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Community and Q&A

Best wall assembly for a Pretty Good House in Zone 7A

Michael Sterner | Posted in Green Building Techniques on

I’ve posted a couple times recently as I question everything about our house design. I appreciate everyone’s feedback. My last inquiry was focused on the execution of the Building Science Corporation double-stud wall: https://www.buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-value-double-stud-wall-construction

Conversations with builders and insulation contractors (regarding this particular double stud wall) have brought to light challenges that can be overcome, but will be labor intensive. For example: 
– Order of assembly, can’t prefab the entire wall and raise as one (taping air-barrier on the exterior of the inside wall)
– Separate dense packing required from both interior and exterior
– Need to “net” the interior and exterior before blowing to allow proper air-movement–fiberboard doesn’t allow enough air to escape
– Need to separate the bays for proper dense-packing
– If we do apply fiberboard first (with air escape holes), we would need to strap down the fiberboard so it doesn’t bulge out

With that, before moving forward, I feel that it is my duty to ask one more time, do I have the right wall system?

I am certain that I don’t have all of the options or pros/cons listed, however, I have distilled this to my front-runners and their main bullet points. I will lay it all out for you.

Wall Assembly Goals: 
– Target R40 insulation / Passive House level airtightness (trying to achieve a load low enough for single point heating system and generally very resilient and durable home)
– Service cavity on interior for electrical, etc.
– Mostly eliminate thermal bridging
– Breathable to maximize resilience of building enclosure (both ways)
– Keep air barrier unbroken and to the inside 1/3rd of the wall per cold climate recommendations
– Easy to understand and build (hard to mess up)


Cost analysis assumptions: All assemblies have furring for rain screen layer. Cost analysis done by breaking down materials costs for a single 4’x8′ wall section. Does not reflect window detailing, does not include labor, etc. No doubt, the labor involved with these assemblies will vary considerably. 

Other notes: Performance is ballpark. Whole wall r-value should probably be considered. Double stud requires service cavity to be insulated, other wall systems include a separate service cavity not necessarily intended to be insulated.

Double Stud Wall (Building Science Corporation version)
Cost: $4.88/sf = $14,042.70 for all walls
Pros:
– Lower material cost
– Likely best overall wall performance
Cons:
– Service cavity must be insulated to achieve r-value
– Potentially higher labor cost
– Order of assembly with OSB on exterior side of interior wall
– Fiberboard will belly out when dense-packing
– Additional steps for insulation contractor (translating to additional cost)
– Modest additional loss of square footage in house
– Slightly increased thickness of foundation to accommodate detailing
2×6 with 4” Exterior Mineral Wool
Cost: $6.01/sf = $17,301.60 for all walls
Pros:
– Exterior insulation, keeps sheathing/framing warm
– Hydrophobic
– Breathable
– Accessible
Cons:
– Requires weather resistant barrier (taped Zip System or sheathing and housewrap) unlike Gutex wall
– Issues with load transfer and compression through material (how to address attaching window eyebrows, porch roofs)
2×6 with 4” Exterior Gutex Fiberboard
Performance: ~R36.6 (as high as R46.6 with SC insulated)
Cost: $6.78/sf = $20,380.00 (includes Gutex shipping) for all walls
Notes: Image of assembly attached. Could use OSB instead of Intello Plus for interior air barrier. Would use dense-pack cellulose or mineral wool batts instead of Havelock Wool.
Another variation of this wall would be the Bensonwood Open-Built wall (2×8 with less Gutex): https://bensonwood.com/building-systems/panelized-enclosures/#wfb-walls
Pros:
– Exterior insulation
– Hydrophobic
– Breathable
– Requires NO additional weather resistant barrier or sheathing
– Seals panels via tongue and groove, no need for lining up ends with studs
– Applies in 1 layer at same time as rain screen furring
– Doesn’t compress like mineral wool
– Likely lower labor cost due to eliminating several steps in other walls (no additional sheathing, two layers of exterior insulation, only 1 wall to frame)
Cons:
– Highest cost
– Inconvenience of obtaining (must order ahead of time)
2×6 with 4” Exterior Foam (for comparison sake though this doesn’t meet my breathability requirement)
Performance: ~R41
Cost: $4.25/sf = $12,232.80 for all walls
Pros:
– Exterior insulation
– Hydrophobic
– Doesn’t compress
– Keeps sheathing and framing warm
– Accessible
– Lowest cost
Cons:
– Does not breath
– Requires sheathing WRB layer
– Puts air barrier on exterior
– Global warming potential

I greatly appreciate your feedback.

Mike

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Replies

  1. Akos | | #1

    Mike,

    Some fiber faced polyiso is vapor permeable, this is what I went with for my high R value wall. Much cheaper than rigid mineral wool and none of the compressability issues. Air barrier bellow the foam, WRB outside of the foam, midi windows. Pretty straight forward build. The cost of the long screws do add up.

    If you want mineral wool, probably significantly cheaper to go with I joist Larsen trusses on the outside of the walls with regular mineral wool batts. 9 1/2" I joist would get you to your R value target. 24" OC spacing plus the thin webs of the I joist would minimize thermal bridging.

  2. Trevor Lambert | | #2

    Noticeably missing from your options is the Klingenberg wall. https://www.greenbuildingadvisor.com/article/the-klingenberg-wall
    If you build it with I joists, you will have individuated cavities that lend themselves to either batt insulation or cellulose blowing. For the latter, you can construct a jig to completely cover one opening and blow into several holes simultaneously. The jig would also have the blow hole(s), so you can have both the fibertec board and house wrap installed prior to blowing. Install furring strips for a rain screen over the house wrap, and the bulging will be reduced to a negligible amount.

    It's probably worth doing some heat loss calculations to determine what r value you actually need to meet your goal. The good thing about the I joist construction is you can increase or decrease the r value just by using a different depth I joist.

    1. Michael Sterner | | #7

      Hey Trevor, love this wall design, however, I am getting feedback from our builder (who has done this wall), that it would be more expensive than any of the options. The other issue, as with thick exterior mineral wool, is transferring the load of the covered porches in our design. Two sides of the house have long covered porches and we'd have to have a different method for transferring load then attaching it to the i-joist.

      Lastly, there are some finishing considerations with i-joists in how the wall is terminated at the foundation that don't jive well with our desired stone clad foundation aesthetic. We looked closely at this because I think it is a great wall system.

      Thank you.

  3. Scott Wilson | | #3

    I've read a few articles about walls and insulation methods in zones 7 and higher and they all recommend putting the insulation on the exterior of the wall sheathing and leaving the stud bays empty so in that case I don't see the point of building double walls.

    I would go the 2x6 with exterior mineral wool insulation route. In looking at the Roxul website I saw in one of their installation manuals their recommendation that when you install exterior insulation that is thicker than 3" you should use a system like Cascadia Clips to help with the installation of the siding and trim.

    https://www.cascadiawindows.com/products/cascadia-clip

    1. Michael Sterner | | #6

      Hi Scott, thanks for your feedback.

      No question that exterior insulation is A way to execute a good building envelope in zone 7+, but is it the best way, all things considered? I guess that is part of what I am trying to figure out. I would welcome reading some articles on why exterior insulation is better when compared with a double wall with infinite drying potential. The other part of this is that in the mineral board and Gutex wall systems (with exterior continuous insulation), it is hard to achieve the same whole wall r-value as a double stud can (and at less cost).

      Doing exterior insulation only and not filling any stud cavities seems like a good idea in theory, but achieving high R-values, transferring loads to the studs, etc. seem to be the challenges.

      While Martin brings up some good points about breathability, it is my desire to have a wall that can dry in both directions. This means installing something like Gutex or mineral wool instead of foam. Taking in all sources I can find, this seems to be the best recipe for enclosure resiliency, until someone convinces me otherwise. I also have an aversion to foam.

      The Cascadia Clips look like a great product for siding and trim, though my concerns with exterior wool and its compressibility have more to do with transferring the load of our lengthy cover porch roofs to the wall without introducing a lot of thermal bridging and other finishing issues.

      Thank you for your time!

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Michael,
    You wrote that you want to make sure that your wall is "Breathable to maximize resilience of building enclosure (both ways)."

    In fact, moisture flow through a wall sometimes contributes to dampness and moisture accumulation. Researchers have found that wall components stay dryest when the wall is covered on the exterior with an adequately thick layer of continuous insulation (which almost always mean rigid foam with very low vapor permeance).

    Sometimes, a wall that can't "breathe" to the exterior stays dryer than a wall that can "breathe."

    1. Michael Sterner | | #5

      Hi Martin,
      Thanks for your feedback. I have come across at least a few articles related to your point, ie:
      https://www.greenbuildingadvisor.com/article/can-exterior-foam-insulation-cause-mold-and-moisture-problems

      How do I quantify or do a reasonable comparison of these wall systems with the understanding that "sometimes" these breathable wall systems could potentially allow more moisture in the building cavity?

      It seems that each of these walls are designed to protect from bulk water and condensation buildup in the form of insulation outside of the vapor/air barrier. Ultimately, I think, even though Gutex and mineral wool are breathable, those assemblies will also ALLOW drying to the exterior. Which seems smart to me, but that is where I need help from experts.

      A foam wrap is the cheapest route from a material perspective, likely still one of the cheapest once you introduce labor... While I do have to consider cost, I am looking for the best, most durable and resilient, high performance wall for my cold climate–how to I distinguish a winner with all of the pros and cons I've already laid out?

      Lastly our builder did a labor estimate on a 8'x20' wall section with 2 windows and a door penetration in the different wall assemblies. The following is what he found:
      2"x 4" double wall with ply on inner wall and fiberboard on the exterior with 1/4" baffles every other stud bay for insulation.
      Labor cost = $31,593
      Material cost = $14,042
      Total cost = $45,635

      2"x6" wall with 4" Gutex and 2"x3" purlin inner chase over Intello air barrier.
      Labor cost = $24,307
      Material cost = $20,380
      Total cost = $44,687

      He didn't estimate labor for the mineral wool or foam installations, however, I think that this shows that I should select based on wall assembly properties rather than cost–they seem to be very similar in cost after factoring in labor.

      Thank you for your feedback.

      1. Doug McEvers | | #8

        I don't know what the labor rates are in Northern WI but I would say it is time to look for a new builder. The cost quotations you have provided for this proposed house are more in line with Manhattan than WI.

        I think you are making this way too complicated for the double wall. I have built a 2x4 exterior load bearing wall with 3/4" fiberboard sheathing and housewrap with rainscreen. A 3 1/2" space between the interior 2x4 wall, and 3 layers of R-15 batts for R-45 total. Do a warm side air barrier, Membrane or the like and some airtight electrical boxes and you have an airtight wall that can dry to both sides. The service chase in my opinion is a complete waste of time, it is not needed for airtightness. I work with builders who are achieving sub Passive House infiltration rates, none use a service chase.

        1. Michael Sterner | | #9

          Thank you Doug. I should point out that the labor estimate is a really, really basic estimate based on a bunch of assumptions. Labor around here is typically going to be $40/hour or so for someone that will have a good attention to detail. This person is also taking their best guess as to what each of these steps will take and likely being very conservative. I am sure that the actual costs will be different.

          I think that if I take it relatively speaking, the point of the estimate at least suggests that I should decide on the wall system I want based on its pros and cons, not on the specific cost of the assembly since they're all going to be relatively close.

          You're probably right on overcomplicating this. I feel like I am asking a question that has been asked a million times. Still, I am preparing to make an investment in what I view as my forever home and want to make sure I make the best decision all things considered.

          I came across this simplified double wall today: https://foursevenfive.com/blog/the-double-stud-wall-simplified-low-cost-high-performance/

          This seems to be a much easier to execute wall that generally has all of the same benefits of the Building Science Corporation wall. You could easily replace the exterior barrier with fiberboard if desired. It could be dense-packed in one fell swoop, it could be built and raised as one, etc.

          In your opinion, is what you describe the best way to achieve a high performing wall given the goals I outlined? Any drawbacks?

          Thank you Doug, appreciate your feedback.

          1. Akos | | #12

            Micheal,

            That wall still has a lot of thermal bridging around the joists/rim board and the roof. Never mind the $$$ in specialty membranes.

            A simple standard double stud wall (wide plywood plate top and bottom) so it can be built and lifted in one shot with exterior plywood/denseglass air barrier interior vapor barrier. You can even tack on a 2x3s on flat on the interior as a service cavity.

            With the interior wall being load bearing, you can overhang the outside wall over the floors to allow for insulation around the rim board.

            Can be either dense packed (better) or insulated with standard batts (more DIY friendly) if the gap between the two walls is a standard dimension.

            If you are worried about the moisture there are a lot of studies:

            https://www.greenbuildingadvisor.com/article/monitoring-moisture-levels-in-double-stud-walls

            Basically the moisture content in the sheathing goes up in the winter, but dries out quickly.

            If the house is air tight with rainscreen, there is very little risk.

  5. Scott Wilson | | #10

    This is the article I was thinking of when I suggested using only exterior insulation and leaving the wall cavities empty, although your area may not get as cold as this.

    https://www.buildingscience.com/documents/insights/bsi-031-building-in-extreme-cold

  6. User avatar
    Stephen Sheehy | | #11

    For what it's worth,here's my wall:
    Cedar shingles, cedar breather, house wrap ( taped), 1/2" advantech ( seams taped)2x4 bearing wall, studs 24" o/ c, 5" space, membrane attached to 2 x4 non- bearing wall 24" o/ c. Drywall. The outer stud wall and space between outer and inner wall filled with dense packed cellulose, blown through holes cut in membrane and holes taped. After plumbing and wiring, fiberglass batts in inner stud wall.

    It was easy to build for my very good contractor who had never done a double stud wall. We hit PH airtightness. It was easy to run membrane long on sides and top to keep it continuous.

    We're in zone 6 Maine.

  7. Michael Sterner | | #13

    Thanks everyone!

    Akos - We would intend to use the interior wall as the load-bearing wall (as you suggest) so that we can properly insulated around the rim joist. As for the cost of membranes, I thought the same thing but once you run the numbers, Intello Plus is really not much more than OSB and it would be easier to install, seal and dense-pack from my perspective. That said, either way would work for the interior vapor/air barrier. Thank you for the link to the article on deep wall moisture.

    Scott - Yes, they're colder than I am here. Either way, in principle, I think it is a good idea. Likely just not practical or necessary for where I live.

    Stephen - Thank you for sharing with your first hand experience. What are the advantages of Advantech? With that, do you increase concern about cold sheathing or does it breath enough that it is a non-issue with an interior vapor retarder and exterior rainscreen?

    What you describe with the vapor barrier on the outside of the interior wall really closely resembles the Building Science Corporation wall–the big difference being that you have a membrane instead of OSB, which is great. You mention it was easy, but curious how finicky it was to get the vapor barrier on the outside of the interior wall? What order did you assemble your wall system in?

    Lastly Stephen, what was your thought process in selecting that double wall system vs. some kind exterior continuous insulation? It is helpful to hear this feedback from someone that recently went through the process.

    Thank you everyone!

    1. Akos | | #15

      Micheal,

      Maybe some guidelines I like to stick by.

      Stick to something that can be built with supplies from a box store/lumber yard/contractor supply. Anything specialty items should be on site well ahead of time with lots of spares. Try to pick details that you can build it with a framing nailer and a circular saw.

      Keep your building shape simple, roof even simpler. If you have to have roof trusses, your roof is not simple.

      Having a larger window/door on on the 2nd floor on the driveway side makes getting materials in easier.

      Any time some says "detail" be ready to spend about $10k. Make sure you want that detail.

      Once you make a decision, there has to be really really good reasons to change the design.

      Best of Luck.

      P.S. OSB and Proclima costs might be comparable. A framer can cover a wall with OSB in no time, it will take way longer to staple/tape/stretch the membrane for someone that has never done it. Never mind the amount of rigidity/strength you loose by not having the osb and dealing with the lack of solid sheathing when trying to mount anything down the road.

      1. Michael Sterner | | #17

        Hi Akos, great tips, thank you.

        I have a very simple roof design–it is a single 10/12 gable, no hips, no valleys. However, I am intending to use trusses simply so that we can blow in cellulose super deep. I am curious what you mean by if you have roff trusses, your roof is not simple?

        Appreciate your feedback.

        1. Akos | | #18

          Micheal,

          Generally you can frame most roofs with dimensional lumber or TJIs.

          The silly decorative roofs currently in fashion can only be done with trusses. That's all I meant. It also encourages builds with unusable attic space, this waste of space really gets to me.

          Sometimes trusses are the better/cheaper solution for conventional roofs, nothing wrong with using them there.

    2. User avatar
      Stephen Sheehy | | #16

      Michael- my builder likes Advantech because it can be exposed to the elements for a long time without any damage. He prefers it to cdx. If necessary, the wall can dry in both directions. The entire North side is covered by an eight foot wide open porch, so not much moisture gets to that wall in any event.

      The exterior walls and roof were framed. Then the interior wall was built on the slab and the membrane applied before the wall was erected.

      In early discussion with our architect, we talked about exterior insulation as an alternative to a double stud wall, but the latter seemed easier. I think it's cheaper as well. Also, the logistics of applying a thick insulation panel, with lots of expensive screws, to the sheathing caused me to opt for double stud.

  8. User avatar GBA Editor
    Martin Holladay | | #14

    Michael,
    In Comment #5, you asked, "How do I quantify or do a reasonable comparison of these wall systems with the understanding that 'sometimes' these breathable wall systems could potentially allow more moisture in the building cavity?"

    Like any other specification decision -- choosing your roofing, for example -- you get educated, you make a decision, and you move on. Every specification has pluses and minuses.

    Double-stud walls are described as easy to build by some builders, but there is evidence that the exterior sheathing in these walls gets damp in February. Since the sheathing dries out in April and May, this seasonal dampness does not appear to cause problems.

    Walls with continuous exterior rigid foam stay very dry, year round, but some builders don't like the window flashing details that these walls require. Both types of wall -- double-stud walls and walls with continuous exterior rigid foam -- perform well.

    In general, you should stop thinking of "breathability" as a virtue.

    If you want to learn more, read "How to Design a Wall."

  9. Michael Sterner | | #19

    Can anyone provide me with some feedback on the Bensonwood wall (Wood Fiberboard Wall 8)?
    https://bensonwood.com/building-systems/panelized-enclosures/#wfb-walls

    - Interior sheetrock
    - 2.5" mechanical/service cavity
    - 7/16" OSB air and vapor control layer
    - 2x8" structural framing 24" on center (with blown cellulose or mineral wool batts)
    - Weather resistant barrier - 2 3/8" of Gutex Multitherm wood fiberboard
    - Rainscreen furring
    - Cladding

    The Gutex would serve as both exterior sheathing and WRB. The wall comes in at about R38 and would be strong, easy to build and detail on the exterior (windows, etc.).

    Are there any concerns or issues I am overlooking? What about flashing for covered porch roof against wall? Does that go below the Gutex or above (since Gutex is both insulation and WRB and there is no other housewrap)?

    I am trying to improve on the mineral wool board exterior insulation so that I don't have to worry about the compression for transferring loads, I also like that with Gutex I can eliminate the sheathing and extra WRB installation...

    My take is if Bensonwood is doubling down on it, it must be a pretty solid assembly...

    Any feedback?

    1. Trevor Lambert | | #20

      Is that R-38 center of cavity? For whole wall, looks more like R-32 to me.

    2. User avatar GBA Editor
      Martin Holladay | | #25

      It's expensive.

  10. User avatar
    Dana Dorsett | | #21

    >"The Gutex would serve as both exterior sheathing and WRB."

    >"Are there any concerns or issues I am overlooking?"

    Without something (else) to provide structural rigidity Gutex can't the only sheathing.

    The 7/16" interior side OSB in the Bensonwood stackup is far more than merely "...air and vapor control layer"- it's what's keeping the place from collapsing in a high wind.

    The 1-9/16" fiberboard is higher density and lower R/inch than Gutex Multitherm, but still probably not sufficient to be the primary structural element against racking forces. The higher density fiberboard with sufficient structure (Celotex, etc) would be about R4 at that thickness (just like SonoClimat ECO4, which is 1.5" thick and a hair over R4.) Gutex isn't sufficient as an exterior side air barrier. It's not clear in the diagram how Bensonwood is making the exterior air tight.

    In a highly air conditioned environment in a high summertime humidity location the high vapor permeance of the fiberboard and low permeance of the OSB puts the OSB at some risk in that stackup. The level of risk would vary with local climate, and to some degree the type of cladding.

    1. Michael Sterner | | #23

      Yes, of course the OSB on the interior serves as much more than the air and vapor control layer–it is the racking strength, as you said.

      Regarding Gutex as an exterior-side air barrier, in this wall I don't think they have an exterior side air barrier–only the interior taped OSB. The Gutex is the exterior insulation and weather resistant barrier.

      I would definitely go for the SONOclimat product (as we were talking about in another post: https://www.greenbuildingadvisor.com/article/design-build-firm-seeks-toehold-in-insulation-market#comment-150287?utm_medium=email&utm_source=notification&utm_campaign=comment_notification&utm_content=view), unfortunately, the 17 hour drive (1 way) is just too much to source a product.

      So, as I understand it, you don't love the Gutex wall. What about 2x6 wall with mineral wool board? What about double stud? Do you have a preference one way or another?

      Thanks Dana.

  11. Rick Evans | | #22

    As I understand it, the Bensonwood wall's air barrier is a layer of zip sheathing on the interior, between the service cavity lumber and the I-joist.

    Im sure this wall would perform well. After all, it isn't that much different than Katrin Klingenberg's design for her Smith House. (I believe her wall has EPS on the exterior rather than wood fiber board?) As far as I know, this house is still standing... :-)

    I am far from an expert on the topic (I work in finance)... but here are the only issues I see with the wall:

    1.) The structural framing is on the cold side of the air barrier. As such it is more subject to the swings in outdoor humidity and temperature. This is probably fine but it would be on the warm side in an ideal scenario.

    2.) With sheathing on the interior, what is connecting the wall to the floor trusses/ rim board? This might be a weaker wall without sheathing to tie into the rim board.

    3.) I would look carefully into wood fiberboard serving as a wrb. There is at least one person from the Building Science Corp that thinks it should be covered with a wrb rather than serving as one.

    I personally like the BSC double stud wall better...

    Good luck!

    1. Michael Sterner | | #24

      Hi Rick,
      Bensonwood has a variety of wall options, but they're not using Zip on the wood fiberboard 8 wall. They just have the exterior Gutex as insulation and WRB. On the inside they have OSB as the racking strength and vapor/air barrier. In that wall, they're not using i-joists, but rather 2x8 framing.

      1. This is the case in many double stud wall designs too. It is, of course, one of my concerns of double stud walls. As far as I can see from the information out there, that is fine as long as the exterior substrate allows drying easily.
      2. Good point
      3. It would, of course, depend on the specific wood fiberboard–most do not qualify as a WRB. It appears that Gutex does.

      I am, without question, not decided... Just have to sift through so many pros and cons. The other one (other than the double stud) that I do think has some real merit is the 2x6 wall with mineral wool boards. Like the attached image or this from BSC: https://www.buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-wall-advanced-frame-mineral-fiber-board

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