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Air barrier with foam insulation.

EJ Palma | Posted in Green Building Techniques on

Just a quick question regarding technique. When building a wall using polyiso or closed cell polystyrene insulation on the exterior wall, where all joints will be taped, siding will be HardiPlank horizontal clapboard 4 inches to the weather: Am I incorrect in assuming that I will need an air barrier over the plywood sheathing before installing the foam sheathing?

After referring to the drawing in the details section, I do not see an air barrier indicated either under the foam insulation on the plywood sheathing, or at the exterior surface of the insulation.

Am I misunderstanding the detail? I thought that I would need to use a Typar or Tyvek air barrier on the surface of the plywood sheathing, and tape the joint laps. Then tape all of the joints on the foam prior to installing the furring strips. Can someone clarify this detail for me?

Thanks for the help.

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure you are using the term "air barrier" correctly. It's possible to create an air barrier with a wide variety of materials, including drywall, glass, concrete, or spray foam.

    For more information on air barriers, see the GBA Encyclopedia and One Air Barrier or Two?

    I think your question really concerns housewrap -- or more generally, what is usually called a water-resistant barrier (WRB).

    The purpose of a WRB is to provide a barrier to wind-driven rain that gets past the siding. Although some housewrap manufacturers say that the product can be used as part of an air barrier system, most builders who strive for low rates of air leakage are using other materials -- for examples, taped plywood sheathing or gypsum wallboard -- as their main air barrier material.

    If your question is, "Do I need a WRB on a house that has exterior foam sheathing?", the answer is, No, you don't need it, but many builders use one.

    Every wall needs a WRB. You can install housewrap or asphalt felt as your WRB, or you can use rigid foam as your WRB. Whatever you choose as your WRB, the WRB must be integrated with window flashings and other penetration flashings to be sure the WRB can shed water.

    More discussion on these issues:
    Location of WRB in Wall Assembly with Rigid Insulation

    What's the best WRB option when using foam to insulate your structure / sheathing?

  2. EJ Palma | | #2

    Thanks Martin in my haste to write the question I used the wrong terminology. My question was about WRB. So the reason that it was not on the detail drawing is that it is not needed. If I want to use a WRB against the plywood sheathing (i never use OSB because I feel that it is an inferior product subject to problems) it is ok though?

  3. GBA Editor
    Martin Holladay | | #3

    Yes, you can certainly install housewrap or another WRB product between plywood sheathing and exterior rigid foam.

  4. Riversong | | #4


    While some builders detail the exterior foam board as the WRB, placing Z-flashings with vertical leg behind the foam for drainage, it makes integrating flashings more complex and problematic and it relies on part of the thermal barrier to also take on the secondary function of WRB. Few materials function as well for multiple purposes as for the single purpose intended by the manufacturer.

    Since the purpose of the WRB is to arrest and drain wind- or gravity-driven rain, the most appropriate location is "to the weather", directly behind the cladding. The most appropriate location for insulation is behind (inside of) the cladding and WRB. Even foam board adsorbs some moisture, and all insulation materials function better when dry. Any defect in the foam-as-WRB would allow moisture to penetrate to the sheathing and would sufficiently slow drying to the outside as to encourage mold and rot, particularly because the foam board raises the winter-time temperature of the sheathing.

    The only scenario in which I would question placing the WRB outside of the foam board is with the use of a fully-vented rainscreen. This leaves the taped seams of the WRB vulnerable to displacement by air pressure differentials and potential failure over time. With cladding sandwiching the WRB tight to the foam board, this will not be an issue, and this is one of several reasons why I discourage the overuse of rainscreen assemblies, which also make integrating flashings more problematic.

  5. EJ Palma | | #5

    Thanks Robert and Martin for your help. Robert not to be redundant, am I understanding correctly that you are suggesting that the WRB go outside of the foam under the firring strip. And secondly are you suggesting not to use firring strip and place the Hardi-plank directly over the WRB. I understand custom flashing details as I build sunspaces and am a glazier besides being a finish woodworker. I laminate my own glazing support bars and always make my own custom flashing details for each job. I am thoroughly impressed by the education that all of you provide to us. GBA is one of the best TOOLs that I have invested in over my 30 plus years in the business. The opinions expressed and the questions asked are always refreshing and thought provoking.

  6. HDendy | | #6

    I have heard builders complain about placing hardi-plank on furring. They said the hardi isn't rigid enough and needs a solid backing behind it to prevent waves. If you're installing with 4" to weather then you might have enough overlap thickness to prevent this (I'm not sure what spacing said builders used, possibly maxed out), but something to think about. That said, I am a big fan of rain screens. As Robert said, I wouldn't be comfortable relying on tape to create a long lasting air barrier. I would shingle lap the WRB and control the air elsewhere in the assembly.

  7. Riversong | | #7


    As Hunter pointed out, there are issues with FC siding over furring strips, but there are also potential issues with FC siding over foam board, unless there is a drainage fabric intervening. You'll have to rely on manufacturer's specifications.

    Ultimately, I'm not recommending either option, since I've written extensively here about the liabilities of exterior foam and the environmental and health liabilities of using petrochemical plastics anywhere in a building system.

    Interestingly, the Canadian NBC standards for exterior/interior R-value ratios for "outsulation" systems to minimize the potential for condensation damage to the sheathing apply only to sheathings with both low air permeance and low vapor permeance - wooden sheathing with gaps is exempted.

    Way back in 1963, Canadian codes began requiring interior poly vapor barriers to prevent the commonplace condensation problems with fiberglass-insulated homes. Then they realized that most of what the poly did was to prevent air movement through the thermal envelope and encouraged air-sealing the poly. As OSB sheathing became popular, they discovered that condensation on the backside of cold sheathings was, once again, creating mold and rot problems - even with air/vapor barriers - and the rigid foam board industry demanded that the codes allow the air barrier to be placed anywhere in the envelope (inside, midline, or outside). Because exterior foam board could raise the sheathing temperature so that it mostly remained above the dew point, the codes were modified to include the out/in R-value ratios for various HDD climates. Studies, however, have demonstrated that if a structure gets wet from a leak event, exterior foam board will prevent drying and, by keeping sheathing warmer, will dramatically increase the potential for mold and rot.

    In other words, poly vapor barriers were required because very poor quality insulation -fiberglass batts - became the industry standard. Then air/vapor barriers were emphasized, both to reduce condensation and to maintain the paltry insulating capacity of the fiberglass. But impermeable poly barriers created problems as well, particularly during summertime AC conditions when vapor drive is reversed.

    Then, as junk sheathing (OSB) became the industry standard, exterior insulation was allowed to minimize condensation on the sheathing, but exterior foam also maintained perfect conditions for moisture problems if a building should ever leak (and they all do at some time).

    Each time we incorporated technological "solutions" to hygro-thermal problems, we created unintended consequences. This is inherent in ALL "advanced" technical solutions.

    So, in the bigger picture (including the global environmental and health crises), exterior foam is no solution to anything - merely one in a long list of technical liabilities.

    What do I recommend? Wooden siding over felt over board sheathing over cellulose-insulated double-frame walls with air-tight drywall on the interior and perhaps vapor retarder primer. Now that's a durable and environmentally-responsible structure - in other words, "green".

  8. David Meiland | | #8

    Edward, if I was going to use exterior foam I would feel compelled to place the WRB outside the foam. This is not from experience, it's strictly from concerns that (a) flashing openings will be a lot harder without the WRB out there for tie-in, and (b) it seems likely to me that water will get thru the foam at some point if there's nothing covering it, and I don't know how long tape will last, especially when stuck to XPS. Once it's behind the foam it needs to come into the house.

    I always use felt on my walls but if I had foam on them I'd probably go with Typar. It seems like 9' wide material would be a lot easier to install successfully than 3' material. You might need to use something like roofing nails to install it. I don't know what else would work if the foam is 2" thick.

    If you're going to use FC siding, I would be mindful of the possible issue of waviness between furring strips and would look into using HomeSlicker or some other continuous material for your rainscreen. It might do a better job of supporting the siding and also help keep the WRB from billowing.

    I think Robert's analysis of the foam and sheathing issues is right on. People are outsulating houses like crazy but I don't think the last word has been written. Personally I could build, live in, and correctly maintain a house with foam outsulation because I would pay close attention over time to ventilation and moisture source control, and would likely prevent any problems, but I think the average mass-market consumer needs a more forgiving wall detail, such as what Robert does. If someone approached me to build them a house with outsulation I would have to think hard about whether or not to do it.

  9. EJ Palma | | #9

    Thanks for all of the help Martin, Robert, Hunter and David. I am not a big fan of FC siding for all of the reasons that you all mentioned. The homeowner had sided his house 2 years ago and there is enough to match the dormers that we are building so I was asked to use it. My preferences would be Roberts' wall assembly with wood siding, as certified wood siding is what I use as cladding. Whether it be clapboard ,shingle, or vertical I am usually matching the existing. I do not use any PVC siding at all, and never have. In this situation though I will consider HomeSlicker or tighten up the centers on the firring strip to 10 -12 inches with WRB over the foam. I am also considering eliminating the foam and using cellulose insulation. Once again thanks for all of the feedback.

  10. BruceF | | #10

    Another scenario....insulating a 100 year old house with ballon framing and plank sheathing. I am hoping to fill the walls with cellulose and apply 1" polyiso to the exterior with cement board siding. I am stuck on wether to use a drainage plain or apply siding directly to the foam. The drainage plain could either be battens or a Homeslicker type product (have never used cement board over homeslicker). And does any body have any info on perm rating for plaster over wood lath. There does not seem to be much agreement on the best way to insulate walls in we say on our job sites - "there are 8 ways to the get to the beach" pick your route.

  11. GBA Editor
    Martin Holladay | | #11

    Bruce F,
    The permeance of plaster over lath is 15 perms. But don't forget all of the layers of paint that have accumulated over the years. Two coats of old paint can have a permeance of 1.5 to 3 perms.

    I wouldn't risk installing fiber-cement siding directly over foam; it makes it too hard for the back of the siding to dry out. If you're tempted to go that route, be sure to check with the manufacturer to be sure it's an approved installation method.

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