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Community and Q&A

Relative value of air sealing at exterior sheathing

hughsdb | Posted in Energy Efficiency and Durability on

I am trying to understand the relative value of air sealing at the sheathing. To do this lets consider cost as the benchmark. I realize that there is value beyond cost but this is an easily quantifiable benchmark and the one that seems most resonate with my clients.

Lets assume that we have two identical houses with 8′ ceilings and 350 LF of exterior wall we are sheathing it with OSB and sealing at the top and bottom plates with caulk and nails at 6″ on center. On one house we are going to spend an extra $700 using high quality tape to seal the seams of the OSB before carefully wrapping with house wrap that is taped. On the other house we are simply using the carefully installed house wrap, which is taped, but no tape at the OSB seams. We are in hot humid climate zone 2.

What is the range of time that I can honestly tell my client that it will take to recoup the $700 investment in tape to seal the OSB?

What is a measure of other compromises that come from not sealing at the OSB?

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  1. Expert Member
    Dana Dorsett | | #1

    Energy costs vary by both location and time. There is no magic number that works.

    It's possible to meet air tightness code by either method, but sealing the sheathing is less likely to be undone during construction or fail over the long term.

    A bead of caulk between doubled-up top plates is necessary, not just under the bottom plate, and even taped OSB needs to be caulked to the top & bottom plates or there will be air leakage.

    It can be cheaper to air-seal the sheathing to the framing with appropriate caulk than taping the seams with expensive tapes. It depends a bit on labor costs and experience, but with a powered caulking gun you can do BETTER than a simple taped-sheathing approach, since sealing the sheathing to the framing at every framing bay cuts down on bypass paths. It won't be anything like $700 in caulk, but it'll probably take a bit more labor (but it's less time than you might think.)

    Air tightness of 3ACH/50 or less has a comfort factor too- it's not just a net-present-value of future energy savings sort of thing. In zone 2A latent cooling loads can be pretty heavy, and an air-tight house means measurably lower indoor humidity / higher comfort during the cooling season.

    It's the end air tightness result that matters the most, but air-tight sheathing is far more rugged than air-tight housewrap, whether it's sealed with goop or tape. If there's a premium paid for getting there by making the sheathing the primary air barrier, the value-added is the long term resilience, not the NPV of energy savings.

  2. hughsdb | | #2

    Thanks for the feedback Dana. Believe me, I work hard to sell value beyond ROI. But I know, especially in my market, it is ROI that people relate most directly to. On new homes we are typically under 1ACH/50. We are currently working to expand our exteriors division and are considering applications for resides. I know that it is not possible to pinpoint hypothetical energy calculations, lifestyle alone is such a big variable, but it is always nice when we can provide some sort of data. Our clients need to be able to base their decisions on something more than the suppositions of the guy selling them something. Unfortunately, there are all too many people out there selling false claims making it harder for us to lead clients to wise information based decisions.

    Are you suggesting sealing sheathing to studs with caulk and leaving the horizontal seam open? If that is the case, on new construction it would seem better to seal the drywall to the framing. I realize that it is best to stop air movement before the wall cavity but at least those seams get taped and penetrations get foamed.

    Thanks for your input.

  3. GBA Editor
    Martin Holladay | | #3

    For a variety of reasons, I think that taping the sheathing seams makes more sense than using caulk. I think that taping is just as fast as, or faster than, caulking; addresses horizontal seams better; and is more likely to result in a lower blower-door number.

    Here is a link to an article with more information on this topic: Getting the Biggest Bang for Your Air-Sealing Buck.

  4. Brian Knight | | #4

    Excellent suggestions for sealing the wall plane but keep in mind this is the easiest area to seal and they are typically areas under more neutral pressure. Its the transitions that have so much room for improvement. Masonry to wood, wall to window, penetrations.. Wall planes and transitions at the top and bottom of the home have more influence.

  5. hughsdb | | #5

    I agree Brian. We are often asked to replace windows in brick veneer walls. Is it better to leave an old single pane window in place or replace the window? Removing the brick almost always takes the project out of consideration.

    Dana and Martin, I still question the efficacy of sealing the OSB at all. I am quite certain that it provides a better air seal, but is it worth the cost? How well does high quality house wrap carefully taped air seal? Are we going down a rabbit hole with our clients money in a quest for air sealing machismo. Don't get me wrong, I am prone to being guilty of this. What is the point at which increased air sealing is not worth the money we pour into it? I realize that this will vary on multiple variables but I feel an obligation to gut check the way we spend our clients money.

    Keep in mind we have to include labor and markup to fully calculate the cost to our clients. It is more than trivial. We need a real and effective improvement, not just in air sealing but in energy savings.

  6. mpg9999 | | #6

    Huber had a study done where a house was sheathed with Zip. They left the Zip seems untaped and installed housewrap with all the housewrap seems taped. They performed a blower door test with both positive and negative pressure. Then they removed the housewrap, taped all the zip sheathing seams, and redid the blower door tests. You can read all about it here: Does Your Air Barrier Work in Both Directions?

  7. hughsdb | | #7

    Thanks for sharing this article Michael. Do you know what house wrap was used or what its perm rating was? The gap on the sheathing seems rather excessive. Do you know of other similar test conducted by a more independent source?

  8. Expert Member
    Dana Dorsett | | #8

    If you're regularly hitting under 1ACH/50 with your favorite methods, there is no benefit to additional or more expensive methods.

    Even the benefit of going from 3ACH/50 to 1ACH/50 isn't necessarily worth spending extra for in a zone 2 climate, even when going for Net Zero Energy.

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