Double stud wall interior vapor barrier with Gussets
I am building a double studded house (9′-9′.6″ walls) in central maine zone 6. I am wondering how you effectively install a vapor barrier on the exterior of the interior wall when there are gussets connecting the interior and exterior walls. The design from inside to outside: 1/2″ Drywall, Double 2×4 studs with total Rvalue of R44, dense packed cellulose, Siga Majrex 200 for a vapor barrier attached to exterior side of interior 2×4 wall, 4.5″ cavity space, plywood gussets, 2×4 exterior wall, 1/2″ plywood sheetings, taped seams for air barrier, Siga Majvest WRB, Waterairvent horizontal strapping, LP board and batten
Questions:
1. How do you have a VB on the exterior side of the interior wall with gussets? I assume the interior wall needs to have the VB installed and wrapped around plate/header and taped prior to standing and nailing into place.
2. With DPC, I would assume with a 11.5″ cavity that DPC needs netting between every other bay to achieve the 3.5-4 pounds per cubic foot? Not sure how this is achieved with a VB on the exterior or the interior wall. And is the Marjex200 sufficient to withstand the installation of the DPC to achieve the pounds per cubic foot?
3. Is the VB needed on the exterior side of the interior wall? Or better to place between the Drywall and studs or not needed at all?
4. Or if the VB is required on the exterior side on interior wall, should blocking be used instead of gussets to prevent twisting, but still an issue with netting bays. My current house, the studs twisted, so blocking or gussets I believe should be used on this house.
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Replies
Sean, I'm also in central Maine--Palermo--and we are in climate zone 6. Your plant hardiness zone might be 5.
I design a lot of double stud walls and although where your designer recommends placing the vapor retarder ("barrier" means that no moisture passes, which is not accurate or desired) in an ideal location from a building science point of view, it creates challenges that I don't think are worth adding. I locate a variable permeance membrane (typically Siga Majrex, available in Portland at Performance Building Supply) directly behind the drywall.
My friend and frequent collaborator Dan Kolbert disagrees and finds that the vapor retarder isn't necessary at all: https://www.greenbuildingadvisor.com/article/a-case-for-double-stud-walls (or here: https://www.finehomebuilding.com/project-guides/insulation/a-case-for-double-stud-walls).
In most cases, the plywood gussets are just there to keep the studs from moving too much, so you don't need many of them. Then insulators just blow the whole cavity with dense-packed cellulose or wood fiber, through Insulweb. Then you can install the vapor retarder over that and install the drywall. I can recommend a local insulator familiar with dense-packing double stud walls if you don't have one.
Micheal
Yes thank you. I would be very interested in an insulator familar with this. If you had other suggestions for anything else I am happy to take them. Located in Waldoboro ME. Looks like the designer recommended Siga Majrex 200VB which I assume is the product by Siga you are referring to as well? I like the idea of it being behind the drywall as seems too complicated inside the cavity and coordinating everyone to do part of their jobs at different times, even though its an ideal location. I read about no VB...thanks for sharing....funny thing is that this in my current house as I overlooked that the VB was not installed on interior of my 12" double stud walls. Been 12 years...no issues I can see, however be nice to peak inside at some point to see if condensation on the sheathing.
1. So sounds like what is listed for my wall cavity looks good...except for location of VB?
2. The gussets, do you recommend 2 for every 2x4 (16" OC) located third from top and bottom or different locations? If not on every 2x4, then is blocking for interior wall added to prevent 2x4s twisting as they dry as the walls are 9-9.5" high? My current house I did not do this right away and they all started to twist and bend...also the code officer came and said anything over 8' needed them anyway...
3. With interior wall on first floor placed on a slab and not nailed to concrete due to it being partly over thermal break is their a diff recommendation for gusset locations such as one at bottom?
There is no "VB" in the name, it's just Majrex 200: https://performancebuildingtapes.com/product/siga-majrex-200-siga-tapes/?srsltid=AfmBOooxmMwy6TLE1NM_HAH8WH1NTatT5Y9yO5eCaaQHXFgxms446Bu_.
1. Yes. I forgot to mention, just be sure to make the sheathing layer as airtight as you possibly can. You only need one really good air control layer in an assembly. Adding more is usually a good thing, but you mainly need to stop air from moving through the assembly. In most cases, drywall reduces air flow from the interior space into the wall assembly, without special measures.
2. It depends what your stud quality is. Usually ten footers are better than eight footers. I would probably install 2-3 per stud, just on one side of the stud, they don't need to be in pairs unless your studs are very twisted.
3. I don't have experience with that. I would not be comfortable having the inner plate not fastened to what it's sitting on. I would consider using a wider bottom plate, even though you'd lose some BTUs, rather than compromise the structure.
Damariscotta is outside of what I'd call central Maine but my go-to companies would probably travel there:
https://heartwoodinsulation.com/
https://www.thebreathablehome.com/
And you probably know of this company, who would certainly go there: https://www.evergreenyourhome.com/. They often subcontract to the first company I listed.
seam918,
The designer sounds like they have never swung a hammer. What does "plywood gussets as required" mean? How and when are you supposed to determine if they are required?
You have nothing but good questions about the assembly and sequencing. I would start by putting them to the designer who specified it.
So with gussets and double stud walls, what have you seen in regards to a vapor barrier? My first floor is on a slab with frost walls. I am thinking with 2x4 walls that gussets would be a good idea. My current house is 2x6 exterior and 2x6 interior with blocking for interior wall only and a VB between drywall and interior wall. That was 12 years ago. Just not sure now.
Sean918,
Unlike air-barriers, Vapour-barriers and vapour-retarders are effective as a percentage of the area they cover. Incidental penetrations or gaps don't matter. So locating them on the inside face of the wall is fine, and a lot easier.
That helps...that makes sense. And I made an error above...2x4 interior and 2x6 exterior in my current house. In your experience.....double stud walls with blown in cellulose, do you usually see 2x4 exterior and interior or 2x6 exterior and 2x4 interior? I will be talking to engineer this week but just curious. And are gussets usually used? Especially on a first floor with slab/frost wall which means the interior wall really cant be nailed to cement as its partly over the thermal break so gussets I assume stabilize it....I suspect should have one placed at the bottom sill area also. Obviously ties the walls together. I just never used in the past, instead blocking only with interior wall. Also...in designer defense, the plans I quoted were a draft and not final plans so I suspect more details with gussets being required is forthcoming.
sean1918,
My (over) reaction to your designer's language came from the frustration I have had dealing with specifications that include either "as required" or "or equal ", which lead to passing the buck down to the builder, when those things should be resolved at the design stage. Similarly, it's a designer's obligation to only draw build-able details. If you look at the sections posted here on GBA by reputable architects - especially the ones with exterior foam - you will find many of them with no way obvious of attaching the various parts together. That also shouldn't occur.
So apologies for my rant - and now it's over I've got to confess that y0u would be better off getting advice on the practical construction of double-walls from those - like Michael Maines - who have much more hands-on experience dealing with them .
My architect and I have designed a similar wall based off of this > https://ca.475.supply/blogs/design-construction-resources/the-double-stud-wall-simplified-low-cost-high-performance#:~:text=The%20wall%20consists%20of%20an,loads%20meet%20the%20design%20criteria.
I attached a screenshot of my drawings showing the gusset details. To maintain the continuous smart VB on the exterior side of the interior stud wall a “nailer” will need to be used to attach the gusset. This will likely need to be done from the exterior (will complicate things in a multi story build). My carpenter and I will be figuring out how to actually build this in the next couple months.
There's no reason to have the vapor barrier on the exterior side of the interior wall. Put it on the interior side.
*edit* -sorry I just realized your response was directed towards the OP…below are my reasons.
It makes it more resilient and will be detailed as an air barrier. The inner stud bay can serve as a service cavity (which will also be insulated later). No plumbing or electrical will penetrate the barrier. Future renovations will also be less likely to mess with it.
It may be a detail that many would never pay the extra labour for, but for me (and many others) I’m good with it.
Deleted
Edit: the comments needed moderation. Probably because of the recent spam onslaught, or maybe because I included a link. Test: I've posted two longish comments on this thread and neither are showing up. Does anyone else see them?
do your longish comments have any links in them? try adding spaces... one time i realized that posts containing b l o g s p o t links were auto deleted.. i imagine there more than just that on gba's filter.
Thanks, they all appeared in the last hour. There were a ton of spam posts yesterday so it might be related to that.
Mike,
Your first post (now labelled as #1) has since appeared. Nothing else.
I see #1 post but nothing else...would love to hear more of your thoughts
Hi Sean, I’m a builder located in southwest Ontario CZ 5/6. We haven’t managed to convince our building inspectors that a vapour control membrane may not be necessary so we chose an vapour variable membrane like Majrex or Intello which satisfies our officials.
Our preferred double wall includes a protected vapour/air control layer as you are describing. After researching various DW assemblies and not seeing a build up that checked the performance and build-ability we developed the
following assembly and found it very builder friendly and straightforward to sequence. Here goes from exterior to interior
Siding (horizontal)
1x3 vertical rainscreen
Zip sheathing (primary air barrier)
2x8 at 24”oc
R32 Dense pack fibreglass BIBS (could be cellulose or batts)
Vapour/secondary air control (ie Intello or Majrex)
2x4 at 24” oc service cavity wall
R14 BIBS*
Drywall finish
The use of 2x8 does add some material cost but little to no additional labour cost. The reason we specified a 2x8 exterior wall was to push the vapour control inboard, similar to the location you describe. Installing the VC on the interior of the exterior wall was very simple and should be very familiar to many builders. The 2x4 service cavity wall is about as easy as it gets and you are only left with a handful of through wall penetrations to deal with (hose bibs, outside receptacles etc)
One of the best aspects of this assembly from a builders perspective is that other than adding an additional wall for services there is really nothing unusual and mechanical, electrical and plumbing trades really don’t need to understand it as they have very little interaction with the air/vapour control layers resulting in easier quality control.
*Our insulator is close by so making two trips wasn’t an issue. We would choose batt insulation for our SC wall scheduling a second trip from insulation contractor was too complicated.