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5 questions: exterior wall insulation

StrongRoots | Posted in General Questions on


My wife and I are thinking of buying a hard-walled “yurt” to put in upstate NY. Looks beautiful, now I am trying to see how green I can make it. Basically, we would have a 35 foot diameter “yurt” that is made up of 4′ x 10′ wall sections that are screwed to each other and then secured to the floor. A cable wraps around the tops of the walls to turn it into integral unit (this is common in yurts). The company sends the walls and roof that we would attach to the floor. I am figuring out the insulation system for walls, floor, and roof, so then I can estimate the heating and cooling demand. For this post I just want to focus on the walls. The company website says each 4 x 10 section is framed using 2 x 6s, 1/2″ plywood shear panels, drain plane type vapor barrier, and 1x 6 tongue and groove red cedar shingles. They can leave the shingles off, and send separately.

Before installing walls I would put in Triple Guard sill sealer in two spots: 1) where floor meets foundation, 2) where yurt wall meets floor. Q1: Bad/good idea? I also wanted to add exterior insulation to avoid thermal bridges etc. I wanted to avoid XPS and EPS because of the GWP and other inputs. Because we’re in a cold climate, I don’t think polysio would be a good choice. If that’s right, I would remove the water/vapor barrier so the wall can breathe, tape the plywood seams (ZIP tape?) and replace with a “smart” membrane like Plus and ProClima DA before finally putting Roxul mineral wool on the outside with vertical furrows. Then attach the cedar shingles to the furrows (3 furrows 2 foot apart on each 4 foot wide wall section). Q2: Is this a reasonable idea? If so, then for my climate to control moisture buildup/”condensation” I am under the impression that the R value for external rigid foam should be at least 11.25 (I’ll round to 12), and that at least 36% of the insulation needs to come from the board ( Q3: Does the same hold true for Roxul CavityRock (~R value of ~ 4.2/inch, so three inches total thickness giving 12.6 total)? Do you all have a preference for CavityRock MD or DD? For the interior of the wall, I was thinking of using Roxul or Owens Corning mineral wool depending on cost and what you all recommend. This will add another ~20 – 30 in R value (budget dependent). Q4: Should the thickness of the rock wool be a full 6 inches, or, is it OK for their to be an air gap with wallboard/sheetrock? I was going to finish the interior with an earth plaster. I believe plywood as an R value of .62, one inch cedar is 1, so final wall value is 1.4+.62+12.6+30 = 35 – 45. Q5: Overall, is this a solid design or foolish? I read here ( that some recommend R40 or even R60 on the walls, but I am well above state codes (20 or 13+5). Thanks for any insight, I just signed up to this website and it is a phenomenal resource.

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  1. GBA Editor
    Martin Holladay | | #1

    First, can you tell us your name?

    Your description is confusing and possibly inaccurate. To provide advice, we need to first clear up some ambiguities.

    You wrote, "The company website says each 4 x 10 section is framed using 2 x 6s, 1/2 inch plywood shear panels, drain plane type vapor barrier, and 1x 6 tongue and groove red cedar shingles."

    1. The phrase "drain plane type vapor barrier" doesn't make any sense. I'm guessing that you are talking about a water-resistive barrier (WRB) like wrinkled housewrap. If I'm guessing correctly, then you're describing it wrong. It isn't a vapor barrier. These WRBs are all vapor-permeable.

    Perhaps you are talking about interior polyethylene, which would be a vapor barrier. But interior polyethylene isn't a "drain plane."

    2. The phrase "1x6 tongue and groove red cedar shingles" doesn't make any sense. I'm guessing that you are trying to describe two layers: (a) 1x6 tongue-and-groove sheathing, and (b) red cedar shingle siding. But I'm just guessing. Perhaps you are talking about one foot by 6 foot panels that mimic the look of red cedar shingles.

  2. GBA Editor
    Martin Holladay | | #2

    You wrote that you want to avoid EPS (expanded polystyrene) because of the "GWP" (presumably, global warming potential). But the blowing agents used to make EPS don't have a high global warming potential.

    For more information on this issue, see Choosing Rigid Foam.

  3. GBA Editor
    Martin Holladay | | #3

    You wrote that you plan to "remove the water/vapor barrier so the wall can breathe."

    Three comments:
    1. Walls don't have to breathe.

    2. The phrase, "walls have to breathe" is ambiguous. Do you mean "walls need to leak air" or "walls need to be vapor-permeable"?

    3. What you are calling a "water/vapor barrier" is probably plastic housewrap, also known as a water-resistive barrier (WRB). Almost all WRBs are vapor-permeable, so they don't prevent a wall from "breathing" (unless you mean "leak air").

  4. GBA Editor
    Martin Holladay | | #4

    Once we clear up the misunderstandings and ambiguities, we can proceed to the advice. Here are the high points:

    1. The most important thing you can do is to seal air leaks. If there is plywood sheathing -- you didn't mention any plywood sheathing in your initial description of the wall assembly -- then it certainly makes sense to tape the plywood seams. [Later edit: Oops -- my mistake. You did mention plywood. Sorry I missed it.]

    2. All walls are required by code to have a WRB. Don't get rid of the WRB. You need it.

    3. You can install continuous insulation on the exterior side of the wall if you want. Semi-rigid panels of mineral wool can be used. So can EPS or polyiso.

    4. If you plan to install cedar shingle siding over rigid foam, you'll have a problem with fastening the shingles. I'm not sure what you mean by "vertical furrows" -- perhaps you meant furring strips? You can't attach cedar shingles to vertical furring strips -- but if you have panels that look like traditional cedar shingles (not real shingles -- but pre-assembled panels), you may be able to install the panels to the furring strips.

  5. Expert Member
    Dana Dorsett | | #5

    "Upstate NY" is a large place, with climate zones ranging from 4A to 7A (at altitude in the Adirondacks). Being a bit more specific about location would be important.

    Using rock batt insulation in anything but standard dimension framing lumber at standard spacing is usually a mistake, practically guaranteed to have gaps and thermal bypass channels for all but the EXTREMELY OBSESSIVE & CREATIVE batt detailer/sculptor. Getting the near-perfect fit at the odd angles of a round building isn't impossible, but it's considerably more time consuming than using blown insulation. Even at a lower R the thermal performance of blown cellulose will usually beat the "as installed" performance of batts in non-standard framing.

  6. StrongRoots | | #6

    Hi Martin,

    Thanks for the fast reply. I really appreciate the informed advice/guidance. My name is Ethan, my wife is Siobhan, and we have a lil boy named Fionnbharr (yeah, she's just a little Irish...). We just signed up and I need to update my profile. To your points (sorry for length):

    1. Re: verbiage. From the company website: "The walls are framed with 2x6 lumber. The framed walls are sheathed with a 1/2" plywood shear panel, then covered with a drain plane type vapor barrier and sided with 1x6 tongue and groove western red cedar siding. " I mentioned plywood in my original post, I checked today and it is cdx. In my post I called it cedar shingles, not siding, I can see why that was confusing. I said furrow instead of furring cause I farm and it just slipped in. My double bad.

    2. WRB/Smart barrier. "Drainage planes can be vapor permeable or vapor impermeable depending on climate" Mineral wool is unfaced. So I was thinking of using an independent smart vapor barrier. I read where “this is the best way to create an airtight envelope for the house” Using a smart barrier that varied based on the humidity made sense to me. "The goal is low permeance in the winter when humidity is low but it’s critically important to block moisture flow and prevent condensation, and high permeance in the summer when humidity is higher and you want drying potential to both the interior and exterior."…”This vapor retarder membrane has a low permeability level in dry conditions, but if the humidity level within the wall gets high, the material will open up to allow the moisture to dry to the other side. The ProClima Intello membrane is notable because it is well reinforced. It will not tear or split from stapling, and this tolerance of handling makes it easier to work with.” That sounded like a good idea that would line up with my choice of exterior insulation. I assumed this would be my WRB and it would go behind the roxul and over the plywood. I checked with the company, I have innie windows I also read I can do furring strips over the roxul: Does that design make sense to you? I will check to see if my local code guy will approve the ProClima as a sub for asphalt paper or house wrap.

    3. Re: exterior insulation, yes, I agree only XPS has a high GWP, but I wasn't a fan of the other inputs like hexabromocyclodecane in EPS. That chemical has been found in the environment and animals, is being limited in the EU, and is a chemical of concern by the EPA. So if I can avoid I want to, it is my understanding the mineral wool is naturally flame retardant. I did read the weblink you sent, choosing the right foam, before posting. This one was also interesting:

    4. So the basic wall design:
    Exterior tongue and groove cedar siding attached to furring strips over Roxul. The furring strips should give a gap b/n siding and roxul at top and bottom for drying and be part of a rainscreen. Behind the Roxul I would put a smart vapor barrier over the plywood. The cdx plywood would be attached to 2x6 studs. In b/n the studs would be more mineral wool batting. Then MgO Jetboard (or drywall depending on cost). The MgO also breathes which would let the interior mineral wool dry out. It also seems to perform better than drywall Then an earth clay to finish the interior wall. The MgO has an R of 1.2, so my final R is 36-46--if I install everything right.

    5. Air leakage vs. breathability and rot. I am sure you can tell I am a dyi’er. Very excited to build our dream home. I have read that some think the seams of plywood should be taped to reduce leaks and others say not to so the wall can dry and avoid rotting. I have read where plywood has negligible vapor permeability. If I did mineral wool batts interior and exterior, and taped/flashed/sill sealed, then wouldn’t this mean the interior mineral wool could only dry to the inside and the exterior could only dry to the outside? Is drying less of a concern than air leakage? Am I overthinking it?

    6. Final question, assume the interior mineral wool is snugly installed b/n studs so there are no gaps, but it is only 5 or 5.5 inches thick and my stud is 6 inches, is it true that dead air space gap b/n the insulation and dry wall is ~ R1, but this should be avoided and the insulation should be flush with the dry wall cause in practice there will be too many leaks? Lastly, I wanted to avoid spray foam insulations, but if I skip spraying around the studs and plywood, will I always have excessive air leaks, or will the exterior insulation make this not a big deal? Is a reasonable middle path to tape the plywood and have exterior insulation as an air barrier, but not spray foam on the inside?

    Thanks again, invaluable advice. Sorry again for the length, trying to be clear and document things (for my own erudition as much as anything else).

  7. StrongRoots | | #7

    Hi Dana,

    Sorry I was writing a response to Martin and didn't see your comment. I will be in climate zone 6, specifically, near Ithaca NY. The walls are made with dimensional lumber. See my above comment on point 4 in response to Martin, and point 6. Don't want to retype for brevity sake. Does that design sound reasonable to you, or a misguided effort. I really want to avoid the chemicals associated with spray foams.

    To quote Riversong (which has to be the best last name I have ever read): "I would suggest considering environmentally-friendly, non-toxic, fire-resistant, insect-proof, rodent-resistant, and mold-resistant cellulose, which is also significantly hygroscopic and assists with natural moisture management (as long as the thermal envelope can breath - i.e. no vapor barriers)," Riversong writes.

    So would cellulose over interior mineral wool be a good idea to decrease air leaks? How would that jive with a smart vapor barrier?


  8. StrongRoots | | #8

    I just heard from the manufacturer, they use Typar DW, not sure how this compares to the ProClima Intello....

  9. GBA Editor
    Martin Holladay | | #9

    Typar DrainableWrap has a vapor permeance of 10 perms. In other words, it's vapor-permeable. If the manufacturer of your yurt is calling this product a "vapor barrier," they are flat-out wrong.

  10. StrongRoots | | #10

    OK, thanks. Sounds like ProClima would be a definite upgrade.

  11. GBA Editor
    Martin Holladay | | #11

    So-called "smart" (variable vapor permeance) membranes are used on the interior side of the wall, not the exterior side. On the exterior, you want a water-resistive barrier (WRB) -- that is, a membrane that is waterproof but vapor-permeable. The Typar product supplied by the yurt company is fine. You don't have to remove it and replace it with something else. It's the right product -- they just described it wrong.

  12. StrongRoots | | #12

    OK, thanks. So should I not bother with ProClima then? Or would this really help with air leakage if installed b/n the interior wall insulation (either mineral wool or sprayed insulation) and MgO board?

  13. GBA Editor
    Martin Holladay | | #13

    It's certainly important to have a plan to reduce air leakage. A European membrane on the interior side of your wall may be a component of your air barrier system, but it isn't the only way to reduce air leakage. Ordinary drywall is an excellent air barrier.

    It seems to me that you are still confused about air leakage, as evidenced by these sentences: "I have read that some think the seams of plywood should be taped to reduce leaks and others say not to so the wall can dry and avoid rotting. I have read where plywood has negligible vapor permeability. If I did mineral wool batts interior and exterior, and taped/flashed/sill sealed, then wouldn’t this mean the interior mineral wool could only dry to the inside and the exterior could only dry to the outside? Is drying less of a concern than air leakage? Am I overthinking it?"

    Reducing air leakage is always good, so you definitely want to tape the plywood.

    Plywood is a smart vapor retarder -- that is, a material with variable vapor permenace. When dry, it has a vapor permance of 0.5 perm; when damp, it has a vapor permeance as high as 20 perms -- higher than the Typar WRB. So it can definitely dry to the exterior when necessary.

  14. StrongRoots | | #14

    Hi Martin,

    I am learning, so there is a 100% chance I am confused/wrong. Just hoping to become less wrong.... Huge thanks for setting me straight. My simple thinking was where two pieces of plywood butt against each other air would get in, taping would keep air out. So the taped plywood would be a decent first air barrier (outside air to inside). In my head, water vapor is the gaseous form of water, and if it is gas, then it is in the air. So if something is truly air tight, then the wall wouldn't be able to dry out if there was water that condensed from warmer interior air getting into backside of the plywood--especially in winter. Could this mean that the plywood would rot on the interior side? I was thinking in theory, drywall would be second air barrier (outside air to inside, and/or, inside air to outside). But if there was interior moisture in the air, in practice, unless I perfectly installed the proclima and drywall it would find a way inside the wall, and then be trapped by the taped plywood. This would then cause rot. So it seemed like there was a catch 22, if you get an air tight house, you may be increasing the risk of winter/seasonal wall rot, but if you have lots of air leaks then you're losing heating/money. If the interior drywall+proclima was perfectly air tight then I guess interior air couldn't get into the wall and then get trapped by the plywood/tape--but I am 100% certain I will not get the drywall perfectly air tight. I like that MgO board has a lower CO2 footprint than drywall, and is more 'people friendly' (I am guessing due to use of fly ash that can contain sulfur and trace Hg but don't know). But I think installing MgO would definitely increase the risk of interior air causing rot since it is vapor permeable.

    You and others have thought about this, and said the trick is the detail

    Rainwater and poorly flashed windows etc seems like just as or more important to get right. Lets assume I get that right. But I am learning and trying to not do something that is well meaning (green non toxic wall with high R) but turns out to be stupid--which according to my wife is not uncommon...

    I am now thinking based on comments from you and Dana:

    cedar siding > furring > mineral wool > Typar > taped CDX > studs > spray cellulose > proclima > MgO board > earth clay. With triple guard/flashing/tape at bottom of wall, window, roof/wall joints etc.

    I see where you wrote builders are hesitant to put a wet material like spray cellulose into walls, and
    Dana wrote on another thread: "The amount of shrinkage of damp spray depends on the moisture content and density when sprayed, as well as the rate of drying. It's typically still tighter than batt installations, and it can be dense-packed with no apparent shrinkage or settling over time, but dense packing is more labor intensive and costs more."

    Based on this article it looks like 25/75 H2O to cellulose is recommended, hopefully that will minimize shrinkage.

    I am thinking if I wait 24-36 hrs then put proclima on followed by MgO board that may let me mitigate any air leaks from cellulose shrinkage.

    Genuinely curious about balancing air tight walls with the need for avoiding rot. Hoping the wall design I listed blocks water (roxul, flashing, typar, triple guard) and avoids air leaks (taped plywood, triple guard, proclima), while also letting the wall dry (mineral wool dries to outside with furring gap, proclima keeps out interior moisture in the winter and lets me use MgO).

    If I am still not getting hearing something or just plain wrong please let me know. Thanks.

  15. GBA Editor
    Martin Holladay | | #15

    Here is a link to an article that explains the difference between air barriers and vapor barriers: All About Vapor Diffusion.

  16. StrongRoots | | #16

    I think you already answered it and I am overthinking:

    "Plywood is a smart vapor retarder -- that is, a material with variable vapor permenace. When dry, it has a vapor permance of 0.5 perm; when damp, it has a vapor permeance as high as 20 perms -- higher than the Typar WRB. So it can definitely dry to the exterior when necessary."

    So if the back of plywood is exposed to water from moist interior winter air, you're saying it isn't a big deal bc the plywood vapor permeance will increase, letting it vent to the WRB and exterior?

    I guess I am wondering if in the winter, the cold external air will have a lower humidity/water vapor capacity than the warmer interior air. As the warmer, moister interior air eventually works its way into the interior of the wall, it will condense and form water on the backside of the plywood.

    In my head, water and plywood don't mix.

  17. user-2310254 | | #17


    You want to avoid moisture condensing inside the wall. You can do that by paying attention to air sealing and making the drywall as airtight as possible. See this article and videos for more information.

  18. GBA Editor
    Martin Holladay | | #18

    More information on this issue: How Risky Is Cold OSB Wall Sheathing?

  19. StrongRoots | | #19

    Thanks for the article and video. You all and this website are invaluable. My biggest mistake is that I tried to reinvent the wheel and build a wall based on my own research and had not read:

    Don't know how I missed that...

    To my biggest point of confusion: I was getting all hung up on thinking interior moist air would condense behind the plywood and rot it out since it would contact cooler exterior air. But since I am installing several inches of exterior mineral wool, this will raise the dew point and decrease condensation. Moreover, even it does condense, Martin was saying (I think) that the CDX can let it pass through to the WRB and since mineral wool is more permeable than XPS, EPS, or faced polysio, water vapor won't get trapped and cause rot--the wall can dry to the exterior. I was thinking water + plywood is bad, but the whole point is venting it out to reduce contact.

    By putting in ProClima, I will be adding in another air barrier, that in theory, should also help prevent interior air from getting into the wall cavity.

    I am a bit confused how something can let in water vapor (e.g. gas/air) and still be considered an air barrier. But I think it is a relative question (so drywall is 50, drywall with paint is 10 perms, and an inch of concrete or block is still 2.4 - 3.2).

    Re: MgO, I have read where it is less energy intensive and contains fewer toxins than gypsum, and doesn't mold as much. Less CO2 intensive. I have also read where being vapor permeable lets it passively regulate humidity, absorbing it when air is high H2Ovap and releasing when air is low H20vap.

    But, this article showed where the ability to absorb water vapor is a detriment, and the hygroscopic property of MgO leads to crying boards and accumulation of water inside the wall.

    Anybody have experience or opinions on MgO?

    I was deciding between spray cellulose or air krete, looks like a similar cost and both are non-toxic. But based on this article will probably stay with cellulose.

    Thank you all.

  20. GBA Editor
    Martin Holladay | | #20

    Q. "I am a bit confused how something can let in water vapor (e.g. gas/air) and still be considered an air barrier."

    A. Please take the time to read these two articles:

    All About Vapor Diffusion

    Do I Need a Vapor Retarder?

  21. StrongRoots | | #21

    Martin, thanks.

    "Vapor diffusion is distinguished from convective vapor transport — that is, water vapor that moves by piggybacking on moving air. With convective vapor transport, the water goes around materials; with vapor diffusion, the water goes through materials."

    Got it, that gets to the difference b/n an air barrier and letting in water vapor.

    Re: vapor retarder and polythethylene, I don't need one on the inside, but adding in ProClima is a good air barrier and in theory can help with any vapor issues.

    How many times do you have to answer the same question? Thanks for being a patient teacher.

    Final question, do you (or others) have any thoughts or an article in MgO vs drywall?

  22. GBA Editor
    Martin Holladay | | #22

    Gypsum drywall is inexpensive and works just fine. I see no reason to substitute an exotic material like magnesium oxide panels for drywall.

  23. Expert Member
    Dana Dorsett | | #23

    For milled 2x6 framing in a zone 6 climate you'll need 3" of exterior rock wool for dew point control at the sheathing layer to be able to use moisture susceptible sheathing such as CDX without a Class-II interior side vapor retarder, even with a rainscreen.

    Using cellulose rather than rock wool for the cavity fill would be more protective of the structural wood by sharing the moisture burden. The microscopic fibers of celluose are hollow, and can take on a substantial amount of moisture as adsorb without damage or losing function. It wicks, stores and redistributes moisture far better than mineral or glass fibers. The difference in "whole wall R" between R4.2/inch rock wool vs. R3.7/inch cellulose is negligible when thermally bridged by R1.2/inch framing. Cellulose would also be more air-retardent than rock wool.

    Air sealing the framing to the sheathing with a bead of polyurethane caulk for the full perimeter nside every stud bay at every is worth it, even if the seams of the sheathing are taped.

  24. StrongRoots | | #24

    Wow. You all rock. Thank you. Really enjoying learning all this. This is gonna be sweet...

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