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What is the best material for an air barrier?

Bill_L | Posted in Green Products and Materials on

Before I can ask my question, I should explain exactly how we’ll be using the material.
I”m in Zone 5 (Massachusetts).
From the inside out we’re in the process of building walls as follows: drywall, 2×4 structural frame, 1/2″ plywood, air-barrier/vapor-retarder material, 10″ lightweight “I-joists” packed with dense cellulose, 1/2 plywood, asphalt-impregnated felt paper, corrugated plastic spacers, cement clapboards. All of this sits on top of a 14″ Thermomass wall (5″ + 4″ + 5″) with EPDM structural gasket (Conservation Technology) below the inner 2×4 sill, and ordinary sill gasket below the outer 2×6 sill.

I considered air-tight drywall, but was concerned about supervising all of the necessary details and the failures that future interior modifications could introduce. The plywood sheathing (that combined with the 2×4 sheathing makes up the structural framing) is penetrated only by window and door ‘boxes’, and is the primary air barrier plane.
The plan is to frame traditionally, cover the outside of the plywood with an air barrier material, leaving extra material at the bottom so it can lay across the top of the foundation wall foam core and be secured between the outer sill gasket and outer sill, before screwing on the I-joists.
The dense-packed cellulose walls will be blown in from the outside, using fiberglass mesh netting, and will press the material tight against the plywood, the window and door boxes, and the top of the foam core in the foundation wall.
I don’t trust tapes or caulking to last long term, and stapling a sheet material over everything seems a lot less labor-intensive than priming the plywood and taping or caulking all of the joints. It also seems considerably less expensive than Zip Sheating.
We have planned to use Certainteed’s Membrain instead of Intello Plus because it came in wider widths and was expected to be less expensive being a US product. Surprisingly, it has not been easy to find locally in Massachusetts, and by appearance, I am concerned about how easy it is to tear or puncture it.
I’m not terribly impressed with its ability to reduce permeability when it is dry, as I expect my drywall paint layer to be my primary vapor retarder to combat diffusion. I really just need it to be permeable when moisture is present and first-and-foremost, to be an reliable air barrier.
I will greatly appreciate any advice for help in choosing a material to use for this, as well as places to buy it locally.
thank you!

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  1. albertrooks | | #1


    If you don't trust tapes. just use a liquid applied membrane at your sheathing layer. The Fast Flash line from Prosoco will flash the window bucks and join with the Cat 5 on the sheathing. It's been used in that detail on many West Coast Passive House projects successfully. Including this one:

    This approach is not inexpensive but the quality is exceptionally high. We sell it at The Small Planet Workshop but I'm sure you can find a supplier in your local market at Prosoco's website:

    The Cat 5 and Fast Flash are in the range of 14 to 18 US perms depending on how thick you lay it on.

    Your drywall will make a poor vapor barrier. However your sheathing will make an excellent one. I agree that laying sheet goods over sheathing is nuts.

  2. Bill_L | | #2

    Thanks Albert,
    I'm not expecting the (painted) drywall to be a vapor barrier, just a vapor retarder. From everything I have read, it seems like it should be sufficient and my primary concern should be air leakage. I also didn't mean to suggest that I'm against laying sheet goods over sheathing. In fact it seems to me like a potentially cost-effective and dependable way to ensure a contiguous air barrier plane. What does seem nuts to me is hoping that adhesive tape will maintain its grip for the hundreds of years I hope this house will stand. The liquid-applied membranes are certainly compelling. I was a very interested reader about the pink stuff until I learned the installers were prohibited by the manufacturer from installing it unless their own fiberglass was going into the wall. I will certainly review the other such products, hoping they are cost-competitive, and appreciate you bringing the idea to my attention.

  3. GBA Editor
    Martin Holladay | | #3

    Most people in your position have decided to trust tapes. Of course, it makes sense to research tapes before you choose a product. Here is an article you might find useful: Backyard Tape Test.

    I can understand your skepticism concerning the durability of tapes. But frankly, what makes you think that membranes that come in a roll will last 100 years? There are plenty of examples of 30-year-old Tyvek that has disintegrated. When it comes to speculating about which materials will last 100 years, we are all just guessing.

  4. jinmtvt | | #4

    Martin : i believe it should be of the most importance to discuss about 50-100 years life of materials if we are to build "green ".

    Most manufacturers don't give ANY info on life cycle of their products in the 20 years + ....

    And since are relying heavily on membranes in our recent buildings and design , it would be nice to know the minimum expected product lives.

    It may have been discussed and i missed it, but i don't recall anything "serious" or "deep" about this,
    other than the recent desintegrated tyvek discovery.

  5. Bill_L | | #5

    Thank you Martin and Jin.
    I was already leery of MemBrain possibly being delicate, and you've helped me to pay attention to that. Not to mention the trouble we've had getting an actual first-hand look at the stuff.
    I'm leaning back toward a sort-of "airtight drywall" approach to the inner plywood sheathing, putting the emphasis on the mechanical connection of the plywood edges to the studs/framing.
    I have the benefit of experience with the framers, and feel that I can trust them to conscientiously apply a contiguous bead/layer of sealant on the studs for each sheet of plywood that they put up. Certainly sealants can fail over time as well, but at least it will be sandwiched between fastened pieces of wood, which may support its longevity and will at least prevent it from simply falling off.
    If time and money permit, I could even add suspenders to this and narrowly spray all of the plywood-to-stud joints from the inside with one of the gap filling materials on the market.
    I will spend my next research efforts looking into a low VOC, very elastic and cost-efficient sealant that is easy to apply to the studs just prior to nailing up each sheet of plywood.
    I will be looking at Sashco Big Stretch (a tube product), and at the Prosoco products mentioned by Albert. I will also consider Tremco ExoAir 230 which perhaps could be applied to the studs with a narrow roller?
    I will appreciate hearing about any field experiences with such products and methods very much.

  6. GBA Editor
    Martin Holladay | | #6

    Of course it is important for building materials to last 50 or 100 years. For those of us who want to be assured of 50-year or 100-year performance, the most certain way to proceed is to use materials that were used 50 or 100 years ago -- and which have proved themselves to be long-lasting.

    Anyone who uses newly developed materials is taking a calculated risk.

    If we are talking about an air barrier material, the worst outcome is increased air leakage rates. That is preferable to structural failure.

    One good air barrier material that has proven itself over long periods of time is concrete. There is some question about the longevity of reinforced concrete -- eventually the rebar rusts, leading to failure -- but unreinforced concrete should last hundreds of years.

  7. albertrooks | | #7

    Here is a complication to your idea of using a sealant at the sheathing to framing layer: The APA strongly advises against using sealant in that location due to the fact that they typically also act as an adhesive and then change the load characteristics in shear. One exception to this is Tremco JS-773 since it's a "non hardening, non skinning" liquid butyl. The ExoAir 230 is not suitable for that application since is does skin and will act as a adhesive.

    Most of our existence is gingerly held together by chemical bond. Including the plywood or OSB that will keep the house standing for it's 300 years... It's all riding on the back of the chemical bond of "glue to wood" to keep the panel together.

    Good quality tape is still (in my opinion) still the simplest "go to" material for turning sheathing into an airbarrier. As with most things there are exceptional products and poor products. Tapes like Siga, Pro Clima, and 3M 8064 are really exceptional. The acrylic based adhesives are quite different than what you normally see. Acrylic adhesives form a "bond" to the substrate that improves over the initial application. Tape gets a "bad rap" from the products that were never intended to be used as an air barrier. As most of us know, solvent based adhesives loose their flexibilty over time and the tape will fail because the backing material comes off. The acrylic adhesives don't contain any VOC's to off-gas and that's why they remain flexible and don't appear to fail in the 20+ years being used as air barrier tape so far. For longevity, I think they (tapes) have as good chance as any of the "other" methods, are simple to apply, and therefore are cost effective.

    I would advise against re-inventing the wheel. A lot of really earnest and passionate people have been working on the "quality and longevity of air barrier's issue" for a long time and have developed good results. The European building culture has been on the AB issue far longer than the US, and at this point, we (North America) are simply riding on their experience. That's why we (The Small Planet Workshop) began importing the products from Siga and supplying them to Passive House builders.

    Your putting some good thought into your air barrier and I applaud your 300 year building longevity goal. I'd also put the same energy or more into "window rough opening flashing". Window flashing failure is far more likely to shorten the buildings life than an air barrier failure.

  8. user-1072251 | | #8

    I'd rely on the plywood taped with SIGA or ProClima tape as Albert Brooks recommended. Acrylic, non toxic and nothing like any tape you've probably ever used. And spend the small additional $ to use EDPM gaskets on the exterior wall - that foam stuff is useless as an air barrier and you do need to keep air out if that exterior I joist wall for it to work properly. So add an exterior air seal fabric - which the SIGA & ProClima dealers also carry. We're finally getting some of the good European products which that part of the world has used for years to build tight houses - they work.

  9. user-1137156 | | #9

    The discouragement of gluing plywood to studs is, to me, very surprising.. Can you further explain the logic as I can not believe depending solely on nails gives a stronger structure in any direction, under any stress. To me the change in panel strength is desirable making the panel stronger not weaker with glue. Is there actual test data to support reduced strength with glue? Floor truss manufacturers OTH all strongly advocate use of both glue and screws.

  10. Bill_L | | #10

    Thank you everyone for contributing your time to this.
    I looked up Tremco JS-773 and didn't like what I read in its MSDS.
    I didn't find APA articles on exactly what Albert was referring to, but I imagine the concern may be with delamination of the plywood or brittleness of the structure.
    I'm hoping that Sashco Big Stretch (which claims 500% stretchability) would not create such a rigid bond as to create a problem when combined with ample nails. That said, I am considering not applying it between the plywood and framing members, but instead running a bead of it along the edge of each sheet of plywood, after having nailed it to the studs and prior to butting up an adjacent piece of plywood.
    I will look further into going over the seams with 3M All Weather Flashing Tape, Siga Wigluv or Siga Sicrall. It appears that the best of the ProClima tapes for this didn't make it into Martin's backyard tape test which, while not an ultimate test, is the best I have right now :) Cost and quick availability will be important factors.

  11. user-1140531 | | #11

    I don’t quite understand the issues being discussed regarding the use of adhesive between the plywood and the framing. If the point is to accomplish air sealing, why place adhesive on the studs that fall in the field of the plywood? There is no joint there to seal.

    Adhesive could be used to seal the edge joints that fall on the stud lengthwise. Adhesive there between the plywood and the stud would prevent air from getting to the actual butt joint of the plywood at that location. You would not need any adhesive there where the edges of the plywood butt together.

    The biggest challenge to sealing would be the plywood butt joints that run crosswise to the studs. Tape may do the job, but I would not use any tape unless I knew it would last for the life of the building. I have no way of knowing that. If I could test the tape for say 30 years and observed absolutely no change or degradation, I would maybe gamble on it lasting for the life of the building, if the manufacturer also promised that longevity.

    But since I have not conducted such testing, and have no time to do so ahead of any future application, I would rule out using tape for the plywood butt joints. But I have gambled on the use of silicone RTV, and have 30 years of observation as a result. My indication is that silicone joints of plywood and other lumber have not failed or degraded whatsoever. Even 30 years of daily direct sunlight and weather exposure shows no degradation of the silicone or its bond to wood.

    So, for sealing the butt joints of plywood, I would consider lapping a ¼ X ¾ wood batten over the butt joint with the batten pressed into a bead of silicone. So there would be a thin layer of silicone under the full width of the batten. The batten probably could be pressed into position and stay put without any added means of holding while the silicone cures.

  12. tommay | | #12

    Bill, if vapor barrier and insulation is your goal and the tape that holds these products together worry you, install rigid foam board with reflective outer paper. You can get it at any home depot.. Then you have a way to make a phsical seal by notching or beveling, edges or removing half a layer on joining pieces Overlap these joints then nail. In my opinion there is never a perfect seal anywhere anyway. especially on this type of application.

  13. user-1061844 | | #13

    Pro Clima tapes have been age tested for 50+ years. The SOLID acrylic adhesive sets when pressurized and the age (artificial and real life) actually shows that the bond between the tape backing, adhesive and substrate actually grows stronger over time (the tape sets by pressure, so doesn't dry out as conventional tapes). The most popular wall build with our materials is 2x4 wall - 3/4" sheathing (OSB/plywood) taped with TESCON Vana, TJI's covered with SOLITEX MENTO PLUS, battens, siding. The MENTO PLUS is both your WRB and densepack netting. A product so strong the densepack lance can't be forced through it, you need to slice the membrane. See attached image from Green Generation that shows this wall under construction.
    In most cases, the builders do use INTELLO Plus on the underside of the ceiling as the airtight layer. Tt is very durable, way easier to put up than sheathing overhead, just make sure to tape and lap it over the top plate before putting your trusses on.

  14. Bill_L | | #14

    Thank you Ron and Tom,
    The issue with adhesive came from Albert Rooks' statement that "the APA strongly advises against using sealant in that location..".  I did not find a particular article addressing such, but can at least imagine that the flexibility (albeit limited) that we get from nailing plywood to studs is better in adverse conditions than the very rigid connection we would get combining adhesive between the plywood and studs.  Furthermore, nails theoretically hold all layers of the plywood, rather than just the adhered veneer, and perhaps the tenacious bond of adhesive on the outer veneer introduces a delamination concern?  Not sure, but all I really need to seal are the cracks between sheets.  Certainly I can run a bead behind each edge (the previous plan was to do just this), but have instead decided to run the bead along each edge just prior to butting an adjacent sheet.  This cuts my sealant volume in half, cuts the framer's caulking time in half, seals the edges of the plywood and barely adheres the plywood to the studs, so I'm quite happy with the approach.
    We are indeed backing up all otherwise-unsupported horizontal joints with 2xs and sealant - thanks for the reminder though.
    In many places we will have an equivalent to battens over the outside of the joints  in the way of the vertical I-joists (which contain the dense-packed cellulose).  Unfortunately, we cant put them in place as we build the walls so that they seat against uncured sealant as the inner structural walls are being built one floor at a time, laid down on the floor, before lifting into place.  I suppose I could ask the framers to leave a nail's diameter between the plywood sheets so that the joints could be caulked just prior to screwing on the I-joists, which will rise the entire height of the exterior walls.

    Thanks for the idea to add a layer of foam Tom, but I'm already pumping 9.5" of dense-packed cellulose into the walls, so an approach that includes more insulation would be hard for me to budget.

    Thank you Floris for your suggestions and for the reminder about connecting the wall air-barrier plane to the ceiling-barrier plane.  We're insulating between the rafters so that the attic space is conditioned, and dealing with the penetration of the ceiling air-barrier plane by the attic floor joists is a big concern.  We've been advised to use 1/2" plywood below the rafters to handle the pressure and weight of the cellulose in the roof.  I'm thinking to have the sheathing around the joist penetrations cut oversized around the joists so that we can fit the nozzle of a spray foam can in there and foam around each joist penetration.

    Thanks again everyone for your time and advice, you've been a huge help,

  15. albertrooks | | #15

    Sorry it took so long... I had forgotten where I found the Adhesive restriction info. It was not the APA.

    I checked with my friendly engineer in the PNW who has helped with most of the modified wall assemblies used in our regional passive house projects Carissa Farkas (I'd highly recommend her!).

    Here is what she wrote back:

    "In the code (SDPWS = Special Design Provisions for Wind and Seismic) section states “Adhesives: Adhesive attachment of shear wall sheathing shall not be used alone, or in combination, with mechanical fasteners. This applies to seismic D and above. The commentary states, “Adhesive attachment of shearwall sheathing is generally prohibited unless approved by the authority having jurisdiction. Because of limited ductility and brittle failure modes of rigid adhesive wall systems….”

    So in essence, any product that binds the stud to the sheathing is not allowed in seismic zone D and above. Obviously all products have different adhesive values and over time these also change. I have not done specific product research for this issue. "

    Hope this helps!!

  16. Bill_L | | #16


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