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Community and Q&A

Whether or Not to Use Exterior Rigid Insulation

dennis_vab | Posted in General Questions on

I’m just about done framing my home and am trying to come to a decision on if I should use 1” of exterior rigid foam. Windows are going to be here in a few weeks and I’ll need to get the window bucks framed for outie windows.

climate zone 6, 7/16” zip, with 2×6 construction, raised heel trusses (for r-60 blown in fiberglass). Stud cavity insulation will be fiberglass batt or rock wool with a vapor retarder

I did a Beopt model a little while ago and I saw only a $5 a month savings if I installed 1” of rigid continuous insulation. From a financial point of view it doesn’t make sense. Does it add any comfort to the home?

I found 4×8 sheets of DuPont branded XPS for $16 a sheet so cost will not be a huge factor, labor is mine.

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Replies

  1. Expert Member
    Akos | | #1

    A well sealed an insulated 2x6 wall is actually pretty decent, not great, but not bad either.

    I'm not sure 1" is worth it unless it is part of code. If you can go up to R11 (2" polyiso), you can skip the warm side vapor barrier, which might be a way to justify the extra labor. The problem is that at that thickness even with polyiso you are looking at screws for strapping which does add a bit of cost. Roofing polyiso available at most commercial roofing places tends to be the best value for rigid insulation, sometimes you can even find it 2nd hand for much less.

    The 2" polyiso would definitely bump up your assembly R value, almost half the losses through the walls which might be enough to let you get away with a smaller HVAC. The cost savings on mechanical equipment could offset some of the cost as well.

    There is also definitely a radiant comfort difference sitting next to an R18 assembly VS and R31 when it is 0F outside.

    1. dennis_vab | | #2

      It is part of code, but I can go with option 2 which is just r23 cavity insulation.

      Anyone else have personal experience with the such a small amount of continuous insulation?

  2. walta100 | | #3

    Climate zone 6 Seemed like 1 inch of foam on a 2x6 wall was not a safe option.

    https://www.greenbuildingadvisor.com/article/how-to-design-a-wall

    Are you sure understand the BEopt report? BEopt assumes you are financing the project at some interest rate and burning fuel that will get more expensive over time at the inflation rate you chose given your local weather. You can enter the cost per sqf of different walls and their R values The program draws a graph the wall on the left cost the least to build the wall on the right uses the least fuel the wall at the bottom is the wall I find interesting in that it is the one that cost the least to build and operate. It is interesting how often the one on the left and the right cost almost the same to own over time.
    Yes the fuel saving when going from R19 to R26 wall maybe small.

    Walta

    1. dennis_vab | | #4

      1” of foam is allowed with a vapor retarder from what I have found. So even a grade A installation with 1” of foam and R21 fiberglass that is paper faced, will be acceptable.

      1. Expert Member
        Michael Maines | | #5

        It's code-compliant but that doesn't mean it's a good or resilient assembly. I would consider 2x4 framing with 2" exterior foam. Or if you are set on using 2x6 framing, use a vapor-open exterior insulation such as mineral wool or EPS.

        1. dennis_vab | | #6

          The house I framed in 2x6 right now. Mineral wool is probably not going to be easy to locate. I can get 2” of polyiso, but I’m not looking forward to attaching it. Do you have any good cost effective ideas on doing a rain screen for 2” of foam? I’m planning to use everlast vertical siding.

          1. Expert Member
            Michael Maines | | #7

            I have only done 2" and thicker exterior foam when I have used it, up to 6" thick on some projects. The methods are the same. There are many threads here on GBA discussing the details.

        2. brendanalbano | | #8

          If dennis_vab really wants to use 1" of foam in a cold climate, rather than 0" or 2+", it's not necessarily out of the question, despite generally being considered to be breaking the rules of good wall design.

          These articles are worth reading and discuss the R-20+5ci wall in cold climates:
          - https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed
          - https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-092-doubling-down%E2%80%94how-come-double-vapor-barriers

          Some ways to add safety to this potentially risky wall assembly from the above article:

          - Use a class II vapor retarder on the interior (kraft paper, smart vapor retarder membrane, etc)
          - Use a bumpy wrb (e.g. Hydrogap) to allow lateral diffusion and drying behind the foam
          - Use a vapor permeable foam like EPS (e.g. Halo Extera, Insulfoam R-Tech, etc). Mineral is even better, but can be harder to find or harder to install.

          However, when breaking the rules, it's best to proceed with great caution! Do read the articles and decide for yourself if you're comfortable with the risks before going this route.

  3. dennis_vab | | #9

    I’m trying to find material to fur the windows out 1” to match the rigid foam. I’m going to use Everlast board and batten siding (doesn’t require a rains screen). The only thing I found so far are 5/4” x 5-1/2” deck boards. I would rip them down to 2-3/4”.

    I’m wondering if I can use two layers of 7/16” scrap zip osb to do the same thing. Does anyone have any input on if this is a good idea?

    1. kyle_r | | #10

      Why not just fasten the windows through the foam?

      1. dennis_vab | | #11

        The zip sheathing will be my wrb. I’m going to tape the foam regardless but I’m not detailing it as my wrb.

    2. Expert Member
      Michael Maines | | #12

      I prefer to keep the window faces/flanges at the WRB plane when possible, since it keeps the WRB and air control layer in the same plane. It's not that hard to make exterior extension jambs and sills. You can also just wrap solid wood around the window openings and detail the WRB over and around that "buck." But you'll probably be happiest with Thermabuck, the product made for your situation.

      1. dennis_vab | | #13

        I installed all of my windows in plane with my wrb (zip sheathing). I have a lot of rejected 1” xps foam that I will be installing over the exterior sheathing. I was going to use Everlast siding but things have changed and will now be using LP siding. I plan to use Benjamin obdyke for a rain screen (the original or the self adhered). What’s the best way to detail the foam and rain screen at the window openings?

        1. Patrick_OSullivan | | #14

          Not super clear to me what you're planning on doing. What will the siding be nailed into?

          1. dennis_vab | | #15

            Siding will be nailed to the 7/16” zip panel. This will be board and batten. Unless you perfectly planned your framing it would be impossible to get board and batten lined up to nail into the studs.

            This is a detail similar to what I am planning to do. 2x6 Studs, 7/16” zip panel, 1” rigid foam, rainscreen, then lp board batten.

            Article where picture is from: https://www.buildingscience.com/documents/insights/bsi-085-windows-can-be-a-pain

        2. Expert Member
          Akos | | #16

          Use thin plywood strips as your rain screen. The siding can still be nailed through the strapping to the sheathing behind with long enough nails. Much cheaper than mesh products and not much more work.

          There are lot of details out there for doing strapping around windows, follow those.

          1. dennis_vab | | #17

            That actually sounds like a great idea. I’ve seen details for the strapping spring windows. My concern is how do I detail it so I don’t get water behind the foam. Especially since the windows are installed and the foam is going to stop just short of the window.

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