Multifamily Passivhaus Project Starts in Oregon
The 57-unit building would become the largest Passivhaus project in the United States, but many challenges lie ahead
A multifamily residential housing project in a suburb of Portland, Oregon, will become the country's largest Passivhaus project should it win certification sometime next year. The 57-unit affordable housing project is being built by Walsh Construction Company.
The three-story building in Hillsboro, Oregon, will have 40 one-bedroom apartments and 17 two-bedroom apartments. The building is the first of what will eventually be a three-building project; when it is completed in four or five years, the entire project will include roughly 150 units. Construction costs for the first phase of the project are set at about $14.5 million, including a little over $9 million in direct construction costs.
It's going to be a challenge for the builder. Mike Steffen, the company's vice president and general manager, said that Walsh specializes in multifamily and institutional projects in the Northwest, including affordable housing projects and housing for colleges and universities. But this is the first time Walsh will seek Passivhaus certification for one of its buildings.
The project has already been pre-certified by the Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. Institute U.S.
A key requirement of the Passivhaus standard is an essentially airtight building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials., tested with a blower door at 0.6 air changes per hour at a pressure difference of 50 pascals (ach50).
Steffen said the company has been careful to design its air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. and document the process with construction drawings in advance of getting subcontractors up to speed. "We've been pretty diligent about it," he said in a telephone interview, while acknowledging that the Orchards project will have to be a step up from previous efforts.
Leakage rates in buildings the company has constructed in the past have been as low as 0.18 cubic feet per minute per square foot of envelope area (cfm/sq. ft.), he said, but to meet the Passivhaus standard that number would have to drop to roughly 0.13 cfm/sq. ft.
On exterior walls, the air barrier will be the 1/2-inch structural sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. with all seams sealed with Siga tape. On the low-slope roof, a peel-and-stick membrane will be both an air barrier and vapor barrier.
Steffen said that a particularly troublesome spot for air sealing is the transition between exterior walls and the roof. On the Orchards project, this area will be sealed with a peel-and-stick membrane that laps up over the outside of the wall, across the top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate. and onto the roof deck. That, in turn, will be covered with the roof membrane.
A parapet wall will be built on top after the air-sealing has been done, rather than created as an extension of the exterior walls.
The construction schedule includes a four-day period when all activity will stop to allow for a preliminary airtightness test, Steffen said.
"Once it's in, I think it's going to be fairly reliable," he said. "We have a lot of confidence that we can do the 0.6 number."
The ventilation system is a hybrid
As daunting as air-sealing a 57,000-square-foot building might seem, Steffen said the ventilation system may be even trickier.
Each of the 57 housing units will be served by a heat-recovery ventilator (HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ), but instead of a single, centralized unit for all of them, or individual HRVs for each apartment, the building will have a "pod" system consisting of three HRV units in rooftop penthouses, each of which serves 19 units.
The system will reduce the air distribution headaches that a single HRV would create, and also makes maintenance much less of a challenge than individual units would. The pod system also is more economical from a fan energy point of view, Steffen said.
Steffen said it's uncommon for market-rate apartments to have fresh air ducted to each apartment; it's even less common in affordable housing projects. So in addition to living in apartments with very low heating and cooling bills, Orchards residents also will have "really great" air quality.
The project will fill a gap in Hillsboro's housing mix
Orchards at Orenco was spearheaded by REACH, a community development corporation that has been in business for more than 30 years, along with a number of other financial partners. Hillsboro is an area that Steffen called the "silicon forest," where there are plenty of high-paying tech jobs and upscale housing. Affordable housing isn't as common.
According to an article in The Oregonian, eight of the first 57 units will go to people with Section 8 vouchers who earn up to 30% of the area's median family income. The balance will be for those making up to 50% of the median family income, which is about $30,000 or less for a single person.
Rents will range from $611 to $733 per month, The Oregonian reported.
Steffen said that construction details that would allow the project to win Passivhaus certification would make it about 11% more expensive than a similar building that was built slightly better than code. Walsh estimates construction costs at $158 per square foot.
If 11% doesn't sound like much given the building's dramatically better performance, Steffen said it's still enough to raise some eyebrows in the region among government officials and conservative watchdog groups.
Funding for affordable housing projects is typically pretty tight, he said, so REACH went to the community and to foundations in an effort to raise the extra money the project required. The benefit on the back side is lower energy bills and "obviously an improved environmental footprint."
"We've certainly been aware of people, especially Passivhaus advocates, either on the design side or the construction side, saying it doesn't cost more to build Passivhaus," Steffen said. "We adamantly disagree with that in our experience with this project, but we still think it's definitely something that should be considered because it may well be worth the additional investment of money."
Slab-on-grade construction, wood-framed walls
The building will be framed with 2x10 exterior walls filled with blown-in fiberglass insulation and covered with 1 1/2-inch semi-rigid mineral wool insulation for a total R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of 45, according to information provided by Walsh Construction.
Here are some other technical details:
- The slab will be insulated with 4 inches of expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) insulation (R-18).
- Foundation walls will be insulated with 4 inches of EPS under the footings and 1 in. of EPS under interior footings.
- The roof will be insulated with 12 inches of polyisocyanurate insulation (R-72) over the roof sheathing.
- Windows will be triple-glazed Euroline 4700 tilt-turn units with vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). frames and argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. gas fill. North- and south-facing windows will have glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. with a solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.) of 0.54, while east- and west-facing windows will have a SHGC of 0.24.
- Domestic hot water will be provided by high-efficiency gas-fired water heaters.
There are no renewable energy systems.
REACH will own and operate the buildings. Ankrom Moisan Architects are the architects of record; William Wilson Architects did the design. Green Hammer provided energy modeling.
- Walsh Construction
Jul 4, 2014 4:00 AM ET