As net-zero energy and Passivhaus-certified houses become more commonplace, it’s not at all unusual to hear of exterior walls rated at R-40 or R-50. But that’s not going to be nearly good enough for Tom Schmidt, who’s building a 3,800-square-foot house in Minnesota.
R-80 is more like it, and the walls need to be “cost-effective” as well as not too thick.
Schmidt’s quest has apparently been prompted by a design that places living space over a garage. According to Schmidt’s Passivhaus consultant, this configuration brings with it some energy penalties and results in the need for additional insulation.
“We have already gone through a couple passes to make it as efficient as possible and are at the point where the only change left that would have a big impact would be to take the garage out from under the living area and have it separate,” Schmidt explains in a post at GBA’s Q&A forum.
“I like the current design (it took us two years to get to this point between the back and forth with my wife) and I want to have the house certified. I could pass on the certification and still have a very high-performing house, but honestly I think it’s cool and want it.”
Schmidt says he has investigated a number of options and is currently leaning toward structural insulated panels filled with expanded polystyrene (EPS).
“Vacuum insulated panels sound interesting, but I don’t know if they are meant for an entire external envelope,” he adds. “The goal is to still keep it cost effective. I just don’t want a 3-foot wall to do it.”
Are Schmidt’s goals achievable? And even if they are, are they reasonable? That’s the topic for this Q&A Spotlight.
Return on investment is clearly not important
GBA senior editor Martin Holladay suggests that Schmidt might want to emulate the details used by Thorten Chlupp (a builder in Alaska who built R-75 walls insulated with cellulose) or the details used by Tom Marsik (a builder in Alaska who built 28-inch-thick R-103 walls insulated with cellulose).
Holladay says that the least expensive way to get to the R-value that Schmidt is looking for is probably with a double-stud wall 22 inches thick that’s packed with cellulose insulation. “Of course,” he adds, “on the day that your insulation contractor comes to do the dense packing, you’ll need several tractor-trailer loads of cellulose.”
What about dense-packing the walls with hundred dollar bills? (That was Stephen Sheehy’s suggestion.) Or, Jesse Thompson suggests, building two rigid geodesic domes, one inside the other, with a vacuum pump that evacuates the space between then.
This is the kind of thing that gives Passivhaus construction a bad name, says Peter L. It’s really about bragging rights and seeing how much money a homeowner can use for construction.
It’s all crazy talk, says Nick Welch. “Stuffing a 3,800-square-foot house with R-80 walls to get a [Passivhaus] certification is like putting five Prius engines in a Hummer so you can drive in the carpool lane,” he writes. “…However, it also piques my morbid curiosity, and I would love to see pics of it actually built this way.”
Schmidt has no illusions about the quixotic nature of his pursuit. “I fully realize I am likely at a point where the [return on investment] is difficult to justify or ever pay itself back,” Schmidt replies. “I am essentially over-engineering something that doesn’t need it to meet the demands of a standard that doesn’t handle my particular design very well. All to satisfy my desire to have a piece of paper that says my house meets a particular standard.”
In reality, Schmidt is chasing the wrong goal
There’s no real reason to design R-80 walls, writes Dana Dorsett, because it should be possible to win Passivhaus certification with walls in the R-50 neighborhood, even in Climate Zone 7. But having conditioned space over the garage probably won’t work.
“The other way to go would be to shoot for net zero energy rather than Passive House, buying your thinner walls with a PV-clad roof,” Dorsett says “It’s probably less expensive overall, and you could probably keep the space over the garage.”
With the cost of a photovoltaic (PV) system now under $4 a watt in Dorsett’s area (and $2 a watt in Germany), and the availability of high-efficiency ductless minisplit heat pumps, there are more appealing avenues that Schmidt might choose.
“In 20 years, when it’s time to replace the heat pumps and/or PV,” he says, “the PV will cost less than half as much, and the heat pumps will have gained in efficiency. The full life cyle cost of heating with ductless heat pumps and $3/watt PV is on par with natural gas in some places, and in 20 years it will be dramatically cheaper than it is today, making the 50-, 75-, 100-year outlook even more favorable for site-produced power and heat pumps than over-the-top super insulation.
“Methinks R-80 is only ‘cost-effective’ in terms of meeting your certification goals, but those certifications have little to do with breaking even on energy costs over the next 100 years, or what’s actually nice for the planet,” Dorsett says.
But if you insist, there are ways to get there
One way of accomplishing the R-80 wall, says Jerry Liebler, is to use a double 2×4 wall filled with mineral wood or blown-in fiberglass. With an overall wall thickness of 19 1/2 inches (plus drywall and sheathing), Schmidt would get to R-82. And, Liebler adds, the added cost over an R-40 wall would be about $2 a square foot.
Whatever you do, writes Jason Hyde, you’re going to need a multi-layered assembly, and no matter what wall assembly eventually wins, it will be thick.
“Build a SIP home and clad it with outstation,” Hyde continues. “Basically a double-stud wall but the inner wall is [a] SIP. This outsulation could be [dense-packed cellulose], blown fiberglass, mineral wool, whatever. The main advantage here is that you get SIPs up quickly (weathered in) and then can proceed with the outsulation. The drawbacks here, in addition to the standard SIP drawbacks, are that you still must frame an outer wall.
“I would have suggested doing what Go Logic has been doing (successfully) and do a stick frame clad with SIPs,” he says, “but to hit R-80, your inner wall would need to be made out of 2x10s or 2x12s.”
If the problem is the garage below the conditioned space, adds AJ Builder, possibly the solution would be to beef up the garage doors. “So, maybe install two garage doors, one behind the other — superinsulated custom doors,” he says. “Be kind of cool to have a stack of doors opening with about a foot of lift delay each… Watching the babies go up and down would make hanging out in front of them in your Tesla a great place to be. Pop a cold IPA.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost has to say:
There are two questions posed here: Can Schmidt’s goals be achieved? And are they reasonable? These two questions may seem mutually exclusive, given the design and the requirements of the Passivhaus standard. But I would argue that Schmidt’s goal of Passivhaus certification is clearly attainable, and that he simply needs to convince his lender and/or the next owner of his home that what he did is reasonable. (Translation: valuable to the next owner).
I have argued before that selecting the interest rate for payback analysis is pretty much a Ouija board exercise for even relatively short-lived goods but completely speculative for long-lived durable goods like buildings. (For more information on payback calculations, see this BuildingGreen blog, or this article by Martin Holladay: Payback Calculations for Energy-Efficiency Improvements.) If the energy performance improvements that Schmidt is considering last beyond his ownership, he just needs to find someone willing to pay for the features he has selected. This is true regardless of whether the feature is an R-80 wall or a granite countertop.
I think that a great way to design, market, or underwrite a “wildly” performing home is to link extraordinary performance with resilience: make the home self-sufficient in the face of extreme events or crises. The “payback” seems like a pretty silly singular rationale for super-high-performance in the face of the grid going down or a hurricane making every home in your neighborhood except yours uninhabitable. BuildingGreen founder Alex Wilson is spending most of this time now on this issue; see the Resilient Design Institute.