Wall Assembly
Below is a wall assembly of one builder we’ve looked for the purpose of doing the framing, windows/door installation and the various barriers (WRB, Vapor and Air). I wanted to know everyone’s thought are for my climate in Ottawa, Canada.
My opinion is I don’t think this is a good wall assembly for the following reason:
The air-barrier seems to be in the wrong location and I believe it should be closer to the middle of the wall assembly. The air-barrier they are using, Agepan isn’t promoted as a air-barrier by the company but as a WRB due to the wax imbedded in the woodfiber. I also haven’t found a project where it was used as an air-barrier which makes me wonder how well the tape will hold on wax. Further they seem to put a WRB on top of the Agepan which is already itself a WRB. It seems a little redundant to than install another WRB (Siga Majvest) over everything.
–Exterior to interior wall assembly–
– Exterior siding
– 1” x 4” strapping (spruce)
– Siga Majvest Water Barrier Housewrap
– Agepan Air Barrier WoodFiber breathable sheathing (3/4”)
– 14″ I-joist wall frame
– 1/2” OSB sheathing Vapour Barrier
– 2” x 6” frame wall
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Replies
Sounds like a Klingenberg Wall adaptation. I am just not sure of the extra 2x6 wall part.
Arnold,
Are you sure they are treating the exterior sheathing as the air barrier? if you hadn't said otherwise I would have assumed the OSB in the middle of the wall was being used as both a vapour retarder and air barrier.
Ottawa is cold (I think my bones still hold the memory of the cold from when we lived there), but is 20" of wall insulation really the optimal amount? How was it decided on? Seems like an awful lot.
The second WRB may be a response to the potential problems of sealing the Agepan. Perhaps they are also wary of relying on tape, rather than laps, for the flashing.
Arnold,
If you have questions about the air-sealing details of this wall assembly (for example, a question about where the air barrier is located, or how air leaks will be sealed), the person to ask is the designer of the wall (or your builder, if the builder is also the designer).
For more information on the Klingenberg wall, see this article: The Klingenberg Wall.
Norman Bunn -- Thanks for letting me know the type of wall assembly is seems to be based on. I will read more about Klingenberg Wall.
Malcolm Taylor -- I confirmed with the builder that the Agepan is the air-barrier. Like you I had also assumed the OSB was the air-barrier.
The amount of insulation in the wall assembly was determined base on PHPP he has done for the model he sells which are Passivhaus ready if one choose to certify.
Martin Holladay -- I had contacted the builder with the same question I posted here but I never got any response after previous back and forth regarding other aspects of the wall assembly.
Arnold,
It's a simple thing to move the air sealing to the OSB. I'd suggest it to the builder. A middle of the wall barrier is a lot more resistant to damage than one at the exterior.
One other small point: OSB doesn't meet the Ontario Building Code requirements for a Vapour Barrier without further coatings.
Arnold,
You want to choose a builder who is capable of answering technical questions. If your builder is unresponsive, it might be time to choose a different builder.
For more information on high-R walls, see this article: How to Design a Wall.
Malcolm Taylor -- I had suggested that to him especially that in Ottawa we can have almost a 100°C fluctuating in temperature from winter to summer which I'm concern how that would affect the air-barrier long term being on the outside wall but the builder never responded back. I suspect for the builder it's much easier and therefore cheaper to have the air-barrier on the outside wall versus in the middle. We plan on using FSC and urea formaldehyde free Plywood to meet LEED V4.
Martin Holladay -- That's the conclusion we have come to and at this point I suspect I will be doing a lot of the work myself.
Thanks again for the response.
Arnold,
From a building science perspective using OSB, or plywood as a vapour retarder in your assembly is a great idea. The Ontario Building Code However requires walls to have a vapour barrier. That means either applying a coating to the sheathing in the middle of the wall, or installing a VB under the drywall. If you decide to go that route, a variable perm membrane makes a lot more sense than poly.
Malcolm Taylor -- I wasn't aware of this aspect of the Ontario Building Code. Would Zip System sheathing be acceptable?
Also would it be worth taping the Plywood even if there will be another air-barrier membrane (Cosella-Dörken,Siga...) in the wall assembly to satisfy the Ontario Building Code?
I definitely don't want the air-barrier right behind the drywall because it's only a matter of time before the membrane get punctured.
A couple of things...
There is no siding on this assembly, just 1x4 strapping over Majvest?
Did you consider and reject a similar more-local (and lower priced) insulating permable fiberboard product such as Sonoclimate ECO 4 rather than the pricey imported Agepan? If yes, what was the deciding factor?
http://www.mslfibre.com/Upload/Documentation/T12670-106_SONOclimat_ECO4_En_08-15.pdf
Dana Dorsett -- There will be siding of course but it's simply not in the wall assembly since it can vary. I also have not really looking into specific products yet as I am still trying to decide what type of wall assembly I would like to use. The wall assembly above is one that the builder in question uses for all of his Passive House build it be custom house or one of their models.
Where available Sonoclimate ECO4 is something less than 1/4 the cost per unit area of 50mm Agepan (the most similar Agepan product it is most similar to). It is likely to be less expensive and definitely higher performance than the 20mm Agepan specified in the stackup.
It is manufactured in Louiseville, Quebec, only ~3-3.5 hours by truck to Ottawa- quite a bit closer than Agepan's plant in Meppen, Germany...
Arnold,
There are two main requirements for whatever you designate as your vapour barrier. It has to be under 1 perm, and it needs to be close enough to the interior to not cause condensation. Unfortunately, OSB, plywood and zip sheathing are all over 1 perm.
You can find this at 9.25.4 of the Building Code. it would be interesting to see how your builder gets around this when using the assembly he favours.
Multiple air barriers always improve a wall. I'd tape the plywood, designate it as your only air-barrier and treat the variable perm membrane as the vapour-barrier.