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Zone 5 – Exterior Insulation

md1986 | Posted in General Questions on

Hey all,

Im in Zone 5, in Western MA, about 20-30 minutes from the VT line

My plan is for 2×6 walls, , cdx sheathing, tyvek, 24″ o/c walls, cavities filled with either fiberglass or mineral wool batts.

First (and hopefully last) time builder.  I keep reading different answers to this, and am still confused.

Is 1″ of exterior XPS in Zone 5 sufficient?  Is it going to cause catastrophic condensation problems?  Some places say 1″=R5 is sufficient, some disagree.
Is it 2″ or don’t bother at all?

I found a great deal on sheets of the 1″ XPS, pink stuff.  I like the sound of just using 1″ because it wont complicate the windows/doors etc as much.   

I’ll be heating with primarily a woodstove and mini splits as backup, if that makes any difference to the equation.


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  1. jollygreenshortguy | | #1

    IRC2021 code, chapter 11, Table N1102.1.3 offers 3 choices for zone 5 walls.
    opt. 1: cavity R20 + continuous R5
    opt 2: cavity R13 + continuous R10
    opt 3: R20 continous

    I've looked into this quite a lot but am not an expert. Hopefully others will chime in. My concern with the first option is that the inner face of the continuous insulation may be too cool and cause condensation inside the wall.

    For unventilated roofs the IRC2021 tells use we need a certain percentage of our insulation to be "rigid board or sheet insulation". R806.5 and accompanying table. For Zone 5 you'd need 1/3 of the insulation (R20 out of R6) in the form of rigid board. This is to control vapor condensation. You want to make sure the inside face of the rigid insulation never gets cold enough to cause condensation inside the roof.

    It's my understanding the physics for walls are basically the same and you should shoot for at least 1/3 of your total insulation R-value to be rigid foam on the exterior side.
    Option 2 above, R10 rigid insulation (around 2") combined with R13 in your wall cavities would do the trick. You could go as high as R19 cavity insulation if you wanted to upgrade, and still be within the 1/3 ratio.

    Personally, I wouldn't go with just 1" of exterior rigid insulation. It seems too risky.

    1. md1986 | | #2

      Thanks for your thoughts,

      Do you think that having 1" of xps makes for a better or worse wall than one without any foam at all, and only batts.

      Do you think any condensation that occurs would be able to dry out, inwards?

      1. Expert Member
        Akos | | #3

        2x6+R5 is a very common assembly up here in Zone 5/6, these walls work fine. It does require a warm side vapor retarder (ie faced batts or smart vapor retarder) whereas with R7.5 or more painted drywall is good enough. Provided your 1" of rigid is not foil faced, you will still get a fair bit of drying through the foam which enhances the robustness of the assembly.

        1" foam does reduce thermal bridging a fair bit and bumps your your wall R value. If the extra cost is not too much, I would definitely go for it. If you have the budget, going up to 2" polyiso halves the heat loss over a standard 2x6 wall but does mean installing strapping for the siding.

        1. md1986 | | #4


          Yes I've heard of people using 1" rigid and 2x6 walls around here, and even less or none, but when I research for myself I see that usually more foam is often recommended. I do understand why. I could probably manage to pay for 2" of foam, but it will complicate other details, which will continue to add more costs. If I knew I HAD to or my walls would turn to soup, then I would.

          Mostly what I hoped to understand, is that 1" is better than none. And if the potential condensation issues (for having only 1") are more or less guaranteed to be a huge problem, or something more negligible.

    2. Expert Member
      Michael Maines | | #9

      The physics are different between walls and roofs, because roofs are subject to night sky radiation which cools them below the air temperature, and because the stack effect leads to higher temperatures and indoor humidity levels near the ridge. The 1/3 on the exterior rule of thumb is outdated; the IRC and various Building Science Corp publications show different "safe" ratios for different climate zones. For example, here in climate zone 6 (Maine), for walls we need at least 33% of the insulation on the exterior, and for roofs we need at least 50% of the insulation on the exterior.

  2. Expert Member
    1. md1986 | | #6

      I signed up to read the article but was still not able to. I'm buying foam today or tomorrow. I'm still confused if 1" of foam will be enough, if I used faced batts?

      Is using 1" an awful idea, or just an imperfect idea?

      1. Expert Member
        Akos | | #7

        No problem with it at all if you are using faced batts. More than the amount of rigid, what makes the biggest difference is the air tightness of your assembly, this is where you want to spend your extra effort. Make sure to tape the seams of your sheathing with a quality tape, seal the sheathing to the foundation and to the ceiling air barrier. Getting your window and door rough opening flashing right is also important.

        I have opened up older walls with thin exterior rigid and the one thing I can note is everything inside was pristine, almost like the day it was built. Older houses with no exterior rigid and the sheathing tends to look on the rough side.

        1. md1986 | | #10


          What if I was using mineral wool batts, which don't come faced?

          1. Expert Member
            Akos | | #11

            You need to install a vaper retarder membrane. The best option is one of the smart variable perm ones (ie Membrain, Intello). Regular 6mil poly will also work, what is commonly used here, but a variable perm ones allow for a bit of drying to the inside as well.

  3. Expert Member
    Michael Maines | | #8

    Table R702.7(3) shows the amount of exterior insulation required in different climate zones and assemblies that are relatively safe from moisture accumulation (aka "condensation")--safe enough that the only interior vapor retarder required is latex paint over drywall (aka "class 3 vapor retarder").

    You can meet energy code requirements with less exterior insulation ( but only if you include a class 1 or 2 interior vapor retarder, which reduces how much moisture can get from the living space into the wall assembly.

    In other words, R-5 exterior insulation over a wall insulated to about R-20 is ok but not considered robust, since it relies heavily on interior vapor control. It would be better to have at least R-7.5 insulation on the exterior, to keep the sheathing warm enough to prevent condensation.

    On another note, XPS is sold as R-5 per inch but over time it becomes R-4.2/in, as air displaces the blowing agents, so to be safe you would need at least 2" of XPS on the exterior. But the blowing agents in XPS are extremely potent greenhouse gasses and should not be something we use if we are being responsible about how we build; there are many options that have much lower levels of up-front carbon emissions and that work just as well. I use exterior wood fiber insulation when possible, or when it's not possible, reclaimed polyiso or rigid mineral wool are also decent options.

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