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Exterior Insulation with Original Framing / Sheathing

bigapplerunner | Posted in General Questions on

Hi All,

We are gut renovating a 150+ year old house in Northern NJ. We’re keeping the original framing and sheathing, but everything else is gone. Given the original sheathing is overlapping boards, we were going to spray 2″ closed cell foam on the interior to air seal, than add 3.5″ rockwool (studs are true 4″ so we’re packing out the walls with 1.5″ strips of EPS/strapping per one of the articles on this website (I forgot the name of the technique but it reduces thermal bridging).

The siding contractors are proposing to add 1/2″ rigid foam on the exterior side of the sheathing because they need a level surface for Hardie planks. We’re speaking to several of them and for some reason they all advise against using 2″ rigid foam on the exterior. While I haven’t confirmed, I suspect contractors use XPS on the exterior, which is denser than EPS?

We know interior spray foam + exterior rigid foam should be a no-no in order to avoid trapping moisture. However, I did want to see whether 1/2″ is not thick enough to be a problem?

I’m hoping we can proceed with the plan to do 2″ spray foam on the exterior + 1/2″ rigid on the exterior (which should also help a little bit with thermal bridging).

Thank you,

EDIT: I should say that my personal preference is to add 2″ rigid foam on the exterior and use fluffy inside the stud bays, however, I do not want to force the siding contractor to do something they clearly are not comfortable with.

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  1. Expert Member
    Akos | | #1

    Spray foam is not cheap, I would ask how much the inconvenience of 2" foam would cost.

    I've had very good luck air sealing older structures with a peel and stick WRB. A bit more work than sheet WRB but not too bad.

    P.S. I think the direct nail limit for Hardie is 1" foam, at 2" you are looking at strapping, which is a good thing as it can make a big difference if paint durability.

  2. dburgoyne | | #2

    Couldn't you look at adding 2 layers of 1" outside, overlapping, then a WRB and 1x4 rainscreen battens with longer screws into studs? Your Hardie boards can attach to the battens. I did this with 2 layers of 1 1/2 inch (3" total) rigid insulation and attached lath and 1" of stucco and it has held just fine and still no cracking 6 years later.

    1. bigapplerunner | | #3

      That would be my ideal scenario but contractors are reluctant and mention Hardie won’t allow more than 1” of rigid foam. I think one of the issues may be that studs are not 16” OC but rather all over the place? Either way, I will dig a little deeper next time I speak with one of them.

  3. matt2021 | | #4

    Thomas, when you say:

    "my personal preference is to add 2″ rigid foam on the exterior and use fluffy inside the stud bays, however, I do not want to force the siding contractor to do something they clearly are not comfortable with.

    . . .you perfectly summarize my predicament. I have not been able to find contractors who are comfortable with external insulation. For that reason, for my small project (a four-season room), I am going to go for Zip System R6 for the walls. The roof remains a problem. I'd like to add rigid insulation above the roof deck, but the contractors are not comfortable with it.

  4. Expert Member
    BILL WICHERS | | #5

    The usual way to go with the thicker rigid foam and hardie plank siding is to install veritcal 1x4s on the exterior over the rigid foam, then secure the hardie plank to that. This satisifies Hardie's requirements for warranty purposes, and also gets you a rainscreen gap of about 3/4" which is a nice plus too. That would be my preference, then batts in the walls in the usual way.

    With the slightly oddball depth you have to work with due to true 4" deep studs, I'd consider either open cell spray foam or low density fiberglass batts for 2x6 cavities and compress them down to fit what you have.


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