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Community and Q&A

Insulating Old Walls Without Sheathing

chenson93 | Posted in Energy Efficiency and Durability on

Context: Live in Benton, AR. Climate zone 3, humid mixed-weather (mid 90s this week but just had a foot of snow in February). House built in 1964. Pier foundation, can walk upright in about half the crawlspace before hunching over some. Cinderwall footing.

Started updating what is to be the master suite. Removed wood paneling to find horizontal tongue and groove system covered with old wallpaper. Boards are about 3/4″ thick. Was able to remove a loose, mostly split board to find that there was no exterior sheathing layer or insulation, went straight to wood clapboard siding.

So my question, should I and if yes, how should I insulate? I’ve seen posts and videos where the tongue and groove or shiplap is the exterior sheathing outside of the studs, but I have yet to see anything where there is only one layer and it’s the interior. I believe the best and most practical option is blowing in cellulose because I do not want to remove every board for practical reasons and I do not want to remove the clapboard and add exterior sheathing for budget reasons. Would blowing cellulose against the clapboard bow and warp it as foam would? Or would I need a barrier of some kind?

Thanks for taking the time to read and answer. I now it’s a long post but I’m new here and wanted to give as much detail as I could think relevant.

EDIT: Forgot to mention my plan is to sheetrock the room

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  1. Expert Member
    Akos | | #1

    I would read through this:

    You can't insulate just by blowing in cellulose, that is asking for trouble.

    Also be careful with removing the T&G, it might be providing the structural bracing for your walls.

    1. chenson93 | | #3

      Thanks for the quick response. That was one reason why I didn't want to remove the t&g. That and the wood is still in great shape.

  2. Patrick_OSullivan | | #2

    Right now you have a very inefficient, but forgiving, wall assembly. Your siding can get wet, but that's okay, because there's so much air moving behind it that it will dry out.

    Once you have insulation, you're limiting that drying potential, and it can have consequences [unless designed for properly].

    This article suggests an air space behind the siding is ideal:

    One of the comments references this study, which shows how blown in cellulose without sheathing can lead to excessive moisture in the siding:

    At a minimum, you have to fully open up one side or the other to make a good attempt at improving this situation.

    1. chenson93 | | #4

      You're right, it doesn't help energy usage much but it sure has kept itself sound and stable throughout the years. I did see that article but was not keen on having to remove all the t&g in order to create that insulation setup.

      I can see from an interior open bay that air flow is coming from the attic. And since most heat is lost up there anyway, do you think air sealing the room well (corners, windows, etc..) and adding r value to the attic instead of the walls would be a solution to avoid removing either the clapboard or t&g, sheathing and insulating, then reattaching?

      I know I'm not going to get anywhere near the r value that is recommended for my climate but something is better than nothing.

  3. Patrick_OSullivan | | #5

    > I can see from an interior open bay that air flow is coming from the attic. And since most heat is lost up there anyway, do you think air sealing the room well (corners, windows, etc..) and adding r value to the attic instead of the walls would be a solution to avoid removing either the clapboard or t&g, sheathing and insulating, then reattaching?

    There are four main things that need to be controlled in the building envelope: water, air, vapor, and thermal. Water, if not managed, will destroy a building, so make sure that's in order. Air is your next thing to tackle.

    You mention "I can see from an interior open bay that air flow is coming from the attic." That is a good observation. But before you go adding insulation in the attic, normally you would work to control the air moving from the foundation to the attic as that, by means of the 'stack effect' tends to move around a lot of energy.

    "Normally", a reasonably robust sheathing layer means you can air seal the tops and bottoms of stud cavities and achieve some improvements in air movement. Your lack of sheathing makes this a bit trickier.

    I think it would be good to understand your goals. Why do you want to insulate, etc.? GBA attracts a lot of people in more northerly locales where the calculus is a bit clearer. Climate zone 3 might not be as straightforward...

    1. kaspergruszczynski | | #7

      Adding to Patrick's comment, for Climate Zone 3 you may not need to insulate much. There is nothing specifying you have to install the insulation in the stud bay. You could, for example, attach the necessary R-value of rigid foam on the inside of the T&G (tape the seams and seal to the floor and ceiling) and then drywall over top that.

      With regards to insulating the attic vs the walls. Any insulation will help your homes thermal performance but air leaking out into the attic is different from radiative loses through the envelope. Sealing and insulating the attic will help with heat loss through the attic but will not change the fact that the walls will feel cold in the winter. That being said, any efforts are better than none. I would recommend you read the other recommended articles and see if that offers any other clarity.

      One last comment, it seems that you are renovating a room at a time so it will be harder to address your homes energy performance, but every effort helps. With regards to the walls, you can of course leave the un-insulated and then insulate when you decide to reside the home.

  4. GBA Editor
    Kiley Jacques | | #6

    Welcome to GBA! Your question is a great one that comes up on a somewhat regular basis, which is what instigated the article you were pointed to. Here are two more for your reference:
    Key Considerations When Insulating Old Walls and

    Intro to the Control Layers of a Wall.

  5. chenson93 | | #8

    Thanks for all the responses everyone. Doing some more research and getting creative, I think I have a new plan. So my idea is to remove the top 3 or 4 t&g from the top of the wall giving me about a foot of stud bay access. Then, I'll take some crawlspace dimpled drainage mats and cut them to fit in the stud bay. From my top opening, I can lower a piece down with the dimples facing the exterior siding, creating the air gap to promote drying. Since I won't be able to nail this in place to the whole stud, I imagine once I blow in cellulose or fiberglass that it will fill the cavity and hold the drainage mat against the siding. I saw a video by Matt Risinger where he used a similar (but surely more expensive) product that created the same effect of an air gap but then he used closed closed foam since he had access to the whole stud bay. It's a dimensional 4" bay so that will give me the 1/2" air gap then 3.5" insulation. It's balloon framing so there's air flow from the attic.

    Please let me know what y'all think of this possible workaround.


  6. Expert Member
    Akos | | #9

    You are in a hot/humid climate. Most of your cooling load comes from air leaks, heat from the roof and from any un-shaded east/south/west facing glazing. Wall insulation hardly affects your cooling load.

    Your time and money is best spent of tightening up your place.

    I think the simplest is to cover your T&G with 1" of foil faced polyiso with taped seams as the air barrier plus a bit of insulation and install your drywall over that.

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