Internal vapor membrane with thick exterior EPS insulation
Hello GBA!
First and foremost, I am building this house in Sweden on the west coast, which would be similar to a mild marine climate in the US (not much snow here, mainly heavy rain/wind driven rain in winters and temperatures around 0C/32F most of winter, some days reach down to -18C/0F (rare cold snap). Summers are mild, mostly around 20C/68F and peaking at 30C/86F.
Now to my problem. I’m finalizing the design of my house and will start to build within a few months. I have one question that bothers me a lot, and that is the need for interior vapor membrane when using very thick external EPS insulation (300mm/~12 inches) with additional 100mm/~4 inch mineral wool/cellulose to boost the insulation value (have not decided on this yet).
The house structure is as follows:
First floor: ICF wall, 200mm EPS outside, 150mm concrete core, 100mm inside.
Second floor/attic: Attic roof truss 1200mm center-center with 300mm external grey EPS, maybe 100mm mineral wool between trusses. Outside the sheathing and membrane there is a 45x70mm/2×3 stud for nailing the furring to (to have a larger target to aim for) and to avoid screw penetrations in the sheathing and WRB. These studs will have a strip of Naideck-style tape (the Siga variant) to seal the nail penetrations when attaching the stud.
The reason for my doubts is that:
-1 I want the drying potential towards the interior since the EPS will act as a vapor barrier, then is it not best to have it fully open to the interior?
-2 I obviously don’t want moisture problems. Been in contact with Siga regarding their Majrex products, which of course they want me to use to ensure airtightness and vapor control. I can understand that it would be nice to have directional flow towards interior only so not much vapor reaches inside the cavity.
-3 (I think) I can get the house airtight enough by using a WRB outside my sheathing but below the foam (I am using outie windows with buckouts) and being detail oriented when taping the seams and penetrations. I will also tape the seams of the exterior EPS layer. Am I delusional and relying too much on one “perfect” layer? Perhaps the taped EPS is the backup here?
-4 I would love to skip the interior vapor layer because that simplifies the installation a lot. I really want to avoid having to tape and seal around each roof truss. It would also simplify the installation of electrical wires and ventilation (ERV ventilation).
-5 Only using membrane on the sheathing would simplify the transition between ICF and roof truss.
The simplest installation is preferred by me, as I will be building this myself. I understand that perhaps the whole idea of this construction is not “simple”, but that’s ok. I just don’t need to spend a lot of time to perfectly tape around each roof truss, it feels like the more details to tape around introduces more problem areas.
From my point of view there are a lot of practical installation benefits with only having the membrane WRB, but I of course absolutely want to avoid air leakage as much as possible.
I have attached a report from Ubakus that shows my intended wall/roof cross-section and materials with two alternatives:
-A1: WRB/membrane on sheating and under foam
-A2: A1+Majrex on the interior underneath gypsum board
Am I just paranoid and worrying about nothing?
Best regards,
Jacob
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Replies
JacobL_SE,
As long as you follow these guidelines and install adequately thick exterior foam, there is no need for an interior vapour-retarder: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
Does your energy modelling support that much exterior insulation? In a relatively mild climate, the benefits of going anywhere near 12" of EPS yields marginal gains.
Malcolm,
Thanks for your response.
I have read that post and assumed it was only talking about a layer of plastic vapor retarder (vapor impermeable material). My thought on that is, yes that is unnecessary/dangerous as it prevents the drying inwards. But with vapor permeable membranes, like the Siga Majrex for instance, their claim is that the membrane allows drying inwards without allowing vapor to pass the barrier to the cavity. So, I guess the question is, vapor permeable membrane like these products, or simple skip the layer entirely?
I have done some simple 2D FE analysis in Ubakus, and those reports are in the attached PDFs. Do you think a batter strategy would be to just have enough external foam to move the dewpoint outside the sheathing and fill up with insulation in the stud/truss cavities? Of course, there is the relationship between external and internal insulation thickness to keep the sheathing above dewpoint.
Jacob_SE,
I linked to that article to give you an idea how much exterior insulation you need to make your walls safe from moisture accumulation - and as you can see you are far exceeding that.
The article cautions against interior poly because you can end up with a vapour-barrier sandwich. That is the danger, not having the interior face of the wall permeable. I don't see what benefit an interior vapour-retarder, variable perm or not, would bring to your wall.
Our code requires a vapour-barrier, and says it must be installed "sufficiently close to the warm side of the insulation to prevent condensation at design conditions". In your wall assembly the interior face of the foam meets that criteria, so no other material is necessary or useful.
Jacob,
If you have an air tight "sheating assembly" then no internal vapor retarder is really required with your insulation layout.
Still - whatever the code and the building inspector tells you would be king.
Then (Yes - i know - bad style - still.):
I have friends who built their swedisch retreat and they ended up with 8" of mineral wool - and were quite happy. Also - you have really deep studs which would not be filled up with insulation. Did you consider to go from 48" OC to 24" OC and a 2x6 stud system? You would gain quite some internal floor space that way!
Your R-value is quite higher than that of an passive house and this can be called overkill.
Reg. UBAKUS - they have a module to estimate the heat load/energy consumption of a house using the saved assemblies. (assemblies, windows, areas, climate, etc.) Did you try that out? It is like a manual BeOpt. I would expect that doing that will give you exceptional numbers - but do you need these?
regards
edit. Sorry - my fault - you attached only the roof so the 12" start to look more normal. Still, the 1130mm spacing looks a bit wide for me and some insulation hybrid below / above the sheathing will save some space. Floor space is obviously not affected that way.
wastl,
Thanks for your reply.
We don't have a specific requrement for a vapor retarder. My thought/concerns was more regarding a vapor permeable membrane, like the Siga Majrex. To use this or not? There are some benefits I guess, but I see more benefits to skip this layer all together.
I agree, going to 24 OC (or more like 600mm center-center) would perhaps be a better choice, it's always nice to get some extra space. I will look into that.
Thanks for the tip regarding Ubakus, I did not know that was possible. Going to check how to do that!
With 200mm of EPS on the exterior, which is a bit shy of 8 inches, you have around R33 on the exterior. With half that much on the interior, I can't see you having any issues, with or without an interior side vapor retarder. If you follow the tables at the links Malcolm provided, you should be safe, just convert your units to ours (25.4mm = 1 inch, 1 inch of EPS is about R4.2, 1" of mineral wool is about R4.3), then follow the RATIOs shown in the table as a MINIMUM and you'll be safe. You can make the EXTERIOR insulation as much thicker than shown in those tables as you want, since more exterior insulation is safer.
You might also want to look into the "perfect wall" concept that Building Science Corp talks about, which is using *only* exterior insulation.
Bill
A variable permeance interior membrane would indeed slow the inward drying rate, though probably not by enough to be a problem, even in a wall with a less permeable vapor retarder, more cavity insulation, and less exterior insulation: https://buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed
If you detailed your interior drywall with two coats of latex paint, it might provide just as good of vapor control as a membrane, but don't quote me on this: https://www.energy.gov/eere/buildings/articles/vapor-retarder-classification-building-america-top-innovation
As others have said, you have a high ratio of exterior insulation. This makes your wall very forgiving. You would only be concerned with interior air leakage and vapor control if you had uncommonly high relative humidity in the wintertime: https://buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation
Your feeling that detailing the exterior sheathing and/or WRB as an air control layer can work just fine. In this case, a self-adhered membrane is the less risky option. And taping the foam won't hurt, but make sure you install it in two layers, and stagger the horizontal and vertical seams: https://buildingscience.com/documents/published-articles/pa-foam-shrinks/view
The main benefit of an interior membrane, and a diminishing one, is that it provides air control redundancy. The comment cannot be linked directly, but Wood Guy is a relatively well known designer with the following to say on the subject: https://archinect.com/forum/thread/140735257/passivhaus-longevity
"The more separate the control layers (WRB, Air barrier, Thermal barrier) are, and the more independent they are from the building's structure and mechanical systems, the easier it will be to repair or replace components when they fail or during renovations."
In this sense, if you have some strong reasons to believe that your taped EPS and your sheathing/WRB air control layers may end up damaged and difficult to restore, then it may be still worth considering the interior membrane.
But as you said, of course the membrane salesperson wants you to buy the membrane.
Jacobl SE,
I would skip the interior membrane as unnecessary given the very high exterior insulation provided you have put a self adhering WRB under it all. My own house is similar in that the windows are bucked out to work with my 6.5" (~155mm) foam layer. The WRB material adheres very tightly and provides a slight sealing effect around fasteners as well. The house is very tight and a witness hole I created shows no moisture issues on the sheathing after several years. My design temp is -26C for comparison.
A good drywall job and paint is the only control I have over air exchange with the wall cavities. Glad for that as trying to maintain membrane integrity during the drywall process would have been a nightmare. The crew was great, but they aren't there to be fussy as they zoom.