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Community and Q&A

Air sealing the building envelope while avoiding moisture issues

Joshua Greisen | Posted in Green Building Techniques on

First off, thank you GBA community for all the support so far in getting my project planning done for my net zero build. I’ve been doing a good amount of research on the best way to provide a good air barrier for my future home. I have significant concerns on a cost effective way to get the proper air sealing in order to hit the goal of sub 1.5 ACH. Below is a quick overview of what the build envelope looks like so far:

Exterior Walls – Stucco, 2.5″ or 3″ inches of exterior EPS rigid foam, rain gap, Tyvek house wrap, plywood sheathing with Roxul or fiberglass batts in the wall cavities, vapor barrier and gypsum panels. The rigid EPS foam will be either InSoFast EX 2.5 Panels, 3″ EPS Type II foam sheathing with taped seams or Insulfoam 3” EPS CI Panels (these come with the OSB sheathing glued to the exterior of the foam) with taped seams.

Roof – Composite shingles, synthetic underlayment, cantilever hip roof trusses, R-60 blown cellulose and gypsum panels.

Floors – 4″ concrete slab with 3″ Type 2 EPS sheathing underneath and stem walls with with 2″ Type 2 EPS using this detail.

Options I have researched so far for achieving high levels of an air tightness include the following list below (please don’t limit suggestions to my list of course):

1) Taping the exterior plywood sheathing seams.
2) Gluing the exterior plywood sheathing to the studs, top plate and bottom plate during installation.
3) Caulk Tyvek house wrap at top and sides of exterior wall, tape at seams (leave bottom un-caulked).
4) Use bottled spray foam to seal the seams between the exterior wall cavities and the studs on the interior side of plywood sheathing (poor mans spray foam).
5) Use an Airtight Drywall Approach – Caulk all interior seams in framing, seal any intrusions to drywall with foam/caulk, foam top and bottom plate seams, seal all electrical boxes/fixtures or use airtight boxes, glue all drywall to studs during installation.

Part of me thinks doing them all would be overkill and may prevent my wall from breathing enough to dry out in the event any moisture accumulates in the wall cavity (since air may be quite stagnant when caught between air barriers like sheathing, WRB wrap, rigid foam and drywall).

Really hoping to hear some advice from anyone who has achieved 1.5 ACH successfully given the build envelope constraints we are working within. None of the options I have listed provide a continuous air barrier except for the Airtight Drywall Approach, the rest seem to be covering for holes/gaps created throughout the construction process.

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  1. GBA Editor
    Martin Holladay | | #1

    Some comments:

    1. The rainscreen gap belongs on the exterior side of the EPS, not on the interior side of the EPS. (When you install a rainscreen gap on the interior side of the rigid foam, the insulating value of the rigid foam is wasted.) For more information, see To Install Stucco Right, Include an Air Gap.

    2. You wrote, "Part of me thinks doing them all [all of the air-sealing measures] would be overkill and may prevent my wall from breathing enough to dry out in the event any moisture accumulates in the wall cavity." Your thinking is confused. Doing all of the air-sealing work may be overkill, but not for the reason you state. Airtightness is good. Leaving a few air leaks doesn't help your wall dry out. Walls don't need to breathe. Walls need to be able to dry out, but not by leaving deliberate air leaks. For more information on this issue, see these two articles:

    Worries About Trapping Moisture

    How to Design a Wall

    3. Of the five approaches to air sealing you list, I vote for #1 ("Taping the exterior plywood sheathing seams"). Of course, you still need to come up with a plan for the floor assembly, the ceiling assembly, the transitions between difference planes, and all of the penetrations.

  2. Reid Baldwin | | #2

    For each assembly (walls, ceiling, foundation), you need to pick one layer as the air barrier and take actions to ensure that layer is air-tight. Any actions you take on other layers are sort of belt and suspenders, which is not harmful but is likely overkill. It is not just a matter of doing extra steps to air seal, but also avoiding things that create air holes. Then, at the transitions between assemblies, you need to ensure that the air barrier layers are properly connected.

    Here is what worked to get 0.82 ACH50 in my house:
    Walls (sheathing is air barrier):
    - taped seams on the sheathing (OSB in my case)
    - caulk between the studs and sheathing from the interior
    Ceiling (drywall is air barrier):
    - no can lights
    - no attic access doors (access is from the garage or the gable end)
    - spray foam over the interior partition walls
    - poured concrete foundation
    - peel and stick over the rim joist connecting the foundation to the wall
    - spray foam on the edges of the attic connecting the ceiling to the wall sheathing

    One comment unrelated to air sealing: reconsider the vapor barrier under the drywall? With your wall, drying is toward the interior by diffusion. Don't do something that interferes with that. You may not need any vapor barrier. If you do need a vapor barrier, consider a smart vapor barrier that still allows drying when needed.

  3. GBA Editor
    Martin Holladay | | #3

    Reid is right about the vapor barrier. You don't want a vapor barrier in that location.

    Here are links to two articles with more information on the topic. (The second article discusses walls with exterior rigid foam):

    Do I Need a Vapor Retarder?

    Calculating the Minimum Thickness of Rigid Foam Sheathing

  4. Tyler LeClear Vachta | | #4

    Just to re-emphasize the importance of that rainscreen drainage gap. You don't want to recreate the stucco and EIFS failures of previous decades. The "To Install Stucco Right, Include an Air Gap" article suggests some rainscreen products; be aware that a couple of those products are vapor barriers, and you don't need additional vapor barriers in this wall system. Another product to consider is Sure Cavity" from MTI which serves as the rainscreen drainage plane but is not a vapor barrier. I suggest using the MTI weep screed also, which has slots designed to actually weep.

  5. Stephen Sheehy | | #5

    Use an epdm gasket between mudsill and top of foundation. Tape the sheathing seams and the tyvek seams. Pay attention to all penetrations, using tape and/or foam.
    Have a blower door test at a point where you can still fix any leaks. Ours is a double stud wall, so not directly comparable to your situation. Our GC hadn't ever tried to do a real tight house, but he and his crew understood the concept and did a fine job of air sealing. It isn't hard, just requires attention. We ended up with .59ach50.

  6. Joshua Greisen | | #6

    Thank you for the responses. I had the order incorrect in my original post in regards to the rain gap. We do intend on having the rain gap in place between the exterior foam and the cladding (in this case a stucco finish system). If we decide on an EPS CI panel solution for the exterior rigid foam (which is likely given the costs), this would put the sheathing on the exterior of the foam as seen here. In this case the rain gap goes on the sheathing since the foam is glued to the interior of the sheathing at the factory. Has anyone used CI panels with foam on the interior, if so what are your thoughts on that solution?

    I think your decisions for air sealing are going to be close to ours. I don't see a better way to air seal the attic using a traditional roofing system. Can you please share the material you used for peel and stick over the rim joist connecting the foundation to the wall?

  7. GBA Editor
    Martin Holladay | | #7

    For more information on the use of nailbase panels for walls, see this GBA article: Nailbase Panels for Walls.

  8. Eric Whetzel | | #8


    We're having good luck with the Pro Clima tapes and their HF sealant from for air sealing. We also considered using the R-Guard series of liquid membranes from Prosoco (it's too cold right now for us to use them). Another option would be products from

    You could also consider using Roxul Comfortboard 80 rock wool instead of the foam for continuous insulation on the exterior of your sheathing. We decided to go with the Roxul for environmental reasons, but also for its unique properties (e.g. fire resistant, it doesn't hold onto moisture, sound deadening, it doesn't lose R-value when it gets cold outside).

    So far we've only used it on the exterior of the foundation, but installation has been straightforward, and no complaints from the guys on the crew (easy to cut, not a lot of debris flying around when it is cut --- even with a table saw).

    We used an EPDM gasket from for our mudsill, as Stephen suggests. It was easy to use and seems much more effective than the typical foam sill sealer.

    I've tried to collect what we've learned so far on our blog. Maybe it can help, even if you end up reaching different conclusions about the details: (Wall Assembly, Foundation Details, Resources Page)

    You're probably already using it, but has a lot of useful info as well --- e.g. use their search function for "air sealing" and a series of articles comes up.

    Good luck with your project!

  9. Joshua Greisen | | #9


    Are you aware of a wall cross section detail for exterior rigid foam with furring strips and stucco? I can't find any details to use an example in conversations with my construction team. All the ones I can find are EIFS...

  10. Sal Lombardo | | #10

    Joshua, I have been beating that dead horse for some time. Detail for 3 coat stucco or "traditional" stucco over exterior rigid foam is complicated and as most have told me (including Martin), risky and some argue, ill advised. So much so, I initially gave up on outsulation. I want to stay true to the style of the house, so dropping stucco is not an option. EIFS is a different material and given its low relative weight, it goes over rigid panels all the time. But 3 coat stucco is very different. So much so, my local engineer will not sign off on a design. I trust him, so for him to tell me find someone else, speaks volumes. Now I am re-invigorated (or just plain stubborn) to try to find a stable, well thought out design.
    I found it is the flashing details and weight of 3 coat stucco (about 11 lbs per sq ft) that complicates the issue. The weight can cause a resultant wall failure phenomenon referred to as creep - see Peter Baker's article:

    Fastening schedules exist to secure furring strips and exterior foam, based on foam thickness and weight of final cladding. This was nicely summarized by Martin previously:

    I found a few contractors who assured me "no problem". But, IMHO, the better ones were reluctant to get involved, "too risky, eventual failure is expected".
    That's said, if you are interested, see my recent post on the GBA QandA.

    Now I am thinking, support the furring strips on a ledge, and maybe keep the foam to the minimum, from: Calculating the Minimum Thickness of Rigid Foam Sheathing

    Would love to share notes.

  11. GBA Editor
    Martin Holladay | | #11

    There are lots of reasons to avoid the use of stucco on a wood-framed building, but many people (for some reason) still want to pursue stucco.

    The easiest approach is to install EIFS. This option is shown in the detail below.

    I don't have a detail showing three-coat cementitiuous stucco over furring strips, but the idea isn't that complicated. For decades, California builders installed stucco over stud walls without any sheathing. They just installed asphalt felt and metal lath on the studs, and then stucco. Seems odd, but it works. If you have vertical furring strips 16 inches on center, the same approach is possible.


  12. Joshua Greisen | | #12


    I have definitely heard the warning on including drainage space for a stucco/EIFS installation when using exterior rigid foam. I think I get a bit confused as to where the recommendation is for the placement of this drainage plane though depending on the chosen finish. So let me reiterate what I think I'm hearing via all the postings and referenced articles, if you could please correct any inaccuracies that would be much appreciated:

    Option 1: If choosing an EIFS finish for your exterior walls, include a drainage gap between the foam and your WRB/sheathing (sheathing should have a WRB covering it of course, like Tyvek).

    Option 2: If choosing a traditional stucco finish for your exterior walls, include a drainage gap between the foam and the first layer of the stucco finish (not sure if this should be building paper, wire mesh or some sort of backer board when this done over a foam product like EPS).

    Please let me know if these are accurate. Thanks!

  13. GBA Editor
    Martin Holladay | | #13

    Basically, you've got it right. When it comes to EIFS, though, you don't have to figure out the details. EIFS can't be installed by a homeowner. It can only be installed by a certified EIFS installer -- your insurance company will make sure of that. The certified EIFS installer comes up with the water management details, not you.

    You will probably be installing some type of paper-backed metal lath if you install traditional three-coat cementitious stucco. As I wrote in my article on the topic, I strongly recommend that you include a drainage gap between the paper-backed metal lath and the rigid foam.

  14. Joshua Greisen | | #14


    Will the local engineers you are referencing not sign off on the design due to the risk of the attached furring strips not being able to handle the weight of the full 3 coat stucco installation? My assumption would be that the furring strips are attached through the 2-4" of rigid foam directly through the sheathing and into the studs. You would think that including more furring strip fasteners would mitigate any concerns. Looking at your post on this topic it seems the risk really is with heavier stone finishes rather than 3 coat stucco unless I'm misunderstanding the post.

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