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Community and Q&A

Best Wall Assembly for High R-Value

MarkMac | Posted in General Questions on

Hi.  I’ve been reading and learning over the last year as we progress our building plans, but only recently joined the site.

I’m hoping to get some constructive feedback on what seems to be the last major decision we face; whether to go with an effective R24 Rigid Foam (2×6 with R20+7.5ci) wall or an effective R28 Double wall (2×6 R-20 + 2×4 R12).    I’m using the Canadian Wood Council Wall assembly calculator as my basis for the effective R-values.

House details
Coastal eastern Canada.
HDD18C is 3,847 – Using Env. Canada 20 yr rec for specific location
HDD18C is 4,050 – using last 50 years.
HDD18C is 4,087 – using last 75 years.
Slab On Grade with 4′ frost wall
2,300 sqft + 500 sqft bonus room over garage
Most windows face east for view
Only one small N. facing window
Attic – R60
Under Slab – 12.5R
R32 under Bonus room
540 sqft of windows in total (13% of exterior wall)
4,100 sqft of exterior wall (3,560 without windows)
Aiming to be 0.5-1.0 air changes
Heat Pump will be sized once final plan is locked down
Heat-Pump Water Heater
10.7 Kw Solar with plan to be NetZero.

Adding more ci becomes difficult, as local rules require wood siding, and going with more ci would increase the siding effort/cost significantly in comparison to the gains.

Using an online calculator with above (R24 vs R28 walls), it suggested BTU heat-loss is 61k-63k/hour with air exchange and windows being the largest losers by far (22k and 27k BTU/hr respectively).  A wall increase of R-4 makes virtually no difference.  In fact, going up to R35 only reduced total BTU loss to 59k/hr.  Maybe I’m off on a few items, or entering something wrong, but proportionally, the wall R-value is not having dramatic swings in total BTU/hr loss.  However, changing the Windows from R1.8 to an R3.5 drops the BTU loss by 16k/hr.

My questions, which I may already be answering myself;

1. How is a double wall justifiable, when it costs more (~$18k), for what appears to be only a gain of R-4.  Even changing from batts to blown-in and getting R35 doesn’t seem to generate the gains needed from a ROI perspective.  Any comfort gains recognizable between the two walls to justify the investment?
2. It seems that I’m better served spending the $18k on window upgrades?
3. The 10.7 kW Solar installed is $18k, which I estimate should be close to NZ without double wall.  So little to offset here with double wall?
3. Am I understanding the rigid foam should not be applied over the double wall?  Or does it depend whether or where an interior air barrier is?  Leaving 1″ of ci would then make it R33, so a little more benefit, however I keep seeing suggestions that ci is not good for Double walls.

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Mark,

    1. I agree that there isn't much difference between the two in terms of their efficiency the way you describe them. One reason for that is you aren't using the benefits of a double wall to your advantage. The idea is that separating the framed walls minimizes thermal bridging and allows the total R-value to be easily increased by widening the gap for very little additional expense. Having one 2-20 and one r-12 wall with no insulation between them removes this benefit.
    2. If the cost difference between them is really that high, then yes the 18K is better spent elsewhere.
    3. Building science aside, I'm not sure it makes sense to construct the double wall, which you are leaning against due to it's expense, and then add another labour-intensive layer of foam over it. It seems like an either/or choice to me.

    1. MarkMac | | #3

      Malcom,

      Thanks. To clarify, I’m using the effective R value from the wall design calculator. It’s setup for double wall, and thus accounting for gain/loss as you build the wall. I’m using a double wall, with thermal break, and two batt’s. The 2x4 wall is r-12 and the 2x6 is r-20. This would be R32, but the calculator factors I’m some loss and suggests the effective R value is 27.8.

      If I’m allowed to post links, the calculator can be found here; https://cwc.ca/design-tools/effective-r-calculator/

      1. Expert Member
        Malcolm Taylor | | #6

        Mark,

        The gains from using a double Wall are in the gap. That spacing is increased or reduced according to what R value you want, and is filled with either a third layer of batts, or the whole thing is filled with dense-packed cellulose. If you don't want too thick a wall I would suggest using 2"x4"s to frame both and increasing the gap you were going to leave between them accordingly - but whatever size gap you decide on needs to be insulated, not left empty.

  2. Expert Member
    Zephyr7 | | #2

    If the walls are built with the studs staggered, then the thermal bridging will be greatly reduced. I agree there isn’t much point in putting rigid foam over a double stud wall — the “double stud” part, when built to limit thermal bridging, doesn’t benefit from exterior continuous insulation the way a regular wall would.

    Solar is often easier to justify if you don’t have net zero as your goal. I like to recommend sizing the system to offset your peak daytime load instead, which allows for a cheaper system and maximizes the benefits of the smaller system. Pair such a system with a time of day utility rate to maximize your overall cost savings.

    Bill

    1. MarkMac | | #4

      Thanks Bill. For solar, the utility company here is a single rate, and they buy back excess at the same rate (Net at end of year).

  3. Forrest Stanley | | #5

    The most cost effective thing which can be done is to use the inexpensive windows
    and install insulated shutters. But you have to be willing to live with the changed aesthetic if you adopt this solution. Shutters can be made several ways - they can be exterior (this is usually a nuisance because you will have to go outside to operate them) - they can be interior, home made or commercial.
    Interior shutters are usually a lot more convenient but have their own set of problems. They often interfere with furniture placement, usually need to be flat against a wall when open , and will allow condensation on the inside of the glass in really cold weather if they are not constructed really well. But, the advantages are considerable. If you were to have them made by your local cabinet shop you could have them approx. 2 inches thick( put 1.5 inches of polyiso insulation in them), with any appearance you prefer, and the resulting R value (of the shutter plus the glass) would be in the range of R-8. They can then be closed at night which is when the really cold temperatures occur. In the past, in my area, this proved to be a less expensive option than ordering triple pane fiberglass windows and has considerably higher R rating - but, you have to like the looks! I have no idea what the costs of might be today in your area. If by any chance you have the skills (and the time) to construct them yourself the costs will become a non-issue. GOOD LUCK!

    1. Robert Opaluch | | #32

      A slightly different shutter option is pop-in shutters. During the warm season, they aren't in the room. During mid-winter, you pop them in place, and during the day put them wherever you wish. Easy to build yourself, and covering can be cloth, wood, or picture frame or wall-hanging appearance. In one instance, someone failed to realize there was a window behind the shutter.

      Shutters also keep out the light to keep the room dark when you are sleeping.

      Yes WAY cheaper than window upgrades.

      One caution is that IF you have really high performance windows, especially facing southward, you need to be careful NOT to leave them in place during a very sunny day, as the heat build-up between the highly insulated shutter and the interior side of the window can be substantial.

  4. Expert Member
    Akos | | #7

    The online calculator is not giving you good numbers, a reasonably sealed house that size your heat loss should be about half. I would check the inputs, sometimes wall assemblies could be off and also most overestimate air leakage rates.

    Even if that is fixed, the difference between and R24 and R28 assembly in terms of energy use is noise, definitely not worth the cost. There could be an argument for an R40+ double stud wall (with no exterior rigid) but even that it is mostly about energy efficiency not ROI.

    Attached garages and bonus rooms tend to be energy pigs and a big source of indoor air pollution. A more compact shape with a detached garage connected with a breezeway tends to be similar cost, much easier to air seal and insulated and more efficient. Even well done, a bonus room will not be as comfortable as the rest of the house.

    1. MarkMac | | #8

      I agree Akos. It seems very high, so I was just looking at it from a relative perspective vs fighting to find where the excess was. Windows and air leakage was accounting for about 70% of the BTU design loss. Probably why even pushing the wall R-value to 35 didn’t even make a significant difference.

      I’ll see if I can figure out where it’s going wrong but with air changes and windows being so large, and the variables straight forward (window area and air changes per hour), I’m not sure why it’s blowing up.

      1. Expert Member
        Akos | | #10

        The air changes in the calculator should be your natural air changes per hour, not your [email protected] Depending on the climate, height of the building, that number is between 16 to 20 times less than your ACH number. Once you fix that, building losses should be more reasonable.

        In a cold climate, most of your energy savings are had with an R20 wall assembly. Going a bit above that just doesn't save all that much energy. This is a bit warmer climate (you are in Zone 6) but still applies:

        https://www.energyvanguard.com/blog/the-diminishing-returns-of-adding-more-insulation/

        Sometimes improving your envelope lets you reduce your mechanical equipment costs, hard to say where the break even point is.

        Your best bet is to build the simplest, well sealed, reasonable R value wall you or your builder is comfortable with.

        1. MarkMac | | #11

          Thanks. I plugged in 0.3 air changes per hour and it dropped the design BTU/hr loss down to 40,000 BTU with only the R24 wall assembly.

          I believe I’d fall in zone 5 given the lowering trend of HDD to 3800 over the last 20 years. I’m using -5F as the design temp.

          My gut keeps telling me, I should easily offset 100% of electric bill with PV so it’s likely a comfort question at this point. More HVAC activity, vs less, depending how far we push window and wall R values.

          Builder is comfortable with either path and regularly builds NetZero and passive. Only odd part I’ve questioned is the idea of foam over the double wall, but seems to be to keep OSB warm/dry, but articles here suggest against that on double walls.

        2. Jon R | | #13

          +1 on making sure software isn't directly using [email protected] for design load infiltration.

          On the other hand, design load isn't about long term averages, where 1/16-1/20 are common values. If your software doesn't do it for you, then you can use this rather complicated Manual J procedure:

          https://higherlogicdownload.s3.amazonaws.com/ACCA/c6b38bda-2e04-4f93-bd51-7a80525ad936/UploadedImages/Infiltration%20per%20Blower%20Door%20Test%20Oct2016.pdf

          Maybe someone has gone through it and can talk about how it compares to simplistic approximations. You don't want to be left cold when it's design day temp (or much colder) and wind+stack effect pressures are causing much more than *average* infiltration ACH.

  5. MarkMac | | #9

    For clarity on the wall design and calculations, attached are the two wall assemblies, with full breakdown of Effective R and pros/cons listed.

    I realize I can leave more of a gap, but currently this is a 9-9.5” thick wall in Zone 5 and I’m already debating if the cost is worth it. So leaving a large gap and adding more does give me more, but I’m struggling to find the benefit now of the double wall with the current price of materials and labor.

    Note: these files are from Canadian wood council, so if there’s any issue in posting, please remove them.

    1. Expert Member
      Malcolm Taylor | | #17

      Mark,

      Have you considered using 2"x8" for framing?

      1. MarkMac | | #27

        Malcom,

        I had not thought about 2x8 framing, but that might be more applicable. With 1.5" of foam, and R28 batts, I'd have eff R-28.5, at a much cheaper construction.

        Looking at the insulation, the increase cost is only $1k, and framing labor should be pretty similar. So it would just be a question of 2x6 vs 2x8 prices. I don't have total volume of material but that should be easy enough to get.

        From running my calculations, going R24 to R28 only saves ~$70/year, so backing into a 20 year return suggests the upgrade from 2x6 to 2x8 would need to be less than say...$2k? Insulation would be half of that, and I suspect the material increase would be more than $1k. Even stretching it to 50 year, the cost would need to be $5k or less.

        I don't know if I'd see any non-financial gains going from 2x6 to 2x8 however. Comfort? Sound? Air tightness?

  6. Jon R | | #12

    When it comes to various energy options and cost effectiveness, you can run BEopt. +1 on not doing anything with a worse ROI than PV solar.

    When it comes to moisture risk, exterior foam is superior to double stud walls. Exterior Rockwool is even better.

  7. MarkMac | | #14

    I have must be missing a key piece of the puzzle? I cannot figure how a double wall makes any financial sense. Even if I consider my double wall cost extremely high, and said it would be half as much, the numbers are still not justifiable. Do the numbers start making a lot more sense when you move to Zone 7? 8? I have to be missing something.

    When I'm running the numbers, the estimated additional kWh/year with an "ok" air source heat pump, the increase is only 550kWh. At the current rate of electricity, that's only an annual savings of $71/year. The ROI for a double wall estimated at $18k would be measured in generations.

    If the additional electricity is only 550kWh annually, I can add one more panel for $800, and call it a day.

    1. Expert Member
      Zephyr7 | | #15

      Double stud walls filled with cellulose are sometimes the cheapest way to get a lot of R value into a wall. In this case, the target is some specified R value and not a price point, so you look to the cheapest way to reach the target R value. Sometimes people want to reduce their energy consumption as much as possible. There have been posts on GBA, for example, with readers looking to build R40+ walls.

      It really ends up being about what your goals are. Justifying the build based solely on cost savings over time isn’t necessarily the only factor, although I agree that going too far overboard on any one part of a project can often be a waste of money since the money may have accomplished more if spent insulating some other part of the project.

      Bill

    2. Richard Levinson | | #16

      At an $18K difference it doesn't make sense at all, but it doesn't necessarily have to cost that much more. To me, that premium sounds like your contractor is comfortable doing a standard 2x6 wall with exterior insulation and would rather not stray from that (particularly for little gain in R value).

      In terms of materials, the costs are likely lower for a double wall (depending on what insulation you use). Just looking at retail prices in Canada, for a 10' long x 8' high wall it'd be something like the following for the materials that differ between the two:

      30 ft - 2x6 plates: $40
      8 - 2x6 studs: $59
      80 sq ft R20: $80
      80 sq ft R7.5 XPS: $178

      Total: $357

      50 ft - 2x4 plates: $42
      16 - 2x4 studs: $68
      240 sq ft R12: $151

      Total: $261

      You could use EPS in the first example which would drop the price more in line with the double wall (though the latter would still be cheaper), but either way you're looking at probably R30 with that double wall vs. R24.

      So it'd really come down to labor and whether any increased insulation levels could justify the difference in labor costs. In your case, it doesn't, but in other cases it might.

      1. Expert Member
        Zephyr7 | | #19

        If the builder isn't familiar with building double stud walls, but is used to putting up some thickness of exterior rigid foam, then the "slows me down" factor is probably a big part of the cost premium for the double stud wall. Builders are reluctant to do stuff they're not familiar with because such things add a lot of "ifs" to a project. "Ifs" can take time to figure out, and since the "ifs" can be unpredictable, the amount of time they take to figure out becomes unpredictable too. That means the builder sees lot$ of potentially lo$t money -- something they can't really afford to do.

        SOMETIMES you can help here by offering to pay time and materials instead of a fixed bid price. The upside here is that the builder won't be as worried because YOU are not paying for any of that "lost time" from the "ifs". The downside for you is that the contactor might not be as motivated to do things at a reasonable pace, since with T and M, a slow project is a profitable project.

        Bill

        1. MarkMac | | #21

          Agree, however I’m confident in this case the builder is comfortable with the double wall. They’re recommended by every other builder as soon as I ask about NetZero or PH. He’s built a few of each and is listed by PHIUS as a recommended builder.

          It all made sense to do a double wall, high efficiency, PV etc. however when running the double wall cost against gains, it’s just so far from a sound investment in my case. I just feel that I’m not evaluating something correctly, and hence why I’m going to look for someone who can run a proper assessment to be sure before we lock in the plan.

      2. MarkMac | | #20

        Thanks. I had not thought to just workout the material cost to be sure the cost was reasonable. Using your material coats of the additional 2x4 wall, with 420’ of wall (so x 42) is $11k in materials before Canadian sales tax. With tax, closer to 13k. Plus labor for 420’ of second wall.

        The exterior walls are mostly 10’ in height, so ~25% more material than your 8’ example. Let’s call it $17k with taxes in for materials, plus labor.

        I’m going to find a local specialist who can truly run the models and help me evaluate the gains/losses for the additional cost. I have a feeling $18k is better spent on higher end windows, and PV array. Based on my anticipated annual kWh usage, a PV setup to cover 100% is also $18k. Without the PV, it would be $2k/year in utility, so much easier to see the return quickly.

        1. Richard Levinson | | #23

          Just to clarify, that 2x4 price of $261 for a 10' long wall was for two sets of 2x4 walls and R36 worth of insulation. So it wouldn't be in addition to your 2x6 wall; it'd replace it. Maybe a better way to express it is price per square foot of wall area using those same prices (before tax):

          2x6 w/ R20 batts + R7.5 XPS (1.5"): $4.45 per square foot
          2x6 w/ R20 batts + R8 EPS (2"): $3.45 per square foot
          Two 2x4 walls w/ 3 layers of R12 batts: $3.25 per square foot

          But obviously labor can change the math quite a bit. And as you and others have pointed out, once you're already at an effective whole-wall rating of R24 and you have an efficient heat source, the returns of paying for additional insulation diminish significantly.

          1. MarkMac | | #25

            Richard,

            After playing with the environmental side of the calculation, I think you’re spot on when you say “ at an effective whole-wall rating of R24 and you have an efficient heat source, the returns of paying for additional insulation diminish significantly”.

            If I change the HDD to 7500, and a design temp of -20, the savings more than double. That said, still not enough between R24 and even R35. However, when I run the numbers using R17 and R35, the savings are hundreds of dollars a year. In those cases, spacing the wall more and pushing to +R40, I can start to see the benefits.

            The idea of a double wall naturally made a lot of sense when reading articles on efficiency, and I naturally assumed in Canada it would be a great idea. It’s seeming the Env side, heat source eff, and what base R value you’re starting from, can skew the value pretty quickly.

            Again I’ll say, maybe I’m missing something, or messing up the calc, but hopefully having a proper assessment done will clarify it all for me.

        2. Jason S. | | #30

          "All energy models are wrong. Some are useful."

          ROI is an impossible equation
          because nobody knows the price of energy tomorrow. You're right to consider the relative order of magnitude of one upgrade vs. another though, as the goal of energy modeling is to make informed decisions. Just don't bank on correct energy use estimates within a margin better than +/-20%.

          The hidden variable with the double wall equation is 'sometimes' a reduced HVAC system capacity and that potential cost reduction.

    3. Joel Cheely | | #34

      You say going greater than R 7.5 on c.i. is not feasible. Why? Are you doing a rain screen with strapping? I used 2 layers of 1-1/2" insulation with strapping and really like it for several reasons; it allows insulation to be continued down to the footing in one continuous run, one layer of 1-1/2 can be replaced with a 2x for detailing of door sills, porches, etc. without reducing required r-value, once the exterior insulation is on, you have an insulated enclosure and can work on interior in cold weather, two layers allows overlapping edges, you're already staged for one layer so second layer is easy, etc. The only extra was the 2x window bucks, which I pre-assembled.

  8. Johngfc | | #18

    Mark - Interesting discussion - thanks to you and respondents.
    https://bsa.ornl.gov/#/ provides good and informative analyses of wall designs, and evaluates them against IRC recommendations for 'safety'. You need to create an account, but it's a gov't site so it doesn't generate junk emails.

  9. Expert Member
    Akos | | #22

    You can get a ballpark of your energy loss through your walls/roof/windows from your HDD.

    BTU lost during heating season=HDD(18C)*9/5*24*WallArea/Rvalue.

    So in your case say for 2000sqft of wall
    For R24 3800*9/5*24*2000/R24=136Therms or 3984 kWh
    For R28 3800*9/5*24*2000/R28=117Therms or 3428 kWh

    A good heat pump will average around a COP of 3.5 over the season, your electrical use to supply the heat lost through the wall is 3984/3.5=1138kWh for the R24 wall or 3428/3.5=979kWh. Based on my local electricity rates, that is $22/year, so ROI measured in centuries.

    You can do the exact same calculation for your windows and roof to see the effect of higher R values.

    Lot of times is not just about ROI when it comes to buildings. For example, well insulated and sealed structures have even temperature and are much more comfortable. High performance triple windows are significantly warmer on those cold clear nights, especially if you have larger windows. Floor heat in bathrooms/kitchen/entrance mudroom simply just feels great. There is no ROI fo these, but I would still include them.

    1. MarkMac | | #24

      Akos,

      I’m starting to think along those lines. The ROI isn’t the decision here, it’s whether there’s other benefits like comfort, even heat.

      Your calculation is basically what I’m doing, but for the total house. I’m pulling total area of surfaces from the CAD drawings. I was even pushing the efficiency of the heat pump down to only 2.5 to see if that could make enough financial difference, but it was minimal.

      Thanks.

  10. MarkMac | | #26

    If I forego the double wall, are there any additional efforts, materials, cost to consider with respect to the tightness of the envelope? I assume the best investment there is experienced labor and techniques?

    Windows and air tightness are the largest two areas of loft in the model numbers I have. So instead of double wall, spending the same or less on upgrading windows further makes sense, but I’m not familiar with the air tightness efforts to know if any decision of A over B can make a significant difference? Maybe that’s a loaded question on a discussion board like this.

    I guess a double wall with the barrier between the two walls, and very limited penetrations is one way…………….

  11. MarkMac | | #28

    Maybe I should start a new topic/question, but I'm trying to look at the option without ROI as the factor. What are the double wall (staggered stud) non-R value benefits that should be considered in addition to investment return?

    1. Sound reduction?
    2. Tighter envelope?
    3. Any comfort difference between double wall and single wall if same R value?
    4. Areas of construction that are simplified?
    5. Some savings in electrical and plumber labor?

    I've read R is R, regardless of foam, fiberglass, cellulose, or how you get there. So what else to consider with Double wall, all things equal in R value.

    Maybe there are negatives also? Larger slab, more concrete to achieve same living area? Added risk to other areas of construction?

    1. Expert Member
      Zephyr7 | | #29

      1- Staggered stud walls do help quite a bit with sound. If your house is on a noisy street or near an airport, a staggered stud will will help keep things quiet within your home. The way this works is that the staggered studs prevent any rigid coupling between the inner and outer faces of the wall, and that reduces sound transfer from one side to the other. Note that for this to really result in a quiet home, you have to look at the entire system, which means sound attenuating windows, good doors, etc. Staggered studs alone will help, but it takes more than that to really soundproof things.

      2 - Not really. There are all kinds of ways to seal up all kinds of walls. If anything, a double stud wall is probably more difficult to air seal just because there are more pieces coming together. If you do airtight drywall inside, and put in the effort to air seal the exterior sheathing, a double stud wall will perform just as well in terms of tightness as any other wall assembly.

      3 - I would say "no" here. The only possible exception would be if you didn't use continous insulation in the assembly and had some thermal bridges as a result, but you are comparing double stud and regular walls with exterior continous insulation so you shouldn't really have any real difference between the two in terms of thermal performance aside from any absolute R value differences.

      4 - You can make the argument that exterior trim details are simpler with a double stud wall than with exterior rigid foam, but the double stud wall is probably more complex to build overall. I think you're just trading different complexities around here and neither wall design is clearly simpler than the other overall.

      5 - Doubtul -- you're still running stuff in a stud wall either way. Plumbing would have more room in a double stud wall, so that's a potential plus, but you really want to avoid putting any plumbing in any exterior wall so it shouldn't be an issue with either type of wall assembly. Electrical is going to run through the studs in the inner side of a double stud wall, so from an electrician's perspective, there is no difference.

      A double stud wall increases the square footage of the floor plate without increasing the useable square footage of the structure. This could be a negative in terms of taxes, or be sneaky if you're leasing the place out since lease rates are typically based on the square footage of the floor plate and not the useable square footage. I doubt you're really looking at much difference here though, especially in terms of any tax issues.

      Bill

  12. Jon R | | #31

    Windows can make a significant comfort difference. Even with some R value difference, comfort with walls and ceiling will be effectively the same.

    Use this to see if windows make much difference for each specific situation:
    https://www.payette.com/glazing-and-winter-comfort-tool/

    > what else to consider with Double wall
    moisture risk.

    1. MarkMac | | #35

      Jon,

      Thanks. I’m starting to lean more towards saving $18k on double wall, and spending more on windows. I had double, low-e already, but maybe there’s other options to consider, even in the hardware.

  13. Robert Opaluch | | #33

    Consider two other items to justify higher insulation and air-sealing costs:
    1. Resilience - When you lose power for an extended period, a well insulated, more airtight, or solar-oriented design will keep the interior warmer during the outage.
    2. Reduction in initial heating equipment costs, maintenance/replacement, and fuel for heating. You might get by with cheaper electrical resistance or heat pump space heating or some combination.

    Most of your windows face east, but do you have a clear view of the low path of the sun on your south-facing or south-east-facing windows? Solar gain can be substantial on sunny winter days, reducing your heating costs and providing more daylighting.

    1. MarkMac | | #36

      Bob,

      Thanks for the comments. All of these ideas are helping me assess the decisions from angles I hadn’t considered.

      I’m the winter, we should have good sun on the east for sure. It’s going to have a 14’ h x 19’ w wall of various windows, with a covered porch. So summer shade, but winter can bank some solar.

      With the east having the views, the south wall layout isn’t living area, and has minimal windows. It’s also the large roof area to make it ideal for PV, and future PV expansion for electric vehicle,

      West has second largest glazing, and also a covered porch. Should have some winter solar gains late in the day.

      1. For your suggestion of resilience, wouldn’t R-value and tightness be the key variables here? If I’m debating between R-24 and R-28, and assuming equivalent tightness, would I see any significant difference in a power loss with only a R4 delta? Also, I’m planning 100%PV tied to grid. I guess I could also consider the increase in cost for solar battery backup, and that would provide a backup when the powers out.

      2. After I have proper modeling done, I’ll be able to size the HP system. Right now I, budgeting a 3 ton air-source HP, for 32k volume of space. If the modeling suggested 2 ton, there’d be some savings, but I’m not sure of the per Ton of HP value?

      Thanks again.

      1. Robert Opaluch | | #37

        Consider putting your heat loss numbers into a pie chart or similar graphic to visualize where your heat loss would occur.

        Not knowing all the details of your proposed building, we might guess that about 25-30% of your wintertime heat losses would be from air infiltration, HRV/ERV and exhaust fans. About 70-75% might be from conductive heat losses through your walls, windows, doors, upper ceiling/roof, and foundation. So perhaps 10-15% would be through all your walls. (You could calculate more precisely, knowing all your proposed R-values.) So would moving from R-24 to R-28 make much of an impact?

        You might reduce the conductive heat loss through walls by about 15% by upgrading R-24 to R-28. That’s 15% less from the walls, but with walls being only perhaps 10-15% of total heat loss of your home mid-winter. That works out to 1.5% to 2.25% of the home’s total heat loss mid-winter by going to R-28 walls. Pretty rough numbers here, but small by these types of estimates.

        Of course, the same holds true about window upgrades. R-4/U=0.25 or R-5/U=0.2 were pretty affordable last time I checked (before recent price spikes), but better spec numbers than that, seemed to drive prices up more substantially, though comfort with triple panes might be worth the investment. Adding cellular window shades, insulated window shades or shutters can add R-3+ to your windows during nighttime hours (which during winter is two-thirds of the length of the day). Very cost-effective.

        Even better is the investment in air sealing. Often tedious work, but usually cheap materials can drive down that 25-30% of total heat losses through infiltration. Be certain that you have at least one continuous air barrier throughout your building envelope.

        Upgrade any bath and kitchen fan and upgrade backdraft dampers, or eliminate some by using your ERV/HRV to vent bathrooms where possible, thereby recapturing some exhausted heat.

        We don’t know your precise location, but if you look it up on WeatherSpark.com, you can see the degree of cloudiness mid-winter, and other nice graphics depicting your local climate. There is more detailed, very useful solar gain data available at:
        http://www.susdesign.com/windowheatgain/index.php
        You will get about three times as much solar heat gain and daylighting on south-facing windows than east or west-facing during Dec-Jan. And almost none on the north side. You can calculate the average BTUs of gain per day for January for the amount of glazing you have facing east, as well as the maximum for a day of full sun. You likely will not be able to get enough solar gain heating to downsize your heating system but you probably will need little or no heating during sunny winter days in the late morning hours.

  14. MarkMac | | #38

    Great idea Bob. I've been using the calculator from BuildItSolar, which suggests the results are likely on the high side.

    Here's the Design Temp BTU loss/hr breakdown using R-24.

    Ceiling Loss 3,825 9%
    Wall Loss 10,219 24%
    Window Loss 12,450 29%
    Floor Loss 1,261 3%
    Slab Loss 2,040 5%
    Infiltration 12,662 30%
    TOTAL 42,457

    If I bump up the wall to an R28, I get the following. While the Wall loss falls in isolation (15%), the drop is not enough overall (3.4%) to make a significant difference

    Ceiling Loss 3,825 9%
    Wall Loss 8,759 21%
    Window Loss 12,450 29%
    Floor Loss 1,261 3%
    Slab Loss 2,040 5%
    Infiltration 12,662 30%
    TOTAL 40,997

    I noted in the original post, I pulled down 75 years of Environment Canada HDD18C stats for the exact town location. As expected, the HDD18C has been gradually decreasing, and in the last 20 years , there's only been one year over 4,000. The Design Temp I've been using is -5F, or approx -18C, which is listed also as the design temperature for the location.

    I'm using R-3.5 for all windows, but suspect we can do better with windows in Canada. Bumping those up to 4.5, reduces the Window component by 22%, and the overall BTU loss by 6.5%.

    Our window layout isn't great for solar gains, as its an E-W lot, and the views are to the East. Garage, Laundry, Bathroom and Closet make up the South facing wall area.

    Direction - (% of total) - SQ ft of Window
    N - 4% - 16
    S - 7% - 30
    E - 68% - 300
    W - 21% - 94

    What are the recommendations for investing in better air sealing? The plan is to try and get as air tight as possible already, but are there key items that make the largest bang for the buck in reducing ACH?

  15. Robert Opaluch | | #39

    You might want to start a new Q&A to get responses to the air sealing question, or search GBA for articles on that topic. Here’s another source JLC:
    https://www.jlconline.com/how-to/insulation/air-sealing-options-good-better-best_o

    Usually taped sheathing (or your ci layer) is used as a primary air barrier for walls. Production builders often use Tyvek or Typar applied over sheathing, but they aren’t as effective as an air barrier. Tape the sheathing joints. Some high performance builders tape each nail head not just at the sheathing edges. You can use tapes, liquid applied sealant products, or specialty gaskets for anything that exits through the walls. There’s lots of videos on the web demonstrating how to seal windows/doors to a sheathing or ci primary air barrier. Gaskets or acoustical sealant seem to be the most popular choices by high performance builders for sealing between the bottom plate and foundation wall. The transition between the walls and top floor ceiling is a critical connection, and I've seen quite a few different solutions. For ceilings as well as exterior wall interior side, there are sheet products. See
    https://foursevenfive.com/products/air-sealing-system/membranes/
    They have lots of high end high performance products, and I think they have educational material on their site also, as well as reviewing articles here on GBA.

    Another approach is airtight drywall for ceilings and walls, rather than using the sheathing layer as the primary air barrier for walls, but it seems tougher to do well. (or do your best at both) See:
    https://www.greenbuildingadvisor.com/article/how-to-hang-airtight-drywall
    and an earlier video series:
    https://www.greenbuildingadvisor.com/article/new-video-series-airtight-drywall

    Better quality windows tend to leak less air too. Some builders actually test their windows for air sealing performance before installing them, to return any that are not performing up to specifications. Better quality doors will have better air sealing, with multiple gaskets at edges and a multi-lock system for door locks, but can cost several thousand each.

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