Where to put an air barrier in this assembly?
Hi all,
I’ve been planning a build for a while now, and my enclosure design has given me the enviable problem of having too many good air barrier options. I could use some help narrowing down which approach is best suited for my enclosure, or if this is a case where I might as well embrace redundancy and get the enclosure as tight as possible.
The basic setup is this: I am leaving the building’s exterior frame fully exposed to the interior, and applying plaster in the stud bays directly to the sheathing (where necessary, service cavities will be made by installing drywall between studs). Insulation will be handled entirely with exterior foam (6-8″ reclaimed polyiso), and windows will be outie. The floor plan is a square, roof overhangs will be added after (as in PERSIST construction), and while there is a small clerestory, there are no other complicating roof details. This setup means there are multiple good locations and methods for incorporating an air barrier into this assembly, so I’ll go through the options I see:
1) The plaster itself: It will be more than thick enough to count as an air barrier according to IECC guidelines, and has the convenient quality of already existing in the assembly. Throw some caulk on the rim joist and other non-plastered areas, and you will make a pretty good air barrier, and one easy to inspect and patch if necessary. The only drawback I see here are the potential for cracks to form on the plaster, perhaps on small scales that weaken the air barrier over time. I am considering using lime plaster, which allegedly is quite resilient to that form of cracking. Note: I couldn’t find specific information, but I presume lime plaster is as effective an air barrier as gypsum plaster.
2) The structural sheathing. As much as I’d love to use only foam and brace using 1×4 let-ins, I imagine a layer of plywood would make a much better plaster substrate (and not react with lime :p). Air sealing this layer would be simple: all it would require is taping up the exterior seams, and caulking up any openings.
3) The WRB. My original plan was to use Ice and Water Shield like traditional PERSIST. Unfortunately, that approach is expensive, so much so that my back-of-the-envelope calculations showed that it would likely be cheaper to remove the last layer of rigid foam and add a couple inches of closed cell spray foam to the exterior instead (having the added benefit of protecting the polyiso from the cold temperatures that can reduce its performance). But since I already have two, much less expensive alternative air barriers, both approaches are starting to seem like overkill, and I might just use a simple housewrap as my WRB. However, this would be the most durable and effective air barrier, and it is hard to pass that up.
Right now, I think the best plan is to primarily air seal via 1, utilize 2 for cheap redundancy, and skip spray foam or IWS. What are your thoughts?
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Replies
Hello there Aedi -
I am pretty sure we need a drawing of this wall assembly to make sure we understand what you are proposing. As written, you seem to be proposing applying plaster to the INTERIOR surface of your exterior sheathing and in between exposed studs, correct? If this is correct, a comment and a question:
1. you can't apply plaster directly to sheathing; you need a bond break to account for the differential movement of a mineral material such as plaster and a fibrous material such as sheathing.
2. What is the reason you would apply the plaster in between stud framing and on the interior of the exterior sheathing--that is certainly unusual!
In response to your other questions:
1. Yes, 3/4-inch thick plaster does qualify as an air barrier material. Reference: "ABAA All About Air Barriers - https://archive.airbarrier.org/conference/Basic%20Air%20Barrier%20Requirements%20-%20Laverne%20Dalgleish.pdf.
2. "best" air barrier system: best for me is where it is easiest to make it continuous (exterior) and rigid (sheathing) rather than flexible (sheet good such as housewraps).
Thanks for the response Peter,
You are understanding the assembly correctly, but I went ahead and attached a diagram anyway. To your points:
1) That is good to know! I had suspected that that could be an issue, but my cursory research had only suggested the use of a metal lathe (as you note, this set-up is unusual, so it was difficult to find applicable info). In that case, I anticipate using a combination of a metal lathe and some sort of fibrous backing that can be wrapped around the frame before the sheathing is attached. I have a material in mind, though its name escapes me at the moment (Edit: Fiberglass tissue. It also appears I can buy a paper-backed metal lathe, should I chose). Regardless, it looks like ought to build some test assemblies before trying it in the field.
2) Two reasons: An irrational distaste for drywall caused by living in too many buildings with leaks and poor moisture management, and an aesthetic that finds beauty in function that leads me to want to show off the frame of the building -- I put a lot of work into designing that frame, after all! I realize this is very unusual, and may very well end up not working. If that ends up being the case, I'll just have to throw some drywall up over the failed experiment, with only minor harm done to my wallet and, perhaps, my pride.
Thanks for answering my question as well. Between you and Martin, it seems the consensus is to rely on my sheathing for my air barrier and WRB. I appreciate the advice.
Aedi,
If you install plaster to the interior side of your wall sheathing (between the vertical framing members -- studs or posts), you won't get a durable air barrier, because of the cracks between the plaster and the framing. These cracks will open up seasonally due to changes in the moisture content of the framing.
The best location for your air barrier is at the sheathing layer. Of course, if you are following the PERSIST method, you will be installing a peel-and-stick membrane on the exterior side of the sheathing, and that membrane will be your air barrier as well as your WRB.
If you aren't following the traditional PERSIST method, and you are thinking of skipping the peel-and-stick membrane, you should tape the seams of your sheathing with a high quality tape. Zip System sheathing is one approach.
Thanks Martin. I was anticipating cracks, but had not considered how seasonal swelling of the framing would affect the plaster. Hopefully that won't become an aesthetic issue! Worst case, I can always throw drywall up after the fact.
Seems like it is worth it to place the drainage plane behind the foam an the structural sheathing then. I'll have to look into the pros and cons of huber's zip compared to Ice and Water Shield, and maybe liquid applied WRBs too.