Denver Builder Embraces ‘Challenge Home’ Standard
A production builder uses standard materials and simple techniques to build high-efficiency homes, some of which are net-zero
A production builder working in Denver's sprawling Stapleton development is buying into the U.S. Department of Energy's Challenge Home standard in a big way.
New Town Builders, which has sold more than 500 homes in Stapleton since it opened in 2001, has pledged to build each of the 250 to 300 homes it will construct there in the next decade to the high-performance, net-zero-ready standard.
The 7 1/2-square-mile development on what was once the Stapleton International Airport eventually will include 4,600 homes, according to the U.S.Department of Energy. Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. is a minimum requirement for houses built there. But last year New Town adopted the more rigorous Challenge Home standard, and this year was declared the winner of the department's Housing Innovation Awards in the production building category. It was one of a number of Challenge Home awards announced at the Solar Decathlon in October in Irvine, Calif.
Challenge Home houses must meet a number of efficiency and quality requirements verified by third-party inspections. Houses built to this standard are 40% to 50% more efficient than a typical new house and have a Home Energy Rating System (HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5.) index in the low to mid 50s, the Energy Department says.
Simple techniques, standard materials
New Town worked with several consultants on the design of the home, using both the HERS software and the Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. Planning Package. Like other high-performance homes, those built by New Town have tight building envelopes and more insulation than conventional houses. But neither building materials nor building techniques are especially exotic.
"I think the remarkable thing about our envelope is that it is simple," New Town operations vice president Bill Rectanus said in a news release from the DOE. "It is built with standard construction materials that are used onsite every day. We are not having to retrain our subcontractors to use some new, high-tech product."
Double-stud exterior walls, consisting of two 2x4 walls separated by a 2 1/2-in. space, are filled with blown-in fiberglass insulation rated at R-36. Studs, 24 in. on center, are staggered to minimize thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. , and walls are built using advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope. techniques to reduce thermal bridging and save wood.
"This wall works well for our climate zone," Rectanus said. "And every framer in town knows how to frame a 2x4 wall, so it helped us reach a competitive price point."
Vented roofs are built with raised-heel trusses with 14 in. of height at the wall plate and insulated with blown-in fiberglass to R-50. Concrete foundation walls, a standard 8 in. thick, are insulated on the interior to R-19. Airtightness on the three-bedroom, 3,560-sq. ft. house cited by the Energy Department was measured at 2.11 air changes per hour at at pressure difference of 50 pascals.
Windows are double-glazed, low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. units with a U-value of 0.28 and a solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. of 0.22.
Net-zero energy design
New Town offers houses built to two different specs at Stapleton. The Solaris II comes with a 2.75-kW photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) array and HERS ratings from the high 30s into the low 40s, the Energy Department said. A zero-energy option on this series (the type of house that won the Building Challenge award) includes a 9.5-kW PV system that brings the HERS rating to zero.
New Town's Z.E.N. (“zero energy now”) series homes have HERS ratings of 40 without a PV system and can become zero-energy with the addition of a PV array with a capacity of between 7 kW and 8 kW.
Other features include:
- Whole-house ventilation provided by a continuously operating fan in a first-floor hallway.
- A tankless water heater for domestic hot water with a PEXCross-linked polyethylene. Specialized type of polyethylene plastic that is strengthened by chemical bonds formed in addition to the usual bonds in the polymerization process. PEX is used primarily as tubing for hot- and cold-water distribution and radiant-floor heating. plumbing distribution system and on-demand hot-water recirculation.
- A forced-air heating and cooling system that includes a 97.4% AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. furnace and a 16 SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. air conditioning unit.
- Low-flow shower heads and drip irrigation.
Z.E.N. homes are available with 3 to 5 bedrooms and 2 1/2 or 3 1/2 bathrooms; the homes sell from the low to mid $400,000s. "We are absolutely competitive and priced right there with the other builders in Stapleton," New Town Builders marketing director Susan Elovitz says.
- New Town Builders