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Community and Q&A

Air seal and insulation for existing 100-year-old exterior walls

Wooba Goobaa | Posted in Energy Efficiency and Durability on

Wall assembly is (outside to in) …  cedar clapboard, tar paper, pine board sheathing, 2×4 wall, lathe and plaster.  IR scan confirms no insulation in the wall cavity.  The walls will not be opened.

BSC teaches me that control layer priority is water, air, vapor, thermal.  Lightbulb for me … air control is more important than thermal (R value).

So what to do with these old walls which have been doing their thing for 100 years?  Would blown-in cellulose screw them up?  Retain moisture in the wall, exterior paint lifting, rot, etc?

A couple tactics I’m considering.

– Aerobarrier the house, improving the air control layer of these old walls.

– Blown in / dense pack mineral wool.  Yes I did mean mineral wool … evidently it can be blown-in (though rarely discussed). Won’t absorb water,  lets vapor pass to dry into the interior.

What do you think?

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  1. Nathan Shirai | | #1

    What city and state are you in, and do you happen to know your climate zone? The colder the climate, the more careful you have to be to avoid screwing up the hygrothermal dynamics of your building (in other words, the way heat and moisture flow) - while drafty and uncomfortable, this type of assembly has beastly drying potential. Insulation and air sealing reduces the drying potential by lowering the amount of heat and air available to the assembly. The complexity of the remedy will depend on the level of risk particular to your building with respect to location and climate.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Wooba Goobaa.

    Dense pack cellulose is probably the most common type of insulation used to retrofit homes like yours. Not only does dense pack cellulose add R-value, it also has some air sealing value. Unless you have leaks from bad flashings, fo example and the walls are getting excessively wet, you shouldn't have issues with dense pack cellulose. Check out this article: How to Install Cellulose Insulation

    Many green builders also prefer cellulose because it is one of the more environmentally friendly options.

    AeroBarrier brings a new option to the table. I have not seen any AeroBarrier work done in any very old houses yet, but would be interested to hear how effective it is in tightening up a home like yours.

  3. Wooba Goobaa | | #3

    Zone 5 Eastern MA.

    Yup been up and down all the cellulose advice and warnings.

    I'm contacting a few AeroBarrier service providers to see what they have to say. I think the house first needs to be brought to a minimum ACH before they will attempt it. NS Build did a piece on an older brick home that was not able to be insulated, but they had some success with AeroBarrier.

    Does anyone on GBA have experience with blown mineral wool into old walls? I see American Rockwool offers a granular form.

  4. Expert Member
    Dana Dorsett | | #4

    >"Does anyone on GBA have experience with blown mineral wool into old walls? I see American Rockwool offers a granular form."

    MA high performance home architect/builder Steve Baczek uses it from time to time:

    My long time biz-partner retrofitted his early 19th century antique in Newton with blown rock wool, but that was ~40 years ago. It's held up well- there was no settling detected in a recent peek with an IR camera when looking for empty bays to be filled with cellulose (several short bays above windows had been missed), but can't say for sure what product that was or what it's R-value is.

  5. Wooba Goobaa | | #5

    A few of the exterior walls have been opened. Unsurprisingly, some surprises revealed.

    All the exterior stud bays have a heavy paper air (?) barrier installed. This will obstruct any attempt to blow whatever into an exterior wall stud cavity. Also note evidence of it having been wet (typical of all stud bays we opened) ... tilting me more towards mineral wool versus cellulose.

    @Dana. Why did your friend go back into his blown rock wool retrofit with blown cellulose?

  6. Jay Thomas | | #6

    Why not just insulate from the outside?

    1. Wooba Goobaa | | #7

      Meaning blow in from the outside? Same issue ... half the stud cavity is not accessible.

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