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Community and Q&A

Insulating Exterior Walls With Interior Sheathing

Diane LeBon | Posted in Energy Efficiency and Durability on

hi all, I have been doing quite a bit of research and reading various articles on this website. We need to replace our existing wood siding, and want to take the opportunity to insulate exterior walls, as there is currently no insulation between studs. Zone 5a. Two story house, built approx. 1860’s. Walls need to dry to exterior, because 20 yrs ago we removed plaster/lathe on interior (in half of the house), installed polyiso and new gypsum board. The remainder of the house is still plaster/lathe on inside. Under the plaster/lathe, or gypsum board/polyiso, is horizontal sheathing boards interior side of 2×4 stud wall. No exterior sheathing, and siding is directly on studs. So, my thought is remove existing siding, install either dense pack cellulose or mineral wool between studs, WRB, then either Rockwool 80 or maybe Gutex, on studs, then wd furring strip rainscreen, then hardiplank lap siding. Thinking we could get away without adding sheathing on exterior since there’s already sheathing on interior side of studs. Assuming this will also work for the front part of the house that doesn’t have the interior poly. We do know we have to do a better job of air sealing and insulating the attic, and rim joist. That is on the to do list as well. The roof is approx. 23 yrs old. It still looks fine but I know within a few years it will likely need replacement so we could take the opportunity to put exterior insulation when replacing roof. Appreciate thoughts on wall strategy with the rockwool or gutex, and wrb recommendations? Thanks

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Replies

  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Diane.

    For the most part, I think you are headed in a good direction with your project. One thing to consider is how you are going to air seal the exterior. It would be a shame to do all of this work and have a leaky assembly. You could try to details a mechanically-fastened WRB attached to the studs as an air barrier, but I think it would be difficult. And perhaps you could detail Gutex as an air barrier, but I haven't looked into that. But this may be a reason to sheath the house. Plywood or OSB sheathing can be a great air barrier with little cost and effort.

  2. Jon R | | #2

    +1 on doing something that provides a very good air barrier.

    Open cell spray foam is a good retrofit air barrier (no ext sheathing needed) and isn't strictly on the cold side (IMO, a less desirable place for a cold climate air barrier). You could use Bonfiglioli strips on the exterior to add wall thickness and reduce thermal bridging (no Rockwool 80 or Gutex needed).

  3. Gabriel Solomon | | #3

    Will you retain your existing windows as "innie" windows? What will you use for a WRB?

  4. Diane LeBon | | #4

    Thanks for the comments and sorry for late reply. I've pretty much come to the conclusion that I should install plywood (supposedly more permeable than OSB) on exterior side of studs. Thinking that along with an adhered WRB such as Vaproshield, I could get water barrier, air barrier and it would be permeable, allowing to dry to exterior. Now if I don't add any exterior insulation such as Rockwool Comfortboard (concerned about tricky details at windows doors, etc and bumping the wall out too much), then do I need to be concerned about condensation within wall, or is that not an issue with the permeability? Also some contractors have suggested using spray foam within the studs, giving me air barrier and insulation. I was trying to stay away from spray foam, and since I have the polyiso on interior side, would the spray foam 'trap' moisture? I've been reaching out to a few contractors so I'm hoping to get this resolved soon so I can get accurate estimates. Thanks so much for your help.

    1. kenmarcou | | #9

      I have a similar house it sounds like, Diane. My mom wants to try to stay there and I’m going to see if I can make that happen. Built in 1840’s in central MA Climate Zone 5. It’s Impossible to heat. No real water control, no air barrier or building envelope to speak of. Roof needs to be replaced because it’s been leaking water and the top floor is damaged. An ice dam tore fascia off this winter. I was thinking of creating an actual building envelope with a water, air and thermal control layer as part of the required roof work! I’d need to take the siding off and cut off the short roof rafter ovehangs (the back/north side is the flatter part of the roof - it’s a salt box or cape w/full dormer I’ve heard it called), get Zip and cover the whole house with it and then seal seams w/tape or fluid applying flashing at the wall/roof transition. From there I’d wrap in insulation (not sure what yet - does it need to be rockwool for a vapor open wall that can dry to both sides??) But instead of using Zip I was wondering if plywood with fluid applied vapor open WRB coating like you’re considering would be a good option too. Have you moved forward with your project yet?

  5. Expert Member
    Akos | | #5

    With the interior foil faced insulation, you only need the exterior to be a bit more permeable. Pretty much anything non-foil faced will work. Plywood, osb, fiberboard, EPS or permeable polyiso will all work just fit. None will not crate a moisture trap.

    You still have to deal with flashing around your windows as leaks there will cause problems.

    In zone 5 with R5 interior insulation and dense pack/batts in the wall you are already getting most of your energy savings. Adding an extra R5 makes little difference on your energy use.

    Having said that, if the window details are not very hard, I would be tempted to go with 1" foam VS OSB/CDX as the cost delta is not that much and you get much more R value.

    Probably the biggest energy improvement is making sure you can air seal the exterior, weather you use foam of OSB, tape the seams with a decent quality tape.

    If your studs are real 2"x4" dense packing might be the best as you have to trim batts to fit which is a lot of work. One extra benefit of dense packing is that it does a great job of reducing air leaks.

    1. Sofiane Azzi | | #6

      Trying to piggy back here if it's ok. I have a similar interrogation: assuming 2x6 studs with r-22 rockwool, how much of a difference is there between additional R5,6 and R9,5 on the interior side?

      I have the option between both, the first rigid insulation being cheap and the second option fairly expensive (kooltherm). Is there a significant difference in terms of comfort and savings between both?

      1. Expert Member
        Akos | | #7

        Depends on the climate and energy costs.

        There is a big difference between 2x6 and 2x6+R5.6, not much bumping that up to R9.5. You can run it through here to see:

        https://ekotrope.com/r-value-calculator/

        A well sealed wall with a reasonable amount of continuous rigid insulation is very comfortable in most climates.

        1. Sofiane Azzi | | #8

          It’s for zone 6, I must have deleted it when editing my post.

          I’ll have a look at the website. Thank you.

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