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Q&A Spotlight

Best Path to Net-Zero Energy

An owner-builder looks for the most cost-effective way to build a net-zero house in Washington State

A planned house in Washington state is a single-story with a floor area of less than 1,000 square feet. Working with a limited budget, the owner is trying to find the most cost-effective net-zero energy design.
Image Credit: Illustration: Joshua Greisen

Joshua Greisen thinks he’s found an ideal building lot in Yakima, Washington, a city in the south-central part of the state in Climate Zone 5B. Now, can he find a design for a zero-net-energy house to go with it?

Working with a limited budget, but on a south-facing lot ideal for passive solar gain, Greisen is looking for a cost-effective way of reaching his goal. “I’m by no means a rich man,” he writes in a Q&A post at GreenBuildingAdvisor, “and can only afford to do what has a return on investment that will be realized within a decade or so.”

The single-story home he’s planning has a footprint measuring 26 feet by 38 feet, about 990 square feet in all, with two bedrooms, a bathroom, laundry, kitchen, and great room. His plans currently call for 2×6 exterior walls insulated with fiberglass batts and a layer of continuous rigid foam insulation on the exterior — probably 2 inches of extruded polystyrene (XPS).

Other features include triple-pane windows with low-e glass, a heat-recovery ventilator, and an air-to-water heat pump with radiant-floor heating and an air handler to provide summer cooling. He’s aiming for a blower-door test result of 1.5 ach50 or better. He’s also planning on 8-foot-tall doors and 10-foot-high ceilings.

Greisen has a number of questions as he searches for the “sweet spot” for R-values in the ceiling, walls, and slab.

“Code [minimum] in Washington state is R-49 in the ceiling, R-21 in the walls, and R-10 in the slab and stem walls,” he writes. “I have read theR10/R20/R40/R60 rule, but that seems overkill, perhaps, given all the other factors in play that I have noted above.”

His research has uncovered a presentation on R-40 walls showing that 2×6 walls with an exterior layer of XPS of between 1 inch and 3 inches is “quite cost-effective,” although he wonders how a ventilated rainscreen would affect the performance of the insulation.

The unusually high exterior doors also are giving him second thoughts.

“Any guidance on these questions would be much appreciated,” Greisen says. “I feel like I’ve been thinking about this stuff too much!”

That’s where this Q&A Spotlight begins.

Don’t skimp on the exterior foam

Greisen recognizes that just about any house can become a zero-net-energy consumer with a big enough solar array. He is, however, “looking to strike a balance and use as little energy as possible based on my design choices.”

With that in mind, Dana Dorsett says that in order to keep the photovoltaic system small enough to fit on the roof of his house, Greisen should be careful to include enough exterior insulation for the climate. Plan on 3 inches of exterior foam in addition to fluffy cavity insulation in the walls, he says.

“Polyiso would be much greener than XPS, due to dramatic differences in the environmental impact in the blowing agents used,” Dorsett says. “Going with 3 inches or 3 1/2 inches of reclaimed roofing polyiso would be about right, or 2 inches of reclaimed goods with 1 inch of foil-faced virgin stock for ease of air sealing.”

Dorsett refers Greisen to a research report from Building Science Corporation on high-performance building, and in particular to a chart showing recommended insulation levels for all climate zones.

“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 Heating Seasonal Performance Factor (HSPF) of about 10, as opposed to 12 now,” Dorsett continues. “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.”

Is R-65 too much in the ceiling?

The insulation recommendations from Building Science Corporation call for R-65 insulation in a vented attic, but Greisen has been told by local experts that given the extra-tight building envelope he’s planning, R-65 probably isn’t necessary.

“You’d be shocked at just how little difference in cost there is between a code-minimum R-49 (R-38 whole-assembly) attic is compared to an R-65 (whole-assembly) attic, if it’s done with open-blown cellulose on the attic floor,” Dorsett replies.

Dorsett adds, “At about 3 cents per R per square foot, once you’re above the truss chords or joist depth, another R-20 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 performance out of fatter walls or higher performance windows.”

What about the sub-slab insulation?

Greisen’s plans call for 2 inches of extruded polystyrene (XPS) beneath the concrete slab. Is that enough?

Dorsett replies: “Three inches of sub-slab Type II EPS (R-12.6) would cost the same or less than 2 inches of XPS, and would outperform it forever. Only 2 inches of XPS (R-10) would be marginal for a heating slab even when new; and as its HFC (blowing agent) bleeds out over a few decades (with its powerful greenhouse gas impact), it drops asymptotically to R-8.4 at full depletion — the same as EPS of similar density.

“If you pour the stem walls in a minimalist ~R-16 (2 inches plus 2 inches of EPS) insulated concrete form (ICF), it’s quicker than insulating it after the fact,” Dorsett adds. “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/stud wall plates where the exterior foam of the stud wall aligns with, or is slightly proud of, the exterior ICF foam to provide a continuous thermal break between the sill plate and exterior.”

With R-8 of insulation between the edge of the slab and the stem wall (or ground) and another R-8 on the exposed exterior of the stem wall, Dorsett says, the unheated slab would be perfectly comfortable even with outside temperatures as low as -15°F.

Bob Irving adds that he would use 6 inches of sub-slab insulation in this instance, and because the cost of EPS is 40% less than XPS, the additional out-of-pocket costs would be minimal.

Reconsider your heating and cooling plan

GBA senior editor Martin Holladay suggests that Greisen rethink his plans to install an air-to-water heat pump system.

“You’ll save a lot of money if you choose a simpler HVAC system,” he writes. “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.”

Given Yakima’s outside design temperature of 11°F, Dorsett says, a house that size built to net-zero standards will have a heat load of less than 10,000 Btu per hour.

“The radiant floor would be less than 5 F° warmer than the room temperature, even at design condition,” Dorsett says. “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 and cool the whole shebang with a 1-ton Fujitsu 12 RLFCD (or worst-case, the 18RLFCD, though that’s probably extreme overkill) mini-ducted minisplit for about one-quarter 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 minisplit (or floor unit) in the great room, as Martin suggests, at about one-fifth (or less) the cost of an Altherma solution.”

Also on the topic of heating, he adds, forget the fireplace — it will simply produce too much heat. If Greisen wants a wood-burning appliance, he should consider a small wood stove instead, preferably a soapstone or ceramic model with some inherent thermal mass.

Irving is another proponent of a minisplit heating and cooling system.

“In the R-40/60 homes we’re building in Zone’s 5B and 6 using air-source heat pumps, a.k.a. minisplits, the comfort level is unlike anything I’ve ever experienced,” he says. “The [indoor] temperatures 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.”

Should foam seams be taped?

Citing a report from DuPont, Greisen also wonders about detailing the layer of exterior foam insulation.

“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,” he asks.

That really boils down to a question of whether rigid foam is used as a water-resistive barrier (WRB), Holladay says.

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

Although any advice coming from the manufacturer of Tyvek, a popular housewrap, isn’t disinterested, Holladay agrees that it’s a good idea to use housewrap as the WRB rather than rigid foam insulation.

“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,” he adds.

Our expert’s opinion

GBA technical director Peter Yost added this:

I decided to check in on this one with Ann Edminster, one of our original Green Building Advisors and a national leader on zero-net-energy homes. Here’s what she had to say:

“Let’s talk windows first. I’m assuming south is up, based on the largest glazing fraction facing that way. That’s fine, but if you want the best daylighting bang for square foot of glazing, a bit of redistribution may be in order.

“The window in the secondary bedroom is pretty skimpy; I would advocate for a second window on the east wall, or at least enlarging the one that’s already there on the north wall. Otherwise, that room is going to be quite dreary.

“You can make up for that addition by reducing one or both windows in the master, which are not crazy-big, but generous in comparison with the size of the room. I would probably keep the south-facing window as is and consider reducing the east-facing one (unless it has a dynamite view).

“The great room windows also could probably be reduced without great sacrifice to the design; a better move might be to turn one of the three windows into a glass door and eliminate the separate door on that wall.

“Now, let’s get that water heater into conditioned space — say, a tankless condensing unit notched into the hallway, stealing a tiny bit of space from the closet area of Bedroom 2. That will put it a lot closer to the hot water draws, too.”

Ann also strongly suggested that anyone interested in net-zero-energy homes check in with DOE’s Tour of Zero and in particular Ted Clifton’s Pacific Northwest projects, such as this one.

Finally, there is the free energy modeling software, BEopt, which is not that difficult to use but is still a powerful hourly simulation designed to “…identify cost-optimal efficiency packages at various levels of whole-house energy savings along the path to zero net energy.” I learned to use BEopt simply by tapping into NREL’s extensive tutorial series and the BEopt forum.

8 Comments

  1. Gideon Brontë | | #1

    blower test
    What is the best way to get an impartial / very qualified test done? A referal from ones electrical company? Im just worried that if i call out of the phonebook Im going to get 'upsold' Thanks in advance

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Gideon Brontë
    Gideon,
    If you visit the RESNET and BPI web sites, you should be able to search for certified experts in your region. Call a few of them up and ask about blower door testing.

    Most of these experts aren't interested in performing energy upgrades on your house, so I wouldn't worry about conflicts of interest. That said, you can ask always them over the phone: Do you perform energy upgrade work? Will you try to sell me on home improvements?

  3. User avater
    Dana Dorsett | | #3

    Wrong climate zone
    "Ann also strongly suggested that anyone interested in net-zero-energy homes check in with DOE’s Tour of Zero and in particular Ted Clifton’s Pacific Northwest projects, such as this one."

    An odd suggestion.

    It's hard for folks outside of the PNW to fathom just how dramatically different the climate is between Clifton's Whidbey Island/Puget Sound location and Yakima, on the eastern slope. It's the difference between very temperate & cloudy zone 4C and sunny dry 5B. The 99% outside design temps on Whidbey Island is about +25F, with a 1% outside design temp in the high 70s (yup, under 80F!)

    In the Yakima valley the 99% design temps are in very low positive double digits to high single digits, depending on exact location, and the 1% design temps are north of 90F. The difference in available sunshine is also gia-normous.

    Using a Net Zero Whidbey Island house as a template for Net Zero in Yakima would be downright SILLY!

    A more appropriate climate, design temp, & sunshine comparison would be Salt Lake City, UT, say Garbett's example on the DOE site:

    https://www1.eere.energy.gov/buildings/residential/pdfs/doe_ch_case_studies/2013hiawinner_garbett_100213.pdf

  4. Edvin Bishop | | #4

    Solar panels aren't all
    Solar panels aren't all they're cracked up to be. I would not push this blindly as the cure for a poorly designed building. Thermal efficiency seems to be pointless with all-glass facades. And what is zero net energy really? It's not building an energy-independent system running on its own but rather implementing something else that in theory offsets fossil fuel production annually or whenever because renewables from the grid aren't coming online fast enough to satiate thirst. You get to a point where people start specifying things like aerogel or other things that make the process a technological pursuit and further waste of resources just to aero out the numerical operational values of one building.

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Edvin Bishop
    Edvin,
    I agree that no one should "push solar panels blindly as the cure for a poorly designed building." Neither this article, nor GBA in general, takes that approach to home design.

    Nor does this article advocate the use of aerogel.

    A net zero house isn't for everyone. But if this design approach is used in a state or region where the local utility offers a favorable net-metering contract, the homeowner can end up with zero (or minimal) energy bills, for an upcharge on the mortgage payment that is less than the energy savings. That can be a very smart approach.

  6. S Maris | | #6

    Many thanks!
    Another excellent and detailed article.
    Much appreciated!

  7. User avater
    Dana Dorsett | | #7

    If you're going to kick the aerogel straw man... @ Edvin Bishop
    If specifying aerogel to meet an arbitrary Net Zero spec is something to be criticized it's important to find at least ONE case where someone is actually doing that. (If you can actually find that case, I'd love to read about it!)

    Net Zero does more than just offset other sources of power, it frees up grid capacity, the flip side of which is that it lowers the capacity requirements of the grid infrastructure. Grid costs are all about peak load capacity.

    Twenty minutes into the future behind the meter storage will be cheaper than buying retail from the grid (it already is in parts of Australia), but a lot depends on how the rates are structured. It doesn't take a lot of grid intelligence to make Net Zero houses responsive to the grid, and demand response (even with batteries at 2016 pricing) is a lot cheaper than serving most of those needs with fast ramping peakers, etc. A very low load house with a power system that can be turned into a sub-minute response short term peak generator (or smart load) can and should be remunerated by other ratepayers.

    So, sure, 20kw of panel to heat / cool an uninsulated dog house isn't a great idea, but distributed power (whether 100 watts or 100 kilowatts) IS a great idea.

    Beginning in 2020 in California the point of Net Zero would be simply to meet code minimum requirements. The grid operators and distribution grid owning utilities can factor the reduced peak capacity requirements into their capital expenditure planning (and they are.)

  8. Chris Gleba | | #8

    Cost
    I spent many man-hours on cost/benefit on our 2012 DER project.

    At the time Roxul CavityRock DD was the best cost/performance for exterior insulation I could find. Based on past quotes 2.5" CavityRock DD (R-10.5) was $0.65/ft2 whereas 2" XPS (R10) was $1.25/ft2.

    Since it is breathable no rain gap nor tape is needed, it is naturally fireproof and there are no blowing agents. More details on our project is in a past GBA article:

    https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/wrapping-older-house-rock-wool-insulation

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