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Q&A Spotlight

Passivhaus Design in Minnesota

Is Passivhaus certification so important that it justifies building an R-80 wall?

Structural insulated panels are one option that a GBA reader is considering for the R-80 walls he will need in order to achieve Passivhaus certification for his new house.
Image Credit: Craig Miller Productions

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.

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  1. [email protected] | | #1

    "According to Schmidt's Passivhaus consultant, this configuration brings with it some energy penalties and results in the need for additional insulation." Please elaborate on these penalties.

  2. GBA Editor
    Martin Holladay | | #2

    Respose to Debra Glauz
    The energy penalty is increased heat flow through the building envelope (floors, walls, and roof).

    The cause of this energy penalty is a stretched-out shape instead of a compact shape.

    The closer one can get to achieving a compact shape (for example, a cube), the better the thermal performance. Bump-outs, ells, and cantilevered floors are a problem, because they increase the surface-to-volume ratio.

    That's the issue that Tom Schmidt was referring to when he wrote, "We designed it to have living space over the garage. My understanding from the Passive House consultant is that based on how Passive House calculates everything, you are penalized by having a tuck-under like we do."

  3. [email protected] | | #3

    Thank you. I understand efficient shapes. I was just making sure there wasn't some obscure penalty for parking a car underneath a living space.
    Sounds like surface/floor ratio is high so super insulating the garage ceiling, along with super insulating walls and ceiling for the living space over garage is not going to get the job done.

  4. user-1119494 | | #4

    Not cost effective, but interesting...
    When folks talk about minimizing surface area and using vacuum panels the punch line "but it only works for spherical chickens in a vacuum" comes to mind. Yes, this is extreme, but if someone really wants to do this impressive insulation level rather than, say, renting a nice shore house on Cape Cod for ten weeks, who am I to judge? And we often learn from principles taken to extremes, so go for it!

  5. ntisdell | | #5

    trade offs?
    With the crazy wall structure i would think the cost would be quite high for this incremental amount of R... I would think that things like windows would be easier to upgrade ? Or better ventilator? etc?

    Or some other dial adjustment?

    Also it seems odd that someone would be so gung ho to be "certified" yet purposely make the house inefficiently laid out? Also seems odd that a few extra wall surfaces (buffered from outdoor conditions by garage walls/door) would cause this dramatic of an increase in needed R... (especially since the garage is likely at least somewhat insulated, (since its a mcmansion...))

    Kind of like buying a Prius to be enviro friendly... but then leaving it parked in the garage and driving your truck every day to work..... :-/

  6. donjahnke | | #6

    I agree it is cheaper to do
    I agree it is cheaper to do PV than add all the R value for a trophy but if that's what the customer wants you build it for him. I would rather see thicker sip panels than stud walls with blow in as it still settles gravity always wins. There are a number of those built in northern MN and an infrared drive by shows that after a couple years they all settle. I wonder if they thought of a sip floor over the garage to separate the spaces.

  7. jklingel | | #7

    Don J: "There are a number of those built in northern MN and an infrared drive by shows that after a couple years they all settle." As far as I know, cellulose packed more densely than natural settling density (1.75 to 2 pcf?) will not settle. Besides, if it did settle and the wall packing were continuous with the lid, it would not matter; you'd just have to go blow a tad more around the perimeter of the lid. That said, settling under a window would be problematic, so pack well to start with.

  8. user-1026988 | | #8

    Wait a minute?
    Hmmmm. When I plug in one of our Certified, foam free (except for sub slab and window install, the two area's that I believe should be vapor closed) cellulose filled projects, in Minn, MN. The AHD stays exactly the same. The Peak load stays the same. The primary energy demand (source energy) rises slightly. From say 99 kWh/M2a to 102 kWh/M2a. Still well within PH standards.

    Me thinks this r 80 thing has nothing to do with PH, and more to do with design/planning/consulting.

    If you want to trade R80 walls for ineffective geometry,insolation,poor glazing specs/ cheap doors whatever. That is fine. Dont have a discussion about PH. These are design issue's. Not PH issues.

    Fairbanks Alaska has 14000+ HDD , Minneapolis is around 7500HDD. Belfast Me. 7500HDD

    I would not look to Thorstons REMOTE walls unless you want a perfect example of how to build an outstanding house in Fairbanks. A 14000 HDD climate that stays light and dark for extreme periods of time.

    R80 in MN? Yes, in the roof. And then some.

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