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1890s house question 2: How to insulate walls?

Joel_K | Posted in Green Building Techniques on

I posted this questions at end of my long “how to insulate new cathedral ceiling in old 1890s house”, but figure that people are tired of that thread so didn’t read the new question:

My helper installed kraft-faced R13 fiberglass bats in the attic I am remodeling.¬†The interior walls were orginally plaster-on-wood-lath and¬†stuffed with blown-in cellulose and/or mineral wool. We will sheath them with drywall, this time, though. The exterior is aluminum siding over clapboard over 1″ x 6″ board sheathing. I am not sure if there is any house wrap or paper over the sheathing or under the alumimum siding.

??? Now that I have read on GBA about advantages of hygroscopic cellulose, would it be better to use dense-pack celluose instead?

??? If we install dense-pack cellulose in the walls, I may install with Insulweb netting and then fasten the drywall. Should I install a smart vapor barrier or just leave cellulose in direct contact with the drywall?

??? Can we drywall with mold-resistant paper drywall? What about fiberglass-faced drywall? I will have to check the vapor permeability ratings of these different types of drywall.

Thanks for responding …

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  1. Expert Member
    Dana Dorsett | | #1

    Most 1890s houses were built with full-dimension 2x4s, which are too deep for the nominal 3.5" loft of an R13 batt, and the bays are narrower than standard 16" on center milled lumber. Blown insulation will always find a near-perfect fit with no gaps.

    Dense packing will make it measurably more air tight, stuffing the cracks in the plank sheathing with cellulose. With the inherent back-ventilation of aluminum siding it won't need an interior vapor retarder in climate zone 5 & lower as long as it's painted with standard interior latex on the interior. In zone 6 & higher a smart vapor retarder or vapor barrier latex primer on air-tight wallboard would work.

    It's possible to dense pack it without insulweb if you install the wallboard first.

    Any type of wallboard is fine if it's going to be painted. The vapor permeance of standard latex paint is much lower than any grade of wallboard, and will ultimately determine how fast moisture can move through the wallboard via vapor diffusion.

  2. Joel_K | | #2

    Dana, if I decide to add a vapor retarder, what kind? Intello? MemBrain? Also, how best to install drywall over the retarder? Because we removed the lath and plaster, which was thicker than drywall by about 1/4", we have to fur out the walls to allow 1/2" drywall to be flush with door jambs. Should I apply the retarder to unfurred studs and then fur out, leaving a thin gap between drywall and retarder?

  3. Expert Member
    Dana Dorsett | | #3

    Even though it's a bit hefty I would expect going with 3/4" gypsum would be easier to install than furring out all the studs. But if you're furring it out anyway...

    If using Intello, it's possible to use the Intello in lieu of mesh for dense packing. If you go that route, install the Intello on the studs, and secure it in place with your 1/4" furring. Tips on blowing behind Intello can be found here:

    If you have 1/4" furring on every stud edge and 16" o.c. spacing of the studs you won't need the counter battening mentioned in the 475 article, but you will need to roll it flat before installing the gypsum.

    MemBrain is unreinforced 2-mil nylon- way to stretchy/flexy to dense pack against on it's own. Install it between the furring and gypsum, and don't dense pack until gypsum is up. Alternatively, blow in mesh held in place by the furring, roll flat, then install the MemBrain.

    If blowing behind mesh or Intello there will be small vertical channels behind the gypsum on both sides of every stud, but not big enough to be concerned about. Both will need to be rolled flat to relieve the pillowing out in the middle of the cavity between studs to get good wall flatness.

  4. Joel_K | | #4

    Thanks Dana, very helpful. We had a contractor review some interior structural changes we wanted to make (to make sure up to code) and he commented that because we removed so much of the interior plaster and exposed all the studs, the building inspector would require us to try to achieve R21 insulation and that, with 4" deep walls, the solution would be closed-cell foam! I dislike foam -- for its toxicity -- so I am considering furing out the walls another 1.5" to make them deep enough for cellulose or fiber insulation.

  5. Expert Member
    Dana Dorsett | | #5

    If you go with rock wool R23s compressed to 5" instead of 5.5" you'd only have to give up an inch of depth to hit R21. Cutting 2" wide strips of 1" foil faced polyiso tacked to the framing edges would cut the R value of the framing fraction by well over half, which is much better performance than R21 or R23 with 5" or 5.5" of all-wood for that framing. You'd have to use longer fasteners for the gypsum board, but not crazy-long. Dense packed fiberglass would give you the same R as the compressed rock wool, but foam edge strips makes it complicated to blow in mesh.

    If you're going to fur out to the full 5.5", use "Bonfiglioni strips" fashioned from 1x strapping and 2" wide strips 3/4" polyiso rather than a milled 2 x 2s. See:

    It's easy to make clean cuts of foil faced foam using a 3-4" steel wallboard taping knife that has been sharpened on the edges.

    A layer of 3/4" or 1" polyiso would roughly double the R-value of the framing fraction, a measurable improvement in "whole-wall R".

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