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Community and Q&A

Net zero construction in Yakima, WA — Climate Zone 5B

Joshua Greisen | Posted in Energy Efficiency and Durability on

Hi there,

I have been a frequent reader of this website over the last couple years and didn’t see an article that addressed a few questions I have run into when attempting to design and build a net zero home in Yakima WA (climate zone 5B).

I do understand that net zero can be achieved in any house with a big enough solar array, but I’m looking to strike a balance and use as little energy as possible based on my design choices, building material decisions and construction techniques. The hard part seems to be finding the line on what is cost effective within the current market conditions. I’m by no means a rich man and can only afford to do what has an ROI that will realize within a decade or so.

Here’s it what our structure looks like so far from an energy conservation perspective:

– Property is a South facing hill slope with no obstructions perfect for passive solar gains.
– The structure is a single story 1000 sq ft apartment with an open floor plan (two bedrooms, single bath, laundry, kitchen and great room).
– The conditioned space is a 26 ft by 38 ft rectangle.
– Continuous insulation for all exterior walls (currently settled on a 2 x 6 wall filled with fiberglass batts and CI rigid foam on the exterior, probably 2″ of XPS).
– Concrete floors throughout with radiant heating via a Daikin Altherma System, which also includes an air handling unit for cooling in the summer (ducting housed with conditioned space of course).
– The air tightness of the conditioned space will achieve 1.5 ACH or better.
– The air quality and circulation will be done via an HRV.
– Atrium triple pane vinyl windows with Low-e glass, all but two are fixed, the remaining are casement.
– The majority of the windows are south facing with a pergola overhead on the exterior to provide necessary shade in the summer to avoid overheating the build envelope.
– The ceiling are 10 ft tall and the doors are all 8ft tall. I know this is not energy efficient but it is a necessary compromise with my wife for all my energy efficiency requirements lol… plus it makes the apartment feel much more spacious.
– Exterior doors are all single fiberglass doors, but they are 8 ft tall and 36 inches wide.

So now for the questions:

1) What is the sweet spot for R-values in the ceiling, walls and slab given this build envelope? Especially in terms of how much rigid foam is need in the slab, stem walls and exterior walls. Code in WA State is R-49 in the ceiling, R-21 in the walls and R-10 in the slab and stem walls. I have read the R10/R20/R40/R60 rule referenced here, but that seems overkill perhaps given all the other factors in play that I have noted above.

2) In my searches I found a great presentation on R-40 walls, comparing different construction strategies and methods (link here). Slide 31 does a great job at indicating what is effective for the money. Based on their results a 2×6 wall with exterior XPS CI anywhere from 1-3 inches is quite cost effective. It’s only beat by a 2×8 or double stud wall with exterior XPS CI. In everything I’ve read a rain gap is needed between the sheathing and foam, how much does that impact the insulating quality of the exterior XPS CI?

3) Given the exterior door height of 8ft I am considering a multi-point locking system, this upgrade is expensive from Therma Tru which is the door manufacturer we are looking at and it also limits lock/handle selection to Therma Tru’s options according to their literature. Is this kind of upgrade going to be worth it?

Any guidance on these questions would be much appreciated. I feel like I’ve been think about this stuff too much!

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Replies

  1. User avatar
    Dana Dorsett | | #1

    For climate zone 5B to hit Net Zero Energy with something that fits on the roof you need to shoot for at least R30 "whole wall", which would be 2x6/fluff + 3" of exterior foam, not 2". It doesn't need be R40, though that would be roughly the center-cavity R. Polyiso would be much greener than XPS, due to dramatic differences in the environmental impact in the blowing agents used.

    Going with 3" or 3.5" reclaimed roofing polyiso would be about right, or 2" of reclaimed goods with 1" of foil faced virgin stock for ease of air sealing.

    Take a look at Table 2 p.10 of this document, bearing in mind that those are "whole assembly-R", with all thermal bridging from framing factored in:

    https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

    That document was done in 2009, when typical better-grade rooftop PV was 15% efficiency (rather than 20% now), and better class heat pumps had a HSPF of about 10, as opposed to 12 now. Between those two factors you can probably cheat the numbers in that table by 10% and still get there if you pay attention to details.

  2. Drew Baden | | #2

    Joshua,
    I apologize for introducing noise here but I'd be very interested in seeing your floorplan if you don't mind posting?

  3. User avatar
    Stephen Sheehy | | #3

    I understand high ceilings, but 8' tall doors? Why? Big upcharge.

  4. Charlie Sullivan | | #4

    I agree with Dana's suggestions on ballpark for insulation.

    Two of my exterior doors have multi-point locks and the air sealing on them is significantly better than the one without. It's also easier to get them working well, without it being too loose or too hard to close. The advantage of multi-point would be more on an 8' door. That said, the air leakage around a decent door without a multipoint lock is not so big as to prevent achieving your air-tightness goals.

    I chose Jeld-Wen doors over Therma-Tru because Jeld-Wen using low global-warming-potential foam in the doors and Therma-Tru wouldn't say what they use.

  5. Steve Knapp CZ 3A Georgia | | #5

    Intus doors are much better quality than Thermatru ar a similar price. They have a standard three-point lock.

  6. Joshua Greisen | | #6

    Thank you for the feedback so far.

    Dana, thank you for the link. In the chart on page 10 I see they recommend attic R-values of R65. What are your thoughts on this number? The local experts I've spoken with are telling me that due to a much tighter than average build envelope in my planned design the requirement for a higher ratio of insulation in the attic is not as necessary as is usually recommended for the typical home. They've gone as far as to say that the same R-values in the attic and walls is sufficient. Curious to hear other thoughts on this?

    Drew, I have attached a screenshot of the floor plan. Feedback encouraged, permits have not been submitted yet :)

    Stephen, I know 8 ft doors are significantly more expensive and require a special order with more lead time. The wife likes them and I had to make a compromise or two for things that don't make fiscal or energy efficient sense. Based on my other options 8 ft doors is a good compromise. I do think 6' 8" with a 10' ceiling seem a bit out of place.

    Charlie, thank you for the input it will help me make a decision.

    Steve, I'm looking into Intus doors and a quote for comparison. How is their window pricing?

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Joshua,
    You'll save a lot of money if you choose a simpler HVAC system. You could heat and cool this house with a single ductless minisplit (especially in light of the excellent thermal envelope that you are planning). That will cost thousands of dollars less than a Daikin Altherma coupled with in-floor hydronic tubing.

  8. D Dorsett | | #8

    Regarding the R65 attic performance. You'd be shocked at just how little difference in cost there is between a code-min R49 (R38 whole-assembly) attic is compared to an R65 (whole-assembly) attic, if it's done with open blown cellulose on the attic floor. That said, as I pointed out initially, with ~20% efficinncy PV and HSPF 12+ heat pumps you can probably bump down 10% (or even move down whole line to the zone 4 numbers in a place as sunny as Yakima) and still hit Net Zero if you pay attention to all the details. At about 3 cents per R per square foot, once you're above the truss chords or joist depth another R20 costs 60 cents per square foot, or about $600 for your whole attic. Even though the performance increase is small, it's still cheaper performance than getting the same peformance out of fatter walls or higher performance windows.

    In your floor plan it doesn't indicate the direction each wall is facing. It would be wise to limit the size of (or completely eliminate) west facing windows, since that injects solar gain late in the day when outdoor temps are higher and the roof deck is still pretty near it's peak temperature, adding a significant fraction to your peak cooling load.

    If you're really going ahead with hydronic floors, install at least 4" of EPS under the slab. If not, 2" would be fine.

    If you build a ~1000 square foot rectangular house to Net Zero levels your heat load at Yakima's +11F outside design temperature is going to be under 10,000 BTU/hr, and the radiant floor would be less than 5F warmer than the room temp, even at design condition. While that's pretty cushy on design-day, at your average mid-winter temperature the warmer floor would barely be noticed. You could probably heat & cool the whole shebang with a 1-ton Fujitsu -12 RLFCD (or worst-case, the 18RLFCD, though that's probably exreme overkill) mini-ducted mini-split for about 1/4 the cost of just a radiant floor heated with with an Altherma. If you limit the size of the windows to the doored-off bedrooms you can do it with a 1-ton or 3/4 ton wall blob type mini-split (or floor unit) in the Great Room, as Martin suggests, at about 1/5 (or less) the cost of an Altherma solution.

    http://www.fujitsu-general.com/us/resources/pdf/support/downloads/submittal-sheets/12RLFCD.pdf

    http://portal.fujitsugeneral.com/files/catalog/files/9RLFCD.pdf

    http://www.centralmaineheatpumps.com/sites/default/files/ec_pro/centralmaineheatpumps/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf

    https://www.master.ca/documents/regroupements/1fujitsu_submittal_asu9rls3h.pdf

  9. User avatar
    Stephen Sheehy | | #9

    Re: Indus doors and windows. Pricing for UPVC windows was pretty good. Windows are really nice, very robust hardware. Doors are also nice and solid. We paid about $2800 per 7' x 38" door, fully glazed. I'd definitely buy them again, but we had a good local dealer, Performance Building in Portland ME. I'd be uneasy if there isn't someone local to buy through. Although we've had zero issues, it is comforting to know there is someone available if anything comes up. In addition, they sent two guys out to show our GC how to install the Windows in the middle of our double stud walls.

  10. User avatar
    Dana Dorsett | | #10

    BTW: With a low heating load that fireplace will become almost unusable- you'll cook yourself. If you want a wood burner, a small woodstove with some inherent thermal mass (such as a ceramic or soapstone stove) would be a better choice. Hearthstone's Bari or Tribute models would work. There are others.

  11. Joshua Greisen | | #11

    Dana,

    Your reasoning on the attic insulation makes sense to me, seems like an easy / cheap upgrade. I'm getting some push back from my GC because the best insulation sub in the area doesn't do cellulose. My GC hasn't done cellulose in any of his recent homes for that reason. I still lean towards blown cellulose in the attic and in the walls based on what I've researched from a feature and ROI standpoint.

    The wall with all the windows is facing North for passive solar gains in the summer. Which makes West the right side of the image I attached above. No windows there just an attached garage.

    If our Altherma bid does come out to be 4 times more that a straight minisplit system we may consider downgrading. The sub we talked with mentioned the same floor temperatures during our discussions but I figured I should check out the cost differences. We love the looks, durability and solar mass of concrete floors but I worry about how cold they may get in the winter when the ground freezes. We currently plan to put 2" of XPS under the slab and around the stem wall.

    I will look into the wood stove options you mentioned. The unit we chose is the Focus 250 with a heating capacity of 800 - 1500 sq ft. I'm sure those numbers aren't for a net zero build envelope course.

    Stephen,

    The closest dealer looks to be Tumwater, which is a couple hours away. Not sure if I consider that local, but I will get a quote and talk with the rep. Thanks!

  12. User avatar
    Dana Dorsett | | #12

    3" of sub-slab Type II EPS (R12.6) would cost the same or less than 2" of XPS, and would outperform it forever. Only 2" of XPS (R10) would be marginal for a heating slab even when new, and as it's HFCs bleed out over a few decades (with it's powerful greenhouse gas impact) it drops asymptotically to R8.4 at full depletion, the same as EPS of similar density.

    If you pour the stem walls in a minimalist ~R16 (2" + 2" EPS) insulated concrete form (ICF) it's quicker than insulating it after the fact. You then float the slab, using the inner EPS of the stemwall to deal with the expansion/contraction of the slab, with the wall-foam continuous with the sub-slab foam. Set the foundation sill/studwall plates where the exterior foam of the studwall aligns with., or is slightly proud of the exterior ICF foam to provide a continuous thermal break between the sill plate & exterior.

    An EPDM sill gasket under the plate will provide s an excellent capillary break between the stemwall & the susceptible wood. If you are in a termite zone you may have to use copper flashing to separate the ICF foam from the wood, which would re-create the thermal bridge, but even then it would not be as severe as the stud plate resting on concrete that was exposed to the exterior.

    With R8 between the slab edge and the stem wall or ground, and an other R8 on the exposed exterior of the stem wall you will have no comfort issues with an unheated slab even when it's -15F outside (has it even gotten that cold in Yakima in the past century?)

    If you have to go with R60-65 blown fiberglass in the attic instead of cellulose it's not really a disaster. Using a chunked up high-density product like AttiCat gets around most of the deficiencies of low-density fiberglass of 25 years ago. Most vendors' products are much improved since then, are less susceptible to "fluffing" by unscrupulous installers of a few decades ago.

    A 3/4 ton cold-climate mini-split like the 9RLS3H or FH09 runs less than $3.5K, (all-in, fully installed) my neighborhood, and a 1 ton is usually less than $4K. I've never heard of even a heating-only Atherma system quoted as low as $15K, but yours could be the first(?). By the time you'd added the chilled water air coil it'll be a bit more expensive. See: http://www.greenbuildingadvisor.com/articles/dept/musings/air-water-heat-pumps

    A 1-ton mini-duct system could run in the $5-6K range, depending on how complicated the duct issues are. With your floor plan it could be pretty straighforward to do a vertical mounted Fujitsu on the other side of the wall from your washer/dryer in the MBR closet, which would also give it ready access to a drain for air conditioning condensate disposal, and access for maintenance & filters, etc.

    With a Net Zero house the heating/cooling balance point will usually be in the 50-55F range, which knocks about a month each off both the beginning and end of the heating season. The heating season for a new code-min house in Yakima would be from roughly half-past September through the end of May, but in a Net Zero house it would be from about mid-October through the end of April.

    https://weatherspark.com/averages/31988/Yakima-Washington-United-States

  13. Steve Knapp CZ 3A Georgia | | #13

    Joshua. My Intus rep was about two hours away, and it wasn't an issue. I discovered Intus too late in the process to order windows, but the rep was great on handling the door. Because we were so far behind schedule, he drove six-plus hours to Charleston to pick up the door and then made an equal drive to bring it to use. He and the contractor's crew install the unit while we watched.

    But do some due diligence on your local rep by all means.

  14. Joshua Greisen | | #14

    Dana,

    I'm curious about your suggestion to use EPS and ICF for the slab and stem walls. My GC thinks XPS is the way to go. Is there an article or document somewhere that demonstrates the concept your are proposing, perhaps with a cross section?

    I'd also like in inquire on the rigid foam material for the exterior walls, what are your thoughts on using EPS there? I have seen some panel systems that include OSB sheathing pre-adhered to the foam with foil faced EPS. I've also seen EPS panels with ridges on the interior side to aide in water drainage that lock together similar to ICF blocks. To be honest, I don't see much information on exterior EPS CI anywhere but I like the consistency of EPS and the price when compared to polysio.

    On separate note, I've done some reading more recently that also questions the taping of seams of a exterior CI. The paper is put out by Dupont, a materials manufacturer, but I was wondering if there's any true consensus on the idea that seams should not be taped and a air/whether wrap should be used under/over the rigid foam (link here)?

  15. Bob Irving | | #15

    You seem reluctant to add additional under slab insulation, but are concerned about how cold the floor will be in the winter. We'd use 6" with your situation, and the cost of EPS is 40% less than XPS so your additional out of pocket costs are minimal. Be sure you isolate the slab from the surrounding foundation, or you'll lose heat to the outside. In the R-40/60 homes we're building in zone's 5B and 6 using Air Source Heat Pumps, AKA mini splits, the comfort level is unlike anything I've ever experienced - the temps are constant and even throughout, and the house will work very well with 100% of the heat, hot water and electrical loads sourced from rooftop solar. I also would not worry about the ceiling height - with the right insulation levels and a very very tight house, it'll make very little difference.

  16. User avatar GBA Editor
    Martin Holladay | | #16

    Joshua,
    Q. "I was wondering if there's any true consensus on the idea that seams [of exterior rigid foam] should not be taped and whether a wrap should be used under/over the rigid foam?"

    A. There are several issues here. The article you linked to (from DuPont Tyvek) isn't really about whether foam seams should be taped. The article questions the logic of using rigid foam as a water-resistive barrier (WRB).

    For more information on this issue, including a discussion of foam shrinkage, see this GBA article: Using Rigid Foam As a Water-Resistive Barrier.

    The reason that it's a very good idea to tape the seams of rigid foam is to reduce air leakage. (Reducing air leaks is always a good idea.) Just because you are taping those seams, doesn't mean that the rigid foam is going to be your WRB.

    It should come as no surprise that a document written by the manufacturer of Tyvek promotes the use of Tyvek. That said, I agree that it's a good idea to use housewrap as a WRB rather than trying to make rigid foam perform as a WRB.

    When it comes to the question of whether housewrap should be installed under the rigid foam or over the rigid foam, the brief answer is that either approach can work. For a full discussion of the issue, see Where Does the Housewrap Go?

  17. Joshua Greisen | | #17

    Bob and Dana,

    Not reluctant at all to add insulation under the slab if that is a more effective solution. At this point I'm leaning towards doing exactly what I believe you and Dana are recommending with ICF stem walls and floating the slab on 2 or 4 inches of EPS. We would then skip the Altherma heated floors and cooling solution and just do a mini-split instead. Dana mentioned in an earlier post on this thread that 2 inches would be fine without the heated floors, if we don't go with heated floors is there and significant benefit to adding another inch or two of EPS under the slab on top of the 3" of EPS we will need to reach WA code of R-10 (assuming ICF stem walls)?

    I've been looking for a good wall and foundation cross section showing the ICF stem walls, floating slab with under slab rigid foam and exterior wall rigid foam CI details. Are you aware of an article that has this cross section or should I draft something up to discuss with my contractor?

    Martin,

    Thank you for the response, I've read through these articles and I'm thinking we'll tape the seams of our rigid foam and use housewrap in between the sheathing and exterior rigid foam.

  18. User avatar GBA Editor
    Martin Holladay | | #18

    Joshua,
    Here is a detail from the GBA detail library.
    The link is http://www.greenbuildingadvisor.com/cad/detail/insulated-basement-poured-concrete-4-interior-rigid-foam-framed-wall

    It's not an ICF wall, but you still may wish to study it.

    .

  19. User avatar GBA Editor
    Martin Holladay | | #19

    Joshua,
    Here is a detail from Quadlock, a manufacturer of ICFs.

    .

  20. User avatar GBA Editor
    Martin Holladay | | #20

    Joshua,
    Here's a detail from Building Science Corp.

    Again, it's not an ICF wall, but you may find it useful.

    .

  21. User avatar GBA Editor
    Martin Holladay | | #21

    Joshua,
    Here is an ICF basement detail from Fine Homebuilding.

    .

  22. User avatar
    Dana Dorsett | | #22

    I'm not sure why the GC thinks XPS is the way to go (drank the marketing Kool-Aid perhaps? :-) )

    At 1.5lbs nominal density ("Type-II") or denser EPS is reasonably rugged, and cheaper per R than XPS, and is well suited to sub-grade applications (most ICFs are EPS, after all). Also, as XPS loses it's climate-damaging blowing agents over a few decades it's performance drops to that of Type-II EPS. The performance advantage at a given thickness is only temporary. Also, under slabs you don't really care about another ~20% of thickness to hit a given (initial) performance point.

    EPS performance does not depend on the retention of blowing agents. It is blown with (relatively low environmental impact) pentane, most of which is gone before it even reaches the distributor, recovered at the manufacturer's site and burned for process heat. Without that dependency it has a stable R-value over many decades.

    XPS is sometimes preferred by radiant heating contractors whose method is to staple the tubing to the foam (rather than tying it to the reinforcing steel) due to it's superior staple retention relative to Type-II EPS. At 2lb density (Type IX) EPS has about the same staple retention as XPS, and is STILL usually cheaper per labeled- R than XPS, but it's sometimes hard to find. Type-II EPS is usually easy to find, since it's a common density used for roofing insulation.

    A minimalist ICF is about R17-R19, and often saves time (= money) over pouring & stripping forms, then insulating later. QuadLock makes a 2.25" +2.25" ICF, which is about R19, SmartBlock makes a 2.125" per side ( ~R18) version that you can even order through Home Depot, though it's probably cheaper through distributors. Some ICF vendors start at 2.5" + 2.5" (R21), but there are a few others that start at 1.75" or 2", which would be fine for a stemwall. When it's 2" or less on each side the blow-out risk is higher when pouring full-height walls (not a problem for a 4-5' stem wall) which is why vendors have trended toward 2+" minimum thickness per side.

  23. Brendan Albano | | #23

    Joshua,

    One reason you might prefer EPS over XPS is that the blowing agents used in XPS typically have a tremendously higher impact on global warming than the blowing agents used in EPS. This will presumably eventually change, but as far as I am aware, at least in the U.S., XPS is still worth avoiding if at all possible.

    Edit: looks like Dana beat me to the punch here haha.

  24. Malcolm Taylor | | #24

    Joshua,
    One potential cost increase in upping the insulation levels in your attic is the work that may be associated with bracing raised-heel trusses in a high seismic zone like WA. Not suggesting you shouldn't do it, just be aware the cost involved may not only be the additional depth of the blown insulation.

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