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Achieving Net-Zero Insulation Levels Without Foam

srasic | Posted in General Questions on

I’m sure this has been asked a dozen times so I apologize for asking again but I can’t find – or maybe can’t understand – what the best framing approach would be for Net Zero.

Our aim is to design a Net-Zero home while balancing cost vs performance.  I’m in Ontario, Canada climate zone 6.  My understanding is that the recommended insulation levels here for net zero is R40.  So the question is how best to achieve this.  The debate about what level of insulation aside, let’s assume we want to achieve R40 or close to it.  For background our original plan was to go with ICF R30 to the roof for a 2 storey house.  But after speaking to a few engineers I’ve been advised that although possible, it may not be the best solution as some of our 2nd story walls don’t line up with the 1st storey.  So we’d have to introduce a lot of steel beams to hold the ICF etc.  Again debate aside, let’s assume we’re now going with ICF for the foundation and stick frame above grade.  I don’t want to use spray foam so our options seem to be:

1. Double studded walls with no exterior insulation.  I’m not a fan of this as from everything I’ve read external insulation is the best approach.  I know I can do double studded + external but then the cost gets very high.

2. 2×6 @ 24″ on centre framing with Rockwool.   If I’m calculating correctly Rockwool at 2×6 24″ is R23.  And their Comfortboard 80 for external is R4.2 per inch.  So with 3″ of external insulation it would only gets me to a total of R35.6.

3.  The third option is to go  2×8 24″ on centre.  Again if I’m calculating this correctly 2×8 @ 24″ is R30.  Plus 2″ of external at R4.2 per inch would be a total of R38.4.  Still not reaching that R40 recommendation.

So now the question is even though the 2×6 model has a lower R value, would it not be better having that extra inch of external insulation vs thicker internal walls?  I’m also trying to balance cost so I would think the 2×8 studs would be more expensive as a total system.  Would the 2×6 @24″ centre with 3″ of external insulation achieve the Net Zero I’m looking for even though I’m not reaching R40?  I know I can keep adding insulation but again I’m trying to balance cost vs performance and I want to stay away from spray foam.

Appreciate any advice.

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  1. Expert Member
    Akos | | #1

    Find out if your engineer is comfortable with using I-joists for studs. Dense packed 9.5" I-joist is close enough to R40 and simpler to build than either double stud or exterior rigid.

    If you want to go with exterior rigid route, I would forget about comfort board. Stuff is expensive, hard to work with plus not the most green. A comfort board is simply a standard batt squished a lot (4x). You get almost the same R value per inch but uses 4x the material, making mineral wool is a pretty energy intensive manufacturing process.

    A good high R value exterior insulation wall would be 2x6 with 2" roofing polyiso. That is an R30 assembly but it is hard to justify going above that based on energy savings, much more foam and your ROI is measured in centuries. Roofing polyiso is fiber faced so it does allow for some minimal drying towards the exterior.

    1. Andrew_C | | #14

      RE mineral wool - I understand that it can be harder to work with because it's compressible, and that it's relatively energy intensive in comfort board form. For those of us that fear ants and termites and fires, it'd be nice if someone designed some insulated standoffs so that we could use regular mineral wool and also address the compression/installation issue. I think exterior insulation is the most attractive wall option, but ants and rigid foam don't seem like a great combination for the long term, even if (big if) the builder does a good job with water management, especially overhangs and flashing.
      [Edit: I remembered this previous article and responses regarding mineral wool installation. A lot of info, although still mostly talking about Comfortboard level mineral wool compression resistance.

  2. seabornman | | #2

    You can't just add up r-values. You should perform an accurate calculation allowing for bridging of framing, air layers, and other materials. That's why exterior insulation is a tremendous asset; there's very little loss to thermal bridging and it can be a great air/water barrier if you choose the right stuff. Go to insulation: a system developed in your native Canada.

    1. srasic | | #3

      I do realize it's a complete system that totals the R value. Just trying to get a gage on what people feel is the most suitable wall assembly for achieving net zero - or close to it - in climate zone 6 but still balancing performance vs cost. Will check out the link you provided. Thanks

    2. Expert Member
      BILL WICHERS | | #9

      The biggest detriment to whole-wall R values is thermal bridging of the studs. This usually knocks off about R2-R3 or so from the R value of whatever you put in the stud bays. R value of continuous insulation can just be added, fasteners don't really do enough to "count" here, so they aren't worth worrying about as potential thermal bridges. I don't bother adding R values for plywood/OSB or drywall, since at best you maybe add R1 there. Convention is to only add R values for actual insulating materials, which is stud bay fill plus any continuous insulation.

      What I usually do for a simple "whole wall" R value approximation is a simple average, which, while not exactly right, gets you pretty close. Assuming a studwall with studs 16" on center, you can think of this as 32 x 1/2" units of "wall filling stuff". There will be a single stud in that, so 3 of those units are full of wood. The remaining 29 units are full of "insulating stuff". Multiply the R value of your "insulating stuff" by 29, assume R1 per inch of wood, so multiply 3 of those units by R3.5 for the wood, add that together and divide by the full 32 units. As an equation, that looks like this (I'm assuming R15 mineral wool for the "insulating stuff"):

      ( 29 * (R15/inch insulation) + 3 * (R1/inch wood * 3.5 inch "2x4" stud depth) ) / 32
      which reduces to:
      ( (29*15) + (3*3.5) ) / 32
      which again reduces to:
      (435 + 10.5) / 32 = R13.9 whole-wall R effective value

      A "real" whole wall R value calculation is more complex than a simple average, but you get close enough with the average, and the calculation is easier. In the real world, you'll have windows, sistered studs, and other things to mess up the theoretical R value anyway, so the relatively small error between a simple average and the "correct" calculated number doesn't really matter much.


  3. DC_Contrarian_ | | #4

    To get R-40 you're going to have to have 10" thick walls, insulation that traps air all has an r-value around 4 per inch. Closed cell foams have slightly better, but if you're not using spray that limits you to polyiso. Going beyond 1.5" or 2" on continuous exterior foam it becomes really difficult to attach the siding, you start needing expensive specialized fasteners.

    The challenge with making a wall that thick is doing it in a cost-effective way. You could just frame it normally but use 2x10's instead of 2x4's, but the 2x10's are about four times as expensive. Three approaches are double-stud walls, the Bonfiglioli wall and the Larsen truss. I think one of those with 1.5" of exterior polyiso will probably be your best choice.

    One of the things people stress here is that the insulation plan should be part of the design of a house, not something you figure out after the house is framed. So you're on the right path by thinking about this now. Having walls that thick is going to create questions about how the doors and windows are installed. You also really want to make sure that your insulation is continuous throughout the building envelope, it's quite common for features like decks and porches to get overlooked in the insulation and create gaping holes. Your insulation is only really as good as its thinnest spot.

    1. seabornman | | #5

      At current prices, the Headlock fasteners I used on my house would cost $320. That's not too much of a burden for exterior insulation. I used 2 layers of 1-1/2" XPS from top of footing to top of truss heel. One beauty of exterior insulation is that once the insulation is in place you're home free. Even in the dead of winter, you can install electrical, plumbing, etc. in comfort.

    2. maine_tyler | | #16

      "Three approaches are double-stud walls, the Bonfiglioli wall and the Larsen truss. I think one of those with 1.5" of exterior polyiso will probably be your best choice."

      You are suggesting exterior insulation with those walls?

      1. DC_Contrarian_ | | #18

        In retrospect I probably got carried away with the exterior foam too. OP said he wanted it.

  4. nynick | | #6

    ... based on energy savings, much more foam and your ROI is measured in centuries.

    You gotta love Akos!

    1. Expert Member
      Akos | | #7

      I would like to say that was meant as a joke, but is about right.

      I'm have about 7200 HDD. Assuming 3000 sqft 2 story house with 9' ceiling, so about the same sqft of exterior walls.

      Assuming R 30 wall you loose over a season:
      7200HDD * 24h * 3000 sqft / R30=172 Therm or 5040kWh

      R40 wall:
      7200HDD * 24h * 3000 sqft / R40=129 Therm or 3780kWh

      So an R40 wall saves me 43 Therm or 1260kWh

      Even assuming heat pump with a COP of 3 with $0.15 electricity, the R40 wall saves $63/year. Using box store rigid price, that extra R10 would cost enough that the savings will not pay for the interest rate, so ROI is never.

    2. Expert Member
      BILL WICHERS | | #10

      Sometimes it helps to extrapolate out to the extreme to make a point :-) Dana once suggested "R one million" insulation in a similar way, which I calculated out as mineral wool 66,667 inches thick, which is a little over a mile thick. Possible yes, but certainly not practical!

      It's always important to remember that whatever we design needs to also be practical to build, and that means with some reasonable cost of construction, and some reasonable physical size and time to completion too.

      I agree that around R28-R34 is the sweet spot for 2x4 walls, since that would be R15 mineral wool and either 2" or 2.5" of exterior polyiso coninuous insulation. With 2x6 walls, you are in the R36 to R42 range with the same amount of continuous insulation, which is about where you want to be if your target is R40, and you're still within the realm of reasonable construction costs, even if your ROI is getting pretty long. I would advise against trying to hit any particular arbitrary R value target, and get close enough within your budget using standard materials and call it good.


  5. Expert Member
    Michael Maines | | #8

    Although R-40 is often a goal in high-performance cold-climate construction, when using continuous exterior insulation, I usually see closer to R-30 as the sweet spot between cost and performance.

    Another approach you might consider is an outrigger system. The Larson Truss is best-known, but the Swinburne truss, developed by architect Robert Swinburne and builder Gero Dolfus, uses vertical 2x6s on the outside of the sheathing (and air/vapor control layers) with any fluffy insulation in the cavities.

  6. srasic | | #11

    Thanks everyone. A lot here to think about. I was hoping to follow the 5:10:20:40:70 model outlined in The Pretty Good House for Climate Zone 6 but it seems maybe reducing to R30-ish with 2" of external is the sweet spot in terms of cost vs performance. Just whether I ended up using 2x6, 2x8, or Larsen truss which I'll have to price out.

    One worry I have about external insulation that ICF did kind of solve for me was that most (if not all) of the local trades in my area are use to conventional min-code construction so I worry about them getting the detailing right. Again ICF poses its own challenges which is why I'm ruling it out. I know I can use Zip-R12 to simplify but structurally I don't like the idea of the sheathing being that far removed from the studs. Having a few layers of external over top the sheathing seems a better approach. If it makes any difference my cladding is traditional 4" brick with possibly some stucco on areas where the 2nd floor walls don't sit on top of the foundation.

    Appreciate all the advice!

    1. DC_Contrarian_ | | #12

      If your trades have trouble with the details on conventional construction, don't think that switching to exotic construction is going to make them any better. I don't really have an answer though, it's a chronic problem in construction that there's very little incentive to do the details right. I know guys who have tried to distinguish themselves by focusing on energy-awareness and they say it's a tough way to make a living.

  7. walta100 | | #13

    Last time I tried the numbers adding more solar had a better ROI than a R40 walls for a net zero house.

    Someone else’s formula for a pretty good house is just that. It is based on someone else’s assumptions maybe their weather or more or less the same as yours. What are the chances that their fuel cost is the same as yours? What are the chances that their guess at inflation is what they would guess today and the same as what you guess now? What are the chances that their local costs to build an R40 wall is the same as yours?

    I think everyone should run the numbers with their variables because an R 40 wall might make sense give the default cost per square foot someone enter in a program 2 years ago but make zero sense if the local contractors now decide to double the cost because they don’t want to build it or the material cost have changed.


    1. DC_Contrarian_ | | #21

      If your goal is net zero then ROI doesn't factor into the equation. For either insulation or solar all you care about is the purchase price.

      The discussion you can then have is whether net zero is worth it -- what's the ROI on doing the extra steps needed to be net zero compared to a ROI-maximizing house? Although I would argue that very few houses are ROI-maximizing for the owner.

  8. seabornman | | #15

    I always see ROI being bandied about. There's no ROI on comfort and quiet. I wouldn't want a leaky, drafty house with enough solar to make it net zero.
    One thing I neglected to point out in using exterior insulation. I used 2 layers of 1-1/2" XPS. Only one layer is required to protect from condensation, so you can freely substitute one layer with 2x framing to let in porches, decks, etc. Or to support door sills. I used 2x4s for window bucks and attached all flashing and siding to strapping. No penetration of Vycor on the bucks

    1. Expert Member
      BILL WICHERS | | #17

      Even a layer of 1/2" continuous insulation will make a noticeable improvement in comfort. 2" would be better, but I doubt very much going over 2" of continuous insulation would make a noticeable difference in comfort compared to only 2". Aside from that though, I agree -- "more solar" is not a substitute for "more insulation", within reason. When you get into the better wall assemblies we've been discussing here though, with walls of R30 or above, more solar is likely to be the better option, because you no longer have improvements to comfort on the "more insulation" side.


  9. boxfactory | | #19

    I can certainly appreciate the value of running a ROI calculation when designing one’s house. Saving money and labor on a stressful project is well worth doing.

    That being said, with an illogically large enough amount of insulation, would there be any advantage in terms of retaining heat in the winter if the power went out?

    Just for the sake of discussion, how much longer would R 60 walls keep a “Pretty Good” sized house reasonably comfortable, as compared to R40? Would the time be measured in days, or hours?

    If it’s hours then I might try and use the software linked above, but if an extra day or two of reasonable indoor temperatures can be achieved, the extra effort / expense could provide some peace of mind.


    1. Andrew_C | | #20

      Your time and money would be better spent on improved air sealing (and flashing) rather than going from R40 to R60.

    2. maine_tyler | | #22

      If you double the whole house R-value, you would cut your heat loss (e.g. btu/h) in half. So going from R40 to R60 would be a factor of 1.5 (or a rate of 0.66 (1/1.5) times less).

      As an approximation (I believe the exact math would be more complicated due to changing delta T's), if it takes 5 days to get too cold inside at R-40, it would take 7.5 days to get too cold at R-60.

      If the house get's too cold in 8 hours at R-40, it would get too cold in 12 hours at R-60. So whether the measure is days or hours depends on whether the baseline comparison watermark is measured in days or hours.

      1. boxfactory | | #30

        This is just the sort of rough info I was hoping for!

        Thank you for the reply,

  10. walta100 | | #23

    Let’s say the upgrade from R20 to R40 cost 7k and saves 200,000 BTUs per year.

    Let’s also say adding 2 extra solar panels and upsizing the heat pump to produce 200,000 BTUs per year costs 2k.

    What is the smart choice? I called it ROI but call it anything you want.

    At some point it makes more sense to stop insulating and start adding solar.

    Yes, I totally made up the numbers.


    1. DC_Contrarian_ | | #29

      The point is, the smart choice doesn't depend on interest rates or depreciation or appreciation or future energy costs. It's just a straight apples-to-apples comparison of which is smaller, $2k or $7k. It's a completely different calculus from trying to figure out if adding insulation today pays off down the road.

      1. Expert Member
        BILL WICHERS | | #31

        I agree. At install time, if you're just comparing one to another, all the fancy future worth stuff is the same for both, so it cancels out and only the initial installation cost matters. If you offset the same 200,000 BTUs either way, then the future impact in terms of energy costs is the same for both and doesn't matter, and the interest costs track linearly for both, so those don't matter either. All that is left is "how much does it cost right now?". It's like having the same number on both sides of the equation for the rest of the future stuff, so those numbers just cancel out.


      2. walta100 | | #34

        DC many people do barrow money and pay interest for upgrades like insulation. Even people that pay cash up front do pay the “opportunity cost” the 7k spent on insulation certainly could have been invested in something and earned a rate of return in the market. That is why I think interest rates do play a roll in this type of decision.

        I think we can all agree energy costs more today than 10 years ago and could increase faster than inflation in the future. If you believe it will in your opinion spending more $ today for the better heat pump will give you a better return given your opinion.

        None of us can be certain of what the future holds but we must prepare as best we can using the data we have and our best guesses at what will happen.


  11. srasic | | #24

    Thanks again everyone. After considering everything I think a 2x4 double studded wall assembly is likely the best solution. Kind of an amazing process as I started out convinced ICF was the way to go, then 2x6 with external, now 2x4 double studded lol.

    As people have rightfully pointed out the R40 is an arbitrary number. I've built 2 homes in the past, both to code, both not comfortable. My wife is insisting this is our last one and it's in an ideal location to make our forever home so the ultimate goal is that it's a comfortable & healthy environment for our kids. If I can hit net zero that would be icing on the cake but not a must.

    I'm still at the stage with the architect where I can make these decisions so I appreciate how helpful this forum has been. A double-stud wall assembly is pretty straightforward for most framers and can be insulate and sheathed normally with a good house rap. It would avoid all the extra detailing such as building out window bucks and long screws that make detailing exterior insulation time consuming and expensive. Plus I'd still get excellent R value with continuous insulation.

    So now the question is how thick a wall.....

    a) I can go 10.5" which is 3 layers of 3.5" rockwool for about R42.
    b) Or 9.5" if I use their 2.5" batt in the middle cavity. They sell this for steel studs and the total wold be around R38. I'm not sure how much the 2.5" batt is as none of the local hardware stores carry it for a quick quote. I'm also not sure if you're suppose to mix insulation meant for wood and steal but I don't see why not?

    As long as the 2.5" thick rockwool isn't a premium and is ok to use with the 3.5" batts it seems this would be the best approach.

    1. Tim_O | | #27

      I think the more important question here is which house rap are you going with? East coast or west coast? Personally, I'm a big fan of Biggie, but I do enjoy Tupac here and there.

      I don't have anything of value to add here, but I do appreciate your threads. I have very similar plans and ideas to you, so I hope you keep the forum up to date on your project as it moves along!

    2. DC_Contrarian_ | | #28

      Usually people go with blown-in insulation with walls that thick, it's easier than trying to stagger layers of batt.

      I find the key to comfort is to have relentless air sealing. Discomfort is caused by drafts more than anything. Air sealing also keeps the house from getting so dry in the winter, and makes the house quieter and less dusty.

    3. Expert Member
      BILL WICHERS | | #32

      I would look at dense pack cellulose for this wall. It will be much easier to install, and probably cheaper too, compared to layers of mineral wool batts. Mineral wool is probably the most expensive material out there to make a batt with.

      BTW, regarding the talk about solar -- insulation pretty much has to be installed at the time of construction, since it's very difficult to add significant amounts of insulation later, with the one possible exception of exterior rigid foam, that can go on with a reside project. Solar can be added realtively easily at any point in the future, so you don't really have to decide on solar right now -- you can always add it later, or even expand a small system later, or even in phases, if you want.


    4. Expert Member
      Akos | | #33

      As others have said, double stud walls are typically dense packed. I would make sure you can find some to do as dense packing is not very common around here before going down this road. Dense packing a thick wall is not the same as a 2x4 or 2x6 wall, you also need a diligent installer that can do higher density to avoid settling.

      A double stud wall with batts is fine for DIY but won't work with normal build flow unless you want to pay either the insulation contractor or framer to come out twice. Fishing batts between studs is very labor intensive, only worth it if you can DIY.

      Depending on how many stories you are looking to build and how tall your walls are, making the load bearing wall 2x6 would be a better option.

      For the 9.5" wall option, I would strongly recommend you look at a single stud wall using I-joists for studs. This can be either dense packed or insulated with batts and less labour than framing up two walls. The thin OSB web of an I-joist reduces thermal bridging and the assembly R value is about the same as 9.5" double stud wall.

  12. nicpottier | | #25

    The best resource I've found for summarizing your options and the best practices around them is amazingly enough a government created document. The housing department for British Columbia has an incredible document covering the three most popular options (split insulation, all exterior, double stud) for high insulation walls here:

    Generally double stud walls use blown in cellulose FWIW.

    1. srasic | | #26

      This is great thanks! A shame Ontario is so behind compared to BC when it comes to building code.

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