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My home was built in 1958, and has a stucco exterior. Sheathing is plywood and the exterior walls are 2x4 with traditional stick framing. We are in a canadian, cold climate.

I ripped out the drywall in one of the bedrooms with the intention of furring out to 2x6 and improving the insulation.

I've noticed something quite odd. On all the exterior walls the double top plate is notched into our unconditioned attic in between every stud!

No wonder my heating bill is so darn high. Given the number of notches, it's like have a small man door just circulating cold unconditioned air into our walls!

Has anyone heard of this before? Why would they have done this? My best guess was to provide air flow behind the sheathing?

I did notice most of our soffit was unvented, which I've remedied, but was speculating maybe this was a way to let the attic breath (despite there being vents, etc.)??

Anyways, unless someone knows of a reason I shouldnt immediately seal them, that is my plan?

top plate with notches

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  • is that picture upside down?
    – jsotola
    Feb 5, 2023 at 4:24
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    Negative. That top section is the ceiling drywall. You're looking at the top plate of the exterior wall.
    – Ray
    Feb 5, 2023 at 5:37
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    You'll get better insulation if you cover the studs with 2" foamboard and drywall, rather than furring the studs out to 2x6 and insulating between. Breaking the stud heat leak path is a major improvement. Alternatively, put 2x2's or 2x3's offset from the current studs, rather than on top of them (that way you don't need long screws to hit the studs through the foam when you hang a picture.)
    – Ecnerwal
    Feb 5, 2023 at 13:42
  • Interesting idea! How do you fasten the drywall through the foam board to the studs - you mention a longer screw...can you recommend one? Would the foam board be straps just covering the stud, or would you run full sheets covering the whole wall space, with conventional insulation still filling the stud space itself?
    – Ray
    Feb 5, 2023 at 19:46
  • @ray: with foamboard over the studs you can 1) install furring strips (1x2) vertically over the foambard exactly along the stud (mark studs on ceiling and floor, use laser level as screwing guide) or 2) instal 2x3 strapping horizontally against stud and horizontal foamboard between straps. No further vapour barrier sheet needed if taping seams. Install fibreglass insulation first between studs of course.
    – P2000
    Feb 6, 2023 at 0:54

1 Answer 1

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The stud space where the insulation will go must be vented to the outside, for moisture control of the space itself and the sheathing behind the stucco.

Since the stucco is gapless and does not breathe, venting such space would require rain capped vent holes, or -as in your case- passages into the attic (whereby no rain cap is required).

With an exterior finish of wood or vinyl cladding, plus a rain shield, venting can be accomplished by a gap-space between horizontally installed sheets of OSB or Plywood that are used for the sheathing.

With your proposed build-out to 2x6, new fibreglass insulation, and a properly sealed vapour barrier, you will be as good as a new build.

If there is no vent for the wall space, this is what one could end up with:

enter image description here

The sheathing, if moist from exterior or interior sources, does not dry out due to lack of venting, since it cannot dry through the cladding. (We don't know the exact cause behind the sheathing failure in this pictures, but it serves to illustrate that unmanaged moisture can be a real problem with expensive structural consequences.

https://www.greenbuildingadvisor.com/article/to-install-stucco-right-include-an-air-gap

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  • Do you know of any diagrams or instructions that specify this? This must be specific to old stucco homes without a air-gapped membrane behind?
    – Ray
    Feb 5, 2023 at 5:49
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    @Ray, I looked but found nothing. It's an important point though: do not simply stucco to replace other cladding, since the venting & moisture management is different.
    – P2000
    Feb 5, 2023 at 8:43
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    More research corroborates what you mentioned. The notches are definitely serving as a connection to led the stud spaces serve as a poor man's air gap for the stucco. One recommendation I've seen is to use Dorkin Delta Dry on the interior to create a barrier between the inside of the sheathing and the insulation. This let's the sheathing and stucco continue to breath, and prevents the insulation from pulling in any moisture. Your answer got me thinking on this, and Matt Risinger on the Build Show and the building science corporation suggest this approach. It's critical to leave an air gap. Thx
    – Ray
    Feb 5, 2023 at 9:07
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    @Ray glad this sent you in the right direction. As an aside, if you haven't yet, also look at the "strapping" technique to enlarge the cavity: 2x3 horizontally against studs, spaced every 16in.
    – P2000
    Feb 5, 2023 at 16:14

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