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Zone 6-7: exterior wall assembly with a high R-value

KrisCurrie | Posted in Energy Efficiency and Durability on

I had asked a question yesterday regarding high R-values in the wall assembly. Thanks for the great answers. After researching more blogs & answers I think i’m still confused on the best way to go in my climate as the risk of vapour and frost ?

2×6 wall 2’o.c. roxul r23 in cav. / ext.-taped air tight 1/2″ plywood/ 2 layers 2″ polyiso taped/ furring 1×4/ext cladding.=r49 — R 43 cold climate? Still shy of R50-60

If I tape the plywood air tight, is there any need for poly air barrier inside?
Could I add a 2×4 service wall with R15 to bump R value- or add another 2″ polyiso on ext.?@$1.25sqft. or soes the cost of getting high R values for 90 days of the year not a good ROI?

Thanks Kris

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure why you think that you need an R-50 or R-60 wall. Such walls have been built, and you can build one if you want to -- but most experts would say that you are investing in expensive insulation that is unlikely to yield you enough energy savings (or comfort improvements) to justify the added investment. There's nothing wrong with an R-35 or R-40 wall in your climate.

    Q. "If I tape the plywood air tight, is there any need for poly air barrier inside?"

    A. With mineral wool between your studs, it's probably a good idea to have an interior-side air barrier. Ordinary taped drywall makes a perfectly adequate air barrier, as long as you use airtight electrical boxes and gaskets or caulk at critical areas. (For more information on this approach, see Airtight Drywall.)

    Most people have moved away from using polyethylene as an air barrier. In any case, walls with exterior foam need to be able to dry to the interior, so the use of interior polyethylene on a wall with exterior foam would be a mistake. For more information on the design principles behind walls with exterior rigid foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. Expert Member
    Dana Dorsett | | #2

    With only 2" of polyiso on the exterior it's better to use a smart vapor retarder detailed as the air barrier. While code prescribes R11.25 min on the exterior of 2x6 for zone 6, it prescribes R15 on the exterior for zone 7. And while the labeled R-value of 2" polyiso may be R12-R13, it's actual performance in that stackup and climate/location will be at best R10 in the dead of winter. If you go with the 4" it would be north of R15, but not more than R20-ish in mid-winter, in which case you won't need an interior side vapor barrier, but it does need to be air tight. A square inch of air leak is worth a whole wall's worth of vapor diffusion through latex painted wallboard from a moisture transport point of view.

    The LAST thing you want to use in the interior for an air-barrier is polyethylene sheeting, since foil faced polyiso is also a true vapor barrier, and you will have built a moisture trap. Using 2-mil nylon sheeting (eg: Certainteed MemBrain) would be very vapor retardent when ever the exterior sheathing is cold, limiting the rate of moisture uptake in winter, but becomes vapor open when the humidity inside the cavity rises above 50% RH, letting the moisture diffuse back out ~10x as fast as it took it in. At about 12-15 cents per square foot material cost you can think of it as cheap insurance.

    The 2x6/R23 + wallboard + plywood sheathing & siding adds up to between R13-R15 "whole-wall R" with the thermal bridging of the framing factored in, depending on siding choices and stud spacing. If you add 4" of polyiso and derate it to about R5/inch for mid-winter performance (it'll be much higher in the shoulder seasons) you're really looking at an R35-ish whole wall figure. That's a good enough wall in zone 6 to be able to go Net Zero Energy with a PV solar array that fits on the house if the rest of the house design is up to snuff. Take a peek at Table-2, p10 of BA-1005:

    (The download link is in the lower right corner on that page.)

    If you build to those whole-assembly performance levels and have a reasonbly efficient shape, buying PV is likely to have a better return on investment than the next R10 of whole-wall-R (which is about what 2" more polyiso or a 2x4/R15 wall gets you.)

  3. KrisCurrie | | #3

    Thanks again for your input Martin and Dana, extremely knowledgeable.

    Martin, I have built a few homes to code over the last 5 yrs and my most recent one-that I'm living in now is cold, full of drafts, low grade glass and doors. It is a beautiful house don't get me wrong @$150/sqft, the same as all the homes built here in Atlantic Canada! I just want something better insulated, tighter and less heating demand. Tired of paying the utility companies a huge chunk every year, rather invest in the home.

    As per your answer... If I go with 4" polyiso taped, 1/2" plywood taped-getting as air tight as possible from the exterior side of the wall - 2x6 wall with Roxul - I don't need poly - just a certianteed membrane on the interior side of the 2x6 before gypsum ?
    Thanks Kris

  4. GBA Editor
    Martin Holladay | | #4

    Q. "I don't need poly - just a CertianTeed MemBrain on the interior side of the 2x6 before gypsum?"

    A. It's not a question of whether you "need" or "don't need" poly. What Dana and I are both saying is that interior polyethylene would be a mistake -- it will impeded inward drying, and therefore make your wall assembly more damp.

    MemBrain is unnecessary but harmless. You can use the drywall as an interior air barrier.

  5. KrisCurrie | | #5

    Thanks Martin

  6. kenorakq | | #6

    Hi Kris, I'm no expert and no I didn't stay at a Holiday Inn Express last night..... BUT..... having followed the energy gurus here for over a year I would suggest a switch to EPS instead of increases in R value as the temperature falls (unlike polyiso which performs worse at lower temps)..

    Its a moot point if you've already purchased the foam though...worth a thought if you haven;t

  7. Reid Baldwin | | #7

    There is a lot of space between code min and R50 walls. The returns diminish pretty rapidly as you add more R-value.

    Don't think of an insulated interior service cavity as an alternative to more exterior foam. If you increase the interior side R value, you need to also increase the exterior R value.

    Have you put much thought into windows and attic and foundation insulation? It is important to keep them sort of in balance. Otherwise, whichever one you don't pay enough attention to will end up dominating your heat loss. Putting U = 0.32 windows in an R35 wall wouldn't make much sense.

  8. KrisCurrie | | #8

    Tim, I did start with EPS above grade for its price but was encouraged to use polyiso because of the foil face?anyone care to debate ?
    Reid,I have tamed down the wall R value to around 46 from discussions today, Basement ICF R20 + 5"eps, Attic R90, as for windows still waiting on some quotes- shooting for around .17&.22 on south.

  9. RD3Sunworks | | #9

    I think you should not be quick to necessarily abandon your R-50 walls, based on the earlier comments. It is highly speculative that the additional insulation is not justified by the cost, and even more speculative that "PV is likely to have a better return on investment than the next R10 of whole-wall-R". There are a lot of variables there.

    If you are doing this analysis yourself, and if you really want to weigh and compare insulation costs versus PV costs, the BEopt software would probably help a lot. I know this: Using BEopt, I was able to accurately model energy use (all electric house) both before and after a deep energy retrofit. I also
    know that BEopt models PV performance as good as System Advisor Model and the Solar Edge Site Designer software. I have not investigated the financial aspects of the program. Not sure, maybe you cannot get BEopt in Canada.I

    Also consider and checkout what other people are doing:
    --I have R35 walls in climate zone 4-5, just barely enough in my opinion. Some people would consider that overkill but it can and does get pretty cold here (design temperature 12F and often in the low 20s or teens at night), but I don't need much added heat--other houses around here do need a lot. And if it wasn't for that R-35 I wouldn't be getting more than 50% of my heating from passive solar design. See my guest blog under Green Homes on this website.

    --Look at Martin's blog on this website , Building a Small House in the White Mountains. They went with R40+ walls, it's zone 6, and I would almost bet your winters are even colder in zone 6-7 than they have.
    --Look at Martin's article in Fine Homebuilding Magazine, Net Zero as the New Normal. Again, zone 6, Vermont, with R40 or R45 walls.
    --I believe in an earlier post, you have already commented that someone is building a house near you with R45 or more walls.

    Above all consider the big picture.--that really is where BEopt can help, to quickly weigh alternatives. Maybe you can get net-zero in your climate with R35-40 walls and have the room for PV. Maybe not--you need to know how much sun you get. But for many of us, net zero is beginning to change a little, and now net zero may include an electric car, which may demand a lower energy footprint (increased insulation, etc) of the house in order to have room for the added PV.

  10. RD3Sunworks | | #10

    Once you get more into the design (or even when all is finished), I wouldn't mind seeing more details that may include: any modeling you've done, projected/actual performance specifications, building techniques etc. I like to see climate/predicted,actual performance specifications for any houses I read about.

  11. KrisCurrie | | #11

    Hi Rick, for sure, I am still R49 for walls -R46 cold climate because of the polyiso. I downloaded BEopt, Ii don't believe it will recognize my location. Just finalizing drawings and sending for CAD first of the week.

  12. Adam Emter - Zone 7a | | #12

    I recently downloaded BEopt and used it for my project. There's definitely a learning curve to it and one of the trickiest parts was getting the weather data to my location. Seemed that I couldn't download the weather data for my location within the software, so I went out to the internet, downloaded the file, put it in the BEopt weather folder and then I was able to select my location. Visit to search for weather data files based on your location. Hope that helps.

  13. kenorakq | | #13

    --Tim, I did start with EPS above grade for its price but was encouraged to use polyiso because of the foil face?anyone care to debate ?
    Reid,I have tamed down the wall R value to around 46 from discussions today, Basement ICF R20 + 5"eps, Attic R90, as for windows still waiting on some quotes- shooting for around .17&.22 on south.--

    I'm uncertain where the need for a foil faced product comes from below grade in your case. I am building in Kenora NW Ontario so a LOT COLDER but also drier than anywhere in the Maritimes (I'm from Parrsboro NS)..

    Having been over (and over and over) the options for exterior foam I have concluded that EPS is superior (in cost and performance) for below grade (and above) foam.

    It (EPS) is less likely to get water logged (than iso or XPS), performs better than iso as it gets colder) and is relatively (its all dependent on who you ask) easy to finish.

    I will be using an acrylic stucco below the siding to about 6 inches below grade and then a peel and stick membrane to the footer.

    my 2 cents worth (although in Canada there are no more pennies so I suppose its my nickles worth)

  14. Reid Baldwin | | #14


    What about the foil facing is appealing to you? A couple R-points for radiant effect? Ability to be a WRB? You might want to look into Insulfoam R-Tech and Insulfoam Platinum R-Tech. Both of these EPS products have facers that may provide what you want. For a given R-value, the EPS is cheaper but thicker than Polyiso. The Platinum is between plain EPS and Polyiso both in terms of cost and thickness.

  15. Expert Member
    Dana Dorsett | | #15

    Reid: Most R-Tech products come with facers bringing it's vapor permeance down to less than 0.3 perms (independent of thickness) which is an order of magnitude lower than latex paint, and not exactly a great drying path. Their most commonly available products are 1lbs per cubic foot nominal density Type-I EPS with facers.

    R-Tech Platinum is an ~R4.5/inch graphite loaded product available in different densities without facers. The 1.5 density Type-II EPS, which should run about 2.5-3 perms @ 1" (couldn't find a hard spec on it), but at 5-6" would be well into Class-II vapor retardency, not exactly a drying path either.

    Without knowing more about the exact location/climate it's not clear if EPS would actually outperform polyiso at a given thickness & stackup (unlikely in the Maritimes), and from a $/R point of view it's typically a wash. You'd pay a $/R premium for graphite loaded EPS, but that would allow for slightly thinner EPS.

  16. KrisCurrie | | #16

    Dana, the climate Here in Charlottetown, PEI has an avg temp of 9.9c , -25c to + 25c give or take a couple rare days. The big thing is the wind, shooting to be air tight from the outside, with X "amount of ext.foam with a WRB. keeping to a 2x6. There is one thing to adding insulation to get the high R but vapor worries me after reading much of what you said? I am pushing 16 hrs on the computer of the last 24 on this situation----What would you do ??? in my case :)
    Thanks really appreciate the knowledge

  17. Reid Baldwin | | #17

    Dana, I was responding to Kris's statement that somebody had encouraged him to choose Polyiso over EPS in order to get the foil facer. He invited debate, so I suggested an alternative. Whether it is a good alternative depends on what Kris hoped to accomplish with the facer. I didn't have the impression that he was counting on any drying toward the exterior.

    Kris, if you are concerned about drying toward the exterior, have you explored rock wool in place of the foam? Hopefully you don't need to reach a decision in the next few days. It does take some time to understand the issues and sort through the options.

  18. GBA Editor
    Martin Holladay | | #18

    Q. "There is one thing to adding insulation to get the high R-value, but vapor worries me... I am pushing 16 hours on the computer (of the last 24) on this situation. What would you do in my case?"

    A. Take a deep breath. You have already gotten advice on this issue. It's up to you to decide whether you want to accept the advice.

    There are two main options. You can choose either:

    1. A double-stud wall insulated with cellulose, blown-in fiberglass, or mineral wool batts. This type of wall needs a ventilated rainscreen gap between the siding and the WRB, and benefits from an interior smart vapor retarder like MemBrain. If you follow the advice on GBA for this type of wall, the wall won't have any moisture problems.

    2. If the published data on moisture accumulation in cold sheathing worries you, you should build 2x6 walls with a continuous layer of rigid foam or mineral wool on the exterior side of the sheathing. Here is a link to an article with information on designing this type of wall: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    You don't have to re-invent the wheel, Kris. Those are your two main options. Choose the one you like, and good luck.

  19. Expert Member
    Dana Dorsett | | #19

    PEI is all a zone 6 type climate based on annual heating degree days, but with a warmer winter, and longer shoulder seasons. A mid-winter binned hourly temperature averaging -10C or warmer is much more temperate than the average mid-continent zone 6 climate, so even the R11.25 minimum in the IRC for a 2x6 wall has significant margin. At 4" with ANY foam you would have PLENTY of dew point margin at the sheathing of a rock-wool insulated 2x6 wall without interior vapor barriers, in fact you may not even need latex paint as a vapor retarder (but I'd paint the walls anyway. :-) )

    Foil facers are dead-easy to air seal using temperature rated foil tapes- easier & cheaper than what it takes to seal bare EPS or XPS. That makes 4" of polyiso an easy choice.

    The the local builders are still freaked by the notion of no filmy plastic on the interior, use MemBrain as the air-barrier, not polyethylene. It can be detailed as an air barrier with the vapour hats etc just as easily as 4mil & 6 mil polyethylene, and it won't impede drying. You don't really need it, but it won't cause harm the way polyethylene would.

    A 2x6 / R23 + ~R16-R20-ish foam wall comes in at a whole-wall R of about R30-R35, which has sufficient performance to be able to hit Net Zero Energy in a zone 6 climate with PV that fits on the roof. If you happen to be on the on the almost R30 end (say, 16" o.c. stud spacing and 4" of cheap Type-I EPS) rather than R35 or so it means the PV may have to be the 20% efficiency range, not 15%, but ~20% is rapidly becoming the new standard "high efficiency " commodity panel. At those wall- R values window performance becomes the major player in the peak load numbers, and the total energy use numbers.

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