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Community and Q&A

Double Stud Wall in Walk-out Basement

HGWNwRfBXY | Posted in Green Building Techniques on

Hello All! I have recently inspected a house that has had a basement renovation completed. The basement was finished with a secondary wall built adjacent to the original foundation/pony wall.
The foundation wall is covered in EPS insulation and appears well detailed with tape and spray foam, as well as being well sealed to the bottom plate of the interior wall and pony wall. The wood framed pony wall has Roxul batt insulation in it. The secondary 2×4 interior wall has nothing in it.
The problem is the original 2×6 pony wall does not have a vapour barrier installed over it. I measured for wood moisture content on the original sheathing (1×8 fir) and its at the red flag zone of about 18% in many locations. The original wall being stucco clad with 1×8 sheathing and ashpalt paper is quite air leaky.

I’m not entirely sure what to suggest to the home owner’s except a complete tear down of the secondary wall or at least a partial tear down in order to install a vapour retarder/air barrier on the original 2×6 wall. Along with drywall to help support the retarder (as well as fire protection) in this leaky wall…Any other ideas? Is there an vapour retarder/air barrier that can be used that does not need the addition of drywall? I suppose one can use plywood…

Vancouver, BC

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  1. GBA Editor
    Martin Holladay | | #1

    To summarize, you observed that the above-grade portion of a basement wall consists of 2x6 framing filled with Roxul batts. The exterior board sheathing has a high moisture content.

    The cause of the problem, your write, is that the wall "does not have a vapor barrier." I disagree.

    In your proposed remedy, you come closer to the truth, when you propose that the solution is "to install a vapour retarder/air barrier." That's the key -- the wall needs an air barrier.

    It's probable that before the basement was finished, there was no interior finish at all on the framed wall. Even if there was a layer of drywall, my guess is that the drywall leaked air.

    The mechanism whereby interior moisture is reaching the sheathing is via air movement, not vapor diffusion. The Roxul batts do little to slow the exfiltration of warm, moist interior air that leaks through the wall, bringing moisture to the studs.

    In theory this problem can be solved by building an impeccable interior air barrier, but in practice that is difficult. Considering the symptoms, the best solution is the use of an insulation type that is not air permeable: spray polyurethane foam or a continuous layer of rigid foam with taped seams.

    Finally, unless you have been using your moisture meter for years and you are familiar with all the ways that these devices can provide misleading readings -- there are lots of them -- I suggest that you don't jump to conclusions about the actual moisture content of the sheathing. The MC may be at 18%, as you report, or it may be higher or lower. The more data you can gather (including exposing some of the sheathing to inspect it), the better.

  2. HGWNwRfBXY | | #2


    :) Agreed on the air barrier vs vapour barrier. Had this basement been inspected during construction it would have had 6 mill poly on the walls, right or wrong.

    Im with you on the need for an air barrier vs a vapour barrier as it is defined by our local building codes.

    Im not a huge fan of spf en masse as I have inspected many installations where the foam has separated from framing members or sheathing. Im also concerned with its possible environmental and human health consequences.

    Unfortunately my testing appears accurate as the sheathing is indeed damp. I have used this type of gear for a long time as a cabinet maker, renovation contractor and property inspector.

    Im sure there would not have been any insulation in the walls originally as well. Most homes of this vintage in our climate did not. Which is why the walls are so nice and dry but not particularly energy efficient! Air barrier on the outside of this one will be tough, so we are likely going to have to retrofit the interior. Im thinking 1.5" EPS in the rim joist foamed in and either Membrain on the 2x6 pony wall or maybe more 1.5" EPS foamed in between studs or over them or sealed drywall over all of it... I get the concepts its just finding the easiest way to install this through the existing secondary 2x4 stud wall which wed like to keep up.



  3. GBA Editor
    Martin Holladay | | #3

    There is one other factor that is likely contributing to the damp sheathing, and that is the unfortunate choice of stucco -- the slowest drying and most problematic type of siding on the market today.

    I always advise anyone considering the installation of stucco to include an air gap between the stucco and the sheathing. I imagine that there is no such air gap in the house you're investigating.

    Here's more information on the current epidemic of stucco problems: To Install Stucco Right, Include an Air Gap.

  4. HGWNwRfBXY | | #4

    Well to be fair it is the original stucco and no there is no air gap.

    Current code does require a rain screen for most types of wall assemblies here in BC. You might recall the infamous leaky condo problem here, I forget how many billions that cost everyone... I know this type of cladding is considered a so called "reservoir cladding" but I have not seen many problems with rotted walls with traditional lime, sand and cement stucco as long as they were detailed correctly.

    Vinyl based stucco systems on the other hand have caused a lot of problems in part because of our continued worshipping at the alter of 6 mil poly...

    Back to this wall assembly, its another reason why I think EPS is ideal as it provides decent vapour permeability and as such some drying potential...


  5. HGWNwRfBXY | | #5


    Researching more about using EPS board in between the studs that the same rules would apply regarding vapour retarders as would apply to insulated sheathing assemblies. That being the case am I reading correctly that I still need a vapour retarder for our marine climate? I am thinking 1.5" EPS at r5.6 and putting the r22 Roxul back in place, having the Roxul batt hang out over the edge of the stud bay is not a big deal in this case, since it will be behind the secondary wall.



  6. user-1061844 | | #6

    Making an airtight barrier on the 2x6 walls with a intelligent vapor retarder would be a good step in the right direction. At we have started to import INTELLO plus which is a reinforced intelligent vapor control layer - open in summer so structure can dry inwards, closed in winter so it is protected. Of course this membrane would have to be tape/adhered in an durable airtight manner to the slab, any penetrations and the floor above, we would of course recommend doing that with the Pro Clima tapes that we carry. A concern would be that you note that the exterior wall is not airtight and thus your Roxul insulation will still get windwashed and might even get exposed to wind driven moisture.

    INTELLO has a class B rating in Germany and we are in the process of getting American testing done. It doesn't accelerate fire but is combustible at very high temperatures.. As it shouldn't be permanently exposed to light/ UV exposure since that will eventually break down any sheet good it would be prudent to cover it with sheetrock eventually. If there are no windows in the basement the material could be left exposed longer.

  7. GBA Editor
    Martin Holladay | | #7

    If you plan to install 1.5 inch of EPS directly against the wall sheathing, you shouldn't install any interior polyethylene. Installing gypsum drywall according to the Airtight Drywall Approach would, of course, be a good idea.

    If your local building inspector insists on an interior membrane, choose MemBrain or (as Floris suggests) Intello Plus.

  8. HGWNwRfBXY | | #8

    Floris - Thanks, your products look great I will add your company to my list of vendors for sure. Its quite difficult to purchase Membrain here dealers need to special order it in.

    Irregardless I was hoping to avoid trying to install a membrane of this type as it would be very difficult to install without taking down the existing wall, studs and all. Which I am trying to avoid as it is a finished area. I was hoping I could remove the upper half of the wall remove the existing batt do the EPS thing and replace batt. As is detailed in the attached drawing.

    Something I could do much more easily than trying to detail a floppy air barrier while working through 16" OC studs... :(

    I could also leave the batt where it is and probably do a decent job putting the foam over top of the 2x6 studs as well. I could then use a thinner EPS like 1" and still provide the air barrier with all seams foamed.

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