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Insulating 100-year-old Stucco

john_m1 | Posted in General Questions on

For Martin Halliday.

You assignment for me was to find out what was behind my walls.
To recap, I have an old stucco 4 square, where the previous owner put blown cellulose into the wall cavities.

I opened the access panel to the bathroom and pried off some lath that wasn’t removed from an early 60s remodel.

What I saw was the cellulose (expected) and some sheathing that appeared to be horizontally oriented.  There was maybe a 1/4 inch gap between the two boards I saw exposed.  I scratched a bit (gently) in that gap, and it looks like there is a second set of sheathing.  I didn’t feel anything rough like the keys from the stucco.  Its a tight space so I can’t view much.

If there is this sheathing and I can vacuum out the cellulose what reasonable options do I have?  Seems like a trade of rockwool for the cellulose, buys me little other than some safety.  No way I am doing closed or open celled foam.  I would do EPS but it would leak like crazy.  If taking the exterior walls down is on the table what does that open up for me?


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  1. Expert Member
    Dana Dorsett | | #1

    >"If there is this sheathing and I can vacuum out the cellulose what reasonable options do I have? "

    Unless there is something going wrong with it, keeping the cellulose (or dense-packing it in place, if it's too low a density and has settled a bit) is the greenest thing to do here. Cellulose is more air-retardent than rock wool, and since it's "sequestered carbon" it has a negative carbon footprint.

    If you're going to strip the stucco there may be a rationale for installing continuous insulation over the exterior of the sheathing to improve thermal performance, but if the stucco is in good shape and the moisture content of the sheathing isn't going off the charts it's usually more environmentally friendly and better bang/buck to focus on improving other aspects of the house.

    1. john_m1 | | #2

      Thanks for your prompt reply. I want to do a blower test, but I think it will be more fruitful after I've done more work. I am busy on the obvious stuff, like EDPM weather stripping around doors and windows and where useful, repairing/replacing storms (about 1/2 wood and 1/2 aluminum)
      Next will be air-sealing the attic.
      In my attic should I cap the cavities?
      Should I consider making the attic conditioned space?

      I am going to keep plugging away.

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