Allison Fleetwood

The value of the achievement of net-zero energy (NZE) greatly depends on climate. Building an NZE home in Los Angeles, for example, is a profoundly different achievement than building one in Toronto. For the California house, the biggest impact comes from the house’s electrical load, while the Canadian house would have a heating load that is greater than the sum of all the other loads. Without any load mitigation, such as superinsulated walls, the NZE house in Toronto would have to produce more than twice as much energy as its counterpart in Los Angeles to meet the baseline requirements of its residents.

My wife and I were inspired when we saw an NZE house in Boulder, Colo., a few years ago, so we decided to build one for ourselves in Boulder, which has a climate much closer to that of Toronto than Los Angeles. It is a great challenge to build an NZE house in the Rocky Mountains, particularly one that is architecturally significant and relatively affordable.

Allison Fleetwood

I established four goals for the design: the house would use no fossil fuels, it would demonstrate that an NZE house can be architecturally distinctive, it would be of modest size, and sustainability of transportation to and from the house would be considered.

No Fossil Fuels

Many different formulas have been used to achieve NZE in Colorado. Some houses burn wood for heating to avoid fossil-fuel usage. Others rely on natural gas for heating and overproduce electricity renewably to offset the carbon dioxide generated by the heating system. Our NZE house uses no gas and will meet the NZE goal without a need for offsets.

Because solar resources in Boulder are so plentiful, we incorporated both a solar photovoltaic system and a solar thermal system in the design of the house. Boulder has more than 300 days of sunshine each year, and the state of Colorado offers generous rebates for solar photovoltaic installations. Through rebates, the state paid for more than half the cost of our 7.2-kilowatt PV array. To save the cost of a battery system and to qualify for the substantial rebates, the PV array was net-metered and tied to the grid. This allows the home to feed from the municipal electrical grid when it is not producing enough energy, as at night or on cloudy days. A net-meter runs both backward and forward; at the end of the year if the house puts more into the grid than it takes out, we will be paid for that energy by the utility.

The living room in the NZE house.
Allison Fleetwood The living room in the NZE house.

PVs are the easy, plug-and-play part of the NZE equation. The technology is very straightforward and all you need is good solar access and a place to install the panels. The hard part in Boulder’s climate is heating and cooling. Our home’s efficient and compact layout is oriented for maximum passive solar gain in the winter. Overhangs shade the windows in the summer and there is minimal glazing on all sides except the south-facing elevation. Most major living spaces face south and have generous glazing and views, and most of the smaller utility, bathroom, and closet spaces that don’t need large windows face north.

The envelope of the house consists of 9¼ inches (235 mm) of R-33 foam insulation in a double-stud wall with staggered 2- by 4-inch (51- by 102-mm) studs made from wood certified by the Forest Stewardship Council. The studs are spaced at 24 inches (610 mm), with a 2¼-inch (57-mm) gap. The windows are R-13 double-heat mirror glass in insulated fiber-glass frames.

As important as a high R-value is airtightness. There are two aspects to achieving this. The first is to seal the house as well as possible. The spray-foam insulation also serves as an excellent air barrier. A blower-door test performed after insulating afforded the opportunity to find and seal any remaining leaks before drywall installation.

Allison Fleetwood

The second step in achieving airtightness is to avoid any negative or positive pressures in the house. These pressures can force outside air into the house through any remaining cracks and expel the energy contained in warmed or cooled indoor air. The solution to this is to have the ventilation of the house run through the heat recovery ventilator (HRV). The NZE house draws exhaust air from the bathrooms and kitchen. Standard bathroom and kitchen fans exhaust the warmed and humid air from these rooms directly outside and create negative pressures. In this case, the exhaust air is ducted to the HRV, which intakes as much air as is expelled, thus avoiding pressurizing the house while recovering most of the energy.

These strategies reduce the heating and cooling loads of the house and thereby reduce the size of the solar thermal system to an affordable point. The house has an array of 140 evacuated tubes that absorb heat from the sun. A 512-gallon (1938-L) water storage tank stores the thermal energy generated by the tubes. The tank is a kind of thermal “battery” whose stored thermal energy is used for space heating and hot water. The fan-coil unit/HRV transfers the heat from the tank to the air stream, which is ducted throughout the house. This combination unit allows incoming ventilation air from the HRV to also be ducted throughout the house. When there is an extended cold and cloudy snap and the water storage tank has cooled down, an efficient electric boiler and electric coils in the hot water tank serve as backups for heating.

Private residence, Michael Kracaeur, Architect, Boulder, Colorado
Allison Fleetwood Private residence, Michael Kracaeur, Architect, Boulder, Colorado

For cooling, which is minimized by the well-insulated envelope and shaded windows, we installed an innovative closed-loop earth tube—an extra loop in the air distribution ductwork that, when cooling is required, diverts the airflow through two tubes that are buried underground around the house at the basement level. The constant underground temperature of about 50 to 55 F (10 to 13 C) cools the air running through the tubes and provides all the cooling needed. In addition, a roof-mounted, solar-powered attic intake vent supplements the ridge and soffit vents and boosts attic air ventilation.

View from the entry to the NZE house.
Allison Fleetwood View from the entry to the NZE house.

Other Goals

Many early attempts at NZE have been marred by a lack of integration of the high-performance components of the house with the architecture. All the features that make this house NZE have been integrated into a complex, modern design. This is achieved without hiding the technology—the solar panels are proudly displayed and integrated into the most visible elevation. In addition, the passive solar south elevation overhangs are carefully designed into the composition of the house. Even though the envelope of the house functions like a thermos, there is a generous amount of glass to bring in light and views.

Some high-performance houses are so huge as to almost erase their benefits because of the embodied energy of the vast amount of materials used to build them. Our NZE house is modestly sized at about 3,000 square feet (279 m2), which includes about 350 square feet (33 m2) for our architectural office. The smaller size not only reduces the embodied energy of the building, but also the energy loads.

Our transportation goal was addressed in three ways. First, we placed the house very close to public transportation and a short drive to the city center. Second, we included a home office to minimize car trips. And third, we produced enough excess renewable energy to power at least one plug-in hybrid electric vehicle.

The architectural office.
Allison Fleetwood The architectural office.


Not surprisingly, a lot of refining is required during the first few months of operating a technologically complex object like an NZE house. We did a fair amount of tweaking for the complex drainback solar thermal system that heats the water in the home. The evacuated tubes are extremely efficient and on the sunniest days the water running through the tubes overheated and produced steam that overstressed the system’s pumps. We replaced four water pumps in a few months, and ultimately we redesigned the system so that the single pipe that provides water running through all the collectors is now split into two, with one pipe run through three panels and the other through the remaining two panels. This prevents overheating and creates less stress on the pumps, with the added benefit of increased energy gain from the evacuated tubes. There have been no problems since, but until the repairs were made, hundreds of kilowatt-hours of energy were lost when the system was down.

Also, the HRV was not properly balanced, which went unnoticed for almost two months before we corrected it. A great deal more exhaust air was expelled than was replaced, which made the HRV very inefficient at recapturing energy and induced a negative pressure in the house. Both of these effects led to significant energy loss.

The PV system’s panels are known for their high efficiency. On two cold, very sunny days—when they reached their maximum efficiency—they blew a fuse. We installed a larger disconnect, which allowed a larger fuse, and that eliminated the problem.

Even though this is a year of tweaking—the initial inefficiencies created a greater draw from the grid and required much more energy than would otherwise be required—we still expect the house to achieve NZE this year. Achieving NZE in a demanding climate such as Boulder’s is significant and, we believe, shows that a sustainable future isn't as far away as we might think.

Allison Fleetwood

Green Team
Architect / architropic, Boulder, Colo.,
Builders / architropic and Morningstar Homes, Boulder,
Structural engineer / Odisea, Boulder,
Mechanical engineer / PCD Engineering Services, Boulder,
Photovoltaic and solar thermal installer / Lighthouse Solar, Boulder,
Spray-foam insulation installer / Colorado Energy Savers, Boulder,
Cabinets / Richard Fakelmann Woodworking, Boulder,
Roofing / Boulder Roofing, Boulder,

Materials and Sources
Photovoltaics / Thirty-six 7.2-kilowatt HIP-200BA3 panels from Sanyo Electric Co., Frisco, Texas,
Solar thermal / ETube Solar Collectors from Lumos, Boulder,
Air handler/heat-recovery ventilator / Nu-Air Enerboss Hydronic Air Handler from Lorax Energy Conservation, Loveland, Colo.,
Backup electric boiler / Thermolec, Montreal, Quebec,
Backup water heater / Marathon Water Heaters from Rheem, Eagan, Minn.,
Solar attic roof vent / Master Flow Solar-Powered Intake Booster Vent from GAF Materials Corp., Wayne, N.J.,
Windows / TC-88 Double Heat Mirror Glass from Alpen Energy Group (now Serious Windows), Boulder,
Insulated fiber-glass frames / Duxton Windows and Doors, Winnipeg, Manitoba, Canada,
Spray-foam insulation / Icynene Insulation System from Icynene, Mississauga, Ontario, Canada, Cabinets / Plyboo from Smith & Fong Co., San Francisco,
Woven bamboo flooring / Ecotimber, Richmond, Calif.,
Cork flooring / Natural Cork, Dalton, Ga.,
Marmoleum flooring / Forbo Flooring Systems, Hazleton, Pa.,
Recycled closet shelving and laundry-room cabinets and counter / Resource Yard, Boulder,

Michael Kracauer is an architect who lives and works in Boulder, Colo. His firm, architropic, specializes in high-performance residential and commercial design.