Zip R9 in Climate Zone 5
I’ve poured through this site and others in an attempt to decide on a wall system for a new house I am building. I’m in zone 5 (12572), the house is a simple gable box (4 walls + raised heel truss gable roof + full basement) with plan area of about 1,100 s.f. per floor – so modest size and simple massing. I am not shooting for any particular performance target, I am looking to get the best building I can for the relatively tight budget I’m on but no particular metric.
I am in the building industry and perhaps unduly biased (negatively) but I highly prefer to keep the detailing/system as simple as possible. I’ll buy a slightly less efficient system if it means its more likely to get done right.
Based on the above I had arrived at: clapboard siding (boral or cedar, tbd) + rainscreen (homeslicker or sim.) + zip R9 + 2×6 with rockwool batt. Air sealing will be the best I can draw and enforce within reason.
Despite a lot of reading I’m finding a hard time getting full comfort with the zip r9. From what I’ve been able to gather my ratio of batt to exterior rigid should be about safe (even with polyiso derated?). In the chance that there is condensation the back face is the polyiso not the osb, the back face of the osb seems relatively well protected. The rainscreen should give me a little extra insurance. I’m an engineer and not worried about the structural issues (or I should say I will sort them out). 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. Is this a safe and reasonable assembly for my zone?
I would have expected this assembly/zone to have been beaten to death but I can’t find any good q&a or info on my setup in my zone. Any input on this assembly or pointing me to an article is appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The IRC prescriptive wall design for climate zone 5 is R13 + R5 continuous exterior insulation. That is a ratio of fluffy insulation to foam of 13/5+2.6. Your proposed wall system is R19 + R9 (significantly better than code). Your ratio is 19/9 or 2.1. Lower is better, so you should be good. Yes, you should maybe derate the insulation a bit, but remember that the Zip-R is actually rated at a full-panel R value of R9.6, with the OSB making up the difference.
I believe the rockwool is a bit higher than R19 (stated as R23) and from what I've read the recommendation is to derate polyiso by about 85% for cold weather, so more like 23/7.5 = 3.0. Is this accurate/problematic?
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. It may work anyway, but at least read below links and then be strict about code interpretation and conservative about the building science. Personally, I'd avoid OSB (cold or cool), use a wall that is > 1 perm to the exterior and use a Class II on the interior. For example, EPS foam exterior to plywood or EPS foam with no wood sheathing.
https://www.iccsafe.org/wp-content/uploads/proclamations/TN06-Vapor-Retarders_pdf.pdf
https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
The above responses are in line with what I've found so far, i.e completely opposing opinions.
I'm a structural engineer by trade and arrived at the assembly by reading and hounding associates of mine. The zip-r was recommended as a starting point by peer/colleague architect that is knowledgeable and uses this setup on some of their projects. 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.
I'm finding it surprising the completely opposing opinions I get on this. 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. 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?
Nat, while the IRC prescribes R13 + R5 for your zone, that is an energy requirement and does not guaranty an assembly free from moisture accumulation. Table R702.7.1 lists what is needed to change from a class 1 interior vapor retarder to a class 2 or 3 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. In your zone 5A, that ratio is about 28% of the total R-value on the exterior. That keeps the condensing surface above 45°F, which may be cutting it close, so it's always safe to increase the exterior-to-interior or exterior-to-total ratios.
With R-23 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. You may be fine with their R-9 product but I would err on the side of caution.
Hi Michael.
Do you feel content to follow the IRC when using ZIP R (as opposed to continuous insulation outside of the sheathing). Will, as Jon suggests above the, the cold OSB create the possibility of absorbing moisture? Or is the inside surface of the foam layer of the ZIP R the only condensing surface to be concerned about?
Brian, I think Zip-R can be used safely; even if the R-value is low enough to allow moisture accumulation at the interior, as long as there are no air leaks the OSB should last a long time because essentially no moisture is getting through the polyiso. 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.
> Table R702.7.1 lists what is needed to change from a class 1 interior vapor retarder to a class 2 or 3 vapor retarder
That's not what it says (it's I/II to optionally III). But +1 on the suggestion to be conservative (eg, use more than min actual exterior foam and/or a Class II interior side vapor retarder even where Class III is allowed).
> essentially no moisture is getting through the polyiso
Even with perfect pass-though air sealing, there are still two mechanisms that allow moisture through the polyiso. Circulating convection from the interior (through the unsealed foam edges) and vapor diffusion. Depending on some other details, it can be much more than "essentially none".
Thanks, Michael. This corresponds to what I had above about derating the polyi by about 85% in which case the numbers look iffy, it sounds like you agree? If so do the following provide sufficient insurance:
1. install interior smart vapor barrier (seems easiest/cheapest)
2. change rockwool to fiberg batts at r19 to improve my ratio in the safer direction
3. flash and batt
Out of the three replies we're at 1 = this is good, 1 = this is terrible, 1 = this is maybe ok but on the fence. I'm finding this very interesting. There must be a huge hole in the market for mass production safe wall assemblies that are easy to build/replicate. I can't imagine the average homeowner wants to remotely delve into these debates.
Hi Nat.
Building science and what high-performance builders actually build is often in conflict which is one of the reasons you will get tons of opinions on questions like this. But done conservatively, many wall assemblies can work.
Two things I would suggest. 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).
Nat, I agree with Brian's three points. While I am all for trying different higher-performing assemblies, I think it's important to be at least somewhat conservative--failures tend to create a lot more news than success stories do. Variable permeance membranes are affordable insurance, but they don't solve everything. I'm not as anti-fiberglass as many "green" builders and designers, and it is lower carbon than mineral wool, but I would recommend erring on the side of additional exterior insulation. Or 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.
Thanks, Michael & Brian.
I wasn't setting out to try and do anything different. I assumed zip-r if anything resulted in a more conventional system - not less.
It sounds like the r value taken at face value is ok on paper, when derated it's marginal. How sensitive is this ratio - I would assume that it already has some safety baked in so if I'm good on paper + a smart vapor barrier.... but it sounds like you still feel that's too risky.
Is zip-r generally not favored by the community? What if I went to the r12? It sounds like it works on paper but there is still hesitancy.
I don't like the concept of exterior insulation. The detailing seems finicky to me for what I consider a rough trade. I spend about 1/2 of my work days on various construction sites and unless you're hand holding or policing I find that any detail that requires repetitive attention to detail is likely to get messed up. Am I wrong on this and most contractors can learn these details without much risk to my envelope? Aren't the door and window details fairly finicky?
Not to hijack my own thread but assuming i'm opposed to exterior insulation and any type of double wall - what if I frame a conventional 2x6 wall with zip sheathing, then I fur out the interior of all studs with strips of 1" insulation + 3/4" furring strip. I can then do R30 rockwool batts (sized for 2x8), and no tricky detailing at the exterior. This sounds like easy work and not reliant on perfect execution. I could probably fab and attach all these strips in a weekend.
> assuming i'm opposed to exterior insulation
Then use a Class II and good air sealing on the interior side and > 1 perm (eg plywood) on the exterior side. This is code compliant and Lstiburek recommended.
Hi Nat.
Some builder have concerns about the structural integrity of ZIP R and the difficulty of installing it right, basically getting nails to drive properly. 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.
I think you are right to be concerned about the complexity of the details it takes to incorporate good water management with exterior rigid foam insulation, but it is going to depend on the contractor. And it is becoming more and more common, so you may be able to find someone who has experience with it.
There's at least one builder who likes the wall system you just described and uses it on all his home and hits great performance numbers. FHB published his approach, which you can read here: Breaking the Thermal Bridge
Thanks, Brian. That article is great - good to know I'm not completely crazy with that idea. The zip-r still seems best (for me) if I could make it work but the differing opinions make me nervous. I don't have the knowledge to make the call and since there seems to be so much hesitation I may go away from it. I'll have to look at some of the Baczek stuff you mention before I let it go.
I like this interior stud break idea. In fact I can't believe no one is mass producing 2x6's with this stuff already adhered. This feels like it would be dummy proof, cheap, and keep the exterior detailing very clean. (Unrelated but one thing that jumped out at me is the 3rd page image in that article - there's no need to end up with three studs below a small window like that, framing clips could keep that entire assembly to one stud - thermal mass mess as shown there).
I like it too. Even though there are a lot of benefits of continuous exterior insulation, I like the simplicity of flashing details without it. If you went this route, consider plywood sheathing sealed as an air barrier. Then, any number of decent WRBs will work.
Looked at some Baczek articles, looks good but it also looks like he's sponsored by huber so that leaves a less than desirable taste.
I'm not sure of the nature of that relationship and I get your concern, but I have know Steve for a long time and he was using ZIP products before he got so involved with them.
Nat,
Over the years a few companies have produced thermally broken studs, and for whatever reason they haven't been able to made a go of it. T-Stud is the latest (and most complex). We will see how they do.
As to the differing opinions on the building science: I would liken it to then various approaches to reinforcing concrete for seismic response that structural engineers take. Because the conditions the structure may face vary, and are to an extent unknown, so to do the responses. That's the same with wet sheathing. The factors that may make the same wall safe or unsafe are fairly complex. It's not surprising you are getting differing opinions. My own approach is to avoid building assemblies where there is any doubt. That's one sure way to remove the risk.
Nat,
I think one of the reasons that you're getting different opinions is that you're pushing the envelope for this material. With FG in the wall and no derating of the polyiso, you look good on paper. But with Rockwool and derated polyiso, you're not so good. You talk about a low-risk design and this might not be it. Adding a smart vapor retarder to the inside can certainly help, and air sealing is a must. Diligent air sealing has lots of payoffs. Unfortunately, it is one of those tasks that is fussy and demands attention. I find that it is one of the hardest things to get builders to do right unless they are already invested in doing it as a business practice.
Personally, I'm not a big fan of Zip-R. From a structural standpoint, I don't like decoupling the sheathing from the framing. In 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. OSB is just so intolerant of moisture that makes me worry too. Essentially, you're doing the wall inside-out. From a building physics standpoint, exterior insulation should really be exterior to the sheathing. I test a lot of houses. Houses with a few inches of insulation on the outside rarely have problems with condensation. Those without it often do have problems with condensation.
If you're really opposed to exterior insulation, continuous interior insulation, or cross-strapping and batts or blown-in insulation can get you similar reduction in thermal bridging, but you have to watch the permeability of all of your layers. Plus, you've got both heating and cooling seasons to deal with. Vapor drive reverses in the summer. Somebody hangs a mirror or a painting on the wall and suddenly there's mold behind it in the summer.
This is why there's no one-size-fits-all approach. We've got 8 climate zones in the country, and moisture levels from desert to rain forest. Nothing works everywhere.
Not looking for a one-size-fits-all approach, not sure how you got that impression. I only need my size.
Thanks, Malcolm - the T-Stud looks neat. I have to think this industry is ripe for some technology. It's hard to believe that people sorting out details in forums and then site building with loads of insulation, taping, caulking etc. is the future.
The consensus seems to be that my zip-r wall should go in the rubbish bin. With that in mind I will explore exterior insulation a bit more to see if it looks like something I can trust a local builder to pull off. Given that all systems have risks associated would anyone mind listing off a few of the main risks I take by doing exterior insulation? I assume that with the number of pieces involved, outie window detailing, taping/flashing etc. my main risk now is poor execution leading to exterior water penetration as opposed to condensation? Is that accurate? I'm assuming there's also a decent labor uptick to sheathe, then wrap, then insulate and tape, then fur, before siding? Is this a week of labor x 2 guys plus materials?
Probably no answer to this but will ask anyway. When we talk about these assemblies being risky is there any relative scale? I'm having a hard time understanding if this means .1% chance that in 50 years you'd have mold/rot or 5% chance in 1 year etc. I can't tell if I'm in a really fancy coffee shop and the baristas are arguing about whether or not the beans should have been pre-digested by an exotic cat and it doesn't really matter to me, or it's a legit concern and I need to order the right brew.
Poor execution of what are more complex details, and a reduced ability of the wall to dry to the exterior would the the two I would worry about most when specifying exterior foam.
A 2"x6" wall that has been thickened, with strapping (a Mooney Wall), or with interior foam strips (Bonfiglioli), or even a double-wall, are all almost risk free as long as you include a rain-screen cavity, and air-seal well. The advantage to all of them is they involve few skills outside the set most builders possess.
The risk is pretty climate dependant. In warm dry areas it's pretty low for every assembly. Cold or wet climates both are much less forgiving. An improperly detailed exterior wall or poorly vented roof here in the PNW will show damage very quickly. A wall with an inadequate thickness of exterior foam and poor air-sealing, or the wrong vapour-retarder, will be similarly problematic in cold regions. I know it sounds alarmist, but the consequences can be pretty severe.
Thanks, Malcolm. Those are what concern me as well and the source of my initial avoidance of exterior insulation. I will delve into the details more to see if I'm its not as complex as I imagine. It is definitely not the standard in my area, i.e. if you take a drive past 20 houses in construction you're unlikely to spot even one using exterior. I've found a few contractors that say they can do it but i'd be seriously limiting my pool, or counting on a learning curve for a newbie.
No one addressed my question about flash and batt. If the main concern is that R9 is marginal wouldn't flash and batt move this well into a safe zone? With an inch I'd be at say R15 even with derated polyiso which should take care of the condensing surface? (Almost looks like I could go to zip r6 if using flash/batt?). I'm in 5A by the way if that buys me any cushion.
If for some odd reason you are set on using a Class III on the interior, table R702.7.1 footnote a) addresses flash and batt. Note that it requires a different (more conservative) R value ratio. Or vented cladding (which you have).
Warmer wood sheathing (the result of exterior insulation) is a good idea. If you don't want to do that, then air sealing (preferably interior and exterior) and perms (interior and exterior) become even more important (see Lstiburek link in #3). Done right, you don't need exterior insulation for a good wall.
Jon,
I'm not trying to be "odd". Clearly I'm asking here because I acknowledge that I don't fully understand the various systems and I'm looking for input from others that have expertise. If it's odd or a bad assembly so be it, I don't have any dog in the fight - just trying to learn about this prior to building a house and clearly it's not as if there is clearly documented agreement for someone like myself to quickly reference.
It's not like I invented some new assembly that I'm hoping has cracked a code. My initial research showed this being used and recommended in many places, for example as noted above by others the many articles by baczek.
Or as another example the below from another gba thread I found on zip-r:
"With sheathing, keeping it "warm (and thus dry)" is not so much the goal, but the means of achieving the goal, if that makes sense. What you don't want is for the rate of wetting to exceed the rate of drying for a long enough period of time to cause damage. Keeping the sheathing warm and dry (on the inside of the foam) is a great way to achieve this goal, but it's not the only way. Zip-R can also achieve this balance, because the foam on the inside controls the rate of wetting from the inside, the membrane on the outside controls the rate of wetting on the outside, and the ventilated rainscreen gap allows the sheathing to dry to the exterior." https://www.greenbuildingadvisor.com/question/zip-system-r-sheathing
So based on my initial research I thought it was at least a system to consider. Since the initial objections were concerned with the ratio and condensing surface I was asking about flash and batt which seems to solve that initial concern. Does it? It sounds like you are saying that zip-r is a fundamentally flawed system regardless of how you stack it up in your overall assembly.
Nat,
I'm in the land (Toronto, Zone 5) of double vapour barriers where code is 2x6 with R5 exterior insulation and interior poly. This is standard construction here, even when done by track builders with minimal attention to details, it works.
I wouldn't sweat too much the ratio and such, as others have said, install an interior vapour barrier, get a decent warm side air seal and you are good.
If you want to save some money, you can go down to the R6 panel, in zone 5 the difference in energy use is probably not worth the extra cost. The yearly energy use between an R28 (R9 zip) vs an R24.5 (R6 zip) is not that much in Zone 5.
If you are tight on the budget, a 2x8 on 24OC wall gets you an R24 wall with standard building materials (R30 batts) and construction. You can get an extra R1 boost by going with fiberboard sheathing.
My biggest beef with Zip is that proper water management requires some attention to details (getting the tape rolled or install liquid flash properly). Call me a retro grouch, but I prefer my WRB lapped and shingled, as long as gravity continues to works, the wall will always drain properly.
Akos,
That's good advice. My reservations about exterior foam are only then it is thick enough that it affects window placement and flashing.
I'll join you in the retro-grouch club.
Thanks, Akos. Very interesting to see the different opinions.
Looks like I'll need to reconsider all options and also do a pricing exercise to factor into the decision. If anyone is aware of a paper or source that has attempted to compare cost of the various systems can you point me to it? I'm sure a lot has to do with how typical the system is for the locale and hence the labor cost (uptick or normal) - but any info out there would help.
There is a lot of analysis out there for high R value walls. At the end of the day, it really comes down to what the trades are comfortable to work with. For example, I find working with 2" to 3" of exterior foam very easy, this extra cost compared with the overall cost of the building is noise, going with it is a no brainier.
Some links:
https://cdn.ymaws.com/www.nibs.org/resource/resmgr/BEST/BEST2_016_EE6-4.pdf
https://nnca.ca/sites/default/files/Optimal%20Wall%20Design%20for%20North%20Feb%2017-2016.pdf
http://third-level.com/wp/wp-content/uploads/2012/09/AIA-Convention-2015-Bryan1.pdf
Based on the feedback above I'm now trying to learn more about exterior insulation. I'm not clear on what the redundancy/path for water is that makes its way into the exterior insulation. I'd assume some exterior bulk water will find its way in. Don't you end up with water sitting on top of a panel somewhere and in contact with your osb for an extended time?
From what I found the idea is that as long as on the interior side you are open you can dry that way. So is the logic that water that intrudes into my insulation matrix (staggered panels) gets absorbed by my osb and eventually diffuses to the interior? Is this deemed much less risky than my original zip-r configuration which works on paper but has some risk of interior condensation?
I was thinking that maybe you need a double rainscreen - i.e. gap behind the insulation such as a dimpled mat but from what I read the answer seems to be no. I'm really struggling to imagine how this works. How does the wrb even help if it's got eps/xps smushed tight to it?
Nat, I'm not sure if you've seen these items but they may answer a lot of your questions:
https://www.greenbuildingadvisor.com/article/how-to-design-a-wall
greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
https://www.greenbuildingadvisor.com/question/r47-wall-and-r-66-roof-mind-the-gap-the-almost-perfect-wall
Also check out the "related articles" links in each article.
> redundancy/path for water is that makes its way into the exterior insulation.
DrainWrap between the sheathing and foam helps remove such moisture out the bottom. Exterior insulation that is vapor permeable (eg, EPS or mineral wool, not foil faced) removes moisture originating from either side.
While I understand the desire for a risk ranking for every possible wall design, it's not available. WUFI (simulation software) is probably closest to that.
Hi Nat.
One of the nice things about exterior rigid foam insulation is that you sort of get redundant WRBs. For example, if you use foil-faced rigid foam insulation and tape the seams, that's a pretty good WRB. Many builders still install housewrap behind that rigid foam and some go so far as to install drainable housewrap behind that rigid foam. There is a slight energy penalty, but the drainable wrap prevents trapped bulk water.
The decision you'll have to make in this scenario is what to flash your windows, doors, and other penetrations to. Here's one article about windows in thick walls: Innie Windows or Outie Windows?. Here's another: Installing Windows in Thick Walls. And one more: Dudley Boxes for Windows.
In this example, you have lots of options for cavity insulation and yes, you would want no more than a class III interior vapor retarder, providing you get the right ratio of exterior continuous insulation to cavity insulation.
Thanks, Brian. To a layperson it sounds like I'd want a drainage mat behind the insulation - seems like the energy penalty you mention would be worth it. I can imagine a foil faced would be a good barrier - but for any water that does make it past isn't it essentially trapped at that point as I assume the foil is more or less close (low perm)? In that case it seems like if I didn't have a drainage mat behind I risk trapping water indefinitely unless the insulation and taping/flashings are perfect. I guess that negates any value in using a standard zip system, if I need to put up a drainage mat anyway might as well get a wrb with one incorporated and not spend the money on zip.
Jon - I'm not looking for any rating as in "this is a 7", I just meant a more general consensus/feeling that the risks associated with this system are less than my initial. Until recently I had never thought about any of this so I'm just trying to get some informed opinions about why/how people have ended up choosing one set of risks vs another (acknowledging that they obviously all contain some risks).
Hi Nat.
As you have now seen, there are lots of ways to design a wall that will work. What I described above is one pretty common approach. The best primary air barrier is the sheathing in this system, which can be sealed with caulk and tape. But the foam with taped seams will help too.
Yes, foil-faced foam is considered vapor-impermeable. In this case the drainable wrap is a good option. So, you won't need to use ZIP. Also, some risk averse builders use plywood instead of OSB sheathing. Plywood becomes a bit more vapor open when it gets wet and handles wetting and drying cycles better than OSB.
Akos - You mentioned that exterior insulation was not a big number relative to overall building cost. I'm looking at this option now and I found one contractor doing it, he showed me a recent house he did that was very small and compact and the exterior + furring etc. was a +$15k. Does that seem ballpark-ish correct or would it be cheaper if it was typical in my market?
Hard to ballpark these costs. My cost was material plus hourly to install, so it was much cheaper than contractor price (1100sqft with 2" of exterior polyiso added $4k in material and hourly cost, also a bit extra in window trim costs).
I would say $15k for a couple of inches of rigid is too high, at that point I would design for a different wall assembly.
Hi Nat-
Surprised that Aerobarrier has not been mentioned in this thread. Air leakage is your biggest enemy both in terms of energy loss and potential decay conditions as it is your biggest source of moisture transfer. If your comfortable w/ ZipR use it, having exterior insulation is huge. Keep your assembly vapor open in both directions, so don't use poly on the interior- I don't know of any respected building scientist that recommends an envelope design with poly. The Aerobarrier will drop your ACH down to passive house standards if that is where you want to go, so be prepared to mechanically ventilate with an ERV/HRV.
Good luck.........
I see that gba did a blog on my q&a. One suggestion for the editors I'd make is that it seems like you should reach out to the original poster for input. A few things I think could have been added in there to make it useful for others in my boat that come across it.
I've not been able to make any further decisions on the zip-r. I had some emails with Huber engineering and they were pretty adamant that this is a safe assembly in my climate zone. I understand their inherent bias but at the same time it's at least another data point and I assume they're in this for the long haul.
I'd like to propose another question in hopes of resolving whether this boils down to zip-r just shouldn't be used, or it's my particular mix/assembly. Keep in mind that I didn't start down this path trying to hit any metric. As an engineer I initially assumed that with a little legwork and mental sweat equity I can/should probably bump up my insulation and air sealing to get a better than code house with minimal uptick in cost, i.e. I figured I could do better than the average builder house by digging into it a bit but I'm not shooting for anything in particular. Turns out I was wrong and it's not so simple. So with that in mind:
- For those opposed to the assembly, would it be more palatable if it was fiberglass batts with the R9. This tips the ratio a bit more to the safe side.
- What if I did 2x4 walls with rockwool batts (r15) with the zip r9?
I think maybe the overall issue is this idea of "dangerously" thin exterior foam. Provided there is a reasonable warm side air barrier, ANY wall with some exterior foam will always preform better than one with no exterior insulation.
With more foam, once you get above a certain thickness, the permeability of the assembly doesn't matter so much. Basically just a more robust assembly. It doesn't mean that a wall with less foam will fail, just means you have to be careful with some construction details.
This is probably a good one to read through:
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed
For your specific question. The 2x4 wall with R9 gives you an R22.5 assembly. The 2x6 with 24OC and R9 is R27.5.
Overall, in zone 5, they are both pretty decent walls, really comes down to your costs as which one to go with.
My preference would be 2x6 as you can build more stories and taller walls.
I brain stormed most of the same considerations Nat did. I decided If I used exterior insulation in zone 5 Iowa, that it must be rock wool due to how ants love tunneling in foam and that it was permeable which allowed it to dry with outie windows..
But due to the deer in the headlights stares Ive received talking to builders in my area about exterior insulation, I decided a 2x6 plywood wall with Bonfiglioli strips and a ACH of less than 1 would be my goal. Exactly how to achieve a super low ACH (best bang for your buck) I hadn't decided on. But Aerobarrier sounded good and I seem to "think" only one super tight air barrier would be best so the walls can dry to the interior.
Typar or other WRB's shed water that got behind the siding, but could perform better at air sealing if they were taped at all seams including the bottom imo. Grace I&WS would be my choice but I don't understand why its only recommended as long as all (or almost all) of the wall's insulation is installed on the exterior side, even if no air barrier is used on the interior side.