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Community and Q&A

Insulation, Sheathing, and WRB for Historic House

Codhod | Posted in General Questions on

Hello, my name is Cody and this forum has been extremely helpful. Thank you!

I’m currently fully renovating a single story (full unfinished stacked stone basement) home built in 1898 in Climate zone 6 (South Dakota) the house is part of a historic district so im limited on cladding choices etc. I have approval to use LP Smooth lap Smartside with a 5.25” reveal.

This is a full renovation, house is fully gutted to the studs, siding removed, new windows and doors on order. So I have options (which is probably the problem I am having!)

True 2×4 exterior walls with true 1x on diagonal sheathing on the outside. (gaps as large as 3/4”)

I’m struggling with what to do with insulation/sheathing/wrb.

I Think I am narrowing it down to my wall stack being sheetrock, 2×4 studs, max rockwool for 2×4 wall, 1x sheathing, 1” foil foam boards taped seams, tyvek wrb, lp smartside nailed through the 1” foam to studs.

am I wrong? Other options? Id like to keep the exterior foam to 1” so I can skip the furring strips required for LP. Ive read that tyvek is ok for rainshield between LP and foam?

do I want a vapor barrier behind the sheetrock?

I realize this is like a bare minimum for insulation but Id like to skip the cost of additional 2×4 on 16” oc furring strips & labor.

Thanks in Advance


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  1. strausjw | | #1

    Hi Cody,
    I think you will have a couple of issues with your plan. 1" of exterior foam will not be thick enough in climate zone 6 to control moisture. Also the Rockwool batts will not fill a dimensional 2x4 cavity, unless you are planning to buy 2x6 batts and squish them down. Some sort of blown fiber may be better for filling these cavities. You would not want a vapor barrier behind the sheet rock if you pursue the exterior foam as is would prevent both interior and exterior drying . Where/What is your air barrier? At the drywall? At the exterior face of the sheathing? There are many articles on this site about minimum foam thickness. Placing the foam on the interior is always an option as well, of course with its own considerations.

  2. Expert Member
    Akos | | #2

    I'm in the land (zone 5) of 2x6 walls with interior poly and R5 exterior rigid and they are holding up just fine. If you up the thickness of the rigid, you can skip the interior vapor retarder but there is really nothing wrong with building a wall with thin rigid.

    Generally the robustness of a wall depends more on how well you air seal the assembly and how careful you are with flashing details around the windows. As long as those detials are sorted, you wall will work great.

    Take a look at table 2(B) here for ratio of exterior insulation and the type of interior vapor retarder you need:

    Dense packing the walls is the simplest way of dealing with non standard stud spacing. The other option is to get 2x6 fiberglass batts and squish them down into the 4" bay.

    I've also had good luck with 4" thick 24" wide Rockwool AFB (very common for commercial construction), turn the batts sideways and cut them to size to fit between the studs. This is much easier and less waste than trying to trim 1" off the side of a standard batt.

    The simplest way to air seal diagonal sheathing is to use a peel and stick membrane for your WRB. A bit more cost and labor to install but you end up with a very tight assembly.

    The other option to air seal is to flash the interior of the stud bays with 1" of closed cell spray foam. Besides air sealing this has the benefit of reducing your stud bay depth so you can insulated with standard 3.5" batts. The extra R value of the SPF plus your 1" of rigid means you can also skip the interior vapor barrier and go with just pained drywall.

  3. Codhod | | #3

    How about skipping the exterior foam insulation and just install new osb /wrb to the diagonal 1x sheathing and then flash and batt the inside walls? I understand thermal bridging is an issue.

    Only reason I would re sheath the outside is so the spray foam would not protrude the large gaps in the existing 1x sheathing.

    This is honestly what started me down the road of exterior insulation, I was trying to figure out how to keep spray foam from pouring out the gaps.

    I also don’t know if it matters but the ceilings are going to be unvented cathedral ceilings with a flash and batt.

    My original thought was to flash an batt the whole house sealing it up. Understanding the thermal bridge issue.

    1. Expert Member
      Akos | | #4

      It is almost a shame to cover up those wide planks!

      You can definitely do flash and batt for the walls. There is no need for OSB on the outside, house wrap will keep the SPF contained.

      If you don't mind a bit of DIY labor, you can reduce the thermal bridging with some foam strips, see here for details:

      You can add enough strips to bring the cavity depth back up to 5.5" after the SPF and end up with a pretty decent assembly R value.

      The only change I would make from the article is to use ripped pieces of Zip R, simpler to install. If you flash the walls with 1.5", you can with R12 ZIP panel strips.

      1. Codhod | | #5

        So the zip r-12 panels are 2.5” thick? That would give me 6.5” of cavity depth,

        Then 1.5” of flash spray foam drops me back to 5” depth for a thick batt?

        1. Expert Member
          Akos | | #6

          There about. 5.5" R19 fiberglass batts easy squish down to 5". The batts squished work out to around R17.

          The 1.5" of cc SPF is somewhere between R9-R11, which is just enough for condensation control of the R17 of batts.

          Including interior finished, siding and air films, this gives you an assembly R value somewhere around R25, which is pretty decent wall. A significant gain form the roughly R14 assembly your existing walls with flash and batt would be.

          P.S. R12 zip might be hard to source, R6 is much more common.

          1. Codhod | | #7

            Just stack the R-6?

          2. Expert Member
            Akos | | #8

            Stacking the R6 is possible but might as well go to the original foam+wood strip idea.

            The simpler is to use the R6 as is, making the overall depth 5.5". This would let you go with 2" of SPF + 3.5" batt, which is still a decent assembly at around R22.

            It would also give you the option to skip the SPF, go for peel and stick to air seal and standard 5.5" HD batts. This would be an R21 assembly, a bit greener as no SPF is needed.

            The energy difference between any of these walls is small enough that I would go for the one that is the simplest for you to build.

    2. severaltypesofnerd | | #12

      So what historic elements will be left of the building, after it's all done? What was the original siding? Interior finish?

  4. Codhod | | #9

    I think I need to stick with the spf since air sealing with tape on all the gaps of the exterior sheathing sounds like a nightmare. Plus the lumber is all rough sawn which getting the tape to stick nicely will be pretty tricky.

    I really appreciate your suggestions. Thank you.

  5. Expert Member
    NICK KEENAN | | #10

    One possibility that hasn't been mentioned is padding the studs on the interior side to make the wall thicker so it can hold more insulation. I've seen three different ways mentioned here: strips of foam insulation on the studs, 2x4's horizontal on 16" centers, and building a 2x4 wall on the inside.

    Old studs are often not straight or even, and need to be shimmed to get drywall to lay flat. A new surface saves that trouble.

    1. severaltypesofnerd | | #11

      Furring out the studs allows reduction of the thermal bride effect. Furring strips can go "the other way" from the studs. Or, use an areogel

  6. user-6184358 | | #13

    The peel & stick membranes are in rolls that are 36" wide x 100 ft long- not really tape. Look up Henry Blueskin Vp100 ---

  7. woobagoobaa | | #14

    Hi Cody.

    Very similar situation to yours but I'm in zone 5. 1910-ish build stripped to studs and exterior board sheathing. Exterior foam not an option. Mix of new and restored original windows.

    I assumed a future assembly failure ... water will get in somehow someway. Tried to make sure the assembly can dry.

    Our exterior wall stack, out to in ...

    - cedar claps over a 1/4" air gap, CorVent-ed at the bottom. I would have preferred Hardie, but hopefully the air gap will give me longer exterior paint life on the claps.
    - Blueskin VP100 as primary air and water barrier. Particularly recommended to me as the tactic to use for gapped board sheathing. I see alot of VP100 in my area.
    - Dense packed mineral wool within my mix of 2x4, 2x6 irregular wall cavities.
    - Membrain (required by town) underneath skim coated blue board walls.

    Underside of the (now) unvented roof now has 6" of closed cell HFO foam ... attic is conditioned space for an air handler / duct work.

    Good luck!

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