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Green Homes

A Prefab Passive House in Michigan

The wall panels were fabricated at an indoor assembly plant and delivered to the site by truck. The panels have 2x12 studs. The interior sheathing is conventional OSB; the exterior sheathing is 2-inch-thick fiberboard.
Image Credit: Kate McDonald
View Gallery 15 images

The A-Haus was assembled with pre-insulated wall and roof panels

This single-family home, nicknamed the “A-Haus,” is the first house in Michigan to be certified by the Passive House Institute U.S. (PHIUS). The striking building is situated on nearly three acres of rural land, bordered by woods and farm fields, in Freeland, Michigan. (To read about a similar project, see the GBA article, “Michigan Gets Its First Passivhaus.”)

The A-Haus was a project of a Detroit-based prefab Passive House component manufacturer called Phoenix Haus. The Passive House method relies on superinsulation, airtight detailing, and thermal-bridge-free construction to achieve extremely low energy use. The home features a continuous fresh air ventilation system using a Zehnder Comfo 550 energy-recovery ventilator (ERV). According to the Passivhaus Institut, the Zehnder Comfo 550 has an efficiency rating of 84%.

The A-Haus has 3 bedrooms, 2½ bathrooms, and a cozy finished third-story loft. “We chose a piece of land that complemented the style of the house,” said Hilde McDonald, owner of Phoenix Haus. “The rolling hills, south-facing exposure and rural setting made an ideal spot for a Passive House home.”

The A-Haus was named in honor of Hilde’s father, Anton Cech. Anton’s story is one for the history books: As a penniless German immigrant, he traveled as a stowaway to Brazil at the age of 16. Three years later he came through Ellis Island with nothing to his name but the clothes he wore on his back. In 1936, in the midst of the Great Depression, he started his own scale business (in spite of having only an eighth-grade education), and lived to share his incredible stories with his grandchildren. He died at the age of 105.

A pergola and an eckbank

Completed in April 2016, the A-Haus features a 45-foot cedar pergola breezeway connecting the home to a two-door detached garage. With a protected courtyard for entertaining guests in the summer months, a refrigerated wet bar, and a corner kitchen eckbank — a tribute to the owner’s German heritage — comfort, beauty and functionality are the main principles of its design. Visitors will also notice small sections of the home’s frame when they look up: three Douglas fir glulam beams are part of the home’s structure.

Phoenix Haus wanted to bring Passive House science to the Midwest and to introduce homeowners to the numerous benefits (for both occupants and the environment) of the building standard. Bill McDonald, principal of Phoenix Haus, conceived the idea of fabricating the wall, roof, and floor panels in a controlled environment as a solution for the high cost and intensive labor often associated with Passive House buildings. “Europeans have been building using components for years,” said McDonald. “I knew we could take that fundamental building science and bring it to the Midwest. Component building is ideal for the Passive House standard because such a high level of precision is necessary and can be easily accomplished in a controlled setting.”

This component building process began behind a computer, using powerful modeling software. A 3D CAD model assured the highest level of accuracy before, during, and after the design process. Once completed, this model interfaced with a CNC machine which cut the lumber for the home. Each step of this process supported the building’s energy performance. After the package of wood parts was cut, it was sent to the Phoenix Haus production facility in Detroit.

The wall and roof panels are insulated with cellulose

A few characteristics make the Phoenix Haus system stand out. The home’s envelope includes a continuous thermal insulation layer that is close to airtight. The envelopes encloses the 6½-foot-high crawlspace as well as the second and third floors. Due to a ledger detail specified for the floor systems, these layers remain continuous and uninterrupted.

The wall and roof panels were insulated at the factory with dense-packed cellulose. The 2×12 Douglas fir studs and the 16-inch-deep TJI rafters provide cavities with room for R-51 and R-71 insulation, respectively. Two-inch-thick Agepan fiberboard sheathing provides some additional exterior insulation.

The wall panels are “vapor diffusion open,” allowing moisture to escape easily. Because the water-resistive barrier (WRB) is pre-installed on the wall panels at the factory, the home was weather-tight within 24 to 72 hours of erection, preventing inclement weather from disrupting construction. All panels are structural.

“Homeowners’ greatest misconceptions come from their belief that pre-fabricated components either limit their ability to be creative or are low-quality — this is totally not the case,” said McDonald. “We can deliver the utmost quality and performance and keep costs low because of our production facility that runs 356 days a year, despite harsh weather, combined with our innovative assembly technique. We’re flipping the pre-fab stereotype on its head.”

Custom designs are possible

The Phoenix Haus system, unlike typical modular or pre-fab construction, does not limit homeowners’ creative wishes during the design process. So long as the Passive House envelope includes superinsulation, airtight detailing, and thermal-bridge-free construction details, it’s up to the clients to choose their desired fit and finish. Phoenix Haus chose fiber-cement siding and classic ribbed metal roofing for the A-Haus.

The A-Haus is one of three Phoenix Haus Design Templates for 2016. The most cost-effective and affordable route, especially for Passive House, comes when using a Design Template approach: pre-engineered and pre-modeled Passive House templates provide a springboard for clients to tweak and change floor plans to whatever they wish. It also saves them time and money, especially by avoiding an expensive custom design fee.

Triple-glazed windows

The A-Haus has a total treated floor area of 3,234 square feet. Windows are Intus Eforte vinyl tilt/turns with triple glazing. The exterior doors are also vinyl units from Intus.

The home itself is oriented to favor southern exposure. Roof overhangs shade the south windows during the summer. The A-Haus is located in Climate Zone 5B, and has (under the PHIUS system) a specific space heating demand of 4.46 kBTU/(ft2-yr).

The home is outfitted with all LED bulbs, a Bosch ventless clothes dryer, a Kitchen-Aid induction stove. Two Mitsubishi MSZ-FE09 minisplits provide space heating and cooling. Domestic hot water is provided by a Stiebel Eltron Accelera 300 heat-pump water heater located in the crawl space. The crawl space has a 4-inch-thick concrete slab over a 12-inch-thick layer of horizontal EPS insulation.

The A-Haus passed its final blower door test at 0.6 ach50.

Lessons Learned

Shipping vertically
On this home, all panels were shipped horizontally or (using industry jargon) “flat packed.” This proved cumbersome and time-consuming — a wide load permit was necessary, slight damage was inevitable during shipping, and the crane operator’s time was not used well. However, it proved to be a great learning opportunity. To fix this, Phoenix Haus now ships everything vertically via a specialized German trailering system. The Germans really do have it nailed down!

Design is everything
Spending enough time during the onset of the project and working with experienced architects, especially those trained in Passive House principles, really means a world of difference when it’s time for construction. These efforts in theory and concept translate to a very well thought-out and well-engineered building. Case in point: window details were severely lacking on this house and a solution for flashing and trim work had to be worked out on-site.

A clearer solution for mechanical systems is desired
Most residential HVAC contractors, especially in the U.S., still use a “cut & paste” approach when it comes to deciding which heating, cooling, ventilation, and hot water equipment to install. I regret not installing a pre-heat ground loop for the Zehnder ERV fresh air intake — this would have reduced our pre-heat function substantially, especially for our cold climate. Also, I’m excited to see what manufacturers are now designing for the U.S. market when it comes to units that combine heating and cooling with energy recovery. The CERV by Build Equinox seems to be the closest right now.

General Specs and Team

Location: Freeland, MI
Bedrooms: 3
Bathrooms: 2.5
Living Space: 3234

Designer: Sedgwick & Ferwerda Builder: Energy consultant: Bill McDonald

Construction

Foundation: Crawl space with ICF walls

Foundation insulation: Under the crawl space slab, 12 in. of horizontal EPS (R-50.8); ICF walls have 2 in. EPS on exterior and 2 in. EPS on interior, plus an additional 6 inches of EPS on the exterior side of the ICF walls; above-grade foam is protected with metal flashing.

Above-grade walls: Factory-built wall panels with 2x12 studs, sheathed on the exterior with 2-in. Agepan fiberboard sheathing and on the interior with 5/8-in. OSB; supplemented by an interior 2x4 service cavity. Panel seams are sealed on the interior with Siga Rissan tape, and on the extrerior with Siga Wigluv tape.

Water-resistive barrier: Siga Majvest with taped seams.

Siding: Horizontal fiber-cement siding.

Wall insulation: 2x12 studs and 2x4 service cavity are both insulated with dense-packed cellulose; fiberboard sheathing provides about R-5; total wall R-value is R-51.

Roof: Factory-built roof panels with 16-inch TJI rafters sheathed on the exterior with 2-inch Agepan fiberboard and on the interior with 3/4-inch OSB, supplemented by a 2x6 interior service cavity; roof panels are supported by a glulam structural ridge beam.

Roof insulation: 16-inch rafters and 2x6 interior service cavity are both insulated with dense-packed cellulose (R-71).

Roofing: Exposed-fastener steel roofing over Siga Majcoat roofing underlayment.

Windows: Intus Eforte vinyl tilt/turns with triple glazing; U-factor = 0.15.

Doors: Glazed vinyl doors from Intus; U-factor = 0.16.

Space heating and cooling: Two Mitsubishi MSZ-FE09 minisplits.

Domestic hot water: Stiebel Eltron Accelera 300 heat-pump water heater.

Below Grade Foundation: R-50.7.

Mechanical ventilation: Zehnder Comfo 550 ERV.

Domestic hot water: Stiebel Eltron heat-pump water heater.

Energy Specs

Space heating demand: 4.46 kBTU/(ft2-yr).

Specific primary energy demand: 15.8 kBTU/(ft2-yr).

Design space heat load: 12,441 BTU/h.

Blower-door test results: 0.6 ach50.

13 Comments

  1. Steve Knapp CZ 3A Georgia | | #1

    Cost?
    Can you share a cost per square foot? Everybody's idea of affordability is different.

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

    Response to Steve Knapp
    Steve,
    Unfortunately, the author was reluctant to share cost data. When pressed, here's what Kate McDonald told me: "Our $/Sf of envelope area was $25.40 – which includes all framing, insulation, weather membrane, taping, sealing, installation as per our system."

  3. Steve Knapp CZ 3A Georgia | | #3

    That seems reasonable for the
    That seems reasonable for the basic structure--especially for a house built to Passive House standards. What say the folks with more hands-on construction experience?

  4. Malcolm Taylor | | #4

    Steve.
    It's really hard to tell. It sounds like it only includes the wall and roof panels, not the structure to support them.The wall sections also show some components to be added on site. Whether these are in that price is also unclear.

  5. User avater
    Kate McDonald | | #5

    Malcolm
    Hey Malcolm....what's included in that price per SF of envelope area does include the walls, roof and floor systems as well as the interior partition walls. The panels are all structural and the home was designed as a timber frame...so those support beams are also built into the price. Hope that helps.

  6. James Morgan | | #6

    3 bedroom?
    I only see two. Clearly the space labelled 'office loft' could easily be converted to a bedroom with the addition of a partition wall, but that would compromise access to the attic stair. And if the third bedroom is counted in the finished attic space the access stair does not appear to meet code.

  7. James Morgan | | #7

    S.F.
    Thick-walled construction and factory panelization would seem to go together 'like peas and carrots' as Forrest Gump would say, and I've read it's pretty much the standard in Sweden and perhaps other European countries. It could potentially do much to mainstream the Passivhaus standard in the US. Just a question though, what counts as 'treated floor area' in this kind of construction? How is it measured?

  8. Malcolm Taylor | | #8

    Reply to Kate
    Thank you for clarifying. That's very helpful.

  9. KEVIN ZORSKI | | #9

    CRAWLSPACE INSULATION
    Could someone please explain why 12" of EPS makes sense under the slab, when only 10" is in the walls of the crawl area. I can see that the horizontal 12" would be significantly cheaper to install than the 10" in the walls, but the 12" still seems like overkill. I would also love to see the framing inside the wall panels, headers,etc.

  10. User avater GBA Editor
    Martin Holladay | | #10

    Response to Kevin Zorski
    Kevin,
    I agree with you that it's hard to justify the investment in 12 inches of sub-slab rigid foam. For more on this topic, see Can Foam Insulation Be Too Thick?

  11. Thompson Johnson Woodworks | | #11

    2x12 studs and heat loss calcs
    Was the choice to use 2x12s as studs for the ledgering of the floor systems? It seems, respectfully, like a huge waste of materials, especially considering only 2, maybe 3, 2x12s are select cut from the largest of SPF logs. Did it aid in speeding up the assembly of panels at the plant? Savings in labor offset the increased cost of 2x12s? It definitely makes sense with regard to compartmentalizing the dense pack cellulose. Traditional double stud walls need netting to create compartments for the DPC. Was this option ruled out for structural and cost saving reasons?

    Agepan lists their 2" panels as R-5.74. I am curious how PHIUS energy modeling dealt with the heat loss through the studs and plates with just R-5.74 of exterior insulation? Is this why the sub-slab insulation ended up at 12" thick? Was extra foam also added to the exterior face of the ICFs? At 3300 SF of conditioned space the heat loss modeling must have been a lot of fun to go through various scenarios!

    Well done!

  12. Bob Irving | | #12

    Does "envelope area" mean the
    Does "envelope area" mean the SF of living space in the heated envelope, or the SF area of the wall and roof surfaces? If it's the former, is that the same as the 3432 SF given, or some other number that might include attic or/and other spaces within the walls?

  13. User avater GBA Editor
    Martin Holladay | | #13

    Response to Bob Irving
    Bob,
    Q. "Does 'envelope area' mean the square feet of living space in the heated envelope?"

    A. No.

    Q. "Or does it meet the square feet (area) of the wall and roof surfaces?"

    A. Yes, that's what it means -- except that it also includes the area of the floor system and the partition walls. (The inclusion of the partition walls is obviously unusual, but that's the way Kate McDonald explains the calculation in Comment #5, above.)

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