More Q&A Spotlight
Writing from somewhere in the Pacific Northwest, Joe Norm is looking for advice on selecting a wall assembly that will perform well without breaking the bank.
“Building in Zone 4C Marine, and looking for a budget efficient wall assembly,” Norm writes in a Q&A post. “Climate here is not extreme, so I do not plan on having a 4-inch foam wrap or anything that colder zone people will do. But stopping thermal bridging in some fashion seems like a no-brainer.”
Norm has ruled out structural insulated panels (SIPs), and he’d like to avoid spray foam if possible. With those conditions in mind, let’s dive into this Q&A Spotlight.
What about a basic 2×6 wall?
Robert Opaluch suggests a straightforward assembly that starts with 2×6 framing and R-19 fiberglass batt insulation. Framing on 24-inch centers would be adequate structurally, but Norm may end up with 16-inch-on-center spacing if the siding he chooses requires more support than a 24-inch o.c. wall can provide.
Other elements recommended by Opaluch include plywood rather than oriented strand board (OSB) sheathing because it’s more water resistant, plus housewrap such as Tyvek or Typar.
“Especially in a rainy, humid marine climate during winter like the Pacific Northwest, a rainscreen assembly is a must to minimize water intrusion and enable better drying potential in your wall,” Opaluch says. “I’d suggest adding a layer of polyiso or EPS foam, then the rainscreen.”
Used foam is inexpensive and it would reduce thermal bridging through the framing. Norm’s rainscreen could be built with 1x material or with strips of plywood.
“Some may suggest a crinkled housewrap product, thereby skipping the rainscreen, but those products cost more than Tyvek/Typar and may not be as effective a rainscreen,” he adds. (For more information on rainscreens,…
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47 Comments
I think a good way to start would be defining the word "efficient". Having a target based on some sort of energy modeling would go a long way to helping define what's necessary for wall system performance.
The air-tightness of the envelope needs to be considered also. If the structure has been designed from the ground up to be airtight, a lower R value structure could work in place of a leakier structure with higher "on paper" R value.
>"I think a good way to start would be defining the word "efficient". "
Others may have a different interpretation, but in this context I'd take "...budget efficient wall assembly..." to mean "measurably better than code without a big up-charge."
A 2x6/R19 wall doesn't even meet code minimum in zone 4C, but it's not an energy hog if done well.
Rigid rock wool is a big upcharge, but rock wool batts, not so much.
Other types of continuous insulating sheathing is a big upcharge, but Bonfiglioli strips delivers a better bang/buck.
Dense packed blown fiber is a big up charge, high density batts of equal or higher R value is cheaper, and will perform if done well..
On the Bonfiglioli method: how does adding the thermal break on the inside of the wall framing compare to using exterior foam or Roxul Comfortboards which would increase the temperature of the exterior sheathing and reduce the possibility of moisture condensing on the interior surface of the sheathing?
Scott,
If you use the Bonfiglioli method, you'll still have cold sheathing in winter. That's not necessarily a problem, however, especially if your wall assembly include a rainscreen gap between the siding and the sheathing to encourage fast drying to the exterior.
What Martin said. In Zone 4C any back-ventilated siding solution (including vinyl siding) is sufficient for wintertime drying of the sheathing.
The interior moisture drives don't actually condense on the sheathing- the moisture is taken up as adsorb, not liquid. In zone 4C the exterior moisture drives are also an issue, not just the interior drives, but vented siding mitigates against damage from either/both.
If you can afford another 15-20 cents per square foot, a layer of 2-mil nylon (Certainteed MemBrain) under the wallboard does a lot to suppress interior side moisture from getting to the sheathing, but also allows exterior drives to dry toward the interior when needed. In this set of experiments done a the Washington State University test facility in Puyallup (deep in the heart of zone 4C) the interior moisture drives were kept artificially quite high to exaggerate the effects. There were both vented and unvented siding configurations, and a variety of interior vapor retarders/barriers tested. See the performance of wall #2, which utilized Certainteed Membrain, as well as the vented cladding variations walls #3 & #10:
http://www.energy.wsu.edu/documents/AHT_ComparingTheMoisturePerformance%20Of%20Wood%20Framed%20Wall%20Systems.pdf
Dana,
Thanks for the link. That's very useful!
Exterior moisture drive to the interior can be an issue with high perms. In such cases, Typar (lower perms) will outperform Tyvek.
Dana: So basically what you're saying is that by using the Bonfiglioli method (along with a product like Membrane) in zone 4c gives you pretty much the same benefits as a typical wall built with exterior rigid foam or Comfortboards (and including a rain screen). The sheathing may be colder from not using an exterior insulation but the rain screen will help mitigate any condensation or moisture problems within the wall cavity.
How does this Bonfiglioli method compare for colder climates, such as a zone 6?
After spending a long time wrapping my head around the "Cold Sheathing Problem", the source of the issue is moisture from the warm humid interior getting into the walls and finding a cold surface to condense on (or adsorb into) during the winter months.
My logical brain then says, if you keep moisture out of the wall you eliminate the cold sheathing issue. Poly does this best, followed by other materials in order of greater vapor permeability. The only issue with saying Poly is best is, if there are air leaks into the wall cavity allowing moisture transfer the Poly wall will dry slower since it only dries to the outside. Unless extreme measures are taken to ensure an airtight vapor retarder installation, a variable permeability material is likely better. BUT, for a given amount of air leakage the variable membrane wall will still end up with the same amount of moisture in it (or more due to higher permeability) as it will not dry to the interior during the winter.
Big thanks to Dana for the link to that study. Finally some science to back up my gut feelings about Poly vs. variable membranes.
The HUGE takeaway from this study (for me anyway) is not so much the differences in building material performance, but the enormous advantage of a ventilated rainscreen. It's pretty safe to say a ventilated rainscreen significantly improves the drying potential (and therefore the durability) of just about any wall assembly.
The Bonfiglioli method will have similar thermal performance to a wall with exterior insulation as long as the overall depth of the insulation is the same, and the thickness of the foam strips is the same as the thickness of the proposed exterior insulation (the thermal break thickness). Example, a 2x4 wall with R13 fiberglass bats and 2" of exterior foam (3.5" + 2" = 5.5") should perform similar to a 2x4 B-method wall using 2" thick foam strips and R21 fiberglass bats (5.5").
I think the B-method wall can make sense no matter what climate zone you're in. It certainly seems to be a budget friendly way to eliminate thermal bridges. The only place it doesn't work is rim joist areas where floors will still be in direct contact with the exterior sheathing, which could be an issue meeting local codes.
>"How does this Bonfiglioli method compare for colder climates, such as a zone 6?"
Thermally it's the same, but in zone 6 you can't get away with JUST the rainscreen to protect the sheathing, skipping the interior side vapor retarder the way you can in zones 5 & lower. Rainscreens provide a lot of resilience in any climate, but the colder the climate the more weeks of wintertime moisture accumulation from interior drives the assembly will experience. Smart vapor retarders are still preferable to polyethylene sheeting in zone 6 though, since most of zone 6 still has a substantial drying season length where the additional drying capacity toward the interior makes it more resilient.
> It's pretty safe to say a ventilated rainscreen significantly improves the drying potential
If there is a vapor barrier outside the sheathing (eg, foil facing), the rainscreen won't have any effect on outward diffusion drying (or inward diffusion wetting).
With a vapor barrier outside the sheathing a rainscreen would still allow the SIDING to dry more quickly and evenly, which protects both the siding and the paint.
Vapor barriers outside the sheathing aren't a generally good idea north of US climate zone 2 except in special circumstances. If the vapor barrier happens to be a facer on insuating sheathing sufficient to protect against interior moisture drives that's fine. Masonry & stucco clad buildings can do better with an exterior vapor barrier in warm/mixed humid climates.
2x4 double framing, offset layout, using 2x8 plates. Single plates if possible. This allow use of smallest fastest growing wood and eliminates most thermal bridging apart from the plates.
Use 1/4" insulating strips between 2x4s on window and doors where framing must coincide. Layout and corners require some thinking, but once you get this technique down, it's not hard at all. Headers have lots of room to be insulated to avoid bridging.
R-30 wall insulation. 80% thermal bridging eliminated. Very comfy living space, avoids issues and costs with rigid foam.
Steve, how much benefit do you really think 1/4" foam is providing?
Your wall isn't really R-30; it's about the same as a wall framed with 2x8s, R-24-ish at center of wall. You have a good point about using smaller-dimension lumber, but the downside is that 2x4s are more susceptible to bowing, so the two layers should be gusseted together, driving up labor costs. Still worth considering, but I save double studs for thicker walls.
Quarter inch foam is about R1, not exactly a thermal break- more functional as a spacer in lieu of shims.
With R15 batts a staggered-stud 2x4 assembly on 2x8 plates hits R30 at center cavity, R19 or so on the 2x4 framing fraction, R8.5 or so on the full-depth framing fraction.
Somewhat lower performance (due to the lower R of the framing fraction) would be 2x8 studs with R30 rock wool batts, which are designed to fit 2x8 framing. That's still a worthwhile upgrade from crummy low density R24-R25-ish batts.
OK, skip the 1/4 strips if this is a sticking point for 'ya. I think it's function was misunderstood. It's just a spacer for ease of framing in the windows and doors. Keeps a thermal break. We stapled on foam sill material where the window and door framing from both sides met, then put the second 2x4 in against it. It's an assembly aide. On doors, we used full 2x8s.
I always thought that two R-15 batts back to back give R-30. Maybe you are assuming using R-13 or else there's some reason that double R-15 does not yield R-30. Anyway R-13 is cheaper and an R-26 wall with probably half or less the thermal bridging of a 2x8 wall is still better.
A 2x8 wall will be more expensive, transmits more sound and have much more thermal bridging loss which does lower the average insulation factor by quite a bit, especially at 16" centers. We have used rigid foam over 2x6 walls to combat that, but I don't have confidence with rigid foam's long term performance and I liked the double wall approach best for the above reasons.
As far as bowing, I don't see the issue. I guess it's about sheetrocking a really nice long wall you can look down? Non-problem in my world and my somewhat limited framing experience. The two double wall (common plate) buildings I've done seem to be performing very well. No gussets seemed to be needed. Quiet and very warm. No foam questions, if you know what I mean.
I am no production framer, but rather a (mostly retired) alternative builder who has worked mostly with Adobe and Straw as well as occasional framed. The client I built the double wall structures for is really happy. When I build for myself again that's what it'll be. A lot easier than Straw Bale and better performing than Adobe. Not interested in single wall stud framing outside of very mild climates. Not for anything I would live in.
Anyway, just throwing it out there to put into the pot of ideas. It was a great solution in practice, but I dunno, may not hold up well in theory !
I'm not sure using small, fast -growing trees is much of an advantage. There is a case for using "junk" species which would otherwise have no value, as the basis of engineered wood products, but frequently harvesting small diameter timber just quickens the cycle, leaving less time for the trees to sequester carbon, form a functioning forest eco-system, and yields very little structurally usable lumber.
How much of a cost differential would there be after factoring in labour and materials between the Bonfiglioli method vs rigid foam or Comfortboard? To me it would be a lot easier to just add rigid foam or Comfortboard to the outside of the sheathing instead of all the fusing with the Bonfiglioli method. Not to mention I now have a better insulated wall with almost no thermal transfer.
The B-method is basically a wall with continuous exterior insulation, just inverted, and should work just as well (minus the colder sheathing, if that’s even an issue). See my response above.
Labor is more, no doubt, but material cost is substantially lower, especially compared to comfortboard. I guess it depends how much extra time it takes.
A modular (pre-manufactured) insulated cladding like the http://www.icap-usa.com system accomplishes all of these goals and lowers installation cost too. The product costs more, but that is offset by the significant reductions in labor costs. It is also assembled in a climate controlled factory opposed to on-site assembly,
Unless the cladding is un-vented and somewhat airtight, how effective is adding that layer of insulation?
"Framing with 2x8s yields R-24.5 for center of wall,” Maines says. “That’s more than 30% better than standard 2×6 construction"
Technically that's correct, but when you look at actual savings it's 30% of a vanishingly small number. The relationship between insulation value and energy/money savings is not arithmetic. At R10 you've reduced energy loss compared to R1 by 90% (U1 vs U0.1), you have to double your insulation to R20 just to gain another 5% (U1 vs U0.05). Yes, R20 is "50% better" than R10, but it's 50% of 10%. So one has to do $ savings to $ cost calculations to determine if it's really worth going from 2x6 to 2x8. I hope my explanation was clear and not too confusing.
R1 is not a realistic starting point. Neither is R10. Since the minimum possible wall is Code minimum, you have to start there.
If you double the R value of a code minimum wall you halve your heat loss. Energy is cheap right now, but that’s changing. For someone building a “forever house”, paying bills on a fixed income is something that a better built wall will make much easier.
>"R1 is not a realistic starting point. Neither is R10. Since the minimum possible wall is Code minimum, you have to start there."
That's right. A code min wall in zone 4C is either 2x6/R20, 2x4 /R13 + R5 continuous insulation, or U0.060 maximum (= R16.7 "whole wall", factoring in the thermal performance of all layers in the wall stackup as well as the thermal bridging of the framing.)
Note, a center cavity improvement of 30% isn't the same as an overall performance improvement of 30% on a whole-wall basis.
>Note, a center cavity improvement of 30% isn't the same as an overall performance improvement of 30% on a whole-wall basis.
But if you have decent details for a 2x8 wall, the whole-wall R-value (or U-factor) would not be very different from a double stud wall on shared plates, which would likely also use 2x8s as window and door bucks, or (2) 2x4s with a 1/4" gap that provides little benefit.
If you step up to a more robust double-stud wall system you can get to R-40 (whole-wall) without much additional effort compared to a 7 1/4" double stud wall.
The Department of Energy tested a 2x6 R-21 wall and found it had an "effective" R-vlaue of R-9 because the studs transfer heat and cold. This is why the IECC and ASHRAE implemented the "continuous insulation" requirement in the building codes. When it is zero outside and 70 degrees inside, the side of the EPS towards the wall never gets below 60 degrees. This means the wall cavity never reaches the dew point for moisture condensation, that is why they removed the rain-screen drainage requirement from the code. The studs are now 60 degrees and the R-21 portion of the wall has increased its insulation effectiveness to where the combination of the R-21 and the R-10 gives you an R-value in excess of R-25.
Les, R-9 sounds suspiciously low--do you have a link to that study? Framing lumber has insulating value; that is, it slows heat flow at about R-1.2 per inch, depending on the species. That's not very good, but in thicker walls it adds up. A 2x8, for example, will perform at about R-9. Averaged with insulation in the framing cavities, the whole-wall R-value should be quite a bit higher, in the mid-teens.
>..a 2x6 R-21 wall and found it had an "effective" R-vlaue of R-9
>When it is zero outside and 70 degrees inside, the side of the EPS towards the wall never gets below 60 degrees.
Check your sources & arithmetic on that- they're off by more than just a little bit.
>This means the wall cavity never reaches the dew point for moisture condensation, that is why they removed the rain-screen drainage requirement from the code.
There has never (ever) been a rainscreen requirement in the IRC (or any local US code I'm aware of.) You have to go to western British Columbia for that.
And the SHEATHING side of the cavity of a code-min 2x4/R14 + R5 wall DOES reach the dew point of the stud cavity air in winter. The sheathing temp will often define the dew point of the cavity air sometimes for days/ weeks on end. But rather than condensing it takes on the moisture as adsorb, not liquid water, releasing it as vapor as temperature conditions change. That's why even with exterior side insulation or a rainscreen needs at LEAST a Class-III vapor retarder (and reasonable air tightness to the interior) to limit the amount of adsorb taken up by the sheathing.
Forgot to say the EPS tested on the exterior of the wall was 1-1/2" EPS 1# density or about R-6.6. For this reason, R-10 is adequate. Although R-10 is standard for ICAP, it offers up to R-21 panels.
>"If you step up to a more robust double-stud wall system you can get to R-40 (whole-wall) without much additional effort compared to a 7 1/4" double stud wall."
Remember the title of this piece is "Efficient Walls on a Budget", which may even rule out a staggered stud 2x8 wall. There are no R40 whole-wall solutions that rhyme with "...on a budget" unless that budget gets better defined.
Also note, in zone 4C there is no lifecycle financial rationale for R40 whole walls. In Table 2 p.10 of BA-1005 suggests R25-ish as the likely limit, at 2009 vintage energy costs and equipment efficiencies.
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
Since that was published heat pumps have gained 15-20% in efficiency at essentially flat inflation-adjusted pricing, and rooftop PV has dropped by over half, with a lifecycle energy cost less than higher-priced electricity markets even without subsidy. The new-improved financial rationality sweet spot ten years later for zone 4C is probably on the lower side of that mark, perhaps a bit over R20 whole-wall. That's achievable with something like 2x6/R23 + 1" polyiso continuous insulating sheathing or 2x4 + 2" Bonfiglioli strip/R23 rock wool. That's a lot cheaper & easier than 2x8 framing, staggered stud on 2x8, or any double studwall approach, and the cost difference may be more rationally applied to a better class heat pump or rooftop PV (or both).
Dana, I agree about R-40 not being for budget-friendly walls, and why I think keeping the labor and details as simple as possible is a smart choice.
Have you ever actually installed Bonfiglioli strips? I have not, but have talked with builders who have, and we all think they are slow and finicky. Same for exterior insulation, at least in zone 6, where it needs to be relatively thick; adding only an inch would be much simpler, especially with an all-in-one product like Zip-R.
The labor for a 2x8 wall is essentially the same as for a 2x4 or 2x6 wall, with the simplest detailing of any system. It's certainly not my favorite way to build a wall, but it's simple. Another factor is the price of the cavity insulation; if this is a DIY situation, rock wool may be worth it, but like most builders I sub out as much of my insulation as I can, and it doesn't pay to have professional insulators install mineral wool. In the northeast, dense-packed cellulose is a better value.
I have wondered for a few years why builders don't just run 1x4 or 2x4 strapping perpendicular to the studs to hang the drywall on, and that would eliminate ~80% of thermal bridging at the studs. Economy 2x4's are super cheap and labor for nailing up 6 boards is minimal. I've been using fiberglass BIB insulation for the last dozen years, so there wouldn't be any difficulty filling voids created by the two orientations of studs and straps. Also, if a 2x4 horizontal strap were added to a 2x6 stud, the total wall depth would be 7" which is an added plus.
I guess I don't want to be the first to try it because I'm not confident that I could predict every pitfall, but I'm tempted to try it. There are plenty of weatherproofing details that can be pitfalls with exterior rigid foam which I'd like to avoid. I've done rigid foam before and I'm not fond of replacing foam with solid backing everywhere I will need solid attachment, and many siders in this neck of the woods have never installed over foam. I can't gamble with new or complicated weatherproofing techniques when the installers often cannot even understand my language and their knowledge is usually limited to whatever the huge production builders spec on their 100 house plats.
This is certainly not a cutting edge proposal, but it's reasonably helpful, quick, cheap, and not too technical for my workers. I'm working in the PNW (Seattle) area too. Maybe I could get some feedback from the readers whether this technique would lower the sheathing temperature past a safe point where mildew or rot may be a threat. Would CDX over OSB help enough to justify the cost? A rainscreen sounds reasonable for moisture control, and it may well be the way of all future building with wood. Thanks
"I have wondered for a few years why builders don't just run 1x4 or 2x4 strapping perpendicular to the studs to hang the drywall on, and that would eliminate ~80% of thermal bridging at the studs. "
They do just that. It is called a Mooney Wall approach, and you can find articles on it here on GBA.
Stephen,
Here are some links to Q&A threads about Mooney Walls:
"Mooney wall for greater insulation value"
"Modified Mooney Wall (SPF)?"
"Rigid insulation on the interior face of the wall"
Thanks for the helpful links. I just finished reading them. I'm going to do it.
What strikes me about the B-method is that it eliminates a well documented method of improving the thermal resistance of exterior walls (namely adding a layer of exterior insulation over the sheathing to help eliminate thermal bridging and prevent moisture from condensing on the exterior sheathing).
I've read at least a dozen articles on this site about how important it is to keep the interior surface of the exterior sheathing warm but now it seems that all that was needed was to include a rain screen detail. The question now becomes (whether you use the B-method or not) if a rain screen is sufficient in most climates to prevent moisture problems in the wall cavity why bother to install exterior insulation at all? It sounds like keeping the exterior sheathing surfaces warm is no longer necessary.
>"...if a rain screen is sufficient in most climates to prevent moisture problems in the wall cavity why bother to install exterior insulation at all? "
The benefit in overall thermal performance of the thermal break over the framing is a primary reason for exterior insulation.
>"It sounds like keeping the exterior sheathing surfaces warm is no longer necessary."
Not necessary, in climates 5 or lower, but it enhances resilience by keeping the drying capacity toward the interior higher. Without sufficient exterior insulation in zones 6 and higher a Class-II or tighter interior side vapor retarder would be necessary even with an rainscreen (and in zone 5 if there is no rainscreen.) In zone 4C the interior vapor retarder is not really necessary, but adding almost amount of exterior insulation to improve the heating season temperature of the sheathing would improve resilience against interior moisture drives.
For zone 4C the IRC calls out R2.5 minimum on 2x4 walls, R3.75 minimum on 2x6 walls. A layer of 3/4" polyiso over a 2x6/R23 studwall would cut the heat loss of the framing fraction nearly in half, and keep the sheathing sufficiently warmer. That's thin enough foam that most siding can be long-nailed into place, and it would outperform a 2x8/R30 rock wool (or R25, with a compressed R30C fiberglass) wall with no exterior insulation.
> if a rain screen is sufficient
You can definitely build a wall with a rainscreen that won't work.
Exterior insulation keeps sheathing warmer which reduces Winter moisture accumulation. Another mechanism that works is to provide for more drying than wetting. Better than either one is both (warmer sheathing and drying to the exterior).
>"You can definitely build a wall with a rainscreen that won't work."
That's right! There are thousands of existence proofs of that thesis in US climate zones 6 & 7.
There are probably some existence proofs in zone 4C too, but those would be rare, if only to demonstrate that the more idiot-proof you make something the more creative the idiots become. The code requirement for rainscreened siding in western British Columbia (which is mostly zone 4C) has surely challenged the creativity of at least SOME of those idiots. :-)
From my perspective here it seems to have completely stumped them. I simply don't see or hear about walls failures. Roofs are still an issue though.
I recently went with Roxul Rockwool, and I priced it against fiberglass batt insulation in a 2x6 wall.
For me in Ontario Canada, it was a no brainer. $1.15 a square foot for Roxul, and $0.99 a sq ft for fiberglass batt. After installing a bit of fiberglass batt in some areas, I believe the upcharge is well worth it. Much easier to cut and install, itchy factor kept to a minimum, just better insulation value overall in the cavity. Sound deadening is also unreal. If you are doing it yourself, spring for a nice insulation knife (I have the Morakniv Crafstmen 13.8 inch, a joy to use on long cuts). You want the true Rockwool/Roxul, as the Dow Corning product is not the same (apparently more brittle and dustier).
Also went with the Certainteed Membrain on the house, as we still require a vapour barrier in my parts to pass inspection. Good stuff, but it is literally 3.3X the cost of 6 mil poly. I will probably do plain poly in the garage.
Dana: Since most rain screens tend to be a rather simple system of either a 3/8 to 3/4" furring strip applied directly to the sheathing (or over a rigid foamboard or Roxul Comfortboard exterior insulation panel) how are people messing it up?
Although in BC, I'm more up in the Rocky Mountains (a zone 6) so I've been planning a 2x6 wall with Roxul Comfortbatt cavity insulation, interior Membrane, 1/2" drywall, and on the exterior 1/2" plywood sheathing, Stoguard Gold liquid barrier, 1 1/2" Comfortboad, 3/4" furring strips and then lapped cedar siding. It seems like a simple, logical wall to build, or am I missing something?
Scott, the most common way I see rainscreen walls "messed up" is with bad flashing details.
Rockwool comfortboard is expensive..but it seems to eliminate the need for expensive smart vapor barriers if you use it to keep the sheathing warm..and those smart control layer products are mind numbing EXPENSIVE. and then the is the labor to tape them. And depending on the siding, the strapping thats holding the siding can be the rainscreen. Also, its R value is higher per inch so that has to factor in. Being vapor open and able to withstand some wetting..does it even need a rainscreen. All these things lower the cost of the overall wall.
>"Rockwool comfortboard is expensive.."
Yup!
>"... those smart control layer products are mind numbing EXPENSIVE. and then the is the labor to tape them."
MemBrain can be had for less than USD $0.17 even in small quantities in the US. Even if it's Mai' Tai's "...literally 3.3X the cost of 6 mil poly..." (which also needs taping) so what? What is the up-charge per square foot for adding rigid rock wool sufficient to not need an interior side vapor retarder?
>" Also, its R value is higher per inch so that has to factor in. Being vapor open and able to withstand some wetting..does it even need a rainscreen."
An R-value per inch higher than what? Wood?
While vapor open rigid rock wool is QUITE air-retardent, and yes, a rainscreen would absolutely be necessary for the rock wool to be able to provide an adequate exterior drying path- there is effectively zero convection vertically through the rigid rock wool. (Really!) Vapor diffusion into a more free-flowing rainscreen would be the ticket.
>" All these things lower the cost of the overall wall."
Show us the math, pray tell.
Re: the Bonfiglioli wall. Any pro or cons to placing the insulation strips on the exterior side of the studs?
User ...471,
A few cons I can think of:
- The exterior sheathing is no longer effectively providing shear resistance for the wall.
- Exterior cladding, or furring, can't get additional supp0rt by being fastened through to the framing.
- The foam puts the studs out of plane with the floor rim-joists, and roof gables, which would have to be furred out to match.
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