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Q&A Spotlight

Choosing a Safe Wall Assembly

An owner/builder looks for comments on his plans to use Zip System insulated sheathing

Zip System insulated sheathing is part of a proposed wall assembly for a new house in Climate Zone 5. The owner-builder looks for comments. Photo courtesy of Huber Engineered Woods.

Wall assemblies that are both well insulated and free of long-term moisture problems are a frequent topic for the Q&A Spotlight. Hasn’t this subject been raked over enough in the past to answer just about every possible question?

Not according to Nat, a GBA reader and structural engineer who is planning his own house in Climate Zone 5. Even after poring through GBA and other websites, Nat finds himself uncertain about the planned wall assembly on his budget-conscious new home.

The house design is simple. A plain box with a gable roof and a full basement, roughly 1,100 square feet per floor. Nat wants to keep detailing as simple as possible. His plans for exterior walls include clapboard siding over a rainscreen, then Zip R-9 sheathing—a combination of OSB and polyisocyanurate insulation—and a 2×6 wall insulated with mineral wool batts in the stud cavities.

“Despite a lot of reading I’m finding a hard time getting full comfort with the Zip R-9,” Nat writes in a Q&A post. While the ratio of batt insulation to exterior rigid insulation appears safe, the insulation rather than the OSB becomes the condensing surface should moisture move through the assembly.

“I see plenty of drawbacks with the Zip-R and I know it’s probably not the best R for my money—but I really like how it seems to involve very little atypical detailing and very easy to make sure it’s well executed,” Nat says. “Is this a safe and reasonable assembly for my zone?”

That’s the key question here, but Nat’s initial post opens the door to other options, such as the pros and cons of an exterior layer of insulation over the sheathing. For more on those topics, use the link above to check the full text of the post.

Let’s start with the building code

Peter Engle starts with the requirements of the International Residential Code for cavity plus exterior insulation in Climate Zone 5: R-13 cavity insulation and R-5 of continuous exterior insulation.

“That is a ratio of fluffy insulation to foam of 13/5=2.6,” Engle says. “Your proposed wall system is R-19 + R-9 (significantly better than code). Your ratio is 19/9 or 2.1. Lower is better, so you should be good.”

Even if the polyiso insulation used in Huber’s Zip-R sheathing is “derated” somewhat to account for changes in its thermal properties in cold weather, the insulated panels should work as planned.

Not so, writes Jon R: “The Zip-R design doesn’t have any ‘continuous exterior’ or ‘over the wall’ insulation and you shouldn’t pretend it does. The OSB will be cold and any air leaking out will condense/sorb on it.”

Although the assembly may work nonetheless, Jon R adds, Nat should read two articles he cites (they may be found here and here) for a better understanding of the building science involved in his plans.

Jon R’s preference would be a wall built without OSB, with a perm rating of greater than 1 to the exterior, and a Class II vapor retarder on the interior.

Opinions are worlds apart

Nat finds this advice typical of what he’s found so far—that is, two widely divergent points of view.

“The Zip-R was recommended as a starting point by [a] peer/colleague architect that is knowledgeable and uses this setup on some of their projects,” Nat says. “After doing my own research it seemed reasonable based on my above points. I recently decided to engage an architect to do an envelope review and his first comments were along the lines of Zip R in this setup will have problems and is not OK.”

Still, Nat finds the distance between the two points of view a little surprising.

“As a structural engineer I often have disagreements with other engineers but I’d be hard pressed to think of a situation where I am at polar opposites on the basics of a fundamental structural system,” he says. “With wall assembly it seems like the building ‘science’ is flavored by a lot of opinion. Surely there is a way for me to determine that this is a safe or not safe assembly?”

A closer look at the IRC

The IRC prescription for insulation levels in Nat’s climate zone is an energy requirement, but it doesn’t guarantee that the wall assembly will be free of moisture accumulation, says Michael Maines.

IRC table R702.7.1 lists what is needed to change from a Class I or class II interior vapor retarder to a Class III interior vapor retarder. “It’s not explicit, but the science says you can extrapolate the ratios of interior vs. exterior R-value in this table for other R-values,” Maines says.

What that means in Climate Zone 5A is that about 28% of the total R-value of the wall assembly should be on the exterior. This should keep the condensing surface above 45°F, he says, but it’s also cutting it close. “It’s always safe to increase the exterior-to-interior or exterior-to-total ratios,” Maines adds.

With R-23 worth of cavity insulation, the minimum safe impermeable exterior insulation is R-8.9. “When this ratio matters is when it’s cold outside, which is also when polyiso’s R-value drops,” Maine says. “You may be fine with their R-9 product but I would err on the side of caution.”

Does it matter where the insulation is?

GBA Editor Brian Pontolilo is curious whether Maines feels comfortable following the IRC requirements when using Zip-R sheathing, rather than having the insulation layer on the exterior of the sheathing (With Zip-R, the polyiso is on the interior side of the OSB).

“Will, as Jon suggests above, the cold OSB create the possibility of absorbing moisture?” Pontolilo asks. “Or is the inside surface of the foam layer of the Zip-R the only condensing surface to be concerned about?”

Maines thinks the Zip-R can be used safely, even if the R-value of the foam is low enough to allow moisture accumulation on the interior. As long as there are no air leaks, essentially no moisture should find its way through the assembly.

But, he adds this: “But just because the OSB is safe doesn’t mean the assembly is safe—you can still get moisture accumulation on the face of the foam facing the stud cavity, at least in theory. I’ve seen condensation in walls insulated with a flash-and-batt system, which is nearly the same thing as the Zip-R approach.”

Be conservative in your approach

Given the dramatically different points of view, Nat has a hard time imagining an average homeowner delving into these issues. But Pontolilo, while acknowledging the differences of opinion, suggests that by building conservatively Nat should be fine.

“Two things I would suggest,” Pontolilo says. “First is that you are not so laid back about the air sealing. If you keep air out of the walls, you greatly minimize the potential for any moisture related issues. And air sealing is relatively inexpensive work. You just need to plan ahead.

“Second, I agree with you that a smart vapor retarder seems like some worthwhile insurance (and can help with the aforementioned air sealing if detailed to do so).”

Along those lines, Maines adds that membranes while variable permeance are “affordable insurance,” they don’t solve all problems. He suggests that Nat consider making the exterior insulation layer thicker.

“Or,” he says, “you may find, as I have every time I’ve tried spec’ing or using Zip-R, that it’s just not the best solution. Exterior wood fiber insulation is vapor-open so you can use any thickness you want, though more is better. Or recycled foam is easy enough to apply over conventionally installed sheathing, without the worries about embodied carbon.”

The structural considerations

With Zip-R sheathing, the interior surface of the insulation is what’s in contact with the framing of the house. The structural layer of sheathing is separated from the studs by the thickness of the polyiso.

“Some builder have concerns about the structural integrity of ZIP R and the difficulty of installing it right, basically getting nails to drive properly,” Pontolilo writes. “But you are an engineer, so I’m sure you can make an educated assessment of the structural aspects of it and a decent framer should be able to figure out how to nail the stuff right.

“Steve Baczek has emerged as one of the leading architects in high-performance home building and building science and he uses ZIP R regularly with no concerns.”

Engle is one of those who questions the structural side of using Zip-R. “Personally, I’m not a big fan of Zip-R,” he says. “From a structural standpoint, I don’t like decoupling the sheathing from the framing. I  know that Huber has the engineering data to show it works, but I’m skeptical. I live in high-wind country, so I worry.

“I also share Jon R’s distrust of the polyiso’s ability to prevent moisture from reaching the OSB sheathing,” Engle adds. “OSB is just so intolerant of moisture that makes me worry too.”

Our expert’s opinion

A few thoughts from Peter Yost, GBA’s technical director:

I hear Nat’s frustration with the apparent inexactness of hygrothermics. It is important to note that how moisture moves in building assemblies, particularly given film theory, is a relatively young science, certainly compared to the physics of structures. That said, below are my efforts to clear some hygrothermal muddiness.

First condensing surface: With air-permeable cavity insulation, the first condensing surface is not the cavity insulation, but the first surface, which is the interior face of the polyisocyanurate rigid insulation.

For an extended and really helpful backgrounder on why moisture accumulates on the interior surface of the innermost exterior sheathing rather than in the cavity insulation, see this Building Science Corp. article by Joe Lstiburek.

I am quoting an early paragraph of this article by Lstiburek to defend some of our hygrothermal muddiness:

“Diffusion is supposed to be easy because all you need to know is one equation—Fick’s Law—and you are done. Ha. Trouble is that the equation is only an approximation, and we can’t really measure the material properties to make the equation work anyway because the properties change with both temperature and vapor pressure. And there are many different materials. Too bad for us that there are temperature differences and vapor pressure differences across assemblies. Too bad for us that there are many materials making up assemblies. Too bad for us that everything varies all over the place all of the time.”

Derating polyiso insulation because of temperature dependency: The best reference on this issue that I could find is BSC’s Info-502: Temperature Dependencies of R-values in Polyisocyanurate Roof Insulation. Some key points:

  • Both BSC and the Polyisocyanurate Insulation Manufacturers Association (PIMA) make a strong point of not extrapolating the testing and results of polyiso roof insulation to walls.
  • There is quite a bit of variation in this phenomenon from manufacturer to manufacturer; we just don’t know which is which. Huber ZIP-R uses Atlas polyiso board, and more than once technical folks at Atlas, off the record, have reassured me that their board works fine, and all of the manufacturers have been working on their formulations to ameliorate this issue.
  • One of the BSC recommendations is to use R5 for any polyiso roof insulation used in a cold climate (PIMA’s Thermal Resistance and Mean Temperature: A Report for Building Design Professionals reports a value of R5.5 for Climate Zone 5 winter value). That means for CZ 5A, 1.5 inches would be R7.5, still meeting code (remembering that we are not supposed to use this info for walls, only roof insulation).
  • This BSC info document is now quite dated (2013).

Yes the OSB in any ZIP-R will be “cold:” This is because it is exterior to any insulation. But:

  • It is not the first condensing surface.
  • It is facing a dedicated air space with both free drainage and ventilation (if the rainscreen is open top and bottom).
  • Any water condensing on the interior surface of the polyiso insulation needs to make its way as a vapor to the OSB. The vapor permeance of the polyiso insulation (according to the ZIP System R-Sheathing ESR-3373) is less than 1.0, making the polyiso a Class II vapor retarder. I would not be worried about interior vapor making its way through the insulation to the OSB. As pointed out earlier, worry more about airtightness, but not with this assembly: the taped ZIP-R creates a formidable continuous air control layer.

Vapor permeance of OSB: Per the APA graph below (resulting from testing by the National Institute for Science and Technology and Oak Ridge National Lab), even generic 7/16-inch OSB has a dry cup vapor permeability of slightly greater than 1.0 perms, more than 2.0 perms at 40% relative humidity and as high as 12 perms at 85% relative humidity. This classifies any OSB as a solid Class III vapor retarder, not Class II. I have seen the results of internal testing of vapor permeance (as expressed graphically as sorption isotherms) by Huber on its panels and in the lower range of moisture content it’s very similar to the APA graph but the curve steepens much more than the APA curve at high RH (85% and up).

Class II vapor retarder on the wall assembly interior: With the right ratio of continuous exterior rigid and cavity R-value, a Class III interior vapor retarder is fine.

Let’s not forget interior wintertime relative humidity: The variables regarding hygrothermal performance of building assemblies in the winter in cold climates includes interior relative humidity. If the winter interior RH is well-managed and/or occupant density is low, there is a lot less risk of interstitial condensation than if RH is not well-managed and/or there are a lot of occupants. A change of 5% in interior RH—say from 40% to 45% results in a change in dew point (at 68°F) of 3.1°F, from 42.8°F to 45.9°F).

Smart vapor retarder insurance: While it can’t hurt to include this in this assembly, I don’t think that it is needed, even as insurance. Having said that, if this interior smart vapor retarder can be economically detailed as also an interior air control layer, that can’t hurt!

Structural comfort with the non-structural polyiso insulation separating the structural sheathing from the framing: Huber did not separate these two components lightly. They chose this configuration so that builders did not need to fuss with exterior rigid insulation details at penetrations like windows and doors. There always are trade-offs. But as with any structural sheathing system, if you get the fasteners spaced right and flush with the sheathing, it’s not hard to get the shear values the lab gets. And they built in the same safety factor on the developed shear that any structural engineer would.

One last thought on this topic comes from Steve Baczek. He notes that in high wind zones, his structural engineer insists on a site inspection of Zip-R panels. In fact, an Evaluation Service Report from ICC-ES (ESR-3373) requires such an inspection for these Huber panels. Steve  believes that any shear panel in a high wind zone should be inspected after installation.

Note from Peter Yost: In the interest of full disclosure: Huber is the lead sponsor of the Homebuilding Crossroads educational series that Steve Baczek and I do. Huber does not provide me or Building-Wright with any other financial support nor does it make any attempt to influence my work at either Homebuilding Crossroads or GBA.

-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.


  1. But Why? | | #1

    An article on creating SAFE wall assemblies uses a picture of someone being obviously UNSAFE in creating a wall assembly...

    1. Malcolm Taylor | | #3

      What's unsafe? Unless the drop is a lot higher than it looks no fall protection is required.

    2. James Someone | | #4

      Seems to be!

      The deck has a rotted joist, the framers wearing a wrist split, and the guy behind him more likely to push him off the deck accidentally than help in a meaningful way.

      1. Malcolm Taylor | | #5

        You guys do much framing?

        - You have no way of evaluating whether what they are standing on is safe from the photo.

        - It's two man sheathing crew. They both position the sheet, then one holds it in place while the other nails it off. That's h0w it's done.

        - One guy has a wrist brace. Probably for carpal-tunnel, something that is pretty common among carpenters. If you looked around the site there is also probably someone with knee-brace, finger-splint, maybe some stitches. If you test the crew they also all will have some hearing loss. So what?

        I'm left shaking my head that either of you would comment disparagingly on something you know so little about.

        1. But Why? | | #21

          BTW, regardless of the height, any fall in that position carries the possibility that he discharges the nail gun into nail gun accidents to see the creative ways and places all you "experienced" framers have managed to "nail" the landing....

          1. Malcolm Taylor | | #22

            Framing nailers have safety noses. Unless he is walking around with the trigger permanently depressed, and modified to fire "bump" he can't shoot anything.

            Let it go.

          2. James Someone | | #25

            Looking closer at the stock photo, in the background a handmade wall bracket can be seen. While this may not be against OSHA reg, it's not best practice either-steel wall brackets and staging exist and can be quickly assembled.

            I've seen a few people forget they're shooting ply nails when building outriggers to install sheathing etc, only to have it collapse when the first OSHA plank gets slapped down on it.

            Great discussion!

      2. BrianVarick | | #16

        Looks like a platform built for a skylifts hooks.

        1. Malcolm Taylor | | #17

          Good catch. I couldn't figure out what that was in the background.

          1. But Why? | | #20

            It is an elevated mobile work platform..Osha regs require railing on that platform and a harness on the worker. Ignoring that, he is overextended beyond his balance point (his toes are extended way over the edge of the mobile platform - your toes are a major part of sustaining your balance) with a heavy object in his hand only contributing to the out of balance situation. It is obviously unsafe, the only question is how unsafe it is based on the fall height.

            At approximately 11:00 a.m. on February 16, 2013, Employee #1, a 33-year-old male with Slone Builders Inc., was performing residential framing activities. Employee #1 was working in the garage area framing up a wall with a pneumatic nail gun equipped with a contact (bump) trigger. The nail gun appeared to misfire/double-tap which caused the employee to lose his balance and the nail gun to come in contact with his face. This caused an unintended discharge of the nail gun. The nail passed through the employee's eye socket and into his brain. Emergency Medical Services transported Employee #1 to the hospital where he later died from his injuries.

    3. James Someone | | #26

      A quick read of Scott Gibson bio doesn't mention of any trades experience. He's in publishing.?

      there s typos also.. Is this real?

      Come on with this smoke screen, enough

      1. James Someone | | #27

        Seems to be about 10 people that keep this website propped up.

        Junk status. Can Scott Gibson pull a 16" layout?

        1. Malcolm Taylor | | #28


          This is a website dedicated to information on energy efficiency through high performance building techniques, with a large library of useful articles, blogs and related news.

          The reason I commented on But Why and your posts on safety was that they are sideshows. Over the years But Why has sporadically posted comments that invariably mock green initiatives or alternate energy. I can't remember him ever contributing constructively to any of the discussions around high performance building, and I'm at a loss to understand why he comes to GBA. You now seem to be happily embarking on the same path.

          Scott Gibson is a journalist. So was Martin. Maybe Scott can't pull a layout. I don't know - and it doesn't matter. So far I don't see any indication you have any useful knowledge about building assemblies for high performance houses. If you are here to learn, great. If you have something to contribute, better still. Otherwise, are you sure you want to stick around?

  2. User avater
    Jon R | | #2

    > I would not be worried about interior vapor making its way through the insulation to the OSB

    People only taping the exterior of SIPs used to think similarly, with sometimes disastrous results. Turns out that moisture flow at the joints (where the OSB/tape IS the "condensing surface of interest") matters. Think about the amount of moisture diffusing through a large area of Class III on the interior and then concentrating in the small cold tape area. Or air flowing from the interior, dumping moisture in the cold joint and then either continuing through a leak or returning to the interior. Zip-R is not hygrothermically equivalent to taped OSB with exterior foam. I'd take additional steps to control the amount of moisture coming from the interior (eg, interior side air sealing and a Class II vapor retarder). But who knows if it will be enough. Summary: don't use 1D mid-panel analysis alone to declare an assembly safe.

    > R7.5, still meeting code

    Except that it doesn't meet the code requirement for "over" the wall. Zip-R foam is inside the wall.

    Huber could pay BSC to instrument/measure houses built with a mix of Zip-R and other common wall assemblies. I find it odd that they haven't. And that they don't publish full perm vs %RH information (a sorption isotherm does not indicate permeability).

    This should be useful to people looking for a simple pass/fail (although I wouldn't use it for the unusual Zip-R).

    1. CarsonB | | #31

      "Turns out that moisture flow at the joints (where the OSB/tape IS the "condensing surface of interest") matters". I've been wondering this same thing but have not found an answer. It seems that there is an inherit flaw where warm moist air from the interior can go through the air permeable batts, goes between the polyiso sheets at the joints, and then hits the back side of the tape used to seal those joints, which is a cold surface and condenses onto the tape/OSB. Does Huber say anything about this? How would would one air seal the back side of the polyiso, spray foam? That seems problematic as the joints cross behind studs. If you need to spray foam the joints it also seems to make the tape redundant.

  3. User avater
    Walter Ahlgrim | | #6

    “His structural engineer insists on a site inspection of Zip-R panels.”

    Let’s worst case if you do pay the structural engineer to inspect and they decide all 120 nails in most panels we over driven.

    What is the remedy? Could an engineer risk recommending add more nails between the ones there now you have nails 1.5 inches on center in compromised OSB. Would that wall now be an become an untested system with the engineer being proficiently responsible for any problem?

    Or will he play it safe and recommend pulling 128 nails from each panel and replace them all.


    1. James Someone | | #7

      Malcolm, the stock photo is bogus. That helper aught to be put on clean up. I can eat a sheet of zip for breakfast by myself.

      You have no evidence to discredit me either.

      Good day!

  4. User avater
    Steve Hall | | #8

    I don't understand the worry about condensation getting through the polyiso to the OSB sheathing when this same surface touches all the studs directly?!

  5. Malcolm Taylor | | #9


    The studs touch the foam on the warm side - hopefully warm en0ugh that the foam doesn't become the first condensing surface. Any moisture that gets to the OSB will find that sheathing close to the same temperature as the outside.

  6. User avater
    Steve Hall | | #10

    Malcolm, Peter Yost's comment in the article is that the polyiso interior would be the first surface, where condensation will occur. I worry a lot less about the OSB than the condensation inside the stud cavity against the structural framing.

    1. Malcolm Taylor | | #11

      Peter seems to be saying that in a wall with permeable insulation, the foam is going t0 be the first condensing surface no matter what. I'm struggling with that. What if there was 24" of foam? Would the interior of it still be the first condensing surface?

      1. User avater
        Peter Engle | | #12

        My (possibly flawed) understanding is that the first condensing surface is the first/coldest/least permeable surface. It's the combination of features that does it. Most of the time, you can guess what that surface is going to be, but as Joe L has pointed out, since it varies based on so many properties, you could be wrong.

        Take the case of a Zip-R wall with drywall, fluffy insulation, Zip-R, perhaps a permeable housewrap because you don't trust Zip sheathing, a rain screen, and siding (Whew!). The polyiso insulation in the Zip-R is the least permeable material in the stack, at just shy of 1 perm. The interior surface of the Zip-R also floats near the dewpoint of the interior air at the design conditions for the house in question. Together, these features make the surface of the polyiso the "first condensing surface." So your answer is, yes, even with 24" of foam, the interior surface would be the first condensing surface - with that much foam, you would never get any condensation but that doesn't matter.

        I share Jon R's concern about the taped seams, but I do point out that there are significant differences between this wall and a SIP roof. First, walls and roofs see much different air pressure drives. Stack effect makes roofs always see positive pressure from the interior. Walls are more variable and closer to the neutral pressure plane. Also, with early SIP construction, there was generally no attempt made to control air leakage at the interior. I think that, for the purposes of this discussion, we've pretty much agreed that an interior air barrier is a good idea. I think that interior air sealing is always a good idea. Finally, a lot of the SIP failures also had other stresses that can be managed. Some of them were in very cold climates, not CZ5. Some of them were also associated with high internal RH, and Scott mentions that as a risk factor in his article. High interior RH can be a problem for nearly any wall or roof system.

        After all the discussion, I'm still not backing away from my first impression. For the OP's conditions, his wall should be safe. He's comfortable with the structural and installation details, and his climate zone is not that harsh. If he takes the further step of installing a smart membrane as an interior air barrier, that makes the wall a bit safer. As long as he reads his framers the riot act about nailing the Zip, he should be fine.

      2. User avater
        Jon R | | #13

        Malcolm: it really is better to use Joe L's term “condensing surface of interest”. Locations that reach condensing temp but don't accumulate moisture are not "of interest". Accumulation is avoided if a) moisture is blocked from getting there and/or b) moisture gets there but keeps on moving outwards (ie, drying >= wetting or flow-through).

        1. Malcolm Taylor | | #14


          That's helpful correction. I'd add to your list of things the stop accumulation that the material is sufficiently warm. That's why the drywall wouldn't accumulate moisture, even if it was backed by an impermeable vapour-barrier.

          To bring this to bear on Steve's posts:

          - If the inner-surface of the foam isn't below the dew p0int of the moist air in the cavity, it wouldn't accumulate surface moisture or wet the studs that are in contact with it.

          - If it is close to the dew point, it as both Peter E. and Jon stress, reducing the moisture that gets to the foam with inner air and vapour-barriers so that the drying exceeds wetting becomes much more important. Otherwise as Steve fears, there could be damage to the framing members.

          - Because the exterior OSB is protected by the low-perm foam, the amount of moisture it accumulates from the interior can be dissipated by the drying to the outside afforded by a rain-screen cavity.


          1. Tyler Keniston | | #15

            I wonder if this is a time when an air-tight but vapor open tape definitively adds insurance? I know that would be odd to pair a non-zip tape with a zip product...

            The seams are the weak points and also the precise location where drying potential matters, and also the point where a low perm product is applied. In theory, the backside of the tape could be considered the first relevant condensing surface.

  7. User avater
    Peter Engle | | #18


    You are on the right track. If the tape is more vapor-open than some other part of the stackup, then condensation on the backside of the tape would be unlikely, regardless of its temperature. This would alleviate some of Jon's concerns. However, the use of a non-Zip product would void Huber's warranty, at least for that detail. That may or may not be a big concern.

    I would think that, if Huber had any evidence of this sort of condensation causing problems, they would address the issue, either through the use of more vapor-open tape, sealing the insulation edges, or other methods. It seems that Huber has some pretty smart engineers working for them and that they are planning on being around for the long haul.

    1. User avater
      Jon R | | #19

      > If the tape is more vapor-open than some other part of the stackup.

      Like a diffusion port. But you need to do a 3D analysis. For example, say there is moisture diffusing in from 32 sq ft of 10 perm drywall. But with 1/8" joints, that same area has only 1.5 sq ft of exposed Zip tape. Not enough drying, even if the tape were 50 perms (does Huber even provide a tape perm spec?).

      Zip-R may work well, maybe even without a Class II and interior side air sealing. But IMO, there is a real lack of good data and analysis proving this.

  8. James Ray Arnold | | #23

    Good day all,

    I’ve enjoyed the insight thus far.

    Rhetorical question:
    Wouldn’t dense packing the cavities with cellulose be a nice pairing with this system, providing some mitigation for changes in the interior RH? I believe I have read a good GBA article illustrating this potential benefit. To be clear, we are in the Pac NW Zone 4, so, our vapor and temperatures dynamics are a bit easier to manage.

    Additionally, I realize that dense packing to 3.5 lbs/cu ft may provide some air slowing, and not air sealing, but our blower door test results always improve when we test again after insulating. This, plus cellulose’s sorption properties, could, perhaps, provide a little extra confidence with regard to the condensation surface at the seams.

    And it’s made from recycled newsprint; not too bad.

    Good luck with your project.

    1. User avater
      Jon R | | #24

      Agreed, cellulose provides hygric buffering/redistribution and this is beneficial when condensation occurs inside the wall. Either at the joints or mid-panel (continuous foam reduces but usually doesn't eliminate reaching the dew point).

      The other extreme would be moisture condensing on Zip-R and not being absorbed by the polyiso or hydrophobic mineral wool. I expect that this leads to water on the base plate and more accumulation in the OSB at the joints.

  9. Michael Pi | | #29

    Is this a risky assembly from the standpoint of insect intrusion? If carpenter ants like damp foam, and moisture condenses on the polyiso, then seemingly this would create an ideal environment. I wonder over 10 or 20 years whether enough damage could occur to the foam to create areas of loose fasteners where the sheathing isn't tightly sandwiched to the studs anymore.

    1. Malcolm Taylor | | #30


      My experience is that carpenter ants like foam period, not just the damp stuff. So I'd suggest the question is whether the foam in the assembly is susceptible to insect intrusion, not whether the moisture content makes it vulnerable.

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