As an architect who is passionate about green building, I have to prioritize between many competing goals to achieve the most sustainable project I can at the most reasonable cost. Since there are so many aspects to green building — energy, water, green materials, etc.— how does one prioritize to make the house the most sustainable it can be? According to building science guru Joseph W. Lstiburek, the answer is simple: It’s the energy, stupid!

Life-cycle assessments have shown that the operational energy a building will use during its lifetime is vastly greater than its embodied energy. Therefore, reducing the amount of energy a building consumes and using renewable energy (instead of burning fossil fuels) for a substantial portion of its energy load is the most sustainable feature for a building. And the most sustainable type of building would be net-zero energy (NZE), which produces, renewably, as much energy as it consumes.

NZE was almost unthinkable just a few years ago. Yet in a very short amount of time we have come to a point whereby an NZE house can be smart, sophisticated and extremely comfortable. Nevertheless, achieving NZE can be relatively easy or extremely difficult, and this all depends on one thing: It’s the climate, stupid!

By architecturally integrating the active and passive solar features of the house in the south elevation, the NZE House proudly expresses what the architect/author likes to call the house: a net-zero energy solar machine for living. Photo by Allison M. Fleetwood Jr.

NZE in Los Angeles is a profoundly different achievement compared to one in, say, Edmonton, Alberta, the northernmost Canadian city. In the mild California climate, almost the entire energy load would be made up of the electrical demand and the water-heating demand. On the other hand, the Edmonton house would have a heating load that is about double the entire energy load of the Los Angeles house on top of this. Without any load mitigation, such as superinsulated walls and limited glass, it would have to produce more than three times the energy as the Los Angeles house and even at a time of year when the sun barely peaks above the horizon!

The Los Angeles house would have few additional restrictions on the building envelope and therefore the amount of glass and transparency. The interior space could flow seamlessly to the outdoors. It wouldn’t need much more than a photovoltaic (PV) array, ENERGY STAR appliances and CFLs to achieve NZE.

But we need to build NZE in all climates. Boulder, Colo., although not as severe a climate as Edmonton, still has a heating load that is more than half that of the Edmonton house. Without any load mitigation, it would need to produce, renewably, more than double the amount of energy of the Los Angeles house without load mitigation.

Putting Ideas to the Test

Clearstory windows provide overhead daylighting in the living room’s “oasis”- a place to escape from direct passive solar rays in the winter months. Photo by Allison M. Fleetwood Jr.

How does one build an NZE house in the Boulder climate, particularly one that is architecturally significant and modern and relatively affordable? We began this house with two basic strategies in mind:

1- Establish one or more renewable energy sources.

Solar is the natural choice for renewable energy in Boulder since it receives more than 300 days with sunshine per year. Colorado also offers generous rebates for solar PV installations, which paid for more than half the cost of our 7.2-kW array. In addition, the utility allows net-metering thereby avoiding a costly battery system. Since no rebates are even offered for off-grid PV, being grid-tied is far more economical.

Photovoltaics are the easy “plug and play” part of the NZE equation. We next had to decide the best source for our space heating and hot-water energy loads.

Additional PV as a heating source might have been a choice, but we were limited by the size of the south-facing roof that could be designed for the site, and a huge array of at least 12 kW would have been required. Additionally, rebates were limited to 10 kW.

The view when entering the house. Even though the envelope of the house functions like a thermos, there is a generous amount of light and views. Photo by Allison M. Fleetwood Jr.

Nevertheless, our lot has excellent solar access, and its orientation allowed us to design the house to accommodate both PV and solar thermal. Solar thermal captures much more solar energy per unit of area than PVs so we could capture more energy in less space. We also found a very cost-competitive Chinese-made evacuated tube panel system.

2. Utilize a careful, sophisticated plan and exterior-envelope design that minimizes the energy loads and, therefore, minimizes the cost of the renewable energy systems.

Boulder’s space heating loads are substantial — about 40 percent of the entire energy needs of the house. Together with the domestic hot-water load, this becomes about 60 percent. Since solar resources here are so plentiful, we also incorporated a solar thermal system in the design of the house.

A very efficient and compact house layout is oriented for maximum passive solar gain in the winter with overhangs that shade the south-facing windows in the summer and minimize glazing in all the other elevations. Most major living spaces face south and have generous glazing and views, and the smaller utility, bathroom and closet spaces that don’t need large windows generally face north. The envelope of the house is 9 ¼-inch Icynene foam insulation (R-33) in a double-stud wall with staggered two-by-four studs at 24 inches on center with a 2 ¼-inch gap. Our windows are double heat mirror glass (R-13) in insulated fiberglass frames.

Air-tightness is as important as a high R-value, and there are two aspects to achieving this. The first is to seal the house as well as possible. The Icynene spray foam insulation is an excellent air barrier. The fiberglass window frames are very rigid and maintain seals longer than other materials. And the most cost-effective thing we did was a blower door test after insulating to find and seal any remaining leaks before drywall installation.

The second aspect is to avoid any negative or positive pressures in the house, which could force outside air into the house through any remaining cracks. The key to this is to have the ventilation of the house, which draws from the kitchen and bathrooms, run through the heat recovery ventilator (HRV). Standard bathroom and kitchen fans exhaust the warmed and humid air from these rooms directly outside and create negative pressures. By ducting the exhaust air to the HRV, which intakes as much air as is expelled, we avoid pressurizing the house while also recovering most of the energy in the exhaust air.

The NZE House has a sensuous, modern interior within a very high performance envelope. Photo by Allison M. Fleetwood Jr.

These strategies allowed us to reduce the heating and cooling loads of the house and, therefore, the size of the solar thermal system to an affordable price point. The house has an array of 140 evacuated tubes and a 512-gallon water storage tank to store the thermal energy generated by the tubes. This energy is used for both space heating and hot water. The Enerboss fan-coil unit/HRV transfers the heat from the tank to the airstream, which is ducted throughout the house. This combination unit allows incoming ventilation air from the HRV to also be ducted throughout the house. For extended cold and cloudy periods, when the storage tank has cooled down, we chose an electric boiler for heating and electric coils in the domestic hot water tank as our backups.

The high-performance building envelope and well-shaded windows also reduced the cooling load to a point where we felt a low-energy solution would suffice — a closed-loop earthtube. This is an extra loop in the air-distribution ductwork. When the thermostat calls for cooling, a damper and fan divert the air through two PVC tubes that are buried underground and run around the house at the basement level. The constant underground temperature (about 50-55 degrees F at that depth) cools the air and has provided all the cooling we needed this past summer. It is important to note, however, that in dry Colorado, which has a large diurnal temperature swing and, therefore, cool nighttime temperatures, a key strategy is to open windows at night to flush the house with cool air and close the windows in the morning to trap the cool air inside.

Other Goals

No fossil fuels at all. The NZE House uses no natural gas and will meet the NZE goal without using offsets. We chose not to heat the house with wood because, even though wood is a biofuel and not a fossil fuel, we still felt it would be best to minimize producing any additional CO2 if possible. All cooking appliances are electric, including an energy-efficient magnetic-induction cooktop.

Show that an NZE house could be architecturally distinctive. Although we were much more constrained by the climate in Colorado than would architects in warm climates, one of the goals was to demonstrate that one can integrate architecturally all the high-performance features into a richly complex, modern design that doesn’t hide the technology. The solar panels are proudly displayed and integrated into the most visible elevation. The passive solar south elevation overhangs are carefully designed into the modern composition of the house. And even though the envelope of the house functions like a thermos, there is a generous amount of glass to bring in light and views.

The powerful angled slabs of Boulder’s Flatirons in the Rocky Mountain foothills provided inspiration for the stairwell’s abstract design. Photos by Allison M. Fleetwood Jr.

You can’t really be NZE unless you are also “Not So Big.” Let’s get real. McMansions have so much embodied energy that this becomes a significant impediment to NZE. The NZE house is modestly sized at about 3,000 square feet, which includes about 350 square feet for the architectural office. A smaller size not only reduces the embodied energy of the building but also the energy loads, making NZE much easier to achieve.

Make transportation to and from the house more sustainable. Buildings consume about a third of the energy that this country uses. Transportation consumes about another third. If we could transform these two parts of the energy pie to a much more sustainable level, we would be well on our way to a fully sustainable world. Because we also address transportation, the NZE house approaches a microcosm of the future we hope to see.

We addressed transportation in three ways: We placed the house very close to public transportation and a short drive to the city center. We minimized my car trips by including a home office for my architectural practice. Last, we expect to produce enough excess renewable energy to power at least one plug-in hybrid electric vehicle. Eventually, we hope to add additional sources of renewable energy to power two all-electric vehicles.

On Track for NZE

During its first few months, the NZE House required a significant amount of tweaking and refinement of its complex systems. The initial inefficiencies made the house consume considerably more energy than would otherwise be required. Nevertheless, the house is still on track to be NZE in its first year, and we expect next year to have enough excess energy to power the plug-in hybrid we plan to purchase.

NZE in such a demanding climate as Boulder’s is a significant achievement. NZE in colder climates would be an even greater challenge. By addressing climate in such an intimate way, I feel that I am part of a movement that is bringing architecture not just back to Frank Lloyd Wright’s organic architecture, which was responsive to locale and climate, but all the way back to architecture’s origins which, like a native plant species, evolved as a response to its environment. In other words, it’s the climate, stupid!

Sidebar: Green Team

Architect: architropic, Boulder, Colo., www.architropic.us

Builders: architropic, Boulder, Colo., www.architropic.us and Morningstar Homes, Boulder, Colo., www.morningstarbuilt.com

Structural Engineer: Odisea, Boulder, Colo., www.odiseanet.com

Energy Modeling/HERS Rater: Sustainably Built, Boulder, Colo., www.sustainablybuilt.com

Mechanical Engineer: PCD Engineering Services, Boulder, Colo., www.pcdengineering.com

Photovoltaic and Solar Thermal Installation: Lighthouse Solar, Boulder, Colo., www.lighthousesolar.com

Icynene Installer: Colorado Energy Savers, Boulder, Colo., www.coloenergy.com

Cabinets: Richard Fakelmann Woodworking Inc., Boulder, Colo., www.fakelmannwoodworking.com

Roofing: Boulder Roofing, Boulder, Colo., www.boulderroof.com

Sidebar: NZE House Material and Sources

Photovoltaics: 36 7.2-kW HIP-200BA3 panels from Sanyo Electric Co. Ltd., Frisco, Texas, us.sanyo.com/Solar

Solar thermal: ETube Solar Collectors from Lumos, Boulder, Colo., www.lumossolar.com

Air handler/heat-recovery ventilation: Nu-Air Enerboss Hydronic Air Handler from Lorax Energy Conservation LLC, Loveland, Colo., www.loraxec.com

Backup electric boiler: Thermolec, Montreal, Quebec, Canada, www.thermolec.com

Backup water heater: Marathon Water Heaters by Rheem, Eagan, Minn., www.marathonheaters.com

Solar attic roof vent: Master Flow Solar-Powered Intake Booster Vent from GAF Materials Corp., Wayne, N.J., www.gaf.com

Windows: TC-88 Double Heat Mirror Glass from Alpen Energy Group (now Serious Windows), Boulder, Colo., www.seriouswindows.com

Insulated fiberglass frames: Duxton Windows and Doors, Winnipeg, Manitoba, Canada, www.duxtonwindows.com

Spray-foam insulation: Icynene Insulation System from Icynene Inc., Mississauga, Ontario, Canada, www.icynene.com

Cabinets: Plyboo by Smith & Fong Co., San Francisco, www.plyboo.com

Woven bamboo flooring: Ecotimber, Richmond, Calif., www.ecotimber.com

Cork flooring: Natural Cork, Dalton, Ga., www.naturalcork.com

Marmoleum flooring: Forbo Flooring Systems, Hazleton, Pa., www.marmoleum.com

Recycled closet shelving and laundry-room cabinets and counter: Resource Yard, Boulder, Colo., www.resourceyard.org

Built-in benches with wood from local recycled trees: T.C. Woods, Lafayette, Colo., www.tcwoods.com

Cement-board siding and soffits: James Hardie, Mission Viejo, Calif., www.jameshardie.com