Portland's Floating Solar HomeWhen my wife Kathryn and I decided to build our next home, we had a number of design objectives. Our primary goal was to build a home that would come as close as practical to net-zero energy performance. Net-zero is the concept that a home produces as much energy as it consumes. The vast majority of existing homes more than 98 percent produce no energy or minimal energy for space heating and cooling, water heating and various electricity demands. Creating a net-zero energy home is a major challenge, to say the least, and is particularly difficult for traditional architectural firms and the construction trades.
Our second primary objective was to achieve net-zero carbon emissions or a carbon-neutral building, in which any carbon emissions produced would be offset by energy sources that generate energy without carbon emissions by an equivalent amount, such as using solar energy to produce electricity. This design objective is somewhat easier to achieve than net-zero energy performance.
In addition to the two primary objectives, our secondary objectives included the maximum use of green, sustainable or renewable construction materials, and an on-site location for a food garden. We used lumber certified by the Forest Stewardship Council (FSC) and a considerable amount of reclaimed wood for structural requirements and wood-trim finishing. For the finished flooring, we used sustainable cork and bamboo.
In constructing our new home, we had one atypical condition: Our building site was a houseboat slip on a moorage on the Columbia River in Portland, Ore., so the home had to be designed as a floating solar home. Except for the homes foundation in this case, a floating wood deck built on steel I-beams anchored to logs upon which the home is built everything above the deck is mostly typical of a house built on land. A key difference was that construction materials were partially selected for their weight. The lighter, the better.
Produce and Conserve
To even approach net-zero energy performance, the house had to be designed to do two things well: produce and conserve energy. Typically, homes require energy for space heating and cooling, water heating, and lighting and appliances. To meet these basic demands, we designed the home to integrate both an active solar system and passive solar approaches. (Our temperate microclimate annually gets around 255 days of at least some sunshine.)
When it came to conserving energy, we faced a completely different challenge, as we had to employ a number of advanced construction techniques and conservation measures to reduce energy use in the home as much as possible. Among other features, the home incorporates advanced wall-framing systems, super-sized insulation, innovative air-quality ventilation systems and highly energy-efficient appliances and lighting.
In addition, we have had to modify some lifestyle habits. For instance, we use discretionary appliances, such as washers and dryers, during the sunny parts of the day whenever possible, since thats when the solar energy system is generating electricity.
To achieve carbon-neutral emissions, we designed the home to use no fossil fuels. We dont use oil or natural gas to cook, heat water or heat spaces. Instead, we use solar energy generated on-site and purchase energy generated by wind turbines for our electrical requirements. A low-emission, sealed-combustion fireplace provides backup space heating in the winter.
To achieve these energy performance objectives within a construction budget of $160 per square foot, we had to integrate the systems wherever possible and obtain multiple uses from single energy production sub-systems of the house. For example, we use the solar hot water (thermal) system for both domestic hot water heating and space heating, and we use electricity generated by the solar photovoltaic (PV) system for cooking, running appliances, backup hot water heating and space heating.
The first step in designing our solar home was to determine how much and where we would get the best solar access, and the best solar orientation for the house. As it turned out, the southeast corner of the slip and the roof were the only good solar access areas because of the close proximity of adjacent buildings (other floating homes) that shaded much of our building site in the winter months.
We also wanted to determine if there were areas on the float where we could grow food to meet our basic needs. Based on solar access, we decided to build both a roof garden (for growing food in the summer) and a two-story greenhouse (for year-around food growing) at the southeast corner of the house. The greenhouse walls and roof are structured with reclaimed timbers and covered with twin-wall polycarbonate (tough plastic) to collect direct solar gain.
An attached greenhouse is a key component of passive solar approaches and is considered isolated solar gain, so in addition to providing a facility for growing food, it doubles as a facility to collect free solar energy for space heating throughout the winter months. Because the greenhouse is isolated from the main house by exterior doors, we simply open the upper level door of the greenhouse on sunny winter days to let the captured solar-produced heat into the main part of the house. The upper-level air return circulates the solar heat throughout the house.
In the winter, the passive solar heating system provides about 65 percent of our total space-heating requirements, mostly through the greenhouse, but also through the south-facing windows in the main house area. On a typical winter sunny day, the greenhouse, with 220 square feet of south-facing window area, will typically add about 6 to 10 degrees to the space-heating needs of the house. In addition to space heating, the greenhouse provides for much of the daylighting for the south side of the house in the winter.
In the summer, to avoid excess solar heat gain, we use a 90-percent shade cloth that completely covers the greenhouse roof and a third of the south walls. We vent any excess heat through an upper-level window to the roof garden, while fresh air enters from a low window on the east side.
Active Solar Systems
Our next step was to determine opportunities for active solar systems. Because we have great solar access on the two-story house roof (the solar access was above the adjacent buildings), we employ two active solar systems: a solar hot water heating system and a solar photovoltaic (PV) system to generate electricity.
The solar hot water panel is mounted on rails on the roof and transfers the heat from the glycol in a 40-square-foot solar panel through a heat exchanger to the water in an 80-gallon preheat tank located in the lower-level utility room. The solar hot water system is about 85 percent efficient, providing preheated water to the electric hot water tank, which reduces the demand for electric hot water heating.
In addition to providing for domestic hot water needs (showering, dishwashing and so forth), the system is also integrated with the space-heating furnace system. Our furnace system is a forced-air hydronic system that uses air passed over hot water coils, with hot water sourced from the solar preheat tank and through the electric hot water heater to provide backup space heating throughout the house.
The solar PV system is also mounted on the roof deck and consists of 10 220-watt panels. They provide about 45 percent of our annual electricity consumption, or around 2,200 kilowatt-hours (kwh) annually. The system is grid-tied, so electricity is automatically exported to or imported from the power grid depending on actual production and consumption patterns.
On an annual basis, about 60 percent of our PV electricity production is exported back to the power grid, with most of the exported electricity produced in the summer months. Power is typically produced and exported during the day, and imported and consumed in the evening. Our local utility has a net metering program, which enables us to offset our imported kwh with our exported kwh at the same price point.
We initially explored a micro-hydro system approach to generate electricity, which would have used a submersible generator to produce electricity from the river current. However, the river current in our location was determined to be too slow (less than two miles per hour on average) for cost-effective power generation.
Our efforts to conserve energy focused primarily on the building envelope (the exterior shell). In constructing the home, we used an advanced, super-insulated wall-framing system that minimizes air infiltration, thanks to its tight construction. The advanced wood-framed wall system consists of 2x4 studs staggered 24-inch on-center on both sides of a 2x8 wall plate to minimize thermal bridging through the studs, while providing an 8-inch wall cavity for insulation to minimize heat loss. The super-sized insulation consists of cellulose (recycled cardboard), which provides R-28 in the walls and R-56 in the ceiling.
To get a tight building envelope with minimal air infiltration, we caulked everywhere, but specifically where the drywall touches the floor and ceiling and between the windows and window framing. In addition, we had blower-door tests conducted by local building-energy-conservation organizations, as well as Energy Star. This helped us zero-in on air leaks in the envelope, such as around exhaust ducts and plumbing penetrations.
After fixing a number of small leaks, we had a natural air change every six to eight hours. Because the house is so tight, we installed a heat recovery ventilation (HRV) system to control and enable the exchange of stale interior air with fresh exterior air to maintain high indoor air quality.
In addition, we use a dual programmable thermostat system. We set the lower-level thermostat on auto-heat, while the upper level thermostat is set on auto-cool. When the temperature in the upper level reaches 72 degrees (in winter) or 77 degrees (in summer), the fan-only operation is triggered to circulate air throughout the house and through the HRV system.
Other conservation measures include the extensive use of CFL (compact fluorescent lamp) lighting fixtures and Energy Star-rated appliances.
Overall, with our combination of passive and active solar system components, we produce about 70 percent of our total energy requirements. The balance of our energy needs about 2,800 kwh annually is imported from the power grid and specifically designated as green-sourced, which means the equivalent energy was produced from renewable wind generation or biomass generation sources.
We have achieved net-zero carbon emissions, but have not achieved net-zero energy performance. Based on our first year of operation, our energy use index (the kilo British thermal units used per year per square foot of heated living area) is 7. While the national average is 46, we need to get to zero. The basic challenge for us to achieve net-zero energy performance is to explore additional sources of energy production opportunities, to include the use of expanded or more efficient PV panels and/or micro-wind electricity generation, and to evaluate different micro-hydro options because of our unique floating-home situation.
Steve Gray is a construction management consultant and a principal in urbansun design, a Portland, Ore.-based firm that offers home design and consulting services for passive solar and sustainable home construction. Kathryn Gray is the principal of urbansun design and is a frequent speaker on passive solar and green design.
Funding Our Green Construction
To help pay for a number of the green features we incorporated into our home, we received a grant from the Office of Sustainable Development, City of Portland (Oregon). The Office of Sustainable Development implemented a Green Investment Fund (GIF) in 2000 to promote exemplary sustainable building practices in residential and commercial buildings. In researching prior GIF awards for sustainability practices, we determined that many of our design and construction objectives were consistent with GIF program objectives.
I developed a grant proposal that identified 10 specific green practices where we could use the grant award to offset some of the extra costs of using sustainable materials and products, such as sourcing and preparing reclaimed timbers for constructing the two-story greenhouse. Our whole-house design approach employing extensive sustainable building materials and practices, coupled with the integrated solar energy systems and our interest in sharing our results and green approaches with the public were key factors in helping us receive the award.
For more information on Portlands Office of Sustainable Development and the Green Investment Fund, visit www.portlandonline.com/osd or call
Creating the Floating Solar Home
The Grays floating solar home, located in a houseboat slip on the Columbia River in Portland, Ore., includes the following features and systems:
Passive solar design with attached two-story greenhouse
Solar electricity generated by a 2.2-kilowatt solar photovoltaic system
40-square-foot solar water-heating system for domestic hot water and space heating
Extensive use of reclaimed wood for the greenhouse structure, stairs and interior trim, as well as sustainably harvested lumber, certified by the Forest Stewardship Council (FSC), for the structure, siding, windows and doors
Super-sized insulation using blown-in cellulose (recycled cardboard) insulation
Extensive use of renewable flooring throughout, including cork for the lower-level floor and bamboo for the upper-level floor
FSC-certified Paperstone countertops
Energy Star appliances, including the refrigerator, dishwasher and clothes washer
Energy Star lighting fixtures with compact fluorescent lamps (CFLs) used in more than 70 percent of the homes lighting fixtures
Kitchen cabinets made with sustainably harvested forest products and engineered wood
Low- or no-VOC (volatile organic compounds) paints and stains used on interior walls and exterior siding
Solar hot-water-assisted hydronic forced-air furnace
Integrated heat-recovery ventilation system
EPA-rated low-emission, sealed-combustion wood-burning fireplace for backup heating
All heating ducts are located in conditioned spaces
Integrated stormwater management system with a rainwater harvesting capability, used for on-site irrigation
Energy performance monitoring system
Neighborhood served by public transit and within walking distance of commercial services.
For more information: Steve and Kathryn Gray at email@example.com or firstname.lastname@example.org, www.sunsmarthomes.com or call