Zero Energy Homes, Made Affordable

How’s this for a challenge? Create a zero net source energy (as opposed to site energy) home as defined by the Department of Energy’s Building America program. Design it to operate in the extremes of Denver’s unpredictable climate, using off-the-shelf, readily available technologies. Keep the mechanical systems as simple and uncomplicated as possible. Incorporate energy-efficiency strategies that don’t require the home owners to be experts in sustainable building operations or conduct any maintenance beyond that required of a ‘normal’ home.

Not overly difficult, you might say? Add that the design must be replicable for future Habitat for Humanity homes, utilize low-cost construction materials, and allow volunteer- friendly construction techniques. Still with me? The clincher: The target market is the affordable housing sector. Impossible, you might say? Challenging yes, but not impossible for an integrated design team of NREL engineers and Habitat for Humanity staff and volunteers. The finished product is a 1,200 square foot, three-bedroom reduced income home that actually produces more source energy than it consumes!

Design Considerations

The combination of energy engineers, a construction manager a real estate development manager and Habitat volunteers on the design helped strike a balance between engineering ideals based on energy modeling, cost realities inherent in affordable housing, and considerations involved in a volunteer construction crew.

Habitat’s volunteer labor advantage steered the design team to approaches that favored low materials costs and high labor costs. While this approach minimizes the substantial labor cost, when combined with the affordable housing requirement it restricts the range of sustainable strategies available. For example, strategies such as structured insulated panels (SIPs) and insulated concrete forms were not considered because of their high cost. Similarly, the requirement for volunteer-friendly construction techniques and ease of replication eliminated the option of using Straw Bale. The design simplicity requirement eliminated the possibility of a combined solar space heating and water heating system. Finally, the zero energy requirement, given Denver’s cold climate and the current high cost of PV systems, required some trade offs that some ‘purists’ might consider controversial.

Design Approach – Envelope

Given the considerations described above, the design team decided to focus first on reducing the home’s energy load as much as possible, and then size the PV system to meet the remaining electricity needs. The first place to look? Yes, you guessed it – a passive solar orientation with a ‘super insulated’ envelope. Starting with a standard Habitat three-bedroom, 26 x 46 square foot design with a crawlspace, the team increased the South-facing glazing area and reduced the North, East and West facing glazing area. Next, a double-stud wall with fiberglass batt construction was selected to take advantage of it’s relatively low cost, volunteer-friendly technique and Habitat’s low construction labor cost. Blown fiberglass installed in the attic achieved an R-60 rating and insulated floors achieved an R-30 rating. While the double stud wall design, with exterior structural studs spaced at 16 inches O.C. might not achieve LEED Homes Advanced Framing Techniques points, the interior studs spaced at 24 inches O.C. certainly meet the requirement. The R-3 fiberglass batts in the exterior wall cavities and the R-13 filling the space between the exterior and interior walls as well as the interior wall cavities definitely help optimize energy performance. An outer vapor-permeable house wrap and fiber cement siding, with and an inner poly vapor barrier plus drywall adds to a very ‘tight’ whole-wall-R value. Blower tests yielded a natural infiltration rate result of 0.15 ACH, a very ‘tight’ indication.

Heating and Ventilation

With the house’s heating energy needs drastically reduced through this super-insulated shell, the design team then focused on the heating and ventilation system. Note that I didn’t mention heating, cooling and ventilation system. Yet another design challenge! Habitat for Humanity Metro Denver has a policy of not equipping its homes with air conditioning. This meant that the final design had to maximize heat reduction (for example by maximizing solar gain) without increasing the cooling energy load.

To supply a proper amount of fresh air to the house while minimizing potential for energy loss, the team opted for an energy recovery ventilation (ERV) system with efficient electronically commutated motors. The system exhausts air from the kitchen and bathroom, and supplies fresh air to the living room and bedrooms. Heat loss from ventilation is reduced because the ERV system heats the incoming air with warmth from the exhaust air.

The design team soon discovered that a very low heating load is a double edged sword. On the one hand very little energy is required to heat the house. On the other hand, most commonly available heating systems are oversized for such low heating needs, and overly complicated or expensive systems cannot be cost-justified. After carefully considering a variety of high-efficiency heating systems, and much internal debate, the team decided to follow a hybrid approach of electricity from the PV system, and natural gas.

Controversial Approach

Some of us who are ‘purists’ may turn up our noses at the thought of a zero energy home using natural gas. However, the economics involved convinced the design team that a hybrid approach was the best solution (see side bar)

The PV system selected by the design team uses the local utility grid for storage, thus eliminating the substantial cost of the storage battery. When the system is producing more energy than is being used, it delivers energy to the grid. When the system produces less energy than it produces, it draws electricity from the grid.

When the system draws electricity from the grid, it is likely drawing fossil-fuel generated electricity. Although a larger sized PV system may minimize the volume of electricity drawn from the grid, the cost of larger systems is prohibitive. The design team opted to include natural gas in order to reduce the size of the PV system by 1.1 kW, making it much for affordable for a Habitat Home. The team designed the system to offset the natural gas used, thus achieving, and even surpassing, the goal of net zero source energy.

The hybrid approach allowed the team to size the PV system that is affordable, offsets the use of natural gas as well as any grid generated electricity, and thus allows the home to achieve (and even surpass) the goal of net zero source energy. The hybrid space heating system combines a pointsource direct-vent natural gas furnace in the dining room and living area, with small baseboard electric-resistance heaters in the bedrooms.

Water Heating

The design team selected a solar water heating system – rather than a combined space/water hearing system – for simplicity, backed up by a natural gas tankless water heater. The team calculated that the 96 square foot collector area and 200 gallon water storage would result in an annual solar-savings fraction of 88%. They opted for the tankless natural gas back up heater after finding that the tankless system uses zero heating energy whenever the solar water tank is at or above 115 degree water delivery temperature.

The Crowning Element

Having reduced all possible energy loads as much as possible, the design team zeroed in on the lighting, appliances and miscellaneous electric loads (MELs). They installed compact fluorescent light bulbs throughout the house, and ENERGY STAR label appliances. This left the miscellaneous electric loads, from TV, hair dryer, toasters, computers, and anything else that could be plugged in by the occupants. Using Built America benchmark assumptions on MELs, the team settled on a 4kW PV system. Because the Built America assumptions on based upon a national average of a ‘typical’
American household, the actual occupant use and local climate may either block the home from achieving zero energy usage, or propel it to the ranks of ‘net energy producer’.

The Verdict

Initial test results were encouraging. From the February to July of 2006, the PV system produced 1,600 kWh more electricity than the house consumed. Factoring in the natural gas used for space heating and water heating backup, the house produced 75% more source energy than it consumed. Although a longer testing period is required, it’s a safe bet to say that the house will be an annual net energy producer rather than just achieve net zero energy user. However, this could change if the occupants begin using more than the average calculated into the Built America benchmark.

And the home owners? While it is true that the house is a net energy producer, they unfortunately are not free from utility bills. There is the monthly charge for the natural gas, as well as fixed charges for the electric grid and natural gas connection fees. From October of 2005 to May of the 2006, the owners shelled out an average of $18.25 per month in energy bills. Because the fixed monthly charges averaged 80% of those bills, in actuality the family used on average $14.60 worth of energy.

For those of us who suffered through $200+ monthly energy bills during that same period, those results are very compelling.


Energy and Atmosphere

Passive Solar Design

o The house was designed with increased glazing area on the long South facing side, and reduced glazing area on the North, East and West facing sides.

Renewable Energy

o 4kW Photovoltaic system using utility power grid storage to eliminate need for and associated high cost of, storage battery


o Raised heel trusses in attic allow 2 ft of blown fiberglass insulation, achieving R-60 rating for thermal envelope top

o Floors insulated to R-30

o R-3 fiberglass batts in outer 2 x 4 structural stud wall cavities, and a second, interior 2×4 stud wall with R-13 fiberglass batts placed horizontally between stud walls and vertically in interior wall cavities

o Outer vapor-permeable house wrap and fiber cement siding

o Inner poly vapor barrier and drywall

Space Heating

o Hybrid natural gas/electric heating system, combining a pointsource direct-vent natural gas furnace in the living room and dining area, and small baseboard electric-resistance heaters in the bedrooms. This combination provides the added bonus of zone heating, as each appliance has its own independent thermostat.

Water Heating

o Solar water heating system with 96 square feet collector area and 200 gallon water storage tank as primary water heating system, with natural gas tankless water heater as a back up system


o Double – glazed, low -e glass installed in South facing windows, with U-factor of 0.3 and SHGC of 0.58. For the East, West and North facing windows, Double – glazed, low – e glass was also used, with a U-factor of 0.22 and SHGC value of 0.27. The U-factors of all windows exceed (ENERGY STAR requirements by 20%).

o ENERGY STARĀ® appliances were installed.


o Compact fluorescent light bulbs deployed throughout the house

Indoor Environmental Quality

Local Exhaust

o Energy Recovery Ventilation (ERV) system with efficient electronically commutated motors, exhausts air from the kitchen and bathroom, and supplies fresh air to the living room and bedrooms.

Outdoor Air Ventilation

o The ERV system heats incoming fresh air with warmth of the exhaust air, thus significantly reducing heat loss from ventilation.
Materials and Resources

o Advanced Framing Techniques: Walls consist of inner 2×4 stud wall, 24 inches O.C.