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.”
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.