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Community and Q&A

Exterior Insulation Thickness

AHTTAR | Posted in General Questions on

I was all set to order ZIP R12 for my current project but I’m having a hard time pulling the trigger.  Seems like just as much of a hassle to install compared to doing foam over standard zip.  Probably ends up being more expensive too.  I also can’t seem to wrap my head around the sheathing still being on the exterior of the foam….I know I’m not the only one.  That said, does anyone know off hand or can point to an article that gives the minimum exterior insulation thickness and type for a zone 6 climate?  I prefer to put most of my insulation budget to the roof assembly.

2×6 walls, standard R21 unfaced batts, zip sheathing, Foam?, 3/4″ rain screen, wood & corten standing seam siding.


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  1. DirkGently | | #1

    Going to guess you would need R-11.25 on outside of sheathing to be safe.

    1. AHTTAR | | #2

      Any favorite product? Thanks

  2. kyle_r | | #3

    Is your home a single story?

    1. AHTTAR | | #4


  3. Expert Member
    Akos | | #5

    That is assuming only painted drywall as interior vapor retarder.

    If you go with a class II vapor retarder (faced batt insulation or smart variable perm membrane), you can go much less. In zone 5/6 with 2x6 R24 walls, this means you only need 24*0.2=R4.8.

    Everybody knows that Zip R is a compromise, not perfect but properly detailed it will make for a great wall. Zip R is a product that is simple to install for trades that have never done continuous exterior insulation. Standard exterior rigid install is not that hard though, so anybody that has gone through the learning curve can do it for less than a Zip R wall.

    1. AHTTAR | | #6

      I'll read that article tonight when I get some more time but I wanted to clarify. You're saying that if I use standard r21 paper-faced bats I would just need a single layer of 2-in poly ISO? This is very different info than I got 7 years ago when I did the last exterior insulation project. It was definitely a no-no to use any paper-faced bats and I think at the time I needed a minimum of r12 exterior. Is there a law of diminishing returns on adding more to the exterior? I feel like there used to be a calculation where it was just a ratio of exterior to bat insulation and you could definitely use too much on the outside. I'm just trying to get into very comfortable territory not crazy comfortable and obviously I want a dry wall assembly. Basically I'm trying to find a balance of comfort and cost. I don't want to go through the extra work of doing exterior insulation if it's not going to be a noticeable performance gain. Thank you

      1. Expert Member
        Akos | | #7

        This has always been the case except it was not very clearly communicated in the code creating a lot of confusion.

        If you want a slightly more robust assembly with a warm side vapor retarder, select exterior insulation that is a bit permeable as it will help with drying to the exterior. You can look at rigid mineral wool, unfaced EPS/GPS/NXG or permeable polyiso.

        In zone 6 an R20 assembly gets you most of your energy savings, so that would be a 2x6 wall with R5 rigid. A bit more never hurts but you quickly hit the law of diminishing returns.

        1. AHTTAR | | #8

          Well this will definitely save me a little money I'm glad I got on here as I do before every build. By robust you just mean durable?

          I suppose a big part of what I choose will be available. A standard paper-faced batt and 1 in of polyiso almost seems too easy at this point. It's hard to imagine that will make for a large increase in performance over just a standard r22 bat and zip taped for air seal as well as other air sealing details. I got a wonder if the exterior insulation at 1 in is even worth the extra time and cost. I'm sure there's an article on here somewhere measuring and quantifying performance. Thanks for all your info you may have saved me a bunch of time and money. And here I was criticizing the building codes for their exterior insulation amounts.

          1. Expert Member
            Akos | | #9

            What is cost effective and high performance depends a lot on local climate, building costs and experience. There is no one answer that works in every situation. If you really want, you can set up your place in something like BeOpt ( and see what works.

            Overall, I think the most important item is good air sealing. No amount of insulation will make a leaky envelope energy efficient and comfortable. A well sealed house is a very comfortable house.

            Going a bit above code on insulation is generally worth it especially some of the low hanging fruit. Going for high density batts (thus slightly higher R value) is a small cost delta plus with HD batts you generally get a much better class insulation install as they are much stiffer.

            Going crazy on exterior rigid is not worth it but within reason it does make a big difference on assembly R value. Usually between 1" to 3" is a good number.

            Most rigid foam installs limits the exterior drying capacity of the wall. On a reasonably well detailed wall with minimal air leaks and good water management details this is not an issue. Selecting a permeable foam just makes this wall more robust as it creates extra drying capacity if leaks happen down the road.

  4. AHTTAR | | #10

    I agree completely that air sealing is the most important thing by far. Its just hard to imagine that 1" of ISO is really going to do much to limit bridging in my climate. I could see it helping in the summer more than winter but the house is faced properly so the walls really don't matter in my opinion, that's where the roof insulation is key. Almost feels like at that point something like a well detailed Zip sheathing and a flash and batt would be better money and time spent. This is just a gut feeling of course. I've done a couple builds with exterior foam but I can't say they were any more comfortable then a house where I meticulously air sealed with Zip and just flashed the base plate and all the zip interior seams then just used faced R21 pink stuff. I don't know if there are any real upgrades for cavity insulation but I'm starting to think if there are, that might be good enough. I know people like Rockwool but its crazy expensive and I don't know that it performs better or not. I'll have to look into this route. I have decided to ditch Zip R though no matter what.

    I'm telling you, I had people on here panicking that I used two layers of 1.5" Iso on one build. They were convinced my walls would be rotting so its interesting to read your info. It was an extremely comfortable house but I believe most of that came from having a slab, air sealing like crazy and 6" of Iso on the roof deck.

  5. Deleted | | #11


  6. AHTTAR | | #12

    Ok, so my county as of Jan. 1 is going with the IRC 2018 codes for wall R values. Lady at the county told me R22+. The only batts I've found over R22 are Rockwool R23 which are spendy. Am I understanding the code correctly that this would pass? I think with Zip sheathing and a 3/4" rain screen this would be a good assembly.

    The other option would be to do R19 or 20 I guess w/ 1" of R5/R6 ISO. The code calls for R20 +R5 so I'm not sure if its R20 or nothing. There are not that many choices for R20. R19 and R21 have always been the standard I knew.

    Again, I'm not all that convinced that 1" of exterior foam is going to do a whole lot. If the Rockwool assembly is allowed under this 2018 code and the price is similar, I think I'd be inclined to save myself the labor and go that route. Please correct me if I'm wrong on how I'm reading all this. Thanks guys.

    1. Expert Member
      Akos | | #19

      I would check with your local lumber yard, they should be able to order it for you. Up here, R22 2x6 fiberglass batts are stock item at the local box store. These are a higher density version of the R19 batts. The nice part is they are also much easier to install as they are much more rigid.

      R23,5.5" thick

      Another option, a bit of a pain to hang siding and tape, but you can also look at using fiberboard sheathing. This gives you an extra R1.3 which would allow for R21 batts.

  7. johngfc | | #13

    Keep in mind that external foam insulation is also a thermal break for all those studs in the walls. If your framing factor is 16%, stud bays are R21 and studs are R7, adding 1" (R5) foam, your wall R goes from 17.7 to 21.9. With 2" (R10) of foam, that same wall is ~ R26. 1" of foam gets you ~ 20% improvement in the insulating value of the wall; 2" about 33%. Without foam, you'd for sure see the cold studs with a FLIR or similar. Our planned build in CZ6 has 2x6 walls with 2" of continuous external foam. Seems like a good investment and something that's not practical to upgrade in the future. (to be clear, the improvements are calculated using U values)

    1. AHTTAR | | #14

      I understand that. I'm not against exterior foam, I'm against too little or too much. 2" is where it starts to make sense in my opinion in cz6. If I'm going to be forced to put up 1", I would just do 2" instead. I've built and lived in a couple houses with 2 and 3" of continuous foam and even with 3" you still get bridging. You may get a 20% gain in Ufactor per your calcs with 1" but you aren't getting anywhere near that in comfort gains. In fact I'd argue the overall benefits of Rockwool over 1" of foam are clear. 2" or more I'd say it starts to be a fight. I'm very much in the camp of putting most of the budget to roof and air seal and if there is leftover add foam to the walls. I would never add it later as you suggested.

  8. dfvellone | | #15

    I received this reply from Dana Dorset to my question about exterior foam in my zone 6 location with 2x6 framing with r20 between studs:
    "Chapter 7 of the IRC calls out R11.25 MINIMUM for a 2x6 wall in zone 6:"

    It was pointed out to me by several folks here that the 2.5" xps sheathing I'd used, because of its loss of r value over time, was minimal.
    The use of a smart vapor retarder/airtight drywall assembly was recommended as insurance.
    Sounds like some of the recommendations here are for using class II vapor retarder in lieu of recommended exterior r value.


    1. AHTTAR | | #16

      Thanks Daniel....Dana has helped me in the past and I'll admit I might only understand some of what he says, too complicated for me. Way back when I did my first one, R11.25 seemed to be the minimum too. To the point of panic from some. Now I'm told I can do R19 + R5 by others. Kinda tough to know what to think anymore really. At this point I'm just trying to figure out if you can pass 2018 code with cavity insulation only or if you pretty much have put 1" of exterior foam.

      1. dfvellone | | #17

        I was in the same boat as you, and after I had installed all the xps, strapped it, and started on my siding, panicked when I was told that I should add more exterior insulation because my approach was minimal. My engineer had originally called for 2x4 studs with cellulose, and 2" exterior xps.

    2. Expert Member
      Akos | | #18

      If you look at the linked to code, it is an exception to a better warm side vapor retarder. The amount of rigid insulation is called for only if you want to use a Class III vapor retarder (painted drywall).

      Code always allowed for less, provided you have a Class II or Class I vapor retarder.

      Up here in the great white north (zone 5 and 6) 2x6+R5 rigid with interior poly has been built for a long time. These walls are holding up just fine.

      You can also read more about this from BSI with a good dose of the usual humor from Joseph Lstiburek thrown in:

      1. AHTTAR | | #20

        Sorry, just saw this. For some reason I don't get alerts anymore. I've decided to go Rockwool and membrain. Now for the roof. What do you think about 4" polyiso, zip deck, Rockwool r38 batts? 4" was borderline in my last house which is in a little colder climate and way less sun. I'm thinking this time do 4" and membrain on the ceiling too if that's ok?

        Is there different math for the ceiling than the walls to use less exterior foam by adding a smart barrier? 6" on the roof just starts adding a lot more design details for facia to make it look my mind anyway.

        1. Expert Member
          MALCOLM TAYLOR | | #21


          One way to get arou8nd the thick fascia problem is to not insulate the overhangs. This is easier to detail with metal roofing:

          1. AHTTAR | | #22

            Thanks Malcolm. I've explored that option as well problem is then you start getting into other details because your fascia is lower than the top of the phone 24 in away. So now you have to either add gutters with a bunch more pitches and complications or build up that section and then you have the same problem with tall fascia. I'm going to notch the bottom of the rafter tails and make the faces step up with a different material just to kind of break it up. I'll check out this link you sent though looks like some pretty cool houses right up my alley. Thanks

  9. AHTTAR | | #23

    I guess nobody wants to take a stab at this one? Can exterior roof insulation thickness be reduced with membrain interior vapor barrier?

    4" Firestone iso r26
    Zip taped
    Rockwool r38 batts
    Taped painted rock
    Wood ceiling


    1. Expert Member
      Akos | | #24

      Zone 6 condensation controlled roofs need 50% of assembly R value as exterior insulation. Unlike walls, roofs are much more sensitive to moisture accumulation, thus the higher ratio.

      There isn't much info out there, so this is mostly my opinion. I think including a variable perm membrane in the stackup does let you push the ratio, how much is a guess.

      Your stackup is closer to 40% exterior, factor in R value loss over time and you are at R22, which is closer to 36%. These numbers are Zone 5 numbers, I think you are pushing your luck a bit.

      Instead of complicating your assembly, I think the simpler would be to use the 4" iso, add in a cover board, go with R28 batts and skip the smart VB. The overall energy use between the two roofs is noise (if I had to guess, maybe 5 therms/year for 2000sqft).

      A glue down cover board is a couple of reasons. Makes re-roofing easier, adds a bit of R value plus it covers the long bolts/insulation washers which is a bit of a thermal bridge.

  10. AHTTAR | | #25

    I'm not sure what you're talking about with cover board. You talking in the half inch or 3/4 in dense board comes in like 2x4 ft sheets? That would cause other issues with the way I fasten the rubber above the foam. It's just held down with the drip edge then it's loose then you have strips fastened with the long bolts and washers. If I did it your way I would either have to put the bolts and washers above the board or glue the entire sheet down which I don't want to do. I'm assuming you just like to do it this way you're not saying to do it to gain our value?

    R 28 is pretty tough to get around here. R30 is easier to get. You're saying to just leave the cavity a couple inches unfilled between the bat and the drywall? Or I guess you don't have to push it all the way up to the roof deck.

    I'll be honest I'm not sure how much of this actually matters. My buddy's been doing flat roofs in the Missoula and Flathead area for decades. We just pulled a 35-year-old roof off a couple of days ago that had nowhere near enough insulation per today's calculations and it looked absolutely perfect. There were literally sections with zero insulation.

    1. Expert Member
      Akos | | #26

      The cover board is mostly for a glue down install. Since you are looking to mechanically fasten, it won't gain you much, the extra cost doesn't make much sense for the small R value increase.

      Yup, don't fill the roof cavity all the way. With undersized batts, you want to install them against the roof deck, you don't want any gap on the colder side of insulation. Gap between the insulation and drywall is fine. HD batts tend to stay put on their own, you can use insulation wires with regular batts or faced batts stapled to the side of the rafter.

      To me roofs fall into two categories. There are roofs that are guaranteed to work, you never have to worry about them. Then there are roofs will usually work, even almost always work, but can also fail. For a new build, why take any chances? Fixing a roof problem down the road is very expensive.

      P.S. With the current EverythingShortage (tm), make sure the bits for the flat roof are available, my local roofing place was out of EPDM lap sealant and insulation washers last time.

      1. AHTTAR | | #27

        Yeah, I only use the board when there is no insulation, like on a covered patio or something. Its spendy for sure, so happy to not use it.

        Do you think I can get away with R30 and 4" ISO? Seems pretty close. I'm still not totally sure why the 50% exterior ratio works but I've always tried to follow it.

        So like this top to bottom:
        4" iso
        Drywall paint/tape
        wood ceiling.

        All ductwork and utilities would be run in soffits/raceways, so the ceiling will be sealed up as tight as possible. Couple of ceiling boxes will be caulked.

        Everything shortage is only gonna get worse I think. Regardless of whether builders stop building or not. My good friend is getting me all the stuff from Firestone and so far they have had no shortages. I agree though, I'm literally planning the entire build months in advance. If I never get on a computer again after this it will be too soon.

        1. Expert Member
          Akos | | #28

          Condensation control comes down to how cold it gets during the coldest 3 months out of the year. In zone 6, this average is much lower thus the increase in exterior R value.

          Go for 4.5" iso with R30. Close enough to never have to worry about it.

          1. AHTTAR | | #29

            I'll see if Firestone makes 1/2", I don't think so. I'll still look for r28 as well, otherwise I'll just do a layer of 3" and a layer of 2" foam.

            Unfaced batts with no vapor barrier?

          2. Expert Member
            Akos | | #30

            My roofing place carries both 1.5" and 3", I think they also have 2.5", no need for 1/2".

            Faced or unfaced batts both work just fine, whichever is easier to get. Except for painted drywall, no additional vapor barrier needed.

  11. AHTTAR | | #31

    Well I appreciate your time. Thanks

  12. Deleted | | #32


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