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Options for high R-value above-grade walls (R-80)?

Haderak | Posted in Green Building Techniques on

To make a long story short, my above grade exterior walls need to be at an R value of 80. This is to meet Passive House standards for a new residential house in a very demanding climate.

I am currently planning on using SIP’s of some sort. But even the best of those only get me up to around R-50 though. Any thoughts on ways to get to R-80 while still being cost effective and keeping the size of the wall to a minimum?

I have found a number of options myself, but was curious if anyone else had thoughts or suggestions to meet such a high R value for a wall.

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  1. dankolbert | | #1

    I'd rather hear the long version.

  2. GBA Editor
    Martin Holladay | | #2

    You are showing remarkable restraint, Dan.

  3. GBA Editor
    Martin Holladay | | #3

    I think that it's fair to say that an R-80 wall will never be cost-effective. (The only exceptions I can think of would be in Antarctica or areas of Alaska where heating fuel is delivered by airplane.)

  4. Haderak | | #4

    Ok, slightly longer version

    I am building a PassiveHouse and LEED v4 certified house in Minnesota. Roughly 3800 sqft. Attempting to be as efficient with the space as possible, so we designed it to have living space over the garage. My understanding from the Passive House consultant is that based on how Passive House calculates everything, you are penalized by having a tuck under like we do. I didn't want to change that part of the design, so it has resulted in throwing the calculations off and needing extra insulation.

    We have already gone through a couple passes to make it as efficient as possible and are at the point where the only change left that would have a big impact would be to take the garage out from under the living area and have it separate.

    I like the current design (it took us two years to get to this point between the back and forth with my wife) and I want to have the house certified. I could pass on the certification and still have a very high performing house, but honestly I think it's cool and want it.

    I have investigated many options, and so far I am leaning to EPS SIP's with the best exterior insulation I can find. Vacuum insulated panels sound interesting, but I don't know if they are meant for an entire external envelope. The goal is to still keep it cost effective. I just don't want a 3' wall to do it.

    Not sure it matters, but this would likely sit on top of ICF walls for the basement. I originally wanted ICF for everything so that I would be safe in the event of a Zombie Apocalypse, but with such high R-values needed, SIP's are more practical. I kept the ICF basement though, although I am on the fence between ICF or SIP for the below grade walls.

  5. GBA Editor
    Martin Holladay | | #5

    How about double-stud walls with a total thickness of 22 inches?

    Of course, on the day that your insulation contractor comes to do the dense packing, you'll need several tractor-trailer loads of cellulose.

  6. Haderak | | #6


    I fully realize I am likely at a point where the ROI is difficult to justify or ever pay itself back. I am essentially over engineering something that doesn't need it to meet the demands of a standard that doesn't handle the my particular design very well. All to satisfy my desire to have a piece of paper that says my house meets a particular standard.

    I am ok with that. I am interested if there are any thoughts to achieve the R-80 in a practical manner that keeps the wall size to a minimum for the cost.

  7. STEPHEN SHEEHY | | #7

    Dense packed hundred dollar bills.

  8. Haderak | | #8

    Double stud walls as you mention are one option. I have been avoiding stick and frame construction because I really like the advantages of SIP's and ICF's. SIP's more so then ICF's.

    I was thinking a 12" SIP panel (I am avoiding polystyrene due to environmental factors) at R48, and than adding either another panel assembly, or enough layers of external insulation to get to R-80. That would result in a wall size somewhere between 18-22" as well, and still have the advantages SIP's provide. That is where I started looking at high R value external insulation solutions beyond panels of EPS.

  9. dankolbert | | #9

    "Any thoughts on ways to get to R-80 while still being cost effective and keeping the size of the wall to a minimum?"

    Every time I read that sentence, I can hear a few more blood vessels in my skull pop.

  10. GBA Editor
    Martin Holladay | | #10

    It really is a remarkable sentence.

    It never hurts to dream.

  11. Jesse Thompson | | #11

    Maybe two rigid geodesic domes built one inside each other with a vacuum pump running all the time? That way you would have a full VIP structure.

    Like Drop City, but this time it will all work out!

  12. mackstann | | #12

    Polyiso is R-6 per inch so that's 13.33 inches of polyiso. You can almost get there with a 12" polyiso SIP.

    I think this is a crazy undertaking for a piece of paper, though. Stuffing a 3800sqft house with R-80 walls to get a PH certification is like putting five Prius engines in a Hummer so you can drive in the carpool lane. Not trying to be an a-hole, it just isn't sensible. However, it also piques my morbid curiosity, and I would love to see pics of it if actually built this way.

  13. GBA Editor
    Martin Holladay | | #13

    Don't forget the dedicated PV array (with one week of battery storage) to run the vacuum pump.

  14. GBA Editor
    Martin Holladay | | #14

    There are two Alaskan builders you might want to emulate.

    Thorsten Chlupp insulated his walls to R-75 with cellulose:
    A Passivhaus Design for Alaska’s Frigid Climate

    Tom Marsik built R-103 walls by making them 28 inches thick and insulating them with cellulose:
    Meet the Tightest House in the World.

  15. jackofalltrades777 | | #15

    This is why some people do not like Passive House requirements. When it is requiring an R-80 wall in a Minnesota climate, people cringe. It will take hundreds of years to see a ROI for this type of wall assembly when using SIPs +. Your OSB SIP walls will rot out & de-laminate long before that.

    People are being cynical here because it's like adding 38" of EPS rigid foam to the slab in a Zone 5 climate. It will never see a ROI, ever, and it's more of a bragging right about how much money the homeowner can burn. People can also view it is a "self-absolution" since one is building a 4,000+ sqft home and they want to feel good about themselves and be "green" so they find ways to forgive their own indulgence.

    If you are going with R-80 walls, I bet the PH Consultant is asking for a R-150 roof, correct?

  16. Expert Member
    Dana Dorsett | | #16

    What I don't get is why the design requires R80 walls, even if it is in climate zone 7 (which is really only norhern MN.) There are existence proofs of PassiveHouse compliant houses at ~R50ish R-values for walls.

    Maybe it's a "My R is biggern' yourn" kind of thing. A guy built a PassiveHouse near me (climate zone 5) with R100-ish walls, just 'cuz he could (being a house building contractor doing a volume business with cellulose installer subcontractors.)

    The way to get to R8 while still being cost effective and keeping the wall thickness to a minimum is to design it out to R55-R60 or something. If you have the PassiveHouse design tools you should be able to make that happen. Getting rid of the conditioned space above the garage would be a good starting point.

    The other way to go would be to shoot for Net Zero Energy rather than PassiveHouse, buying your thinner walls with a PV clad roof. It's probably less expensive overall, and you could probably keep the space over the garage.

    The definition of "cost effective" can only be in terms of comparative cost between solutions. It's pretty clear that the last R25-R30 or so will not be cost effective against energy costs over a nominal 75 year lifecycle, even if all of the energy savings were made up with photoelectric panels (without subsidy), leveraging the space heating at a paltry 2:1 with heat pumps. The going price in my neighborhood is currently under $4/watt, and in Germany it's $2/watt. Better-class ductless heat pumps would leverage the power at 2:1 even when it's -10F outside, and 3:1 when it's +10F. In 20 years when it's time to replace the heat pumps & /or PV the PV will cost less than half as much, and the heat pumps will have gained in efficiency. The full lifecyle cost of heating with ductless heat pumps and $3/watt PV is on par with natural gas in some places, and in 20 years it will be dramatically cheaper than it is today, making the 50, 75, 100 year outlook even more favorable for site-produced power & heat pumps than over-the-top superinsulation.

    Read the first chapter of this document as a sanity check on the cost effectiveness of R80 from an energy use perspective:

    Methinks R80 is only "cost effective" in terms of meeting your certification goals, but those certifications have little to do with breaking even on energy costs over the next 100 years, or what's actually nice for the planet. Most analyses insert an energy price inflation for longer term, but the cost trajectory for PV over the past 30-40 years implies that there could be broad energy price DE-flation beginning as early as the post 2030 time frame (per the Sanford Bernstein investment bank's analysts, among several others.) That would make expensive R80s walls (and possibly R50 walls, but I'm not willing to go there just yet) essentially a "stranded asset", that would literally never recover their cost, even in a US climate zone 7 climate.

    BTW: This week Kepler Cheuvreux (another investment bank) published a bit of analysis titled "Toil for oil spells danger for majors" (google it) that demonstrates $2.20/watt PV (higher than the current cost for utility-scale solar, though they were projecting that as the cost in 2035) would be dramatically cheaper for the personal automotive sector than $75-100/bbl oil (which is the current cost of what it takes to extract shale oil or bitumen from sand.) This blog covered it briefly- the full text is even more interesting:

    Energy price inflation cannot be presumed over the lifecycle of buildings built today, despite the creeping energy price inflation that has endured for a generation after the first oil price shocks of the 1970s. Peak oil notwithstanding, we would seem to be at a disruptive tipping point in the energy markets. (Greener will likely turn out to be cheaper.)

  17. user-1137156 | | #17

    Double stud, 2x4 walls spaced 12 1/2" apart filled with mineral wool or dense packed BIB (Spyder) Over all wall thickness is19 1/2" + drywall + sheathing cavity is r82+. . Make the inner wall structural with OSB sheathing on it's outer face creating a good air barrier and service cavity filled with mineral wool bats. Cost added over r 40 wall about $2/wall sq ft.

  18. Expert Member
    MALCOLM TAYLOR | | #18

    Come on. You guys made this up because it's Friday night and you are bored and too tired to go out.

  19. user-626934 | | #19

    I suppose it's OK if someone wants to try jam a square peg into a round hole on their own house....but it seems kinda unfair for folks on the forum to knock the round hole for being round. ;-)

    More information on Thorsten's first version of the "Arctic Wall" - I think he has since thickened the exterior truss and eliminated the 2x6 stud cavity insulation.

  20. Airithol | | #20


    You appear committed to an R80 wall assembly, so the idea of multiple wythes should not alarm you - we really can't get there without at least two frames. Understand that whatever wall assembly you decide upon, it is going to be thick.

    I mention this for two reasons. First, because you intend an ICF foundation wall. Your above grade walls are going to be significantly thicker than any (typical) ICF wall. While I am sure your designer has a structural solution, this issue indirectly affects the cost (effectiveness) of the wall. The thicker your wall, the thicker your, solution.

    Second, whatever insulation you pick, it needs to be contained by something, usually a set of walls. This why people are suggesting double stud walls. The only exception really, is rigid foam (REMOTE/PERSIST), or SIPs.

    Since you seem to like SIPs, and are resistant to conventional double stud walls, what about a hybrid?

    Build an SIP home and clad it with outsulation. Basically a double stud wall but the inner wall is SIP. This outsulation could be DPC, blown fiberglass, mineral wool, whatever. The main advantage here is that you get SIPs up quickly (weathered in) and then can proceed with the outsulation. The drawbacks here, in addition to the standard SIP drawbacks, are that you still must frame an outer wall.

    I would have suggested doing what Go Logic has been doing (successfully) and do a stick frame clad with SIPs, but to hit R80, your inner wall would need to be made out of 2 x 10/12s...

    A few other points.

    PHIUS, if I recall, is shortly coming out with revised requirements for certification - climate appropriate ones. If your time frame permits, you might find that R80 is unnecessary.

    Cellulose really is the cheapest way to achieve R80, other factors aside. Given the amount of super insulated homes built in the NE, finding an experienced installer should be relatively easy (compared to where I live where cellulose is still considered a 'green alternative'...).

    SIPS allow zero drying potential so plan accordingly.

    Like other posters, I am skeptical that R80 is really required here. Without knowing all of your design assumptions and inputs, we can only guess. Unlike me (a guy designing his own home) many of the posters here are professionals that have been involved in multiple passive house projects.

    Finally, Tom, you made my Saturday morning. Zombie Apocalypse! If you really have survivalist tendencies, or have other reasons for heightened security, keep in mind that windows will likely be your main point of attack. Automatic (with manual override of course) steel plated thermal shutters could solve your security issues, and your R80 wall issues. Ask your PH consultant what impact +R20 for window area, say 45% usage time, would do to your space heating load.

    I am not sure if I had a laugh at your expense, or if you had a laugh at mine...


  21. wjrobinson | | #21

    80" TVs in 10 of the rooms... would be all the heat needed? But then their is a limit to plug power in Passive House right?

    OK.. real solutions... the garage door must be what is throwing off the calc. So, maybe install two garage doors one behind the other, superinsulated custom doors, be kind of cool to have a stack of doors opening with about a foot of lift delay each.... watching the babies go up and down would make hanging out in front of them in your Tesla a great place to be, pop a cold IPA....

    Martin, this math test... may keep me sharp into my old age... buggers... wanted to get Alzheimers and just forget everything sooner than later. 8+6.... how about 37 13/16" + 19 1/32" - 5 1/2? Or maybe some diffyQ so we can solve Dana's Mass verses insulation dilemmas? As we age,,,, maybe 1+ double digits

  22. STEPHEN SHEEHY | | #22

    AJ: probably need more than 10 tvs. The new ones are insanely miserly when it comes to power use and don't throw off enough heat.
    Maybe easier to just add a dog or two.I assume the Passive House program allows you to input various breeds so you can really nail the heat loss number. My lazy old golden retriever puts out 80 btu per hour, but a frisky border collie is probably twice that. The cats are useless.

  23. jackofalltrades777 | | #23

    The comments all seem to share the same sentiment; R-80 for a wall in Minnesota is absurd and the consultant who recommended this should be let go and find a different energy consultant. It's as simple as that.

    Unless you want to spend huge amounts of unnecessary money and have a wall assembly that is 24" thick, rethink your approach and strategy. If you got money to burn then think about maybe building a small home for a homeless mother and her kids. Doing that will have a lasting impact on peoples lives and on humanity. Much more so than stuffing your walls with R-80 insulation so you can have a piece of paper that says PH Certified.

    Is going through the huge unnecessary expense of an R-80 wall really worth that much to have a certificate hanging on your wall and collecting dust that says "Certified Passive House"? Don't get me wrong, I am all for building a PH design but there comes a point when certification and reason part ways. For me it's clear that time is when the consultant tells you that you have to incorporate an R-80 wall assembly in order to get a PH certificate.

  24. [email protected] | | #24

    If living space over the garage is the issue, instead of concentrating attention on the walls did you try beefing up the garage ceiling insulation along with elimination of thermal bridging through the ceiling?

  25. charlie_sullivan | | #25

    If you ave doing a double wall with cellulose between, it's the double wall that's expensive, not the cellulose. So the cost of R-80 isn't that much more than R-40. The details of attaching it to the foundation and doing the windows and doors would be some extra cost and difficulty, but even a 12" thick wall is thicker than the foundation wall, so you've got some of those challenges in any case.

    I think that Tom had originally rejected that because he didn't want the walls that thick. If cellulose is too thick, and SIPs are too expensive at R-80, it seems like the only remaining option to consider is a combination of the two, if you stick with R-80.

    But I agree with Debra, that working on the garage ceiling could be more fruitful, if that's the weak point. The cool thing about that is that there are no windows in it, so you don't have the extra cost of doing that detail, and you don't have the aesthetics of a window in a thick wall to consider.

    Or take another look at the windows. If you are doing a double wall construction, doubling up the windows too? An "innie" and an "outie"?

  26. heatherjdb | | #26

    There is very little discussion of the impact of building size- 3800 sf is a huge house. Families of 5 lived in 1200 sf in the 1950's.

  27. dankolbert | | #27

    Please don't ruin this by being reasonable.

  28. jinmtvt | | #28

    Ahahahah but seriously , is this post for real on the week after both PH conference reports ??

    If it is, Tom , you will unfortunately not find any reasonnable info on your question, here, in this time.
    Maybe if you try again in a few weeks to let the heat cool off.

    3800sq ft. PH.
    To be curious, what is the total budget of your house project ??
    This kind of huge dwelling should be penalized for space VS occupancy.

    The only way i would try to achieve such a "special" target , keeping in mind that a huge project like this needs to be very long lived to even payback its deed to the environment,
    so i would only consider ICF or other non damageable structures.

    ICF such as Quad-Locks with all of their additional EPS coats on,
    reach approx R50.
    You still then have both sides to cover with some more insulation since you're left with fastening strips on both sides.
    I would use Polyiso on the interior side since we all know now that is performs to its best when it is used on the "warm" side, and add some EPS boards to meet your target on the exterior side.

    ICF on that type of construction , simplifies the "sealing" operation quite drastically, thus makes up for the additional labor involved in setting the forms and pouring concrete.
    Then , using that thick of insulated forms makes the form very stiff when pouring , thus easier to brace and ends up with straighter walls wich needs less corrections.

    and PLEASE,
    if you DO go on with this project, use Recycled insulation boards
    or be ashamed :p

    Dan you are hilarous :)

    Dana : did you start that thread ?? :p

  29. getmeoutofgba | | #29

    I live in northern MN, climate zone 7 and am a certified PH designer. I'd like to back up from the "R-80" wall discussion to point out that we know little about the house, other than its 3800 square feet and tuck under garage. PH is not a prescriptive approach. Much can be done with integrated design, to potentially reduce the requirement for R-80 walls. Is an R-80 wall the result of tweaking everything else (window spec, sizes and layouts, optimized solar overhangs, etc., and roof and foundation insulation levels? There are standard extra thick ICFs with R-38 or more for below grade.

    But back to the walls, I don't think SIPs are a silver bullet. But if you want to use them, look to the Bagley Classroom Passive House for an example. It is in Duluth. The walls are 16" SIPs (R-64). The building is small. If this building can be PH certified with 16" walls, surely Tom's house can get there with R-64 walls. Again, I'd say get back into PHPP.

    Before my esteemed colleagues at GBA tease me for offering this "advice," I want to add that I don't believe in heroic efforts to make the PH equation work just for its own sake, but I do believe in answering a question as thoughtfully as I can.

  30. GBA Editor
    Martin Holladay | | #30

    No one should tease you. Thanks for sharing your expertise and your helpful advice.

    I also offered Tom Schmidt some advice (back in Comment #14), concerning other cold-climate homes with walls in the R-75 to R-103 range.

  31. Haderak | | #31

    I appreciate all the advise. I am in the stage of design where I am still doing tweaks and incorporating the various sub-systems into the design and how that impacts PH. At the end of the day I am just a home owner that is interested in building science, I am far from an expert in any of these particular fields. I do a lot of research to learn as much as I can, but obviously I lack the experience and training many of commenters have.

    I naively assumed that with SIPS that are at around R-50 in a single panel, going up to R70 to R80 range would be relatively straight forward. Turns out that doesn't seem to be the case.

    I have been reviewing all the feedback and information posted, which I appreciate greatly.

    It still isn't completely clear what the house design will land at in terms of the recommended R value for the wall to meet the PH metric. The approach so far was to keep the wall size as the dependent variable and adjust all the other factors.

    I have also been using the comments to better understand the other environmental aspects of some of the building materials (EPS, PolyIso, concrete, etc) in helping me make more informed choices on the building materials to target.

  32. DEnd2000 | | #32


    Since you are going to have an attached garage I encourage you to think about the air flow between the house to the garage. Attached garages are a possible cancer risk. Homes with attached garages are associated with 3 times higher indoor benzene levels than in homes without attached garages. ( because of this you really want to have any possible air flow going from the house to the garage and not the other way around.

    When suggesting this to several people I have met some resistance, because they think that proper air sealing between the garage and the house is enough. Quite frankly if there is a pressure difference between the spaces (and there will be) no amount of air sealing is enough to stop air movement. If you look at this thread ( ) you will notice soot in the insulation between the garage and house and will notice the airflow from air leaks on the garage side. Granted the soot is likely from the previous owners idling a vehicle in the garage, but the same air flow would happen even if only running the vehicle during entry and exit. Benzene and other pollutants are there all the time not just when the vehicle is running, they evaporate and disperse from the gasoline and other oil products stored in the garage.

    I am basically a nobody (I am registered as a nurse, though I do not currently work as one) but I do have recommendations:

    #1 Do not have an attached garage.

    #2 If you must have an attached garage (hey they are pretty darn nice to have), then you should ensure that you always have a lower air pressure on the garage side across all connecting surfaces between the garage and the house.

    I do not know what an appropriate CFM rating should be, but this is something that your energy rater should be able to figure out on their final blower door test. They would do this by setting up their blower door in a window or outside door to the garage and using a manometer to record pressure differences between the garage and house at several points along any adjoining surfaces. (several points at ceilings and corners where air leaks are likely to happen, any doorways between the spaces, and several points ie low, high, mid on adjoining walls, these measurements would be taken on both the garage side and house side of the adjoining surfaces) They should take these measurements for several household operating conditions as well. One set at no mechanical air flow in the house, one set with a likely high exhaust air flow (range hood and a bath fan, plus ventilation if you are running an exhaust only ventilation scheme), one set with the exhaust air flow still running but add in the HVAC at full tilt too, and one with just the HVAC at full tilt. With each set of measurements the energy rater would adjust their blower door CFM to the minimum required to meet negative air pressure at all points on the garage side. I would use the highest cfm number as the minimum continuous rating for a garage exhaust fan.

    This ventilation for your garage your want to be continuous. The reason is the difference in purpose between it and ventilation of your house. With ventilation for your house you want to provide fresh air, for the garage this is only part of the reason. The main part is to provide negative pressure air pressure in the garage in relation to the house. If you put the garage ventilation on a timer then you are only providing the negative pressure part of the time. Garage pollutants are there all of the time, that will likely only reduce indoor pollutant levels and not take care of the source pathway into your home.

  33. dankolbert | | #33

    Tom, you get extra credit for coming back. I don't think anyone doubts your sincerity, but if you really are still in the design stage I would strongly encourage you to re-think your approach.

    If your goal is to demonstrate that a big, not-well-thought out house can meet the PH standard, you're doing fine. But if you want to build a high-performing house that takes advantage of best practices, there are far better ways to do it. Size, layout, materials, etc. all come into play.

    I guess I have a few questions for you:

    Who designed your house (Not a name, but was it you? An architect? A designer?)

    Did you reject smaller versions? Why?

    Where is budget on your list of priorities?

    How many people will be living in the house full time? Part time? Are you retired or would you like to retire in this home?

  34. [email protected] | | #34


    Have you considered injected foam R501? It has a slightly higher R-value that SIPs and can be injected into the walls. I have used it (brand name Retrofoam at in my home that I am selling here in Colorado Springs and I am considering using it again in Indiana, where I am moving.

    I am obviously not as knowledgeable with energy efficiency unlike many of the other members here, so I thought this might be an out-of-the-box suggestion you might consider researching. It is about 9% higher in cost, but the injection capability into the walls made it ideal for our use.

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