Zero Energy Homes, made affordable


How is it a challenge? Create a source of zero net source energy (as opposed to site energy) as defined by the Ministry of Energy's Building America program. Design it to work in the extreme conditions of Denver's unpredictable climate, using technologies available on the market. Keep mechanical systems as simple and simple as possible. Incorporate energy efficiency strategies that do not require homeowners to master sustainable construction operations or perform maintenance work other than those required for a "normal" home.

Not too difficult, will we say? Add that the design needs to be replicable for future Habitat for Humanity homes, use low-cost building materials and enable user-friendly building techniques for volunteers. Still with me? The tire: The target market is the affordable housing sector. Impossible, could you say? Hard yes, but not impossible for an integrated design team of NREL engineers and staff and volunteers from Habitat for Humanity. The finished product is a 1,200 square foot, three – bedroom, low – income home that actually produces more energy than it consumes!

Design Considerations

The Combination of Energy Engineers, a Construction Manager The Real Estate Development Manager and Habitat Volunteers helped to strike a balance between the ideals of Based on energy modeling, the costs of affordable housing and considerations related to a volunteer construction team.

This favored low material costs and high labor costs. While this approach minimizes the substantial cost of the workforce, when combined with the requirement for affordable housing, it limits the range of sustainable strategies available. For example, strategies such as structured insulated panels (SIPs) and insulated concrete forms have not been taken into account because of their high cost. Similarly, the requirement of user-friendly construction techniques for volunteers and the ease of reproduction have eliminated the option of using straw bales. The requirement of simplicity of design has eliminated the possibility of a combined system of solar heating and water heating. Finally, the requirement of zero energy, given the cold climate of Denver and the current high cost of PV systems, required compromises that some "purists" might consider controversial.

Design Approach – Envelope

Given Given the above considerations, the design team decided to focus first on reducing the load As much energy as possible from the house, 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 isolated" envelope. Beginning with a standard three-bedroom and 26 x 46-square-foot home with crawlspace, the team has increased the south-facing glazing surface and narrowed the north-facing glazed area, l & # 39; east and west. Then, we chose a double post wall with a fiberglass structure to take advantage of its relatively low cost, its user-friendly technique for volunteers and the low cost of construction labor. ;Habitat. The blown glass fiber installed in the attic was rated R-60 and the insulated floors were rated R-30. While the double stud wall design, with the outer structural studs spread to 16 inches OC It may be that you do not reach the LEED Homes advanced framing techniques points, the inner studs being spaced 24 inches OC certainly meet the requirement. The R-3 fiberglass mats in the cavities of the outer walls and the R-13 filling space between the outer and inner walls as well as the interior wall cavities definitely help to optimize energy performance. A fiberglass exterior cladding and an air permeable fibrocement facing, with an internal poly-vapor barrier and drywall, add to a very narrow R-value of the entire wall. The blower tests gave a natural infiltration rate of 0.15 ACH, a very "tight" indication.

Heating and Ventilation

With the home heating energy requirements greatly reduced The design team then focused on the heating and ventilation system. Note that I did not mention the 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 provide a good amount of fresh air at home while minimizing energy losses for an energy recovery ventilation (ERV) system with efficient electronic switching motors. The system vents air from the kitchen and bathroom and supplies the living room and bedrooms with fresh air. The heat loss due to ventilation is reduced because the ERV system heats the incoming air with heat from the exhaust air.

The design team quickly discovered that a very low heating load is a double-edged sword. On the one hand, very little energy is needed to heat the house. On the other hand, the most commonly available heating systems are oversized for such low heating requirements, and overly complicated or expensive systems can not be justified in terms of costs. After carefully examining a variety of high efficiency heating systems and a lot of internal debate, the team decided to follow a hybrid approach to PV system electricity and natural gas.

Controversial Approach

Some of us who are "purists" can turn a blind eye to the idea of ​​a zero energy house using gas. natural. However, the economic aspect convinced the design team that a hybrid approach was the best solution (see sidebar)

The PV system selected by the design team uses LAN for storage, eliminating the substantial cost of the battery. When the system produces more energy than is used, it provides power 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 that it will produce electricity produced from fossil fuels. Although a larger PV system can minimize the volume of electricity drawn from the grid, the cost of larger systems is prohibitive. The design team chose to include natural gas to reduce the size of the 1.1 kW photovoltaic system, making it an affordable price for a Habitat home. The team designed the system to offset the natural gas used, thus achieving, and even surpassing, the goal of zero net source energy.

The hybrid approach allowed the team to size the PV system which is affordable, offsets the use of gas as well as all the electricity generated by the grid, and thus allows home to achieve (and even exceed) the goal of zero net source energy. The Hybrid Space Heating System combines a direct vent natural gas furnace in the dining room and living room, with small electrical resistance radiators in the rooms.

Water Heating

chose a solar water heating system – rather than a combined system of hearing the # Space / water – for simplicity, supported by a natural gas tankless water heater. The team calculated that the 96 square foot collection area and the 200 gallon water storage would result in an annual solar savings fraction of 88%. They opted for the tankless natural gas heater after finding that the tankless system uses no heating energy each time the solar water heater reaches or exceeds the 115 degree temperature.

The Crowning Element

After reducing as much as possible all possible energy charges, the design team focused on the lighting , appliances and various electrical charges (MEL). They installed compact fluorescent light bulbs throughout the home and ENERGY STAR labeling devices. This left the various electrical loads, television, hair dryer, toasters, computers and everything that could be plugged in by the occupants. Using Built America's benchmark assumptions on MELs, the team installed on a 4 kW PV system. Because the assumptions of America built on the basis of a national average of a "typical"

US households, the actual use of the occupants and the local climate can either prevent the home from reaching zero energy consumption, or propel it to the ranks of 'net producer of electricity'. energy".

The Verdict [19659002LesrésultatsdespremierstestsétaientencourageantsDefévrieràjuillet2006lesystèmePVaproduit1600kWhd'électricitédeplusquelamaisonconsomméeComptetenudugaznaturelutilisépourlechauffagedeslocauxetlechauffaged'appointlamaisonproduisait75%plusd'énergiedesourcequ'ellen'enconsommaitBienqu'unepérioded'essaipluslonguesoitnécessaireilestprudentdedirequelamaisonseraunproducteurd'énergienetannuelplutôtquedesimplementatteindreunconsommateurd'Howeveritislikelythattheoccupyingoccupantswouldusemorethantheaveragecalculatedinthebuilt-inAmerica

and the owners? While it is true that the house is a net energy producer, they are unfortunately not exempt from utility bills. There is the monthly fee for natural gas, as well as the fixed costs for connection to the grid and natural gas. From October 2005 to May 2006, homeowners disbursed an average of $ 18.25 per month in energy bills. Since the fixed monthly fee averaged 80% of these bills, the family used an average of $ 14.60 in energy.

For those of us who have suffered monthly electricity bills of $ 200 and up, these results are very convincing.


Energy and Atmosphere

Passive Solar Design

o The home was designed with increased glazed area on the long south side and reduced glazing area on the North, East and West Versions.

Renewable energy

o 4 kW Photovoltaic system using an electricity storage system to eliminate the need and high cost of storage battery


o Raised trellises in the attic 2 ft blown fiberglass insulation, rated R-60 for top of thermal envelope

o Insulated floors at R-30

o Fiberglass mat R- 3 in the outer wall cavities 2 x 4, and a second, interior 2×4 s wall in tud with fiberglass mattresses R-13 placed horizontally between the walls of the posts and vertically in the cavities of the interior walls

o Envelope Exterior vapor permeable and fiber cement siding

o Polyester vapor barrier and drywall

] o Hybrid natural gas and electric heating system, combining a direct vent natural gas furnace in the living room and kitchen. dining room, and small electric resistance heaters in the bedrooms. This combination offers the added bonus of zone heating, with each unit having its own independent thermostat.

Water Heating System

o Solar water heater system with a 96 square foot collection area and a 200 gallon storage tank with a natural gas-free water heater as a system


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

Home Appliances

o ENERGY STAR® Appliances Installed


o Compact Fluorescent Light Bulbs Spread throughout the House

Indoor Environmental Quality

Local Exhaust

o Ventilation at Energy recovery (ERV) with efficient electronic switching motors, evacuates the air from the kitchen and bathroom and provides fresh air to the living room and bedrooms [19659002] Outdoor Ventilation

o The ERV system warms the air fresh air, thus greatly reducing the heat loss due to ventilation.

Materials and Resources

o Advanced framing techniques: The walls consist of a 2×4, 24-inch O.C interior wall


Source by Daniele Loffreda

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