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Air barrier and insulation – Zone 3A Atlanta

pofe333 | Posted in Energy Efficiency and Durability on

Good morning all. I’m loving this site, so much good info here. It’s really helped in choosing the right contractors for a few projects now.

I’m hoping someone can help shine some light on some ways to best approach air sealing and insulating our 1950’s home in Atlanta as we replace the siding. We’ve opened up some walls and look to be faced with a few setups:

1. On the original house – From outside to inside looks to have aluminum siding —> paper/foil barrier —> wood siding —> plywood sheathing —> tar paper (on INSIDE of wall) —> wall framing

2. On the Addition – From outside to inside has aluminum siding —> paper/foil barrier —> diagonal 1 x 6

3. On the enclosed Sunroom – Not positive, but likely similar to the addition with masonite siding instead.

Long story short, we’re removing all exterior products, replacing any rot and re-siding with Hardie over a WRB.

Question is, if the sheathing is good and we air seal from the interior…

1. what should we do with the tar paper facing the inside, cut it out?
2. the old siding was nailed directly to the sheathing with a ton of nail holes through it and the tar paper now. How should we approach sealing those up?
3. would spray foam be the preferred insulation here to both air seal and insulate and take care of #2?
4. if spray foaming, should it be done prior to siding install so as not to expand through the gaps and push towards any newly installed siding?

Sorry for so many questions, I’ve read so much at this point I’m actually beginning to confuse myself. Hoping someone can weigh in with opinions. Thanks in advance!

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name?

    It's unclear whether you are working from the outside, from the inside, or both.

    If you're installing new siding, I assume that you are working from the exterior. So how are you going to get access to the stud cavities to install spray foam? Are you demolishing the interior drywall, too?

    In general, nail holes in sheathing are not an issue.

    Nor would I worry about a layer of asphalt felt between the plywood sheathing and the studs. It's unusual, but it's not a big deal.

  2. user-2310254 | | #2

    It would help to have some pictures. Pofe333, I thought you might be describing fiberboard or a similar asphalt coated siding material. On the sunroom, you should rule out asbestos before doing any demo.

  3. pofe333 | | #3

    Hey guys, thank you for the replies. My name is Evan.

    We've recently contracted with a company who will be demoing/installing the siding so their work will be done from the exterior. The plan for us then is to subsequently remove the drywall from the interior to air seal and insulate as it turns out to be much less costly than removing and reapplying all exterior sheathing.

    I could snap some pictures tonight and post back if it would help as mentioned above. I'd love to know why the tar paper is on the interior of the wall. We've had some musty odors and I'm assuming somewhere between the multiple barriers there has been some slow drying materials.

  4. GBA Editor
    Martin Holladay | | #4

    You're basically doing a gut rehab + siding replacement. That gives you the opportunity to rethink everything.

    The best possible approach is to install an adequate thickness of rigid foam on the exterior side of your wall sheathing. For more information, see these two articles:

    Roofing and Siding Jobs Are Energy-Retrofit Opportunities

    How to Install Rigid Foam Sheathing

  5. Expert Member
    Dana Dorsett | | #5

    Current IRC 2015 code minimum for walls in zone 3 for 2x4 walls is R13 (cavity) + R5 continuous insulation:

    That results in about a ~R15-R16 "whole wall" value after factoring in the thermal bridging of the framing, and adding in the thermal performance of the wallboard/sheathing/siding, etc.

    According to Table 2, p10 of this document, bumping that to R20 whole-wall is likely to be financially rational:

    Rigid foam isn't cheap, but using reclaimed roofing polyiso at
    In Atlanta you can probably get it from these guys (at about half-price) any day of the week:

    They're selling bunks of 24 sheets of 2" x 4' x 8' for $400 is <$17/sheet.

    Assuming R11.5 and 32 square feet per sheet that's less than 5 cents per R per square foot, which is comparable to or cheaper than what you'd be installing in the wall cavities. Assuming even a fairly heft scrap rate for windows doors and a few damaged sheets that might come in at 6 cents /R-foot after scrap.

    If you're not swapping out the windows and installing new window flashing, if the existing window flashing is good it's probably best to use a crinkle type housewrap (eg Tyvek DrainWrap) and place it between the structural sheathing and the exterior foam, carefully lapping the flashing to the house wrap.

  6. pofe333 | | #6

    Sorry for the late response on this, work has been crazy.. Attempting some pictures below and have been doing some talking and digging around in the meantime.

    From who we've talked to so far and what we're getting into I think we might be most comfortable on this house shooting for a level of "good to better" by leaving the existing sheathing in place and airsealing where possible. We've had enough redo's with this place. Mostly not comfortable with knowing that it would be executed correctly and wind up causing other issues. .

    What's stuck out to me as a safe option so far is to:

    a) tape the plywood seams on the exterior as well as sill/foundation area (Siga) before new wrap is installed
    b) foam/caulk penetrations and gaps on the interior side of the wall for the area sheathed with diagonal 1x6
    c) remove tar paper on inside of the wall and add mineral wool between that and the drywall to provide some insulation and noise reduction to the exterior
    d) caulk drywall seams

    While not cutting edge, I feel like that puts us in the "better" category. Penny for your thoughts?

  7. GBA Editor
    Martin Holladay | | #7

    It's your house. There's nothing wrong with your plan.

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