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Exterior Foam on Double-Stud Wall

mhenson | Posted in Energy Efficiency and Durability on


I have broken ground on a new house and have/had double stud walls (2×6″ load bearing walls) as the configuration.  After reading some of the new reports on moisture accumulation in the exterior OSB/ZIP panels.  I decided to switch to two inches of sealed taped EPS on the exterior, furred out to allow air flow behind the siding and the rest of the insulation in the wall.  My framing contractor is not a fan of this over the double stud wall.  Time is of the essence, can someone set me straight on what is now considered to be the best method?  Especially if neither of these is best!

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  1. Expert Member


    The risks of moisture accumulation in the sheathing of double stud walls appears to have been mostly theoretical. It doesn't seem to be occurring in practice.

    A few things can help keep these walls safe:

    - Good air-sealing.
    - Cellulose insulation to buffer and redistribute moisture.
    - A rain-screen cavity to aid drying to the exterior.

    Whether either (or some other) option makes sense is very dependent on your climate zone.

    There are quite a few good articles on double stud walls n GBA. This link is to one you may find useful:

  2. mhenson | | #2

    Our climate zone is 5b elevation 10,500ft in Colorado. I have done a lot of reading and had settled on double stud walls until I read a couple articles that said they may be risky. We live in a fairly arid climate (high altitude desert), but at over 10,000ft it is wetter than most of Colorado. I guess this is more of a technical question. If you were building your dream home and the last home you intend to own. Would you build double stud walls or use exterior insulation and why?

    1. Expert Member
      MALCOLM TAYLOR | | #11


      I would build a double-stud wall and size the gap appropriately for my climate. Although cellulose improves the robustness of the walls, I would skip it and rely on keeping the ratio of interior to exterior perms correctly balanced, and include rain-screen to keep things safe. Apart from settling cellulose also makes any future renovations much more difficult.

      Both double-stud walls or exterior foam can be detailed so they are simple or complicated. To me that makes more difference to their longevity than which one you choose.

  3. Expert Member
    BILL WICHERS | | #3

    I would use exterior insulation, but that's mostly because I'm most familiar with exterior rigid foam. There are others who prefer double stud walls. I think exterior rigid foam is probably a bit safer from a moisture prespective since it keeps the exterior side structural sheathing warmer (warmer usually means drier), but there are a lot of people who have built double stud walls without any problems.

    I think if you dense pack cellulose in your double stud walls, the moisture buffering ability of cellulose will probably be sufficient to deal with any minor moisture that might slowly diffuse in and out of the wall over time. I don't think double stud walls are inherently more risky here.

    BTW, I think pretty much anywhere in Colorado is going to be quite a bit drier than most of the areas many of the other builders on GBA work. I would especially consider Malcolm's experience here since if double stud walls are safe in his PNW/Vancouver region, they're likely to work pretty much anywhere else from a moisture perspective.


  4. tech1234 | | #4

    I have built a few double stud wall houses in a "riskier" climate (NH) and I think they are the best wall assembly we have right now for high efficiency wall assemblies. I have also done foam wrapped builds and I think double stud walls are a better option. CDX or RTD "real" plywood is a much better sheathing option and when combined with dense pack cellulose, air sealing interior and exterior, and a rain screen the sheathing wetting risk is near zero

    1. tech1234 | | #5

      CDX/RTD have much better perm ratings to osb/zip. Which will help with drying as well as being more tolerant of wetting if any were to occur. There have been some double stud wall houses fitted with sensors that show very little wetting risk.

  5. mhenson | | #6

    This isn't directly related to the question asked, but... Everyone in the double stud wall camp recommends cellulose. I love everything about it except the settling. For those of you that have experience and assuming sprayed, dense, cellulose what have you done to mitigate settling? Especially at the top of the wall where it could mostly defeat the entire assembly?

    1. maxwell_mcgee | | #7

      what about a product like hemp-wool or blown in hemp fibre?

      Similar buffering capabilities to cellulose. Are there drawbacks?

      1. brendanalbano | | #13

        One drawback to a material like hemp-wool or blown-in hemp fibre is simply that it is uncommon. Can you find a local contractor that has worked with it before? Is there a local distributor? Is there a reliable source of information about it's long term performance other than the people selling it?

        You can find articles on Green Building Advisor where people have installed moisture sensors in cellulose filled double stud walls and monitored their performance. With a more novel material like hemp-wool, it's much harder to find that type of information to give you confidence in the assembly.

        That's not to say you should never use novel building materials, but I try to avoid using an uncommon building material in situations where a common one is adequate.

        It's likely that hemp insulation would also work great in a double stud wall, but if cellulose also works, why fix something that isn't broken?

    2. tech1234 | | #14

      Dense pack cellulose does not settle. The moisture buffering of the cellulose is one of the keys to making this wall system safe.

      1. Expert Member
        MALCOLM TAYLOR | | #15


        Can we can say that for sure? Revisiting Thorsten Chlupp's dense-packed house found some settlement. We don't know how common that is.

        I'm also not sure we really know how critical cellulose is in keeping these walls safe. It certainly helps, but I haven't seen any data to show that similar walls insulated with batts become risky.

  6. gstan | | #8

    Colorado is pretty much a "Goldilocks" climate for super insulated walls of any
    type - I have designed, built and lived in three of them - one in Durango, Colorado
    and two more in Flagstaff, Az. (7000 Ft. Elev.) - my brother presently lives in
    another outside Woodland Park, Colorado. Total time actually living in them
    exceeds 26 years - Never the slightest hint of a problem with any of them.

    My advice: Skip the foam! If you don't think your R-rating for the walls is high
    enough with your present design then go to mineral wool insulation - if that
    still isn't high enough consider placing a foam layer in the center between the
    double walls (polyiso is the foam of choice for this - one of my houses was
    built this way - there are many constructed this way in Canada - It Works!)
    Last if you really want high energy efficiency install insulated shutters inside
    on all windows & close them at night - this can be done after the house has
    been completed and you are living in it. GOOD LUCK!

  7. mhenson | | #9

    It seems mostly unanimous that the walls should be double stud. Thank you all for the input. I am using cellulose in the ceiling, but am hesitant to use it in the walls. If it settled even three inches over the next few years all the effort put into making this house efficient would be out the walls and expensive to fix.
    I am going to create a post about the entire project give details on the entire building plan and see what the community has to say.

  8. frankcrawford | | #10

    In walls you use DENSE packed cellulose installed to the correct density by a skilled and experienced installer, that is how you ensure the dense packed cellulose does not settle. You can take cores of the cellulose and check that it is the correct density, for thick walls this is generally 4+ pounds / sq ft. LOOSE blown cellulose is used on flat ceilings as you don't care if it settles. Same product different installation techniques and reequipments. Make sure you hire a contractor experienced in DENSE packed cellulose. Don't put foam over cellulose on the exterior as it reduces the permeability (ability to dry out) of the wall assembly. You want air tight but vapour open.

    1. Expert Member
      MALCOLM TAYLOR | | #12

      "Don't put foam over cellulose on the exterior as it reduces the permeability (ability to dry out) of the wall assembly"

      And to maintain a safe ratio you would need an absurd amount. Double-stud or exterior foam is an either/or choice.

      1. Robert Opaluch | | #35

        And Malcolm you have pointed out previously that its extra work doing both double stud and exterior insulation for one wall. Pick one or the other, not double work. There's enough work building a house already.

  9. onslow | | #16


    I have to question your being in CZ 5B at 10,500ft. I live in SW Colorado at 8,000ft and my HDD range is around 7200 by chart and functionally a bit higher since I like to keep the house at 68 average. My air temps go down to -10 occassionally, but the local design temp is -16F since the county elevations range up to 12,000 plus feet. You might want to read this GBA link:

    Regarding the double wall vs. foam, Malcom is correct that it is an either or choice. If you have already commenced with the double wall framing then you need to go through with it. You will need to attend to air control from the inside as closely if not more than your exterior WRB.

    I threw in with foam and a friend went with double wall. We are both happy with the energy results. My friend is a bit suspicious that settling has occurred in parts of his walls, but the energy hit seems to be low. That said, my concern would be that potential moisture build up may occur in parts of the walls where settling has changed the dew point profile.

    To your more generalized question of which choice to make if starting over, the TL/DR part.

    I enjoy the same low humidity environment you speak of and agree with the observation that double walls will likely not see the cyclic build up of moisture recorded in certain homes in humid environments. I recall at least one article showing sheathing moisture content approaching 23% in winter months. The summer did see a drop of levels in the sheathing, but I remain a bit skeptical of long term success if average humidity in the summer remains above 35% average. Something many parts of the country endure with little relief likely coming with warming trends and rain pattern shifts.

    That particular home may have had interior leakage points that were allowing too much moist air to get into the walls. This brings up certain practical matters about construction. I generated a fair amount of friction with the crew over detailing the WRB and other points. Everyone was onboard with the concept of energy efficient construction, but just the same, getting past the "close enough, we need to get this done" thinking can be tough. Finding a truly competent cellulose installer will be the biggest problem you have. Not having every trade poking holes in your interior vapor barrier will be the next great challenge.

    Interior moisture load will be your greatest issue to control since a double wall system puts the dew point in the cellulose mass. The buffering "feature" of cellulose does have limits, as well as a dispersal time that may not successfully moderate moisture build up in pockets of thin (or absent) insulation due to settling. Holes poked in the interior vapor control barrier aren't going to be helpful. Imagine the combination of settling and control barrier holes in a bathroom or laundry room.

    I chose to go with deep foam over a Henry Blueskin WRB and tight sheathing details. This approach puts the dew point for almost any interior humidity level outside the sheathing into the foam most of the year. My vapor open interior approach meant I could use batts in the walls and the underside of my unvented exterior foam roof. Once I had detailed all exterior framing to minimize air infiltration, I had no worries about what the electricians, plumbers or drywall crews did. Not needing an interior vapor barrier made life much easier for me. Bothering with special electrical boxes and gaskets was not required either.

    Was it more expensive? Hard to say, since I only built the one way. The foam was certainly more than cellulose per R largely due to labor. Still I am happy - just like my double wall friend who lives at 9,500ft.

    One thing you will need to consider carefully at 10,500 ft is your heating choices. While the new ASHPs are leaps and bounds ahead of where they were in 2014, you do have to factor in about a 30% de-rating to account for the much lower air density at your elevation. Same applies to the jetting of any fueled appliances or furnaces/boilers. Even in the relatively low humidity environment we enjoy you will likely experience frost cycling of ASHPs. Not sure how much an energy hit that is not having used them.

    From my 2014 perspective and limited ASHP choices, it became more sensible for me to use cove heaters throughout the house. Not baseboard or wall heaters. The cove heaters have proven themselves quite capable with my above normal insulation levels of whole wall average of R34 - roof R52 - windows R6.6 - basement exterior R15 walls and R15 slab. Some think cove heaters look odd or won't heat well. I find they fade from awareness and have the advantage of individual room control, virtually silent operation, freedom from animal hair, no furniture placement issues, and due to initial outlay a much better deal than 2014 ASHP options. I am also free of maintenance and snow problems. They are very slow to bring the house into a steady state, but that only occurs once a year when I finally convince my wife that we must close the windows.

    It is true that I lack AC as a side benefit of using an ASHP, but at our lower altitude we just vent out the house at night and close up in the morning. This has worked for 6 yrs and I hope the climate stays the same. I am not entirely hopeful, as the weather data over the last 20 years shows a distinct upward trend. At your altitude it should take even longer to get hot.

    Last note, get the best windows and don't make too many operable. A different friend warned me after his nearby build. The winds are just too much here.

    1. Expert Member
      MALCOLM TAYLOR | | #17


      A really interesting and comprehensive reply!

    2. maine_tyler | | #21

      "That said, my concern would be that potential moisture build up may occur in parts of the walls where settling has changed the dew point profile."

      Obviously settling would be bad thermally speaking but I'm not sure how it would lead to increased moisture accumulation risk. It seems to me it would be the opposite. That part of the wall essentially becomes uninsulated and has the 'furthest out' dewpoint (most energy throughput).

  10. maxwell_mcgee | | #18

    To the original poster, can you please elaborate on your comment re: moisture issues with Zip/OSB?

    Is there a specific set of issues that you're referring to? Or just general questions around their durability/resilience?

    Curious because I'm planning a build soon with Zip-R likely to be the sheathing/"outsulation" choice, but I'm not completely sold on this system myself yet.

  11. Expert Member
    BILL WICHERS | | #19

    I've been reading these replies that are concerned with settling and had a thought (that I haven't tried, so it's a "maybe this helps" idea, with not guarantees): If you put some horizontal blocking into the double stud wall, being careful not to bridge all the way between the two sides of the wall, this might help to limit settling somewhat. The horizontal blocking would act to split the wall into several, shorter, vertical cavities instead of one big one from top to bottom. The blocking would help to carry some of the load of the cellulose above, which would limit the amount of compressive force excerted on the lower levels of the cellulose fill. This would help to mimize settling.

    If I were to try this, I'd probably put in three equally spaced horizontal pieces, to split the wall roughly into thirds.


    1. Robert Opaluch | | #36

      Good idea, to reduce settling by breaking up the tall pile of cellulose inside the wall. Maybe just use a membrane vs wood blocking, to reduce the thermal bridging through the wall from wood members. Dense packing requires membranes to limit the volume to successfully dense pack to highest densities.

      Another suggestion: Use some foil faced polyiso at the top of the interior of the wall before blowing in cellulose. That way, if there is settling, you have a radiant barrier which will reduce the heat loss to the top plate(s) of the double wall (or any window sill plate), where a settling gap might appear later. You also add polyiso insulation directly attached to those plates as well, to reduce the effects of any air circulation in that gap that may appear.

  12. gstan | | #20

    Why all this blather about cellulose? The man already said he didn't favor it in walls in
    post #9 above (and I wholeheartedly agree with him) I also agree with BILL WICHERS
    in post #19 above but there are better solutions than cellulose anyway. For those of
    you with a real interest I suggest taking a look at the work of Natalie Leonard, the
    Founder of Passive Design Solutions in NOVA SCOTIA, CANADA.
    It takes a little sleuthing to find her wall designs, but
    you will often find a double wall with a layer of Polyiso sandwiched
    in the center and one of the fluffy insulation products in the stud walls on either side.
    She claims to have designed in excess of two hundred units (I have been involved in
    the design and construction of two houses with this wall type). This is undoubtedly
    about the best double wall that can be easily constructed with today's technology.

    1. Robert Opaluch | | #37

      If you consider polyiso, think about the construction sequence. I thought it would be a great alternative until I thought through the construction sequence and building inspections. Normally you would put up double walls, then do electrical and plumbing, then inspections, then insulate after framing/rough electrical/plumbing inspections. Polyiso is rigid so looks like you'd have some constraints about adding the inner (double) wall later to make polyiso installation easiest, and not being able to inspect framing on the outer wall after adding polyiso and the inner wall. Double walls also involve fire breaks every 10' or less, and window and door framing that span the inner wall. How do you sequence that with no inner wall in place? I'm not saying it can't be done without serious attention to the construction sequence, and cutting and fitting polyiso in places. Just that it seems that its way easier to use blown-in or flexible insulation alternatives.

      I figured that adding another inch or two of inner wall space with dense pack cellulose, or saving an inch or two of inner wall space with polyiso, results in comparable R-value.

  13. onslow | | #22

    in reply to Malcom #21,

    My reasoning is based on settlement likely being partial rather than a total loss of cellulose coverage from inside to sheathing. If the settling resulted in a level collapse of material then the interior warmth would probably reach the sheathing at a sufficient temperature to avoid condensation on the sheathing. The risk of course would be dependent on exterior temps and air/moisture exchange rate into the wall. This harks back to my comments about hidden damage to the vapor barrier from construction activities.

    My past experience with blown cellulose suggests that the placement density is very dependent on how well the machine "fluffs" the feed stock as much as the installer's skill. Even in a dense pack wall, core sample variations will show up. Loose blown attic cellulose often settles into a landscape of denser mounds and fluffier valleys not directly created by lazy placement. This is why I would suspect that a failure in dense pack walls would also show up as a valley style slump rather than a perfectly flat slump. The possibility of some insulation remaining against the sheathing would prevent sufficient warming from interior losses resulting in potential condensation inside the wall.

    Admittedly, it is speculative as to how harmful random hidden gaps in a double wall assembly will be over the long haul. The buffering ability of cellulose is still present. For mhenson's and my environment we are ahead of the game for seasonal drying time. However, I grew up back east and the typical humidity levels spring, summer, and fall would make me a bit more nervous. If the cellulose in a double wall did settle a uniform amount across all bays, I would be more concerned about total energy losses first.

    Given that the temperature profile through an 8-10" dense pack wall inherently puts the dew point in the wall over the heating season it might well be more relevant to look at the colder insulation below any settlement gaps. The seasonal cycling of moisture into the total insulation mass and redistribution of that moisture by diffusion may already have been analyzed in depth, but I haven't sought out that info being a "foam guy". I have however pulled out lots of very damp feeling insulation during basement renos. Not a very fair comparison, since the concrete is often more of a problem than the room moisture. Still, we do advise people to not use vapor open insulation in basements on the assumption that room moisture will get to the wall and condense. ( And thank you for #17.)

    To Mr. McGee's question #18,
    The Zip OSB is sealed by the foam on one side and a permeable coating on the face. The taping of seams completes the effective encapsulation of the OSB with controlling elements. (might want to seal the bottom and top raw edges) Rates of moisture absorption are controlled unlike "raw" OSB and I believe the adhesives used in it's fabrication are also different. Claddings in combination with rain screen or drainage mat materials should allow for proper exchange of moisture loads over the seasons. Download the tech papers and review them. There have been discussions about proper installation and engineering when using the very highest R sheets, so search GBA for more on that.

  14. mhenson | | #23

    Very helpful inf0rmation, what I am currently thinking, based on the comments above:
    Cellulose isn't a big risk, but why do it if there are better options.
    I am looking seriously at HD R21 fiberglass (probably unfaced) between the 2x6 studs.
    1-2 inches of Polyiso in the void between the two walls R6-R12 and then HD fiberglass batts between the 2x4" part of the wall R15. That would give me R42-R49 walls and still be in budget.
    Please shoot holes in this idea, I thought Polyiso in the middle might create a problem, but it looks like it doesn't. Obviously the exterior wall would be well taped and sealed.

    1. Expert Member
      MALCOLM TAYLOR | | #25


      I think a double-stud wall with rigid foam in the gap would perform very well. It would be both expensive, and from a sequencing of construction perceptive difficult to built though, and I'm not sure it has much of an advantage over using a variable perm membrane in the same location.

      1. mhenson | | #27

        Hi Malcom,
        Please explain the variable perm membrane. My main concern was getting enough R-value in the available space. Maybe it would be better to go with HD fiberglass batts between the 2x6" studs, then cut foam strips to put between the 2x6" studs and the 2x4" studs and fill between the 2x4" studs with fiberglass batts. I read an article on that very thing and it looked good. That would also give me a +/- R42 wall. R21 for double batts and foam in the gap between the studs. Still a lot of work to cut and install the foam strips.

        1. Expert Member
          MALCOLM TAYLOR | | #32

          The beauty of double-stud walls is that you can have any amount of R-value you want by adjusting the thickness of the wall, without adding a layer of foam. With cellulose that's easy, batts require a bit more attention to making the gap a standard size.

          Some double walls bury the air-barrier / vapour-retarder closer to the middle, which makes it less vulnerable to damage. Using either foam or a variable perm membrane there means the wall functions well in both cooling and heating situations.

          There are a huge number of viable variations possible for double-stud walls. What makes sense depends a lot on who is doing the work. One possibility is adding 2" foam strips to the studs and plates between the two walls and using 6" batts in both. I doubt you would ever find a contractor willing to do that. Similarly, it you decide to use a continuous layer of foam between the two, that probably means the insulation contractor needs to return twice to add cellulose or batts. Again something they will be reluctant to agree to unless they receive more compensation.

          1. StephenSheehy | | #34

            We put a membrane attached to the outside of the interior stud wall. Outside that is around 8 inches of dense packed cellulose. Inside the interior stud wall we used fiberglass batts. One big advantage is the elimination of penetrations in the air barrier from electrical receptacles and switches.

  15. MartinHolladay | | #24

    M. Henson,
    Here is a link to a relevant article: "Exterior Rigid Foam on Double-Stud Walls Is a No-No."

    1. mhenson | | #28

      Thank you Martin,
      I have dropped the exterior insulation, and am now focused on the double stud wall solution. Not only because it seems to be a better solution, but my plans submitted for the building permit included the double stud wall, before I started quadruple thinking everything.

  16. mhenson | | #26

    Sorry for the double post, but:
    As I said, they are getting ready to pour the stemwall in ICF blocks, so there is still a little time to get this right. I just read a couple articles on the sustainability of fiberglass and they were very positive with modern manufacturing and plant based binders. But, you guys know the facts please post. Also, EPS may be better than Polyiso in this case, as it doesn't off-gas and doesn't lose R-value over time. Is there a good reason to go with rock wool (or something else) vs fiberglass. Is it smarter to go with the Zip System or CDX for the siding (I have read so many articles with differing opinions that it just becomes confusing)? Also the inspector has told me if I go with taped CDX I have to do a Tyvek wrap over the it.
    All my contractors want to go with Smartboard, So far I am insisting on Hardie Plank because of the fire danger when you live in a forest. All the windows are Alpen triple glaze/argon filled as they are designed to be installed at this altitude and they will be fully detailed and sealed with Siga Wugluv (assuming it gets here in time).
    Thank you all for your patience! I have been working on this design for over a year, read at least a hundred articles. The information density is so high and balancing budget, sustainability, R-value/U-value, code/inspector requirements, subcontractor capabilities, and practicality is very daunting. The other thing that has put me off cellulose is there are no competent contractors in the area, period.

  17. mhenson | | #29

    Now I feel like an idiot. Of course!
    unfaced fiberglass between the 2x6 studs, then a semi-perm membrane stapled to the studs. Then faced fiberglass attached to the 2x4" wall with rigid foam between the studs and I am done. No sag, and an R42 wall. I am following one of the many excellent articles to get the truss to stud wall interface right so no worries there.

    1. Expert Member
      BILL WICHERS | | #30

      Yes, you can do that, but normally you’d use unfaced batts for the inner layers and only have faced batts on the side facing the living space.

      I would probably use netting between layers if you want extra support. The Kraft facing isn’t particularly strong and is prone to tearing.


      1. mhenson | | #31

        Thank you Bill, I edited the question as the light bulb finally lit! All the confusion is gone. As you can tell I am not an industry professional, but a computer network engineer trying to build a house right.

        Thank you everyone for helping me get my mind around the obvious!

  18. mhenson | | #33

    As an FYI I found bulk 2 in and 2.5 inch closed cell backer rod in both rolls and precut lenghts for cheap and will stuff that between the studs.

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