zip tape that have been left outside for a decade.

“He says the tape is perfectly fine and the edges have swollen a little with full exposure to rain, ice and snow,” Alex P says.

Our expert’s opinion

GBA Technical Director Peter Yost adds this:

I like to keep the air barrier and the water-resistive barrier together so that at penetrations I can seal both in the same plane or location.

On the topic of ventilated rainscreens, for all but the driest of climates, I strongly prefer a free-draining and airflow-promoting gap (open top and bottom) between the cladding and the WRB. The one situation that no code-approved test for WRBs requires is water held in tension between the claddings and the WRBs (no gap between them). I have seen this water held in tension result in wet sheathing with a variety of different WRBs.

Quite some time ago now, Paul Fisette—now retired, but then head of the Wood Technology and Construction program at the University of Massachusetts, Amherst—worked up a test for water held in tension. You can read that report here.

Paul was a “wingnut” long before I codified that term on BuildingGreen and then with Martin’s help here on GBA .

On a side note, does WRB stand for “water-resistive barrier,” “water-resistant barrier,” “weather-resistant barrier,” or “weather-resistive barrier?” In the 2018 International Residential Code (IRC), these terms are used interchangeably: water-resistive barrier 30 times, water-resistant barrier six times, weather-resistant barrier three times, and weather-resistive barrier twice. That is just odd.

Even more odd is to use the terms “resistive” and “resistant”—both meaning some level of opposition—with the term “barrier,” which means an absolute stop or block. In our industry, we certainly struggle with water and weather, even in our terminology.

37 Comments

  1. User avater GBA Editor
    Brian Pontolilo | | #1

    Peter,

    When I first started to write this article about WRBs, I was using the acronym WRB to stand for “weather-resistive barrier.”

    https://www.greenbuildingadvisor.com/article/the-complicated-role-of-a-water-resistive-barrier

    It quickly became clear that “weather-resistive barrier” and “water-resistive barrier” are often used interchangeably, in code language, by building professionals, and by manufacturers. And the products often can do more than just keep water out of your walls, which as you point out, has benefits.

    However, I learned enough in my interviews—including my interview with you—to change the article so that “WRB” stood for “water-resistive barrier.” The thing is, it’s important to be very specific about the four different control layers in a building assembly—water, air, vapor, and thermal. Unlike the water-resistive barrier on walls, which may terminate at the top of the wall beneath a roof overhang, for example, an air barrier is a continuous assembly that must connect from one plane of a building to the next. This means that you may be able to use a water-resistive barrier as the air barrier on the walls, but that is only one dimension of the air-control assembly.

    Even when we do use one product for multiple controls, it’s worth speaking of the controls individually, I think. And since water comes first in terms of prioritizing the controls, I went with “water-resistive barrier," in the article.

    1. User avater GBA Editor
  2. User avater
    Jon R | | #3

    > there are better choices for an air barrier than a staple-up housewrap.

    The below link suggests the same for a water barrier (fully adhered outperforms staple-up and is used in commercial and institutional buildings):

    https://www.buildingscience.com/documents/insights/bsi-067-stuck-on-you

    Makes sense to me - a small hole in an adhered membrane doesn't allow much water flow.

  3. Paul Pfeiffer | | #4

    In "Super House" Donald R. Wulfinghoff makes an argument for plywood over OSB that is somewhat independent of their abilities to withstand periodic wetting. I believe it goes something like this: (1) we don't know how long adhesives will last, but wood/nails have been proven over hundreds of years (2) OSB's strengths depends entirely upon adhesives, but nailed-up plywood's strength will remain even if adhesives fail. Note that I believe he thinks in terms of 100+ years.

    Can anyone speak to this? Are there actually very good reasons be believe modern adhesives in OSB will (or can) last for as long as nails and wood?

    1. User avater
      Jon R | | #5

      > but nailed-up plywood's strength will remain even if adhesives fail

      So the claim is that 5 sheets of 1/8" wood with no adhesive are as strong as the same 5 sheets glued together? Not true. But no doubt that plywood is generally more durable than OSB.

      1. Paul Pfeiffer | | #8

        You are probably right, but it's not immediately obvious to me. Tell me if I'm crazy: If sheathing is for shear strength (and not bending strength), and shear strength is proportional to cross-sectional area, then unglued plies and glued plies are (almost) equivalent (due to negligible glue thickness). However a friend has pointed out that unglued plies would be more susceptible to splitting at the fasteners, which would ruin the wall's shear strength. OSB could also split at the fasteners, or the strands could separate. So perhaps it depends on which one of those things happens first on whether plywood would outlast OSB or not, from a glue-lifetime perspective.

        1. Malcolm Taylor | | #9

          Plywood and OSB have to have strength in both shear and bending. The panels are all marked for directional strength, so for floors and roofs there is an arrow showing the correct orientation between supports.

          Neither Plywood or OSB would have any useful structural usefulness if the adhesives failed. Laminated layers act in an entirely different way that unattached ones, sharing their strength.

          Widening out the topic a bit: We simply haven't seen any evidence that plywood will be the thing that limits the lifespan of a house. In fact we haven't really seen that any building material (unless it is defective) has much of an influence on the useful lifespan of a house. If the enclosure is maintained - primarily the roof - then the house will probably last until other factors make it undesirable to live in.

          To me, one way that makes sense to divide up the elements that go into a building are to look at the ones that need periodical replacement - like roofing, glazing, etc, - and to detail them so that this replacement is easy and doesn't affect the rest of the building. I can't see adding plywood or OSB to that list.

        2. Tyler Keniston | | #34

          Theoretically and simplified: there is some sense to thinking shear or racking forces are being taken care of by unbound plies. Consider that two beams side by side - supporting a load from above - need not be joined to add their strength (linear relationship) IF the load above is equally distributed and twisting tendencies are removed. Two beams stacked vertically do need to be joined for an exponential strength increase (as with ply in bending).
          The reality with ply in shear resistance, however, is that the glue will add strength (better yet, it relies on the glue): each ply is so thin that twisting and off axis forces will be of concern; each ply may not have consistent grain (think footballs, checks, cross grain, etc); and load transfer will be more reliant and concentrated on fastener locations.
          Summary: I do think ply may have some advantage in this neighborhood (meaning if we had a total glue failure of our sheathing product), but whether this is of significance in the big picture..? My gut tells me that glue failure, whether OSB or ply, will not be the first/only thing to take a structure out of commission. Malcolm makes great points on that above.

    2. Malcolm Taylor | | #6

      Paul,

      Plywood is made by laminating thin layers of wood peeled from logs. OSB is made from compressing chips of wood. Both are entirely dependant of adhesives for their strength and longevity. Short of substituting solid lumber for the panel goods on roofs, walls and floors, there is no way round relying on adhesives in modern construction.

      1. Lance Peters | | #17

        An extreme example, but an example none-the-less. A deck of cards could be plenty strong with all the layers glued together.

        Without the adhesive it's 52-pick-up. :-)

  4. Kameidopapa | | #7

    In the Pacific northwest region, plywood is usually considered more durable since it retains less water and dries faster than o.s.b. Just look at the perms of zip wall when the o.s.b is included.
    I also want to challenge the idea of sheet w.r.b.'s being less than other products. As a long observer and tester of air weather barriers in the northwest, I can attest to buildings regularly beating the 0.25 air leakage rate using Tyvek or Vapor Shield. Tyvek is less forgiving than some of its competitors so an installer does need to know what they are doing, but they can achieve very low leakage rates consistently with sheet applied membranes. It is also good to remember that "staple" application is usually limited to temp/ initial installation. The rain screen battens are securing the wrb to the wall. Thank you

    1. Lance Peters | | #18

      I wonder how these 0.25 ACH50 homes would fare in a two-way blower-door test like PH requires where the WRB isn't being sucked tight against the sheathing? Also, I can see siding battens increasing the attractiveness of Tyvek, but most homes don't use battens under the siding (even though we know they should be).

      I don't doubt the airtightness of Tyvek as a material, but I do doubt the ability of most building crews to apply it in a detailed manner that rivals taped sheathing.

      1. Malcolm Taylor | | #20

        Lance,

        Maybe that's a good way to look at WRBs. Rather than a blanket endorsement of one over another, the best choice may depend on various other considerations. Housewrap is probably a poor choice without rain-screen battens. One that relies on adhesives or tapes may not make sense in a climate where much of the construction takes place in wet conditions, liquid applied would be a bear to get right on dirty sheathing. As long as the choices if installed properly yield a good result, local conditions and practice might dictate which makes sense.

        1. Lance Peters | | #21

          Excellent points!

  5. Joe Norm | | #10

    Thanks for the write-up.

    It still does not clarify whether the more expensive wraps are any better than Tyvek.

    If there is a good reason to by Vaporo-Shield, Mento, etc. then I am not opposed to spending the money. But if Tyvek(or equivalent) does the same job then what is the point?

    Tyvek and their tapes are available everywhere. This is half the battle. If I am going to special order something from Germany it better offer a justification for the hassle.

    1. Malcolm Taylor | | #11

      Joe,

      I'm with you. While it may make a lot of sense to spring for expensive European variable perm vapour-retarders, I don't see much benefit to using an exotic WRB over the readily available ones sold here.

      1. John Clark | | #13

        Is there a difference in the exposure/warranty period? IIRC residential Tyvek is warrantied for 7 yrs where as a product such as ZIP is 15 yrs or more for the original homeowner.

    2. Jason Ruel | | #14

      My take-away was that if there isn't much performance difference between all of them, then focus on the sealing above all else: tape the seams and apply whatever WRB you want properly, consider a rainscreen for ventilation, and carefully plan bulk water movement.

      If Tyvek (et al) have the weakness of staple holes, then tape those. Added labor, yes, but you could upgrade to a self-seal or liquid applied instead if that's the issue.

    3. User avater
      Jon R | | #16

      > more expensive wraps are any better

      A necessary step is to define "better". Ie, tests and conditions that people agree on.

      Intuitively, I'd say that a lapped and fully adhered membrane is best.

  6. John Woloshyn | | #12

    Malcom ,
    You consistently provide some of the most well thought out advice from an extremely reasonable and knowledgeable point of view. I always try to read your comments and I find them to be grounded in real world experience with a profound understanding of building dynamics and issues tied with an amazing insight into the balance between performance and expense and ease of assembly. Not to mention your ability to communicate. Thanks for all your input on GBA.

    1. Malcolm Taylor | | #15

      John,

      Thank you for the kind words. I'm glad if my posts are helpful. I take more from GBA than I give, and am grateful to all the rest of the posters for their insights. I've learned a lot.

    2. Lance Peters | | #19

      Ditto John. Malcolm is a real asset to this site. Thanks Malcolm!

  7. Ed Dunn | | #22

    I have always wondered about the effect of rain screens on energy efficiency. It is my understanding that air moving against a surface can move heat from the surface and the area below it so we want to usually avoid this situation. Rain screens will obviously create a situation where air is warmed and inside heat is moved out and up behind the siding. What is the trade off/cost benefit to using rain screens?

    1. Malcolm Taylor | | #23

      Ed,

      The R value of most siding is negligible (Hard-plank is R0.5). You could say a rain-screen moves the outdoors in from the siding to the sheathing, so it might decrease the total R value by that amount.

    2. User avater
      Jon R | | #24

      Offsetting what Malcolm said, a rain screen will reduce wind velocity which increases the R value of the relevant air film. And reduces wetting (which can effect R value).

      1. User avater
        Dana Dorsett | | #25

        What Jon R said!

        And...

        ...a foil-faced exterior foam is being used (with the WRB between the foam & sheathing) the low-E foil next to a rainscreen gap adds ~R1 of thermal performance compared to putting the siding tight to the foam.

      2. Malcolm Taylor | | #26

        So fair to say it's a wash? Or not significant enough to worry about either way?

        1. User avater
          Dana Dorsett | | #27

          >" Or not significant enough to worry about either way?"

          It's "within the noise" of the error bars from a load or annual energy use calculation point of view on even a code-min house. The quality of the air sealing &/or insulation installation has a much bigger effect.

  8. Heather Thompson | | #28

    C'mon! Can't we just call it a WCL instead of WRB? The 4 golden principles: water, air, vapor, thermal - 4 Control Layers. Clean up the vocab & any subject starts to become easier to learn.

    As far as the topic of this post: the original question plus the comment section, in my opinion, have been a particularly good one. All smart people trying to figure out the hardest conundrum of all: quality, time, cost - pick 2. You can have all 3 but understand what that really means, scope change. The basic project management triangle (PMT).

    "Joe Norm", if that is your real name, you are already in trouble if you are in the process of framing a house & haven't decided upon a WCL/ACL. Or you have accepted you chose all 3 in the PMT, in which case, post often on GBA! Your project will be fascinating to follow!

    Find a builder you are comfortable with. If they are on board with your goals, you will have a successful outcome. If your 2 or 3 choices from the PMT are met with opposition, keep searching.

    1. Joe Norm | | #30

      Heather-

      I am the builder, and the designer. And I am keeping a full time job on top of the build.

      I am amused when people suggest I am "in trouble" (I've gotten that a couple times here now) for not having all the details sorted out and learning and designing as I go. How else is one supposed to do it? I am not retired nor do I have enough money to hire a green building professional to map out every last detail.

      The topic of WRB's ( or should I say WCL) alone is vast and changing by the day it seems as new products arrive. It's difficult to sort through all the information and come out with any clarity. Sometimes I feel more confused than when I started with the amount of options that become known.

      That said, I have learned soooo much in the last few months posting here and reading everything I can. Thanks so much to everyone who contributes, its a really great resource!

      As for my choice, I am heavily leaning toward the taped plywood seams and tyvek drain wrap, or Commercial D if I can source it easy enough. The longer exposure time is a plus as I will be going slow with foam and siding.

      1. User avater GBA Editor
        Martin Holladay | | #31

        Joe,
        Q. "I am amused when people suggest I am in trouble... for not having all the details sorted out and learning and designing as I go. How else is one supposed to do it?"

        A. Finish designing the house before you begin construction. For more information on this topic, see this article: "Plan Ahead for Insulation."

        1. Joe Norm | | #33

          Martin,

          I agree, planning ahead is key to success, just not always possible.

          I read the article you linked and was curious about the comment on windows. I was actually planning on installing my windows, then butting the foam to them. I was going to install a piece of metal flashing to be able to then butt the siding to.

          Does this seem like a reasonable plan? Would it be better to install the windows after the foam for less thermal bridging through the window frame?

          thanks

          1. User avater GBA Editor
            Martin Holladay | | #35

            Joe,
            Your approach to window flashing doesn't sound typical. I'm not sure whether it will work based on your description.

            Before windows can be installed in a rough opening, the following steps are essential:

            1. The builder has to identify the water-resistive barrier (WRB). This could be the rigid foam, or it could be the housewrap, or it could be the Zip sheathing. Any of these WRBs can work -- but you can't install your windows if you don't know where your WRB is.

            2. The window rough opening needs to be flashed, and the window flashing needs to be integrated with the WRB.

            If you plan to install exterior rigid foam, your window flashing details need to be well understood and planned for -- before the windows are installed, and before the rigid foam is installed.

  9. Charles Campbell | | #29

    I sort of thought Martin might have chimed in by now. Wasn't he testing tapes in his back yard?

  10. User avater GBA Editor
    Martin Holladay | | #32

    Charles,
    This article tries to discuss many issues -- air barriers, water-resistive barriers, and sheathing choices.

    In general, I don't think the brand of housewrap matters much, as long as it is well installed, and as long as the building has a ventilated rainscreen gap. Careful installation matters more than the brand.

    Housewrap is not an air barrier. Tapped Zip sheathing or taped plywood sheathing is an air barrier.

    Zip System tape performs well.

  11. Kameidopapa | | #36

    As a follow up to my initial post and Lance's comment. None of my experience is with residential ach testing. Only 2-way ace whole building blower door tests. Also if I wasn't clear, sheet product doesnt matter if the installer is experienced. If the installer is less adept at installation the more expensive products have more forgiving install options.

  12. Aerobarrier | | #37

    In regard to your concerns of providing a structure with the absolute best air barrier, please consider Aerobarrier. We have the ability to seal a building envelope to meet code or passive home levels. It DOES NOT matter what your exterior sheathing may be for Aerobarrier to be successful. We can complete the process within a day and guarantee results. The product is a "misting caulk" that can seal any gap from the size of a human hair and up to 1/2". We use a blower door to promote positive pressure and emit Aerobarrier to find the points of air infiltration. The product is Green Guard Gold Certified and has withstood a synthesized 50 year test. I am happy to answer any questions. http://www.rmaerobarrier.com

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