The zHome was the first net zero energy townhome complex in the United States, and two years of real data shows the homes actually produce as much energy as they use.
Ichijo USA of Seattle constructed 10 high performance homes in the zHome community in the City of Issaquah in 2011. These homes have HERS Index scores ranging from 0 to minus 1, and use local materials, employ water reduction strategies, and ensures indoor air quality.
The project was launched to spur the market toward deep green housing for the average person. As such, zHome was built to rigorous environmental benchmarks of net zero energy use, a 70% reduction in water use, a 90% construction recycling rate and the use of only low- and non-toxic materials among other specifications.
All zHome units that went up for sale eventually sold at slightly above standard prices for the time and area, despite going on the market at the tail end of the Great Recession.
In a white paper titled "zHome: Setting a National Net Zero Energy and Green Building Precedent," Built Green, the Seattle-area green building program of the Master Builders Association of King and Snohomish Counties, the actual performance of the homes in energy and water usage was revealed.
Excerpts from the report below outline the successes and challenges of the program.
According to the report, based on utility data, we have found that zHome achieved its benchmark of net zero energy, and, in fact, releases excess energy onto the grid, producing 3.5% more energy than is consumed. Over a two-year span, the eight units for which we have data and the community trellis generated an excess 562.8 kWh of renewable energy for the grid on top of what zHome consumed. When the net energy use is instead calculated on a rolling annual basis, zHome still comes out positive, producing an excess 160 kWh per year on average. Energy efficiency helps make this possible. The highest consuming zHome unit only uses 759 kWh per month, significantly less than the Issaquah average of 838 kWh. Additionally, despite a recent drought, the average zHome resident uses 16.07 gallons of water from the utility per day, which is well under average — depending on the baseline comparison, either approaching or exceeding a 70% reduction.
The project was the result of a coalition of partners led by the City of Issaquah in concert with Port Blakely Communities, King County, Built Green, Ichijo USA, Puget Sound Energy, and the Washington State University Energy Office.
The project was made possible through an innovative development deal brokered by the City of Issaquah in collaboration with Port Blakely Communities, which provided a site for both an affordable housing community (now the YWCA Issaquah Family Village) and the zHome project. For zHome, the builder was given land at no cost in exchange for meeting strict sustainability benchmarks as well as providing for partnership project management costs, absorbing market risk and reserving one unit as an education center for up to five years.
The primary benchmarks included: zero net energy use; 60% reduction in water use; locally produced, recycled content; FSC certified and low toxic materials; construction recycling; meeting high indoor air quality standards; and minimal stormwater discharge.
zHome was conceived as a net zero development. Each individual unit was modeled to achieve net zero through energy efficiency gains with solar photovoltaic (PV) accounting for the remaining energy consumption.
Net Zero Achieved
Over the two years for which we have post-occupancy energy data (April 2013 – April 2015), zHome achieved net zero energy use. zHome’s net consumption over this two-year period was -562.8 kWh, or in other words, 562.8 kWh was generated above what zHome consumed and was put onto the grid. For perspective, 500 kWh is equivalent to 180 pounds of carbon dioxide emissions in Washington State.
These data account for eight of the ten living units and the community trellis (which provides energy for outdoor lighting and the geothermal pumps). Given that the total energy taken from the grid for these units over the two year time period was 162,169 kWh and that 562.8 kWh were generated in excess of what was consumed, this means that zHome exceeded net zero energy by 3.5%, or almost $54 of electricity savings.
However, there was substantial year-to-year variability. Over the year-long period of April 2013 to April 2014, zHome did not achieve net zero; instead consuming a net of 1,636.4 kWh. This was reversed from April 2014 – April 2015, when zHome contributed 2,199.2 kWh onto the grid. These two years average out to a net production of -281.4 kWh.
Additionally, the data can be examined in rolling years by calculating the net use and production from each month forward for a year. By doing this, we are able to analyze all possible year-long time frames. Over the 14 periods we can apply this calculation, the eight units we have data for plus the community trellis reached net zero or better than half of the time. When averaged, the yearly net consumption is -160 kWh. Given the data, it is a safe statement to say that, overall, zHome achieved its net zero goals.
It is quite clear that zHome is much more efficient than the average dwelling. Even though there was significant discrepancy between units when comparing net consumption, they all perform quite efficiently. The unit with the highest average consumption is still 10% more efficient than the city average.
These efficiency gains were achieved through both high performing appliances and design elements that induce more efficient occupant behavior. Examples of high efficiency appliances and systems include ground source heat pumps for heat and hot water, R-38 wall insulation and R-60 ceilings, a SIP (structural insulated panels) roof of R-55, low flow water fixtures (which save hot water and thus energy), LED and fluorescent lights, and ENERGY STAR appliances. Homeowner action is what ultimately leads to the amount of consumption; thus design aspects such as black/white switches to prevent phantom loads and ample natural lighting to reduce the need and desire to switch on indoor lights help reduce energy consumption further.
The size of each solar PV array was fit to the unit size, with arrays ranging from 4.8 kW to 7.2 kW. All installations performed better than their target. This is true even though the rated yearly production accounted for the estimated solar access of each roof where the panels would be installed.
The zHome data illustrate how energy consumption and production are almost perfectly in opposition to each other over the year.
Given that zHome averages a yearly net production of 160 kWh per year and exceeds zero net energy by 3.5%, the models that predicted zHome would reach net zero were incredibly close to the mark, especially given that energy modeling is imperfect. Though zHome’s yearly average is close to net zero, in reality, its year-to-year numbers vary, with a 3,835.6 kWh difference between the two full years we have data for. What accounts for these yearly fluctuations, and for the discrepancies between reality and models?
Apart from the intricacies inherent in modeling complex systems such as buildings, the main element that leads to differences between models and reality is human behavior. Within zHome units of the same size, which were built and modeled to the same specifications, there are differences in energy consumption that stem from the differences in occupant behavior.
Though building a zero net energy development was the primary goal of the zHome project, reducing water use was an important aspect of the project as well. A goal of reducing water consumption by 70% was set. In addition to water-saving fixtures, such as efficient faucets and toilets, rainwater is also collected in cisterns (cisterns vary in size from 1,100 to 1,700 gallons) and then used for flushing toilets and doing laundry. After zHome was developed and Built Green’s Emerald Star certification was finalized (largely based on lessons learned from zHome) a water efficiency gain of 70% was approved for the Emerald Star criterion as well.
Average water consumption in the Issaquah area is 47.9 gallons per person per day. The Emerald Star checklist uses a baseline water consumption of 67 gallons per person per day — from which a 70% reduction must be made.
zHome water consumption comes both from the water utility and from rainwater harvesting. Analogously to our energy calculations, water consumption from the rainwater harvesting system will not count toward household consumption like water from the utility does. Unlike in the case of measuring energy use, however, we do not know exactly how many gallons per day of rainwater are used since that is not tracked by the utility. But, it is possible to estimate these numbers based on research on toilet and clothes washer use (which zHome utilizes rainwater for).
According to over two years of utility water data (from November 2012 to May 2015), average per person water consumption across the nine units we have water data for was 16.07 gallons per day from the utility. Estimated rainwater consumption is 11.46 gallons per person per day,7 leading to a total per capita per day consumption of 27.53 gallons — well below the average consumption of 47.9 gallons per day in Issaquah.