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Community and Q&A

Wall cavity insulation

Mill_house | Posted in General Questions on

I live in climate zone 7 (Canadian prairie).

I’ve been doing some demo work on the inside of my old story and a half house (1940ish).  No eave extension, no soffits.  

I see there’s no insulation on the main floor – just some wood chips settled at the bottom of the stud bays and some silver/black/silver paper and thick cardboard type sheet stapled across the wall, topped with plasterboard.

The house exterior is original stucco.  I can’t tell if the stucco has felt under it.  Based on the areas of stucco that are chipped, I don’t see any evidence of felt.  The house has multiple air leaks and the flashing above the new windows is old.    

I’d like to add insulation to the stud bays.  My approach would be to add roxul then poly, then drywall.  I am concerned that moisture would still be allowed to travel through the stucco and sit in the stud bay leaving it damp and the house not smelling so good.  

Couple of questions:
If I choose to leave the stucco as is – how should I finish the inside in terms of insulation, poly/no poly, etc?
I understand there’s some kind of flashing detail called dam flashing (?) where the flashing curves up at the ends and prevents water from dripping down the sides of windows. I’m wondering if that can be installed after the fact – perhaps as an add full length strip under the original flashing?
Exterior details such as slight cracks between the stucco and the windows, stucco and chimney, should be sealed with some kind of exterior caulk, correct?
And lastly, ripping off the stucco down to the sheathing would be a big expensive job.  Would reassembly be: something like house wrap, rainscreen gap, then stucco or fiber cement siding?  Do the windows stay in place during this process or do they need to be removed to have the house wrap done?  And then interior wall assembly could be the regular old school batt insulation, poly, drywall.  

Thank you,

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  1. GBA Editor
    Martin Holladay | | #1

    Lots of questions. You may want to talk to a local remodeling contractor about all of your options.

    If your house has no asphalt felt (or water-resistive barrier) behind the stucco, your walls are indeed at risk for water-entry problems. How acute these problems are depends on the width of your roof overhangs, how exposed your house is to wind, and the annual rainfall in your location.

    In general, caulking exterior cracks is not a good way to solve water-entry problems in walls.

    Removing the stucco and installing a proper WRB, followed by a rainscreen gap and new siding, would indeed be the best way to proceed. You're right that this work can get expensive.

    If you can afford to do the work, it's a good time to replace your existing windows with new windows installed in properly flashed rough openings.

    Your choice of insulation isn't very important. A wide variety of insulation materials will work. Whatever you do, pay attention to airtightness every step of the way.

  2. Mill_house | | #2

    That's the problem Martin. I come to this site and read all kinds of things that make sense. But when I talk to contractors about ways I might want to do things - based on what I've read here - they look at me like I've got two heads. So with GBA introducing concepts/ideas that i certainly haven't heard before, i'd like to get some info from the source prior to having additional discussions with contractors.

    I will continue to read articles here and try to learn as much as possible. However, in the meantime, even though this is not my preferred solution, I'm wondering if you could let me know what makes the most sense if the stucco remains as is:
    Could I insulate with batt roxul insulation in the stud bays with poly over? If using poly is not a good idea should I use membrain instead?


  3. GBA Editor
    Martin Holladay | | #3

    Q. "Could I insulate with batt Roxul insulation in the stud bays with poly over?"

    A. Yes.

    Q. "If using poly is not a good idea, should I use MemBrain instead?"

    A. Of the two options, MemBrain is more forgiving. If it were my house, I'd install the MemBrain.

    Remember, materials selection is only a small aspect of a good installation. More important is the quality of the work. The mineral wool batts need to be carefully fitted to the stud bay widths, and trimmed to fill all voids. The MemBrain needs to be installed with attention to airtightness. Ideally, you'll use airtight electrical boxes, or have a plan to limit air leakage at electrical boxes. All of these details matter more than the choice between poly and MemBrain, or the choice between mineral wool and fiberglass.

  4. Expert Member
    Peter Engle | | #4

    Insulating behind an existing stucco wall, especially one with no felt and/or sheathing is a very high risk operation. Even more so with a house that has no eave extensions or soffits as you desribe. As the house stands now, the wall cavities probably get wet every time it rains and only dry by virtue of the heat and airflow through them. If you eliminate the heat and airflow, the walls can rot very quickly.

    All stucco leaks. Stucco without felt or any sort of seals and flashings around windows leaks even more. Unless you are in a near desert area, I would not recommend insulating without remediating the stucco. The stucco is 70 years old now. It's lived a good, long life and has provided good protection for the house as it was built 70 years ago. We've learned a lot in 70 years, and maybe now it's time for an upgrade.

    In any major renovation project, you should start with the outside rather than the inside. Once you've got a watertight, durable and properly insulated shell, you can feel comfortable that your investment in the interior won't go to waste.

  5. Mill_house | | #5

    Thanks for the answers. Because this is all new to me and because anyone I’ve talked to here has not suggested checking the stucco out etc I’ve aleady begun demolition on a few rooms. And winter is coming. Zone 7 gets really cold. So it looks like I’ll have to do something to get by this winter. Can’t leave the walls open.
    If I go the route of installing roxul and membrain will that system work when the outside walls get down (wrb, rain screen and cladding)?
    Does Hardie board or equivalent allow install in the case of no eave extensions (meaning how much space does the board need at the top of the wall in order for it to be sealed)?
    I feel like I’m in for a miserable next 12 months.

  6. GBA Editor
    Martin Holladay | | #6

    In my very first answer, I pointed out that your walls are at risk of water entry. Peter Engle is giving you similar advice.

    It's possible to do the work in stages -- to insulate now, and to address the lack of a water-resistive barrier (WRB) next year.

    Roof overhangs are protective, no matter what type of siding you choose to install -- stucco, HardieBoard, or other types of siding. It's possible to extend your roof (using either framing and sheathing, or stiff metal flashing) if your roof overhangs are skimpy. For more information, see "Every House Needs Roof Overhangs."

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