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Green Building News

Phoenix Gives Net-Zero House Plans to Anyone Who Wants Them

In a bid to reach carbon neutrality, the city sponsors a contest for architects and then makes the plans available for free

Home NZ was developed by Marlene Imirzian & Associates Architecture and provided to residents of Phoenix, Arizona, as part of an effort to build more energy efficient housing. Illustration courtesy of Marlene Imirzian & Associates Architecture.

Building a net-zero house in Phoenix, Arizona? The city will be happy to provide you with a full set of plans at no cost.

The offer is part of the city’s plan to see all new buildings net positive by 2050, according to an article posted at Architectural Record, and follows a design competition in 2017 challenging architects to develop plans for a near net-zero house suited to the city’s hot, dry climate.

The winner was Marlene Imirzian & Associates Architects, which developed plans for a 2,185-square-foot, three-bedroom home. Home NZ, as it’s called, has a HERS score of 30, making it 70% more energy efficient than an average built-to-code home. With the addition of a modest solar array, the house would be net-zero.

The house can be built for an estimated $344,000 (not including the contractor’s overhead), and is designed to fit on a 60-foot by 110-foot lot. The design is orientation neutral, meaning it would perform as intended no matter which way the building lot is oriented.

According to the description posted with the plans, the single-story house takes its design cues from mid-century modern residential architecture. It includes a large front porch and a built-in garage. Operable, exterior shades can be left open when the sun isn’t shining and closed when windows are in direct sunlight, reducing cooling loads. Shades are designed to prevent 95% of direct sunlight from reaching window glass.

Other building features include:

  • Structural insulated panels (SIPs) for the walls and roof. Wall panels are 9 1/2-inch thick (R-45), OSB over polyisocyanurate insulation. Roof panels are of the same construction, 11 1/4 inches thick (R-70).
  • A reflective cool roof to minimize heat absorption.
  • Passive cooling by means of a solar chimney at the top of the house that can flush out stale air and introduce cooler, fresh air at night.
  • LED bulbs, a smart thermostat, and wi-fi enabled energy management.
  • Heating and cooling with a Carrier air-source heat pump. Whole-house ventilation with a Zehnder energy-recovery ventilator. The variable-speed air conditioner limits starts and stops for higher efficiency.
  • Double-pane windows.
  • Estimated annual energy costs include $286 in service charges, $240 for cooling, and $597 for lights and appliances.

The house plans are available free of charge, but they are provided without a warranty, and the homeowner assumes all liability. Also, the city requires that anyone taking the plans agrees to have them reviewed by a local professional architect and/or engineer as well as a licensed contractor before construction starts.

Winner almost didn’t enter contest

According to Architectural Record, Imirzian nearly ruled out her participation in the 2017 contest despite the $100,000 prize that would go to the winner. She wasn’t familiar with the HERS rating system—a key component of the competition rules—and she would be required to develop all of the construction documents and get them approved by the city’s planning office if she won.

“I said, ‘Forget it, I’m not doing more free work,’ ” she told the magazine. Later, she warmed up to the idea of entering, in part because it would give her a chance to learn more about energy-efficient design. Two weeks before the contest deadline, she contacted a HERS consultant in town, then rounded up an engineer, a construction company and a consultant.

Imirzian’s design was one of nine submissions. It took three people in her office 600 hours to develop construction documents and get city permits. She spent another $45,000 on consultant fees but didn’t get the prize money until the planning office had issued final permits. After the city uploaded the documents, Imirzian was asked to update them to conform with changes in the city’s building and energy codes. That took another three months of work.

The three-bedroom design is suitable for any climate zone, according to the architect who designed it. Floor plan courtesy of Marlene Imirzian & Associates Architects.

In a telephone call, Imirzian laughed when reminded of her initial hesitation to get involved. Although her practice includes residential design, commercial and institutional projects are the firm’s bread and butter.

“I probably shouldn’t have said that,” she said. “It’s a competition and it’s time, and I was just being honest about the evaluation. But the thing that was very compelling for me is that I was very interested in seeing what could be done to show that there are very reasonable and simple ways to do very cost effective, sustainable single family homes. It would be a very important message to get out.”

She said that to her knowledge no one has actually built the house she designed. That’s not as important to her as demonstrating that highly efficient houses don’t have to be complex, difficult to build, or expensive. The $344,000 price tag, she said, was probably in the lower range of a comparably sized custom house in the Phoenix market.

The $100,000 prize helped, but, Imirzian said, “it was not a profit maker, let me say that.”

“The level of ease, hopefully, of implementing this in someone’s new home is what I hope is the message people get,” she said. “Even if they don’t copy the house or use the house plans, what we’re trying to do with the graphics and the explanations is to show how simple assemblies and approaches to whatever project they do could lead to very significant performance results.”

On the road to carbon neutrality

The Home NZ project is the brainchild of Mark Hartman, the city’s chief sustainability officer who describes his job as helping to make Phoenix “the most sustainable desert city on the planet.”

Building houses that use less energy in Phoenix’s baking heat (it was 109° there the other day) is part of a city-wide effort to not only become carbon neutral but also provide a healthy ecosystem for its 1.6 million residents in many other ways. The package was approved by the Phoenix City Council in 2016. (You can read more about the effort here.)

“There’s this misconception that green homes cost a lot more,” Hartman said in a telephone call. “But if you design right from the beginning, the home could be efficient, and that means you need smaller [mechanical] equipment inside. The goal was to build a sustainable home that costs the same as current construction.”

The city used a $100,000 grant from the U.S. Department of Energy to establish the design prize, and set aside some additional money to cover permitting fees for people who decide to build Home NZ. Fees will be lower because the final plans—when they are posted—will not need plan review.

Plans posted now at the city’s website are preliminary. A last minute check of the HERS score after Imirzian & Associates updated the plans found it exceeded the 30 target by a couple of points, so architects went back and tweaked the specs for windows. Final plans should be available soon, Hartman said.

There have been 40 downloads so far, but Hartman had no idea yet how many of those people (which included reporters) will actually be interested enough to build the house.

“I would love to see 50 of them built,” he said. “That’s kind of my goal. That would be great, then maybe we could do other plans. I’d love to see homes like this become a standard, to have people realize that it’s possible.”

One obstacle is builder reluctance to try new things. For example, Hartman learned that windows with a lower solar heat gain coefficient are widely available and essentially the same cost as the windows that Phoenix builders typically use now. Ordering the windows would help cut cooling costs for homeowners. But there’s little demand for them. He hopes an upcoming publicity push with builders in the city to promote Home NZ will help change that.

Even some of the architects who took part in the design competition originally doubted it would be possible to get the HERS score down to 30, Hartman said. “No one thought you could hit HERS 30,” he said. “There was no way that it could happen. Even the participants said that until they actually started working on it and brought a team together. They were surprised—we really can do this.”

-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.

47 Comments

  1. Hugh Weisman | | #1

    I downloaded plans and gave them a very quick once-over...Nice little house, but with very different basic construction than the "typical" American home.. For example, steel edge framing supporting flat SIP roofs with little or no intermediate framing. We've designed multi-million dollars homes without M/E engineer consultants and some with. But for a $350K home, the ME's are incredibly complex and detailed....a sub-contractor would be likely to jack up their bid just for the time it would take them to fully read and digest the plans....LOL.

    1. User avater
      Austin Trautman | | #39

      The architect is used to more complex commercial designs. It shouldn't be difficult for them to swap to a simplified ducted minisplit and HRV/ERV system if a client desired something high performing but easier for a contractor to digest and install competently.

  2. Malcolm Taylor | | #2

    "The three-bedroom design is suitable for any climate zone, according to the architect who designed it. "

    That' seems a bit problematic. It's either optimized for the hot desert, or it's a generic plan that should be.

    1. User avater
      Dana Dorsett | | #3

      No kidding!

      I'm sure there are cheaper (and lower-R) ways to hit Net Zero in Phoenix -than R45 SIP walls and R70 SIP roofs!

      1. KurtGranroth | | #21

        I'm in the process of building what I hope will be a Net Zero home in the Phoenix Metro area and so I've spent an appreciable amount of time using BEopt to model potential aspects of the plan just to see what makes the most sense. According to BEopt, increasing the r-value of walls beyond R-13 hits diminishing returns extremely quickly. My house will have an an r-value in the mid-20s and BEopt insists that increasing it more than that will result in energy savings in the double digits of kWh PER YEAR. Hardly worth it.

        I will admit that that's not an intuitive result to me. The heat is so oppressive outside, that it feels like wall insulation would make a bigger deal. But I guess even in extreme cases where you have indoor temps at 70 degrees and it hits 115 outside, there is still "only" a 45 degree differential... far lower than the 70+ degree differential you can see in some northern climates in the winter!

        The high r-value in the roof does appear to matter, according to my modeling. I'll be shooting for around R-50 and the numbers suggest that it will be worth it. An increase to R-70 would likely not be worth the extra expense... but if you are making it from SIPs already, then that might just come along for the ride.

        (I had a paragraph here about thermal mass, but that was because I kept substituting ICFs for SIPs in my mind when I read the article... SIPs have very little thermal mass compared to ICFs so the paragraph made no sense)

        1. User avater
          Dana Dorsett | | #22

          >"I will admit that that's not an intuitive result to me. The heat is so oppressive outside, that it feels like wall insulation would make a bigger deal."

          It's the windows, not the walls that make or break it. A code-min window has a U-factor about 3x that of a 2x4/R13 wall. Even on merely conducted heat a square foot of window passes three times the amount of heat of a square foot of 2x4/R23 wall. But that's not even factoring in the radiated heat gain coming through the windows.

          And the solar gains trough windows are from more than just direct-sunlight - backscattered light from light colored landscapes still adds up.

          >"The high r-value in the roof does appear to matter, according to my modeling."

          That's fairly intuitive, considering the very high summertime sun angles at 33 degrees north latitude. The direct solar gain of the roof and the peak roof temperatures are quite a bit higher than is experienced by the walls, which natural convection cool to within a very few 10s of degrees of the ambient air temperature even when the sun is hitting them. Peak roof temps can easily be more than 50F higher than the ambient air temp even on sloped roof, let alone flat roofs. Utilizing high solar reflective index roofing and using roofing with some thermal mass to it (such as concrete tile) can improve the cooling season performance by quite a bit. White TPO membrane roofs work pretty well for keeping temperatures bounded on flat roofs.

          1. User avater
            Austin Trautman | | #33

            Your BEopt assessment is more or less correct. Due to our "low" delta T temps in Phoenix super high R values aren't necessary.

            What IS super helpful though is paying attention to thermal capacity, not just R-value. We have side by side testing of a high thermal capacity system (Gutex) at R-5 FAR outperforming a low capacity (foam) R-10 system.

            We also have multiple homes built and tested at net-positive energy and HERS scores under 30 before adding solar. The trick is understanding all the pieces working together, not just the ones that affect HERS scores or typical energy modelling. In the end, our climate is unique in non-obvious ways but not that difficult once you understand it all.

            It's not rocket science but it is building science.

          2. User avater
            Jon R | | #36

            > (Gutex) at R-5 FAR outperforming a low capacity (foam) R-10

            But if I'm looking at the right version, Gutex is still much more expensive per R value equivalent. Perhaps cellulose is the greenest/$?

          3. User avater
            Austin Trautman | | #38

            Yes Gutex isn't the least expensive insulation (about $0.27/R/SF). On a strict $/energy performance it's not your top choice. There are other benefits to consider though such as moisture, mold, durability, sound transfer and human comfort.

            We love cellulose. It's cheap, carbon negative, locally sourced, has decent thermal capacity, low chemicals, etc. A competent dense pack installer is key and a proper interior barrier to keep it dry year round is a worthwhile insurance policy.

            Don't be afraid to combo insulations either, as long as you understand the building science of what you're doing.

    2. Jason_Cole | | #4

      "Suitable for" and "optimized for" are two completely different measures of detail. According to the article, it also performs as designed regardless of solar orientation. Basically the city wanted a generic plan that could be implemented regardless of lot location/orientation, and this delivered it.

      Regarding Dana's comment below, $350k for a ~2000sf home is $175/sf. That's not unreasonable given construction costs before energy efficient upgrades (rigid or spray foam, above code min wall assemblies, reliable air sealing, etc) can easily hit $150/sf.

      Quick zillow check shows plenty of 3/2 homes around 2k sf on the market for $320k-$350k. I can't imagine any of these coming in with a HERS of 30.

      1. Malcolm Taylor | | #7

        Jason,

        I don't want to pick on one small statement as a way of discrediting what may be a well designed and functioning house for Phoenix, but the design is not suitable for most other climates. It wouldn't work well in either the wet PNW here I live, any cold climate, or a hot humid one.

        I'm also a bit troubled by the process. Why would designing and preparing the contract documents for an energy efficient house of this size take 600 hours of the architect's time alone - for work that then required another three months to reconcile with the applicable codes? Without a more detailed explanation, it just doesn't make any sense.

        1. Andrew C | | #8

          Regarding how much time it took to design and then reconcile with code -
          The architect said that they don't do much residential and weren't familiar with HERS, so they are early in their learning curve. If they stuck with it, I imagine that they'd speed up quite a bit.

  3. User avater
    Jon R | | #5

    The price (without contractor overhead?!) and the energy savings are both estimates. Hopefully they will build a few and carefully track the real numbers. I expect that it is insulated way beyond rational.

    A goal of net zero energy buildings is cost inefficient, market distorting and ineffective. Far better to look at larger areas (eg, a net zero state).

    1. Hawksnestbay | | #28

      When you download the plan, the cost estimate states that it DOES include contractor overhead. Custom in the Valley is that the Most Desirable orientation is t0 face the most amount of glass t0 the north. No builder in the Phoenix area includes insulated glass in the specs. Extra only. I am a fan of the Idea of an off the shelf plan, but I am reminded of the appraisal concept of , "Cost does not equal value."

  4. Craig | | #6

    While it's stated these plans are good for every climate zone, I disagree. Layout, heating system, roof slope, are just a few of the details that will not work in northern climates.

    1. User avater
      Dana Dorsett | | #9

      >" Layout, heating system, roof slope, are just a few of the details that will not work in northern climates."

      OK, now I had to take a look:

      https://www.phoenix.gov/sustainabilitysite/Documents/PHX_Sustainable_Home_Design_Overview.pdf

      What do you mean? I LIKE having a parapet to capture the nor'easters so that I can crawl up a ladder and inspect the roof top;e snow drift from the interior from that central daylighting tower (which of course would never leak, because it's built like an aquarium)! :-)

      But then again, even with that bit of sun-shade overhang I'd still be able to get the inside-the-drift view from the first floor glass that goes down to near-grade when the average snow depth in the yard is four feet (happens once or twice per decade), no ladder needed.

      A few years ago we got ~10' of snow in 6 weeks- even sloped roofs had deep enough drifts and cornices to need attention with a shovel. In one snow trap roof valley I measured it at 7 feet as I shoveled my way across. Having no slope to help and having to heave it over parapet would have been easily twice the work and then some. A flat roof in this climate CAN work, but only if it's dead-flat (so that it gets wind-scoured rather than piling up drifts), and rated for the snow load. The Home NZ house's roof, not so much.

      "Taking cues from the legacy of the 1950’s case study homes - the start of what is now commonly referred to as midcentury modern dwellings..."

      That's a bad cue to take if going for efficiency in any climate.

      The "... highly efficient Solarban 70XL glazing ..." would also be a poor choice in any heating dominated climate, but that could be swapped for a more appropriate higher-gain U-0.28ish low-E double-pane without affecting cost, though it's pretty silly to have R45 walls and that much R3.6 glass, even in zone 4 or 5.

      http://www.vitroglazings.com/getmedia/5fde19d1-aa54-4f24-987b-6bf5e7e928a9/Vitro_Product-Data-Sheet_Solarban-70XL_Feb2017.aspx

      The overall glazing/floor ratio looks too high (as it is in almost any "midcentury modern" house to make Net Zero in a cold climate without MUCH higher performance glass than that.

      Also, the canine security system shown on p.8 of the PDF wouldn't really cut it in climate zone 7. (The Pit Bull would be down right MISERABLE much of the year, and may rightfully refuse to go outside! :-) )

      1. Malcolm Taylor | | #10

        Dana,

        Whatever the merits of your other arguments, you are not allowed to criticize the Case Study Houses. They are the architectural equivalent of holy relics. For modernist designers, unquestioning belief in their purity is an act of faith that trumps any regional concerns.

        1. User avater
          Dana Dorsett | | #13

          :-)

          Somewhere I have pictures of the incredible duct sculpture somebody installed atop my in-laws' midcentury modern in an (unsuccessful) attempt to cool it with 7-8 tons of rooftop AC (two commercial building type rooftop units) prior to them buying it. I've seen hamster mazes that were simpler! (The purists would have had to PhotoShop that out! ) And that house is in a zone 5A climate with a 1% outside design temp of only 83F/ 28C- can't imaging how badly it would have fared in sunny Phoenix in summer.

          In winter the place WAS miserable ( at a design temp +5F/-15C), and I got tapped to help them make the decision as to whether to fix it's problems or just sell it and move on come spring. After advising on the available options and approaches they kept it. Maybe I should even write up a blog article on what it took to tame that beast into something sorta-reasonable.

          It's now quite comfortable, if still lossy compared to current code min. It involved swapping a lot of glass for something better, and building a very low-slope gable roof (with 6" of exterior polyiso) over what had been a low-angle butter-fly-to-flat roof to create a very low-height micro-attic to accommodate the new right-sized mechanical systems & ducts. It still looks like a mid-century modern, but the load numbers have been cut by some gia-normous fraction- more a measure of just how miserable the "before" picture was than the efficiency of the "after" picture. The hamster maze is gone, as is the noisy of the ridiculous cooling approach, and it no longer stinks of hot bitumen on hot sunny days.

          1. User avater
            Michael Maines | | #17

            Dana, I'd like to see your article on your parents' house. A few years ago a friend of mine was considering buying a classic Paul Rudolph house in Cambridge, MA but only if I could get it close to Passive House standards. There was no way that was going to happen, with metal trusses running through single-glazed walls. Though it looks like whoever bought it was able to improve it: https://www.dwell.com/home/rudolph-house-7f368bda.

        2. John Clark | | #15

          OMG.. ROFL!

      2. Craig | | #11

        I didn't even look at the drains in the roof to see if they heat traced them... But I guess with R70 it shouldn't melt, so no worries of freezing right?

        I was loosely excited for a good NZ ranch home design, but sad to find out anything north of CZ4 is likely to have to make changes to one or more items. In my zone 7, it's pretty much only a floorplan to me.

        1. Malcolm Taylor | | #12

          A floor plan, which while nice, is very similar to a number of other mid-century modern inspired ones: https://www.dezeen.com/2018/12/06/edward-ogosta-overhauls-california-bungalow-with-clarity-and-restraint/

  5. Al Cobb | | #14

    Building this one may be difficult when no SIP manufacturer's in the country make SIPs with PIR as a core. Several use PUR and that core material is available at a maximum thickness of 7-1/4". I love that they have specified SIPs as it's my material of choice. However, this one is not likely to happen.
    Secondly, I'd welcome the opportunity to see the Rem/Rate file to see how a score as low as 30 was calculated. In terms of "pure" energy efficiency, I've never seen or heard of a number this low.
    Please speak up if you know of another project that has a documented number this low without "alternatives". I'd be happy to be proven wrong.

    1. User avater
      Michael Maines | | #18

      Al, Foard Panel offers a PIR core SIP with an overall thickness up to 12.25": https://foardpanel.com/specification-documents. (click on "SIP data sheet").

      1. Al Cobb | | #26

        I guess I should have qualified my statement with "Code Approved"
        You are correct Foard does make that panel.

        1. User avater
          Michael Maines | | #27

          Al, the flame spread and smoke developed numbers are better than code minimum. Why wouldn't it meet code? Are you assuming or do you have evidence that says the panels weren't tested at the thickness intended for use?

    2. User avater
      Dana Dorsett | | #30

      >"Several use PUR and that core material is available at a maximum thickness of 7-1/4" "

      At any thickness the extremely high global warming potential HFC blowing agents used for all currently available PUR SIPs makes them the opposite of "green".

      The comparative verditude of SIPs (including PUR SIPs) comes up on this forum from time to time. eg:

      https://www.greenbuildingadvisor.com/question/double-stud-vs-sip-wall-if-they-were-the-same-price-for-the-same-r-value-which-is-better

      >"I love that they have specified SIPs as it's my material of choice."

      I actively hate that they specified foam core SIPs- it's the opposite of "green building".

      All foam insulation comes with a significant environmental footprint (it's not just greenhouse gases or embodied energy/carbon), and any time the primary insulation is foam it's at-best in the yellow part of the spectrum, not green, even when using more benign blowing agents. At higher R it gets worse, not better, since the marginal environmental cost becomes even further out of balance with the marginal performance improvement.

      Suffice to say that despite their advantages from a construction schedule and air sealing point of view I'm not a fan of foam core SIPs (or ICFs for that matter) due to the higher environmental costs. YMMV.

      There is at least one manufacturer of wheat-straw core SIPs, (Agriboard) but the thermal performance per inch of thickness is nowhere near that of foams, though the much higher thermal mass gives them an interesting dynamic-R-equivalent performance boost in climates with high diurnal temperature swings (though as with ICFs the marketing claims are a bit over the top.) I'm not aware of anyone experimenting with rice hull insulation in SIPs, but if reasonable binders could be developed it might be pretty good, with high fire resistance and significant thermal mass effects too, though probably no more than R3/inch, making for fatter walls.

      At about R18 (steady state, not dynamic) the 7-7/8" thick Agriboard straw-core SIP would probably have worked pretty well in this design in a Phoenix climate due to the thermal mass benefits, and, probably well enough to still hit Net Zero with very few changes to the rest of it. Unlike EPS, straw insulation is sequester carbon, and the embodied energy and other environmenatl aspects (included end of life disposal issues) are also very favorable by comparison.

      http://www.agriboard.com/carbon/Embodied%20Energy-AgriBd.pdf

      The density is quite high compared to foam core SIPs- they're really hefty suckers! (Not that SIPs are installed manually no matter what core.)

      1. User avater
        Jon R | | #31

        > foam core SIPs- it's the opposite of "green building".

        But it has "net zero" in the name - it must be green :-).

        1. User avater
          Dana Dorsett | | #32

          Oh yeah... I guess it must be green then- my bad! :-)

          I really hope that design does NOT become widely adopted without modification. An R45 polyiso SIP wall is just plain ridiculous for Phoenix's Zone 2B climate, even if it does meet the construction budget.

          Builders in Austin TX (Zone 2A) are hitting Net Zero with 2x6/R20 walls, no continuous insulation, or 2x4/R13 plus an inch of continuous foam.

          If a SIP solution is desired, the ~8" Agriboard SIP beats the performance of 2x6/R20 walls even on steady state whole-wall R-value (even without factoring in mass effects) and is ~1.5" thinner than the polyiso SIPs specified in the design.

    3. User avater
      Austin Trautman | | #34

      We built a couple of homes in Phoenix about 5 years ago that are under a HERS 30 without solar. One of them is 1,500sf and Living Building Challenge Net Zero certified with a 3.6kW system.

      In the end, though the HERS score only tells a tiny part of the story. Through real-world testing, we have found that thermal capacity in our climate is at least as important as R-value, especially over R-15 or so. An R-25ish wall with minimal framing (think advanced framing and designing to standard spacing), no framing thermal bridges, a tight envelope, well-installed glazing and balanced ventilation gets you a healthy, efficient home.

      I have other inherent issues with SIP panels but their lack of thermal capacity already rules them out for me in Phoenix or similar climates with large daily temperature swings. We have real-world testing to show foam being outperformed by materials such as Gutex at half the R-value. On extreme temperature days really interesting things happen.

      1. KurtGranroth | | #42

        Very cool to hear that there are building science nerds in Phoenix. I've had a very difficult time finding Phoenix-specific green building practices.

        Regarding the "thermal capacity" of Gutex, can I assume you are referring to "thermal mass" in the building context? If so, then do you have results posted or published somewhere on your tests? I've consistently heard that thermal mass plays a very tiny role in the heat bubble of the Phoenix metro area; save a slab floor which has an even bigger thermal mass attached to it.

        1. User avater
          Austin Trautman | | #44

          We, or at least I, are (am) here. There's a lot of unscientific hearsay floating around Phoenix for sure. And we are (or at least were earlier this year) the cheapest market in the country for sprayfoam. You can guess at what impact that has had.

          You're on the right track. Thermal mass as a term tends to have a bit of a different connotation as it's usually used for uninsulated materials, like concrete. But yes, it's the same. You are correct that thermal mass alone with little to no insulation (think a typical Phoenix block house) won't make a comfortable home.

          The really interesting materials have both significant resistance (r-value) and significant thermal capacity. See the link below for the specific heats for some common materials. Notice the units are J/kgC, or in other words, the amount of energy needed to change a given mass of the material by 1 degree Celcius of that material. Gutex isn't listed on there but it's 2,100 j/kgC. Cellulose is around 1,400 and fiberglass around 850. Then you need to consider the density of each material, where it's placed in the wall system and the typical thicknesses. All of those parts can have a significant impact. Remember, it's about how the system performs as a whole.

          So....in the extremes if you compare Gutex rigid board to fiberglass batts you get 40 times more thermal capacity at the same R value. Gutex over cellulose is about 6 times the capacity.

          https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html

          1. User avater
            Jon R | | #45

            ORNL (Fig 7) found single digit % energy savings for concrete/ICF walls in Phoenix - not even close to cost effective. An equal R value of cellulose holds perhaps 1/30th the heat of ICF concrete and say Gutex is 1/5th - so I'd expect negligible % savings (from thermal mass) for either. Is there some good publicly available data showing that whole house/whole season thermal mass effects of anything other than concrete is significant? Or that any of it is cost effective?

            My view - thermal mass has minimal effect on net energy use. Only include it if it's nearly free.

          2. User avater
            Austin Trautman | | #46

            Good question, that's a useful and often sited study. I agree on ICFs, they do give you tons of mass but the data (and common sense) would suggest it's actually way more than needed. There are many other issues and costs associated with ICFs as well. The carbon impact alone makes it a non-starter for many, if lowering the lifecycle carbon impact of the building is the type of thing someone cares about.

            If you really get into the weeds this whole conversation is about thermal diffusivity. When mass and resistance work together then you get "magic". https://en.wikipedia.org/wiki/Thermal_diffusivity

            Here's a great experiment from my friend Lucas at 475 done in Tucson. The datalogger info comparing R5 of Gutex to R10 of XPS is somewhat shocking. Again, it doesn't HAVE to be about one specific material but it does show the physics in a digestible way: https://foursevenfive.com/blog/gutex-vs-xps-insulating-in-a-hot-house-world/

            Again, from Gutex but the math is accurate. You can see a basic chart on the thermal diffusivity of different building materials. https://gutex.de/en/product-range/product-properties/insulation-in-summer/

            In the end, it's about the simplest and most elegant solution for a given structure. If correctly assembled the cost add of building a high-performance home should only be 5-10% above standard construction while giving exponentially better comfort, durability and health. The energy savings are a nice bonus.

  6. But Why? | | #16

    All of these comments and not one mention that ITS UGLY. There's only a limited number of people who will except ugly in the name of efficiency. Phoenix better buy a list of Prius owners from Toyota if they want to market this mess.

    1. User avater
      Michael Maines | | #19

      Hmm... despite its deficiencies, I think it's an attractive house with an interesting floor plan. But I also like my Prius.

      1. Charles Campbell | | #25

        I agree the house is ugly but I don't get why people think the Prius is ugly. They also say the Chevy Bolt looks "dorky" or "beaten with an ugly stick". Those cars look fine to me.

  7. Debra Coleman | | #20

    While I do like the floor plan and some of its features, I see nothing that makes it more suitable for net zero than any other one-story house design. Hard to believe it would work equally well with any orientation. Surely the long side oriented east west would be better. And in colder climates that could use a little direct solar gain, the dining/living area on the south would help. I have a preference for conventional construction, sloped roofs, attached overhangs, proper window placement based on orientation and floors plans with the long axis oriented east-west. While this home certainly could be Net Zero, I am more concerned that it could be misleading that flat roofs, SIP panels and exterior shades are what make the home Net Zero. Also, the discredit to orientation is troublesome to me.

    1. Antonio Oliver | | #41

      It's clear to me that the flat roof is partly what allows the house to work with any directional orientation in that solar panels aren't biased toward a roof slope. They can easily be directed southward regardless of how the house is oriented. In addition, a lot of houses in the dessert west have flat roofs anyway; I suspect because the amount of rain is relatively low for one reason. In addition, with the sun screens blocking solar gain to the windows, I can see how there would be little reason to orient the house with a long east/west axis...assuming those sun screens are an integral part of the design and are carried over with the design if built in a colder climate zone.

      1. Malcolm Taylor | | #43

        Antonio,

        If you omit the sun-screens on the windows you have what looks like a typical mid-century modern inspired house, with extensive glazing, that is very popular among architects right now. Rather than ruin the aesthetic, they have added the sun-screens to adapt it to the less forgiving desert sun, leaving the occupants to stare at opaque walls several feet outside the windows. Wouldn't it have been more appropriate to design the glazing with both reference to orientation and climate?

  8. Patrick Stuart | | #23

    Having personally submitted several designs in architectural competitions (including a Habitat For Humanity prototype with energy efficient features), it’s not uncommon for the winners to never get built. Many times it’s due to unforeseen costs by the designer, lack of funds by the organizer, practicality, bureaucracy/politics, aesthetics, or a combination thereof. Which is unfortunate. But oftentimes, the real value of a competition is in getting a necessary conversation going.

    As a former design director for a local non-profit, we presented several designs and ideas to review boards in low-income areas, many of which never went beyond schematics. Some of those areas I can now drive through and see development and opportunities that weren’t there before. Progress has to be built, and in my viewpoint we were simply a necessary block in the foundation. Maybe this net-zero house gets built, maybe not . . . but I already see it spurring other people to make something better. :)

    1. Malcolm Taylor | | #24

      Patrick,

      There are a number of very capable architects with extensive experience in energy efficient residential design - some of whose work has been featured here 0n GBA. In retrospect it seems too bad the City of Phoenix didn't commission one of them to prepare several more appropriate prototypes.

      I take your point that just initiating a conversation around these issues may be useful, but what worries me is that energy efficient houses may become associated with outrageous consultants fees and exotic construction techniques.

    2. User avater
      Austin Trautman | | #35

      Well said. I'm a building science nerd in Phoenix and we have designed and developed who homes with similar goals to this competition. I have to say I'm thrilled to see the conversation finally starting!

  9. KurtGranroth | | #29

    Two things about this jumped out at me that I definitely will try to track down more details on.

    First, it has a solar chimney to pull in "cooler, fresh air at night" with a convection stack. Those definitely can work in desert climates. I'm skeptical that it would work at all in the Phoenix metro area during the summer, though, since the urban heat bubble here ensures that the night air never gets "cool". Typical lows in the summer are in the mid to high 80s. So there would be a couple months in the spring and fall that would come into play... but honestly, during the those months people often don't use A/C anyway since it's nigh perfect weather all the time. If I'm wrong and it IS worth having just for those months, then I'd like to know now because maybe I can incorporate a similar concept in the net-zero-capable house I'm building.

    The second curiosity is the reference to passive cooling here: "During the hottest days of the year, the house utilizes two-pass high efficiency equipment without compressors utilizing an adiabatic cooling process to condition the space". Adiabatic cooling? Isn't that just a swamp cooler / evaporative cooler? Or is there some part of this that I'm missing?

    Very interesting concept in any event.

    1. User avater
      Austin Trautman | | #37

      Skip the solar chimney and build a tight envelope. Your payback will be much greater and comfort improved over the year. It's not hard to get enough night cooling in Phoenix on those cool days just by opening some windows. Also, today's high-efficiency mini splits (even the ducted ones) give nearly free cooling on those cool days, if that's your thing. Also, look into Mitsubishi units for the Phoenix climate. They do really well when it's over 105 outside. Many high-efficiency units can lose as much as 40% of their rated capacity at 105+, right when you need it.

      Skip the complicated mechanical systems. Minisplits (ducted or ductless) and an HRV or ERV are your friends. Feel free to contact us for some guidance if you need it. We can also help with the HRV vs ERV selection in our climate. We have years of real world use and can give you the finer points that we have learned.

    2. User avater
      Jon R | | #40

      > a swamp cooler

      In conditions like right now, one can use a simple humidifier for cheap cooling. But OK, it wouldn't amount to much before interior humidity got too high.

      1. User avater
        Austin Trautman | | #47

        Not a bad idea Jon, in certain situations. We've found that with an HRV and properly sized high-performance mini-split for cooling there are indeed times when you can have quite low humidity. If you were someone who preferred 60% RH and 72F instead of 40% and 79F you could maintain that swap. Basically you keep adding humidity into the air and the heat pump keeps sending it down your condensate line. Now make sure you capture that and do something useful with the water :)

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