A Net-Zero Townhouse in Oakland
General Specs and Team
Location: Oakland, CA
Living Space : 1561 sqf
Cost (USD/sq. ft.): $165/sqf
Cost per square foot excludes land, sitework, and foundation.
Foundation: sealed conditioned crawl space
Foundation insulation: 1 1/2" XPS at foundation perimeter
Wall framing: 2x6 OVE framing
Wall insulation: 1" XPS sheathing (R-5) + R-19 Icynene
Windows: Serious Windows with Heat Mirror glazing (U-0.23, SHGC 0.42)
Ceiling insulation: R-32 Icynene
Roofing: Modified bitumen
Space heat and cooling: Goodman 16 SEER/9.5 HSPF air-source heat pump
Domestic hot water: AirTap heat-pump water heater (2.11 EF) backed up by 40-gal. electric water heater
HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. Index: 0
PV system: 5.4-kW; expected output 8,285 kWh/yr
Heating degree days: 2,310
Cooling degree days: 79
- Drainwater heat-recovery device
- Energy dashboard mounted in kitchen
- Low-water-use plumbing fixtures
- Dual-flush toilets
- Drought-tolerant landscaping plants
- A portion of the roof drains to a drywell to recharge ground water
Indoor Air Quality
- Zero VOCVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production. paint
- Interior finishes with no added formaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen."
- A variety of mold-prevention strategies
- Mechanical ventilation: Airiva HE100 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.
- MERV 13 air filters
- Exhaust fan in garage
Green Materials and Resource Efficiency
- Located close to public transportation
- Concrete includes fly-ash
- Resource-efficient framing
- Some recycled cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection.
- FSCNonprofit organization that promotes forestry practices that are sustainable from environmental and social standpoints; FSC certification on a wood product is an indicator that the wood came from a well-managed forest. flooring
- FSCForest Stewardship Council. An independent, nonprofit organization that promotes responsible forest management through the use of a third-party certification process. FSC certification includes a chain-of-custody requirement that tracks sustainability of wood products from growth to end use. Ipe decking
- Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. appliances
- Mostly Energy Star lighting
- LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. Platinum (99.5 points)
- Green Point Rating = 206
- EPA Indoor airPLUS
- Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners.
- DOEUnited States Department of Energy. Builders' Challenge
Factory-built modular units were quickly craned into place
Not many projects achieve HERS 0, especially on a small urban infill site. But that's the score achieved by the Lancaster Live/Work Townhome, a recently completed (Fall 2009) project in Oakland, Calif.
The Lancaster townhouse was assembled from four factory-built modules produced by ZETA Communities, a new company producing factory-built modules for net-zero energy homes, mixed-use projects, and public facilities. In addition to targeting net-zero site energy use, the Lancaster project is participating in the Department of Energy’s Building America program and has certifications completed or pending for LEED Platinum, Energy Star, Energy Star Indoor Air Plus, GreenPoint Rated, and Builders' Challenge.
It’s not just about energy for ZETA’s founder and CEO, Naomi Porat
Porat set out to create an affordable, deep-green housing solution, primarily for urban infill sites. She worked closely with green architect Dan Smith of Daniel Smith and Associates (Berkeley, CA) and energy/building science consultant John Straube of Building Science Corporation (Somerville, MA) to optimize the energy systems and overall resource efficiency. BSC's participation was funded under the Building America program.
The all-electric Lancaster project has a hip, urban sensibility, consistent with its mixed residential-industrial-arts neighborhood setting; many community services are within walking distance. Most of what makes it innovative, though, is under the hood – in this case, in the insulated, sealed, and conditioned crawl space (the cleanest one I’ve ever had the pleasure of crawling through) and centralized service core.
Heating, cooling, and clean air
For starters, the crawl space is home to a 16 SEER/9.5 HSPF air-source heat pump. It also serves as a tempering space where the air flowing through the air handler interacts thermally with the insulated, concrete-floored crawl space. The tempered air in this “thermal basement” enters the lower story via grilles in the first floor/basement ceiling; the second floor is fed by conventional ductwork. The return system draws air from both the first and second floors.
Heat pumps can provide cooling as well as space heat. Oakland’s mild climate and the building’s high-performance enclosure and “thermal basement” will minimize cooling needs. In addition, a residential “economizer” — a large outside air duct — will be used to cool the house with outdoor air ventilation when conditions permit.
The air handler is set to operate periodically to mix the air, helping to provide a consistent temperature and air quality throughout the home. Outside air is supplied by an Airiva HE100 heat-recovery ventilator (HRV); the exhaust ducts are connected to the bathrooms, and outside air is supplied at a register near the second-floor return. The HRV is designed to provide bathroom exhaust ventilation via a pushbutton timer and can also be set up to cycle on an hourly basis. When placed in service, the Airiva HRV proved to be a troublesome and poorly constructed unit, so it would not be recommended for use in the future.
The absence of fireplaces and the selection of interior finish materials without VOCs or added formaldehyde safeguard indoor air quality. A continuously operating exhaust fan in the garage prevents vehicle exhaust and other emissions from entering the living space.
An electric AirTap heat pump water heater (2.11 EF) is located at the rear wall of the garage, and a drainwater heat-recovery system in the plumbing service core will provide further energy savings. The service core, next to the mechanical room, carries all the plumbing lines (insulated, of course) in straight vertical runs for efficient hot water distribution: the kitchen and both bathrooms back up to the core, keeping horizontal runs short.
Much of the effort to minimize energy use went into designing and building a high-performance enclosure. The home consists of four modules that were assembled at the site on a crawl space foundation. Framed in the factory from (some) 2x4 and (mostly) 2x6 FSC lumber and non-FSC I-joists, the modules minimize thermal bridging and lumber use by using 24-inch centers and 2-stud corners throughout.
The buildings are insulated with a combination of low-density spray polyurethane foam and cotton batts, with spray foam sealant used liberally to minimize infiltration. The exterior walls include a 1-inch (R-5) continuous layer of extruded polystyrene (XPS) foam insulation over the framing; the XPS greatly reduces thermal bridging through the studs and reduces the risk of condensation within the walls.
All walls are detailed with a ventilated rainscreen that greatly improves moisture management. There are two types of cladding: fiber-cement lap siding and fiber-cement panel siding.
The windows are Serious Windows with Heat Mirror glazing (U-0.20, SHGC 0.42). The windows have two layers of glass with a suspended film (comparable to triple-glazing).
The modified bitumen “cool” roof is topped with a 5.4-kW photovoltaic system. The array is sized to provide net-zero performance, assuming careful management by home occupants. The home is currently functioning as a sales model, however, so occupied usage data are not yet available.
More green features
All appliances are Energy Star, as is most of the lighting. Some LED fixtures are included.
The plumbing fixtures (including the dual-flush toilets) are all low-flow. The drought-tolerant, native-and-Mediterranean plants further reduce water demand, and a portion of the roof drains to a drywell to recharge ground water.
Fly-ash was substituted for some of the cement in the concrete used for the foundation. The outdoor deck boards are FSC Ipe. Countertops and interior doors incorporate recycled content.
The project’s energy performance is being continuously monitored. A TED 5000 electricity dashboard is mounted in the kitchen to provide real-time feedback to the occupants and to measure both energy usage and energy production. In addition, BSC, NREL, and PG&E will continue to monitor home energy performance for one year to collect data required for ongoing R&D and to help improve performance of future units built by ZETA.
Hindsight is 20-20
ZETA’s management team offered a number of observations based on their pilot experience tackling the net-zero-energy challenge:
- Assembling an integrated, multi-discipline design team from the beginning is critical to ensure that the design meets energy, cost, and fabrication requirements and to avoid errors, issues, and costs in the field.
- Define your green priorities early on. There are many trade-offs between energy efficiency, embodied energyEnergy that goes into making a product; includes energy required for growth, extraction, and transportation of the raw material as well as manufacture, packaging, and transportation of the finished product. Embodied energy is often used to measure ecological cost., and indoor air quality, and each decision also affects cost. For example, no single insulation product meets all objectives. Some with better insulating values are more expensive or have greater embodied energy. Others have lower embodied energy, but may be more vulnerable to mold and moisture problems, etc.
- Start with a tight, well insulated building enclosure. This is the most cost-effective energy-efficiency strategy.
- Controls are important, to ensure that strategies requiring the lowest energy inputs (e.g., ventilation) are employed to achieve thermal comfort first, before higher-energy systems (air conditioning and heat) are turned on.
- Design is only part of the equation. Implementation, performance, and behavior are also extremely important. Energy dashboards are critical to influencing occupant behavior and decreasing energy usage of systems that are occupant-controlled. ZETA plans to monitor for one year (with the help of DOEUnited States Department of Energy., NREL, and PG&E) to assess actual performance.
- Consider source versus site energy. Especially when it comes to heating, whether to favor electricity or natural gas depends on the project’s goals.
Ann Edminster is the owner of Design AVEnues, a green building consulting firm in the San Francisco Bay Area. She is also a member of GreenBuildingAdvisor.com's team of advisors. This case study is one that Ann researched for her new book, Energy Free: Homes for a Small Planet.
- Toshi Woudenberg
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