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Community and Q&A

2×8 staggered stud walls with exterior insulation

MStaudaher | Posted in Green Building Techniques on

I am in the planning stages for a new ~3900 sq ft single story home in central Oregon (Climate Zone 5B) I want to build starting in the early summer (2019).  I plan to use advance framing techniques.  I also would like to have R40 walls and plan to have a sealed conditioned crawl space for plumbing, limited ducted HVAC (for mini-splits), electrical and possibly independent duct work for an ERV or HRV.  

The wall assembly that I would like to use is as follows from outside to inside: fiber cement siding; 1×3 furring strips for a ventilation space and attachment points for the siding; 2″ of taped XPS foam exterior insulation; Tyvek drain wrap, OSB with the joints, nail and other penetrations sealed with Proseco joint and seam sealer; a staggered stud space (2x4s 24″ OC on 2×8 top and bottom plates); stud space filled with dense packed blown in fiberglass, possibly a smart vapor retarder (MemBrain or Intello-plus); dry wall and paint.  I plan to be careful with air sealing.

My question is related to the thickness of the exterior insulation in my climate zone.  Because of cost and detailing, I would like to avoid using more than 2″ of exterior XPS insulation.  When I reviewed IRC tables N1102.5.1 and R702.7.1, they only mentioned minimum R-value for foam sheathing/vapor retarders for 2×4 and 2×6 walls (R5 and R7.5 respectively) in my climate zone.  

Since I plan to use a 2×8 effective wall thickness, do I extrapolate the table to indicate that the minimum R-value for a 2×8 wall should be R10?  If so the 2″ of XPS that I would like to use would seem to work.  Also, if that is the case would I even need to use a smart vapor retarder at all.  Would there be any benefit to do so?

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  1. brendanalbano | | #1

    Quick comments:

    XPS should be avoided due to the super high global warming potential of the blowing agents used in making it. Use Polyiso or EPS instead.

    The ratio for exterior to interior insulation can be extrapolated from the table by turning it into a ratio. You need 30% of your total R value to be the rigid insulation in climate zone 5. Here's an article that covers the details:

  2. gstan | | #2

    There is a far more rational way to construct a double
    stud wall - We're going to put a really good air/moisture
    barrier in the middle so it can dry to both outside and
    inside and then insure that the surface of that moisture
    barrier always remains above the dew point!

    Outer stud wall - ordinary 2x6 with batts (mineral wool?) -
    then fasten 2 or more inches of polyiso to the inner face
    of the 2x6 studs - then construct a 2x4 inside wall against
    the inner face of the polyiso. The inner 2x4 wall becomes the
    service cavity and sheet rock hanger - the polyiso (Foil Faced)
    is the air/moisture barrier and is protected from physical
    damage by both inner and outer walls and from R-value drop
    by the insulated outer 2x6 wall - YES! you do need to tape
    the seams on the inner side of the polyiso and seal it to the
    floor and ceiling with some combo of caulk, tape, or spray-on.

    With 3 inches of polyiso the total wall thickness will be 12
    inches (stud face to stud face). The "whole-wall" R-value will
    be around 36. Want more R-value? Fill the service cavity with
    batts or blow in for around R-45, Or increase the thickness
    of the polyiso, and/or use 2x10 for outer wall. With this wall
    design at 12" and the service cavity filled, the inner foil
    surface of the polyiso will remain above the dew point for any
    temperature difference (delta T) between interior & ext. less
    than about 88f(49c). (and int humidity levels less than 50%,
    which is normal when it's this cold).
    In other words, it would have to be colder than about -18f(-28c)
    to reach the dew point in the wall. If it commonly gets colder
    than this where you live you need a higher R-value (thicker wall).
    Emphasize commonly here - many days/nights, not just a couple
    every few years. 2" polyiso will keep the interior of the wall above
    the dew point down to about -10f which might be adequate in your

    CAUTION! Do not even think about an insulated 2x6 interior wall
    (service Cavity) or placing extra (fibrous) insulation between the
    interior wall and the polyiso - although it's a cheap way to raise
    the total R-value, it would also lower the temp at the interior
    foil surface of the polyiso - likely below the dew point. NOTE!
    just leaving the inner 2x4 wall empty will keep you above the dew
    point for nearly any outside temp (-70f(-57c),no problem) but
    the lower R-value will increase energy use unless you add more
    polyiso. There just ain't no free lunch!

    There are some unstated assumptions here, of course, such
    as a working heating/cooling/vent system, good choices for
    exterior siding, sheathing and rain screen, a preference
    for living at normal temp/humidity levels, etc.

    Know that materials cost of 3"polyiso (R-19) instead of say
    3.5"Roxul(R-15) in a double wall will add around $1000 to a
    2000 sq. ft. house (2019).But, then subtract the cost of
    the now unnecessary vapor retarder and add peace of mind.

    Why fumble around with fragile vapor retarders, exterior foams
    vulnerable to insects and/or fire, awkward and labor expensive
    siding installation over foam, or, worse, insulation prone to
    settling over the years? If you live in a climate harsh
    enough to justify double wall construction -this is about the
    best compromise available with current technology.

    1. Expert Member
      MALCOLM TAYLOR | | #3

      User ...182,

      There are all sorts of variations on double walls that have the vapour and air barrier buried close to the middle of the wall. Some use membranes, some use plywood or OSB. Some move the control layer to the outside of the inner wall for continuity, some have no exterior sheathing. What are the advantages of your proposed assembly over them? Have you actually built using this wall or is it a theoretical solution?

    2. MStaudaher | | #7

      Thanks for the comment.

      In the wall assembly that you described could you use two 2x4 stud walls separated by 2" of polyiso with the exterior wall containing Roxul R15? If you reduce the exterior wall thickness from 2x6 to 2x4 and use the polyiso between the two stud walls, is there enough insulation in the outer wall to prevent the polyiso from losing R-value when it is cold in the winter?

  3. GBA Editor
    Martin Holladay | | #4

    M. Staudaher,
    Two relevant articles you might want to read:

    "Combining Exterior Rigid Foam With Fluffy Insulation"

    "How to Design a Wall"

    1. MStaudaher | | #8

      Thanks for the information. I have referenced a link to an article entitled "Evaluating R-40 Above Grade Walls for a Production Built Zero Energy House."
      This is from an organization called IBSCOS and was presented at the 2010 Building Enclosure Science & Technology Conference in Portland. It discusses the performance and testing of different wall assemblies and it was the source of the 2x8 staggered stud wall I mentioned in my post. If you have any time, would you please look at the article and give me your feed back. I also would be interested in your opinion regarding the double stud wall separated by rigid foam that was referenced by User-6935182.

  4. MStaudaher | | #5

    Thanks for the comments. I have not built the wall yet. I've read the articles mentioned above and still have a couple of questions. First, in my location, Sisters, Or., there are 6494 heating days (HDD) per year. When I look at IRC Table entitled "Acceptable Insulation Component R-values and Insulation Ratios for Use with a Class I or Class II Vapor Retarder" it shows in climate zone 5 with 9000 HDD the minimum insulation ratio should be 0.2. That means with cavity values of R13, R15, R20 andR21, in the table, that the R-value of the continuous insulation should be 2.6, 3.0, 4.0 and 4.2 respectively. Based on a ratio of 0.2, it looks like a 40 R-value wall using this table, would require a minimum of R8 exterior insulation. Since the table assumes maximum heating degree days of 9000 and Sisters, OR has approximately 6500 it seems like R8-10 exterior insulation should work.

    In the article Brendan referenced it said the ratio should be 0.3 (I assume that is without a class I or II vapor retarder). In that case, the exterior insulation would need to be a minimum of R12.

    Does this mean that the addition of a smart vapor retarder to the interior side behind the drywall allows for a minimum insulation ratio of 0.2 instead of 0.3 in the article Brendan referenced?

  5. walta100 | | #6

    To my ear it sounds like you are trying to combine 2 good ideas. You are doubling your problems and expenses for marginal benefits. If you want a R40 staggered stud wall make it thick enough be R40 and skip the foam, Or build a single stud wall and add enough foam to get to R40 and skip the staggered studs. My guess is the price per square foot for simplified R40 walls will be about 1/3 less than your wall.


  6. Expert Member
    BILL WICHERS | | #9

    I agree with the others that it would probably be easier and cheaper to either go with a “normal” double stud wall thick enough to reach the total R value you want without exterior foam, or build a “regular” stud wall with extra thick exterior foam.

    As an example, a 2x6 wall with R23 batts and 3” polyiso would give about R37.5 whole wall performance. Using 3.5” polyiso would get you up to about R40.5 and would still be a more commonly built wall configuration. Note that with extreamly thick exterior foam, exterior details get more complicated. Usually over about 3” starts to be considered “really thick” in this case.


  7. KeithH | | #10

    Some DIY opinions:
    Thick foam:
    Everyone who says more than 2" thick foam is a pain is right. If a big part of your labor is a carpenter with a nail gun, he's likely to complain above 1" of foam as he wants 1.5" of nail in the stud. Doing it with screws yourself is laborious and expensive. don't forget that thivk foam layer is going to prevent drying to the exterior.

    using polyiso: it's an interesting idea if you can avoid cutting much polyiso. Once you have to start fitting it and foaming it, install cost will go up a lot. I've done a smidge of cut and cobble: really slow!

    'fragile' air barriers: 2 ideas to consider: pro clima intello plus isn't fragile (but it is expensive); taped plywood isn't fragile. Plywood has the disadvantage of off gassing but the advantage that it can play a structural role in terms of racking and shear strength. Osb plays the small role but isn't vapor permeable even when wet.

    Larsen trusses: I'm curious if you considered an empty 2x4 wall with Larsen trusses. I've never done them but would personally investigate them on a future build. Just a statement from ignorance about an option you didn't mention for achieving high R walls.

    1. MStaudaher | | #11

      I thought about Larsen trusses, but was concerned about the cost and complication. When I read the article I mentioned
      it looked like the staggered stud wall on 8" plates with 2" of exterior foam was a good choice from a constructibility and performance perspective in my climate zone. Because of the layout of my house plan, I really didn't want a 12" plus wall (8-9" not counting exterior cladding and interior drywall would probably be the maximum without further increasing the square footage or causing problems with the interior dimensions). I was intrigued by the double stud wall with polyiso between the studs mentioned by user-6935182. If I took that approach and used 2x4 stud walls with R15 equivalent (dense pack fiberglass or Roxul) in the cavities, it looks like would have a 9" R40-43 wall without using exterior insulation. I would be interested in any comments on either the article I referenced and/or the 9" double stud wall configuration referenced above.

  8. gstan | | #12

    Here are a few thoughts and calculations for your
    proposed 9" (2x4,polyiso,2x4) wall in Sisters, OR.

    First lets talk about the "whole wall" R-value - With
    mineral wool insulation, a 2x4 wall will only have an
    R-value of 12 (if and only if) the installation of the
    mineral wool is a perfect job. This is because a 2x4
    wall will be around 25% wood - here is the calc.
    wood: R-val = 3.5 (1 per inch)
    mineral wool: R-val = 15 (4.29 per inch)
    whole wall: R-val = (3.5+3x15)/4 or
    (1/4th wood+3/4ths mineral wool)/4 = 12
    Fiber insulations nearly always have some gaps after
    install - its never a perfect job - a really careful good
    job will probably get you up around R-11 - you are
    proposing two 2x4 walls so you should end up with around
    R-22 from those plus 2" polyiso (R-13 when brand new) for
    a total whole wall value of 35 in a 9" thick wall. You
    can't get a "real" value of R-40 out of a 9" wall unless you
    use all closed cell spray foam in it (talk about expensive!)

    Sisters has an average low temp. in mid winter of around 20f.
    The all time record low is in the -23f range but that doesn't
    happen often - so lets assume a design low of say zero and lets
    assume your going to maintain 70f inside with 45% humidity.
    checking a Dew Point Calculator on the web you will see that
    the dew point inside the house will be 48f. Now lets calc the
    actual temp on the interior foil surface of the polyiso in the
    wall when its zero outside. Here is the calc.
    interior 2x4 wall: R-11
    total wall: R35
    percent of insulation on the interior side of the foil: 11/35=31%
    delta T (inside temp minus the outside temp): 70f
    temp drop at foil surface: 70*31%=21.7 or about 22f
    temp of the foil surface: 70f-22f=48f (inside temp minus the drop)
    So, you'r good down to about zero with this wall. The only cost
    effective way to be sure to stay above the dew point in a 9"
    wall at temperatures lower than zero and/or higher interior humidity
    than 45% is to lower the R-val of the interior wall. Here is how!
    Replace the mineral wool (in the interior wall only) with cheaper
    fiberglass batts - new interior wall R-value will be about 7
    and total wall will be around R-31. Re-do the above calcs and
    we can show temperatures above the dew point clear down to around
    -28f outside - You decide! Is it better to have the highest possible
    R-val or assurance of never having condensation in the wall?
    There just ain't no free lunch!

    About R-val drop of the polyiso in this wall - a similar calc
    will show that the exterior inch of the polyiso will be in the
    mid 20f range when it's zero outside therefore: the total wall
    R-val will drop by about 1 or 2 - not worth worrying about!

    Having gone through all this seat-of-the-pants engineering I can
    unequivocally say YES! your proposed wall should work just fine
    in this climate - with either option.

    Last, consider that with this wall design it becomes possible to
    erect the outer 2x4 wall and then complete a weather tight house (as
    one big empty room - no interior walls). Then do all the rest of
    the work inside, out of the weather, including finishing the double
    wall because the inside 2x4 wall of the double wall is merely another
    interior wall! Although, not every contractor will like this construction sequence.

    1. MStaudaher | | #13

      Thank you for the detailed reply. I have a lot to think about.

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