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Q&A Spotlight

Choosing the Cheapest Path to Net Zero

An owner-builder hashes over the key details: ventilation, windows, heating and cooling

To build a net-zero energy home, owners and builders often weigh the advantages of such features as a heat recovery ventilator (left), a heat-pump water heater (center) and a drainwater heat recovery system (right).
Image Credit: Green Energy Futures / CC BY-NC-SA 2.0 / Flickr

Writing from northeast Ohio, a reader with the screen name “User-6877304” — let’s call him Steve — is seeking comments on his plans to build an affordable net-zero energy home. The house, to be built on a 30-foot by 50-foot slab-on-grade foundation, seems to have many characteristics of a “Pretty Good House” — that is, it’s well insulated and ventilated but not attempting to hit the Passive House metric.

Steve plans to install R-10 rigid insulation beneath the slab. The house will have double-stud walls insulated with cellulose to R-40 and a raised-heel truss roof insulated to R-60.

“A simple gable metal roof,” he writes in a Q&A post at Green Building Advisor. “Solar panels. Basic interior finishes: nothing custom.”

His questions boil down to a consideration of specific features: the best heating and cooling option, windows, drain-water heat recovery, and whether a heat-recovery or energy-recovery ventilator is a good investment.

That’s today’s Q&A Spotlight.

Start with this document

Dana Dorsett suggests that Steve start by reviewing the recommendations in a Building America report authored by John Straube in 2010 and updated the following year. (For more information on this report, see R-Value Advice from Building Science Corporation.) Because the report is a few years old, Dorsett points out, the efficiencies of photovoltaic systems and heat pumps have changed somewhat, meaning that Steve can probably hit net-zero performance with the recommendations for Climate Zone 4 rather than Climate Zone 5 where Steve is actually going to build.

But, he adds, it’s important to remember the effect of thermal bridging.

“Note, those are ‘whole-assembly R-values,’ not center-of-cavity R-values, factoring in all the thermal bridging,” Dorsett say. “An R-25 wall could be a 2×6 / R-20 wall with 2 inches of exterior polyiso foam. An R-30 wall would take 3 inches of exterior foam. A double-stud wall with a foot of cellulose also comes in the mid 30s, despite a center-cavity R-40+.”

As to the specifics, Dorsett makes these recommendations:

(1) Heating with minisplit heat pumps will be “far cheaper” than beefing up the size of the PV system enough to handle the load of electric baseboard heat, and the same system will be able to provide high-efficiency cooling as well.

(2) Triple-pane windows would make the house more comfortable than double-pane windows, but in Steve’s climate a double-pane window with a U-factor of 0.25 is “probably going to be the sweet spot for net zero.”

(3) Drain-water heat recovery requires at least 4 feet of vertical drain below the shower in order to work, making it an option for a two-story house built on a slab but not a one-floor design.

(4) An ordinary tank-style electric water heater is probably the best bet. On-demand electric water heaters draw a lot of current, potentially costing “tens of dollars” for a 15-minute shower. A heat-pump water heater would potentially work, providing Steve had a place to put it.

Is “standard construction plus solar” an option?

A reader with the screen name “Anon3” suggests that if Steve’s goal is the cheapest route to net-zero energy, the answer is probably “standard construction + solar panels.”

“The trades (even illegals) just cannot compete with the efficiency of automated Chinese solar factories,” Anon3 says.

But “What’s ‘standard’?,” asks Andy Chappell-Dick.

“Anon3, you may not be aware what ‘standard construction’ is in Ohio,” he writes. “Our dated state building code is not enforced in many places, and contractors haven’t adopted any energy improvements on their own. It would take a lot of solar panels to zero out a build like that. [Steve], find a contractor soon that understands what you want and is willing to work with you toward your goals. Many of these early design decisions will have a major impact on final cost (and the number of solar panels you’ll need.)”

Anon3 replies that if Steve could save $20,000 by using standard construction techniques, he could use that money for a 20-kilowatt PV system. “That’s a lot of electricity,” he says.

“Where in Ohio can you get solar PV installed for a buck a watt?” asks Nate G.

“But PV [is] not a buck a watt (yet), and won’t be any time soon,” Dorsett adds. “The national average cost of small roofop PV in the U.S. is about $3/watt [according to the Open PV Project run by the National Renewable Energy Laboratory]. In Australia it’s closer to $2/watt. If you can wait another 15-20 years it may hit $1/watt, though.”

The real cost of solar

Although Anon3 writes that the cost of a PV module is already below $1 per installed watt (actually more like 60 cents a watt), others point to the difference between the cost of a PV module and the cost of an installed PV system.

“You appear to be confusing the retail price of the panels with the installed price of the complete system,” Nate G says, adding that a website Anon3 has referenced sells a complete 20 kW system for $31,000, not the $20,000 that Anon3 referred to. “And regardless,” he adds, “retail or wholesale prices (panels or full system) are completely irrelevant to this discussion unless [Steve] is planning to install the PV array himself.”

What if Steve installed the solar system himself to save money?

“Tread very carefully here,” Dorsett says. “There are a lot of regulatory hoops to jump through, and they vary by local codes, and by the local utility. An experienced local installer will have already starred in that movie, and is more likely to get the immediate approval from the local regulatory and utility officials. Even if you do it perfectly (and assuming it’s even legal for DIY grid-attached solar without the necessary licenses and credentials), you will receive more scrutiny than the contractor, and are likely to be waiting in longer queues to be allowed to hook up.”

Further, solar contractors can get better quality materials than someone building a single system. “It’s doable,” he says, “but study hard and think about it before diving in.”

Investing in heat-recovery ventilation

A key question for Steve’s house, as it would be for any tightly built home, is what type of ventilation system should be installed. Heat-recovery and energy-recovery ventilators are a common choice because they salvage some of the heating or cooling energy even as they introduce fresh air into the house.

Steve wonders whether he could skip the extra expense of buying an HRV or ERV and compensating for the loss of energy by buying more PV capacity. Maybe, he says, he could get by with one or two Panasonic WhisperComfort fans, which are advertised as “spot energy recovery” units.

“ERV cores are prone to freeze damage in your climate,” says Dorsett. “For a 1500 [square foot] house with a relatively open floor plan you can probably get by with a single pair of ductless Lunos HRV and a Panasonic WhisperGreen bath fan with the optional humidity/condensation sensor for ventilation.”

No, replies GBA senior editor Martin Holladay, an ERV should not be a problem. “Dana Dorsett is rarely wrong, but on this point, he is wrong,” Holladay says. A few inexpensive ERVs (including the Panasonic FV-04 mini-ERV) are inappropriate for cold climates, but most ERVs are designed with controls to address frost build-up. Holladay says that when “installed and operated according to the manufacturer’s installation instructions,” an ERV will not be damaged by frost.

Holladay thinks that in Steve’s climate zone, a larger PV system plus a simple ventilation system is a better investment than an HRV or ERV, although Steve may have reasons other than cost to prefer an HRV or ERV.

Holladay refers Steve to a GBA article called “Are HRVs Cost-Effective?” “Note that since John Semmelhack performed the calculations discussed in that article, the cost of PV has dropped, making it harder to justify an HRV or ERV on cost savings alone.”

And while Dorsett raises concerns that an exhaust-only ventilation system could increase radon levels in some areas, Holladay says that is not necessarily the case. (For more information on this issue, see “Exhaust-Only Ventilation Systems and Radon.”)

What about the windows?

Triple-pane windows can be a lot more expensive than double-pane windows. Are they worth it?

Relying on the “Pretty Good House” threads he’s read, Andrew C believes that triple-pane windows would be “overkill” for most houses.

“Get good double-pane windows, and used fixed versus operable where you can,” he says. “Spend more time and attention to your air-sealing details to get more bang for your buck.”

Nate G agrees with Dorsett’s initial suggestion: double-pane U-0.25 windows will indeed be the sweet spot. “Just fine and not too expensive,” he says.

Jon R put it this way: “I’d say that not having triple-pane windows is a matter of turning up the heat on the coldest days and does not require tolerating lower comfort. This one can be treated as a solely economic issue (cost of window upgrade vs. cost of increased energy use).”

But to Stephen Sheehy, there’s more to it: the comfort factor. “Windows with a better U rating will not just save energy,” he says. “There is a definite comfort factor for anyone sitting near a window on a cold day. The warmer the interior glass layer, the more comfortable. Whether better windows are worth the extra money is of course pretty subjective.”

Our expert’s opinion

GBA technical director Peter Yost added these comments:

Steve has gotten great advice on a number of fronts for his high-performance home. I have a few additional thoughts:

(1) Double-stud walls in Climate Zone 5: Dana Dorsett references a great Building America resource, but let’s remember some key hygrothermal issues for double-stud wall assemblies. Given how cold the sheathing can get in the winter, airtightness is much more important for a double-stud wall than for a conventional wall. Many builders using the double-stud wall system select the most moisture-tolerant sheathing they can afford — in my neck of the woods, southern Vermont, some builders use board sheathing. Also, maintaining reasonable interior wintertime relative humidity (around 35% or less) is important. Finally, use a smart vapor retarder on the interior to control winter interstitial moisture content and maintain good drying capacity to the interior.

(2) Exhaust-only ventilation: Joe Lstiburek very reluctantly changes his building science assessments — not necessarily a criticism — but he has unequivocally eliminated exhaust-only systems for whole-house ventilation for “tight” new homes. In his assessment, he considers how this ventilation approach can aggravate radon. In my own home, adding exhaust ventilation to the basement moved radon levels from 6 picocuries per liter (pCl) to 13.

The next most affordable approach is a central-fan-integrated supply system. This of course would require Steve to have a central forced-air HVAC system, which is not an option he is currently considering. In terms of “affordable” balanced mechanical whole-house ventilation systems, you’ll find a good discussion here.

(3) Minisplit vs. electric-resistance heat: If you get the loads low enough in this home, electric resistance heating is a real option. This comparison needs to include layout and the configuration/use of spaces, because minisplits are better-suited to open layouts and fewer or single-zone configuration, while electric-resistance heat is well-suited to individual room control. And this comparison must consider what type of whole-house ventilation will be included. And, oh yeah, this decision also depends on how you will be making domestic hot water.

(4) Windows: I know there is no way to “cost-effectively” rationalize Passive House tilt-and-turn triple-paned windows. But their engineering — beefier frames, more and better gaskets, sturdier hardware — makes them so much more durable and easier to operate that their total benefit makes them worth considering. It could be that if or when you go to sell this house with these superior windows, you can find the buyer who is willing to pay a premium for the house because of them. Then you don’t have to completely burden the window selection with just a payback analysis; you get your return based on the extra value conveyed from you to the next owner.

For more information, check this article at BuildingGreen.

8 Comments

  1. Davor Radman | | #1

    Triple pane price
    Since it was mentioned, what is the price difference between double and triple pane windows on average in USA?

    In Europe, they make us do all sorts of extreme things for the sake of energy efficiency (though I don't necessarily mind that always), so triple pane, where I live in central europe, is like 1.2$/sqft over double pane. For the house I am planning, with ~2500 sqft of windows, it comes out to ~300$.

  2. User avater
    Dana Dorsett | | #2

    A few comments for Peter Yost
    This house is in US clmate zone 5, which is warmer than southern VT, and I was recommending 2-3" of reclaimed foam on the exterior of a single studwall as being a cheaper way to hit the necessary performance level than dense-packed double-studwall. If he went through with the double studwall approach a smart vapor retarder would definitely be cheap insurance, but the cold sheathing risks in his location aren't nearly as high as even the warm edge of zone 6.

    While triple-pane windows are nice, specifying them on a house based on a perceived resale value at some undetermined date is a pretty lame, and definitely NOT "Cheapest way to build net zero house?", the title of his original thread.

    It's good to know that I'm not crazy to assume there can be increased radon levels with exhaust only ventilation schemes in tight houses.

  3. Bob Irving | | #3

    I agree with Peter Yost's
    I agree with Peter Yost's comments on windows: we've used primarily good quality vinyl triple glazed windows or European style Tilt-Turns on the NZR houses we've built. Both work well; the European are 30-35% +/- more money (depending on how much fixed glass - which is relatively inexpensive with European style windows). But they aren't apples to apples. With typical vinyl windows, the lifetime is shortened by deterioration of the vinyl from sunlight and the glass-vinyl seal which is affected by the different rates on expansion between vinyl and glass. In contrast, the European windows have typically a better U value, but they are much more rugged, with better window seals which separate the glass from the vinyl, a tighter and more secure locking system and a more secure operating system. in other words, they're much better windows for not that much more cost.

  4. User avater
    Dana Dorsett | | #4

    Still not the cheapest path to Net Zero Energy (@ Bob Irving)
    In zone 5 you don't need triple panes to hit Net Zero, and it's not the lowest cost upgrade path. A purty-good U0.25-ish window will do.

    Comparing a vinyl triple pane to a European model on price/value isn't even relevant to the discussion. A Euro tilt & turn triple-pane will be SUBSTANTIALLY higher cost than a fairly decent U0.25 double-pane, not 30-35% more expensive.

    The original thread wasn't about the best value or best performance, only lowest cost path to net zero, in a cool but not too cold climate.

  5. David Landry | | #5

    Lunos HRV in our Net Zero home
    I am living in a 1700 sq.ft. operational Net-Zero home here in Southern Oregon Zone 4, (4600 HDD) that I designed and moved into 16 months ago. We have 10" double walls, 14" raised heel trusses, on a thermally isolated finished floor slab with 4" of EPS sub-slab. We went the extra mile and used Prosoco fluid applied WRB and tested out at .06 ACH50. Solar System is 6.7kw. The EPS score was 1. With such a tight shell we installed 2 pairs of Lunos HRV units (approx. $2k). These maintain good air quality as measured by CO2. One pair would have been quite insufficient. We have only 2 primary occupants. A single mini-split is more than adequate to condition our 3 bdrm. home. Temperatures are remarkably even throughout. I actually need to shut the master bdrm door on winter nights to try and cool it off a bit so that we sleep better. I can't tell you how comfortable our home is year round compared to our previous home built 35 yrs. ago.

  6. User avater
    vap0rtranz | | #6

    Frost / freezing ERV research
    I found the CCHRC folks did tests on ERVs in Alaska with their findings posted so wanted to share after reading Dana v Martin's comments:

    http://www.cchrc.org/energy-recovery-ventilators-ervs-cold-climates

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

    Response to VaporTranz
    Vapor,
    For more information on this topic, see "Preventing Frost Buildup in HRVs and ERVs."

    1. User avater
      vap0rtranz | | #8

      I'm a Prime subscriber now. 1 month of reading and articles like that are too good! ty

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