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Will moisture be a problem in this wall?

KDmVtuQnhe | Posted in Green Building Techniques on

I am part of a volunteer fire dept in the NEK of Vermont and we are building a new firehouse. We want to build it as energy efficient as possible since it gets used seldom (hopefully). Just a little different from most homes around since it’s only heated to 55 not 70.
Our plan is the following from inside to outside

5/8″ Sheetrock
Poly Vapor Barrier
2×4 stud wall with fiberglass insulation
Double layer of 2″ Green Guard staggered joints
2×4 stud wall with fiberglass insulation
5/8 Advantec Sheathing
Typar Housewrap
Vinyl Siding

Do we need an airspace between the vinyl siding the sheathing? Is there risk of moisture in the outer most wall that would negatively impact the fiberglass?

I’ve looked at some of the Dew Point calculators online and I don’t really want to spend a month investigating this.


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  1. user-901114 | | #1

    Interesting concept. The vinyl siding should be it's own airspace since it's not nailed hard against the wall and may have drain holes. The moisture risk in the exterior wall fiberglass comes from flashing details that could be missed or sloppy j channel work. This insulation will also be cold which can't help it perform very well. I wonder how you're dealing with the rim joist and roof wall intersection. How wide is the foundation?

    The builders I talk to up here in zone 6 nh are going with 100% outsulation with polyiso. Saying that cold fibrous insulation is weak.

  2. GBA Editor
    Martin Holladay | | #2

    Hi Dana. I live in Wheelock, Vermont. Where are you?

    I'm glad you are advocating a well insulated wall; that's important. However, your suggested wall assembly could be improved. If you care about a high-performance wall, here's my advice:

    1. Choose a strategy that includes careful attention to air-sealing.

    2. Choose an insulation material that performs better than fiberglass batts, since fiberglass batts are the worst-performing insulation available.

    3. Choose a wall assembly that addresses the "cold sheathing" problem. (If your Advantek sheathing is cold all winter, which it will be, it can accumulate moisture and become susceptible to rot.)

    4. Don't include interior polyethylene.

    I think the two best-performing wall assemblies for our climate are these:

    1. 2x6 walls filled with dense-packed cellulose insulation, and sheathed on the exterior with at least 2 inches of polyisocyanurate rigid foam (more foam is better).

    2. A 12-inch thick wall framed with double 2x4s, insulated with dense-packed cellulose, and sheathed with plywood or diagonal board sheathing (to avoid the cold OSB problem).

    In either case, you want to install the interior drywall according to the principles of the Airtight Drywall Approach. This requires gaskets or caulk and the use of airtight electrical boxes.

  3. Starbright Steve | | #3

    Martin, I like option 2 and I understand the moisture performance of real wood (diagonal board sheathing) but won't plywood trap significant moisture with 12 inches of cellulose between it and the heat.
    One big weak point in the thermal boundary of a building like this is the overhead doors . Are there any better options for big entry doors?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Won't plywood trap significant moisture?"

    A. Plywood is more vapor-permeable than OSB, so it can dry to the exterior at a faster rate than OSB. A lot depends on the siding. Stucco limits drying to the exterior, and is therefore a bad choice unless there is a ventilated air gap between the sheathing and the stucco. Vinyl siding is a good choice, since it is well ventilated and allows fast drying to the exterior.

  5. wjrobinson | | #5

    The Advantec sheathing and the foam sheathing are vapor barriers and need to either be together or one eliminated to be the absolute safest build. But if you have good overhangs and a great builder you could be fine. There is much more to a project than just a quick look at the order of components in a a wall assembly. Get an architect and or superinsulated builder/designer on board. You can't just ask a few questions online and go build, which I am sure you know.

    Oh and as Martin stated no to poly which would be even a third vapor barrier. Air barriers are what you want that are open to vapor getting out of your assembly but stop air from moving and transporting the moisture.

  6. KDmVtuQnhe | | #6

    Hello Martin,

    I'm right next door in Sutton. Thanks for the advice. Our department has a lot of building experience but not a whole lot of design experience. With all the new materials out there in the last 10 years it seems that there could be a few things to watch out for. It seems we've been pushed towards spray in foam but since we can't do that ourselves and several of our members have a very negative experience with it (or rather the application of it) we'd like something we can do.

    The goal of the wall construction was to have all the plumbing/electrical and other utilities in the interior wall to just about eliminate all protrusions thru the double layer of foam. The double layer of 2" green guard/blueboard would really be the airtight barrier. There aren't too many windows so it should be pretty efficient. I really didn't realize that Advantec was a vapor barrier but it makes sense. So we'd want the plywood because it does allow moisture to move thru it better. Can you explain why the interior poly isn't a good idea? I'm a little concerned because there will be a lot of moisture in the building since the trucks may be wet.


  7. wjrobinson | | #7

    Dana, if you have high humidity inside you will need dehumidifying. Also you may want the poly in your case or better yet put your foam inside under fiberglass based drywall and use exterior sheathing that is permeable. Then your wall can dry to the exterior but have a way to stop truck related moisture. There are engineered plans for high moisture builds like pool enclosures. If you could get engineered plans, that is what you really should do.

  8. GBA Editor
    Martin Holladay | | #8

    I'm glad to hear that Sutton is getting a new fire station. (We've been talking about building a new municipal building / town garage in Wheelock for years, but nothing's been decided yet.)

    Lots have studies have shown that the main way that interior moisture gets into walls (and sometimes causes problems) is through air leaks, not vapor diffusion. That's why energy experts are backing away from polyethylene -- it really isn't necessary, and it doesn't allow the wall to dry to the interior. So air sealing the interior finish material is much more important than having a vapor barrier.

    If you want the advantages of a sheet-style vapor retarder on the interior, consider using MemBrain. It is a so-called "smart" retarder with variable permeance, so it allows a wall to dry to the interior during the summer, but still acts as a vapor retarder in the winter. It costs more than polyethylene, but it performs better.

    Minimizing penetrations that cause air leaks is a good idea. Keeping electrical and plumbing runs in a separate wall (a so-called "service cavity") without any insulation, inside of your thermal boundary, is an excellent idea.

  9. user-1075855 | | #9

    Martin, you're advising against polyethylene because any moisture that might get into the wall needs to escape, right? Why do you then recommend installing the "interior drywall according to the principles of the Airtight Drywall Approach"? Wouldn't that trap moisture?

    Also is there a reason you're recommending cellulose instead of other products (such as bluejeans)?

  10. GBA Editor
    Martin Holladay | | #10

    Jay Sheth,
    Taped drywall is an air barrier, but it is not a vapor barrier. Water vapor can pass through drywall by diffusion.

    If you want to use cotton batts (I assume that's what you mean by "bluejeans"), go ahead. There are many types of insulation out there, and almost all of them work if they are installed correctly.

  11. wjrobinson | | #11

    Martin, really, you're saying airtight drywall is all one needs for a high moisture build?

    I must get around to posting all the moisture rot pics from walls built with ridgid foam that were destroyed by humidity and moisture in 5-10 years only from new.

    I think your advice is fine for normal home moisture loads only.

  12. GBA Editor
    Martin Holladay | | #12

    Q. "Martin, really, you're saying airtight drywall is all one needs for a high moisture build?"

    A. I don't know what a "high moisture build" is.

    However, I do know that the Airtight Drywall Approach works. Gypsum drywall can certainly be a good air barrier, as long as you pay attention to the usual seams, intersections, penetrations, and electrical boxes.

  13. GBA Editor
    Martin Holladay | | #13

    I'm confused, AJ. My comment about drywall was directed at Jay Sheth, who asked a question about vapor diffusion through drywall. My comment was not directed at the original poster, Dana Patoine. I don't think that Jay Sheth was concerned about wet fire trucks.

  14. wjrobinson | | #14

    You do know what a high moisture area build is Martin and are not spec'ing such. The poster desires to roll wet firetrucks in. That is not a normal environment of reasonable levels of humidity. An enclosure design more like that of a modern engineered pool enclosure structure is what makes sense. Here in my towns there are new firehouses everywhere. I bet the plans and help and sharing of what has worked is much more what this project needs not just you and I differing.

    Just saying and moving on.

  15. wjrobinson | | #15

    May have to read whole thread tonight.... If I oops'd, wouldn't be the first time.... All I know is what I see.... Moisture and blog built structures make me wary.

  16. jklingel | | #16

    Dana P: Another permutation of the cellulose/double-stud wall is to put the plywood, goo'd and taped, on the outside of the inner 2x4 wall. There, it is warm and acts as an air barrier. On the outside of the exterior 2x4 you can use Tyvek or a Siga wrap material (Majcoat?). Follow that w/ 1x4 furring and your siding. I think if you search here (or google) for the Sunrise house you will see the details thereof. The plywood then extends over the roof to have a continuous air barrier.

  17. wjrobinson | | #17

    John Klingel is in the right track for this particular thread. We are posting about a high moisture environment. If Jay wants high moisture advice.... Perfect. If not, start a new thread.

  18. user-1075855 | | #18

    Thanks Martin. That clears up my misunderstanding.

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