Net Zero Energy Manufactured Homes May Be on Their Way
The manufactured-housing industry may have a reputation for efficiency in terms of cost and time savings, but when it comes to energy efficiency . . . not so much. That could be changing with recent construction and testing of the nation’s first manufactured home to meet the high performance requirements of the DOE Zero Energy Ready Home (DOE ZERH) program. With support from DOE, Clayton Homes’ Southern Energy Homes division, the Tennessee Valley Authority, and the Systems Building Research Alliance, building scientists from the Levy Partnership and the National Renewable Energy Laboratory (NREL) set up a 15-month study to compare the real-world performance of the DOE Zero Energy Ready home with that of two other manufactured homes—one built to the criteria of Energy Star Certified for Manufactured Homes and the other built to Clayton Homes’ standard construction package.
All three homes were constructed by Clayton Homes’ Southern Energy Homes division and were set up on the lot of the company’s Russellville, Alabama, production facility. The 1,200 ft2 three-bedroom, two-bath, double-wide manufactured homes had identical floor plans and identical vinyl siding and dark-colored asphalt shingle roofs. Under the hood, however, these homes were three different animals.
Table 1. A Comparison of Energy Efficiency Features in Three Manufactured Homes
Table 2. Costs and Savings for the DOE Zero Energy Ready Manufactured Home
Heating and Ventilation Plan for House C
House A was built to the best-practice thermal specifications of the HUD Manufactured Home Construction and Safety Standards. This is the same HUD code to which 65,000 manufactured homes in the country are constructed each year. Unfortunately, the code hasn’t been upgraded since 1994. The HUD code specifies an overall insulation U-value based on the home’s location. Russellville is located in HUD Thermal Zone 1, where it coincides with the International Energy Conservation Code (IECC) Mixed-Humid Climate Zone 3A. To meet HUD code here, a home would need a minimum of R-12 insulation in the ceiling, R-9 in the walls, and R-9 in the floor. House A was actually built to Clayton’s standard construction insulation package, which is a step up from HUD code, with R-22 blown fiberglass in the ceiling, R-11 fiberglass batt plus R-1 sheathing in the walls, and R-14 fiberglass blanket under the floor. House A was equipped with an electric furnace and a split-system air conditioner. The energy efficiency features of all three houses are shown in Table 1.
House B was built to the Energy Star Certified Manufactured Home programs, which included an improved thermal envelope and a conventional split-system heat pump. House B was insulated with R-33 blown fiberglass in the ceiling, R-13 fiberglass batt plus R-1 sheathing in the walls, and R-28 fiberglass blanket under the floor.
House C was built to the strict performance criteria of DOE ZERH. As the first manufactured home ever built to the DOE ZERH criteria, House C earned Clayton Homes a DOE 2014 Housing Innovation Award. It meets all of the requirements that site-built homes must meet to qualify for this high-performance home-labeling program. The home is built to meet all of the air sealing and construction quality requirements of Energy Star Certified Homes Version 3.0. It also meets the indoor air quality (IAQ) requirements of EPA’s Indoor airPLUS program, and the hot-water distribution requirements of the EPA WaterSense programs. The DOE ZERH program also requires homes to meet the insulation requirements of the 2012 International Energy Conservation Code (IECC). The program is called Zero Energy Ready because with the addition of a few solar panels, a home built to the program criteria should reach true net zero (a home that produces as much electricity as it consumes in a year). In this case, solar panels were not installed, because of the home’s location.
House C was insulated with R-13 fiberglass batt plus R-5 rigid-foam extruded polystyrene sheathing in the walls, an R-28 fiberglass blanket under the floor, and blown fiberglass in the low vented attic; this last varied from a dense-packed R-44 at the eaves to a whopping R-55 at the center of the attic.
Another innovation in House C was the use of a ductless mini-split heat pump and transfer fans. The super-high-efficiency, mini-split heat pump has a Seasonal Energy Efficiency Ratio (SEER) of 22 and a heating season performance factor (HSPF) of 12—far above the minimum federal standard for cooling-and-heating equipment of 13 SEER and 7.7 HSPF, respectively. The ductless heat pump mounts on an interior wall and provides heating as well as cooling. To move the conditioned air into the bedrooms when bedroom doors are closed, holes were cut in the walls and small fans were installed to pull conditioned air into the bedrooms. Door undercuts and transfer grilles provide a path for air to move from the bedrooms back to the return side of the centrally located heat pump. The other two homes use ducted heating-and-cooling systems with the ducts located outside under the floor where they are exposed to the heat and cold of outdoor conditions. Because the ductless mini-split heat pump in House C is located entirely within the thermal envelope, the system does not sustain the heating and cooling losses that occur when air leaks through the registers, or when heat is lost through transfer from ducts located outside the conditioned space.
The researchers identified a number of ways in which mini-split heat pumps can cut costs and improve energy efficiency in manufactured homes.
The ducts for traditional systems must be assembled and installed in the field, but ductless heat pumps can be fully installed in the plant. The system’s small compressor is mounted on an exterior wall and connected to the inside unit at the factory. This eliminates the possibility of mismatched equipment and allows for refrigerant charging and commissioning at the plant. This in turn reduces the likelihood of callback complaints and greatly improves the odds of achieving a quality installation.
Mini-split heat pumps provide both heating and cooling in a single, wall-mounted device. This makes installation simpler and saves space—a plus for small floor plans.
No holes need to be cut through the floor for ducts. This saves construction time, reduces opportunities for air to leak out and pests to get in, and gives homeowners more options for room layouts by eliminating floor registers.
In large homes, the outside compressor can be connected to several indoor air handlers for zoned heating and cooling. Compressors located outside operate much more quietly than unitary air conditioners installed inside.
Ductless heat pumps are available in small capacities, and nearly all high-efficiency models incorporate inverter-driven compressors and variable-speed fan technology that can meet the low-load heating and cooling needs of small homes at much higher-rated efficiencies than traditional air conditioners and furnaces.
Window improvements were another source of energy savings. While House A had the industry standard for this climate zone—single-pane windows with metal frames— House B had double-pane vinyl-framed windows with low-e coatings to help reduce heat gains and losses, and House C had even higher-performing double-pane low-e windows with an argon gas filling the space between the panes to further reduce heat gain and loss. All of the appliances in House C were Energy Star rated, and the majority of the lighting was energy-efficient CFLs.
All three homes were built with wood-framed walls using 2 x 4 studs spaced 16 inches on center, and all three homes incorporated some advanced framing techniques, a common practice in this cost-conscious industry to reduce the amount of lumber used. These lumber-saving techniques, which also leave more room in the walls for insulation, included single studs around windows and doors, open headers over windows on nonbearing walls, and two-stud, rather than three-stud, corners.
Standard-practice air-sealing techniques were used in Houses A and B. These techniques included foaming ceiling penetrations, caulking under bottom plates and between the top plates and the ceiling, and installing a marriage line gasket. All of these techniques were also used in House C. Additional techniques used in House C included foaming around window and door rough openings, patching or foaming all floor penetrations, taping rigid foam sheathing joints, caulking foam sheathing to top and bottom plates, and spray foaming along the marriage line. While each of these steps may seem inconsequential, together they can significantly reduce heat loss by cutting air leakage. This is borne out by the blower door test results. House C had a tested air leakage rate of 3.85 ACH50, while House A had an air leakage rate of 4.7 ACH50, and House B had an air leakage rate of 4.6 ACH50.
To help ensure good IAQ in House C, Clayton Homes incorporated all of the air quality and moisture management requirements of Energy Star and EPA’s Indoor airPLUS program, including mechanical ventilation and installation of only low- or no-VOC paints, finishes, cabinets, and carpets.
The three homes were extensively monitored over the 15-month period from May 2014 to August 2015 by the Levy Partnership and NREL. Monitoring included air temperatures and relative humidity in each room, the attics, and the crawl spaces; status, power, and energy consumption of all equipment; moisture content of wall framing; and weather conditions. Short-term testing included blower door tests, duct leakage tests, ventilation flow measurements, transfer fan flow measurements, and coheat testing in combination with tracer gas tests to measure thermal envelope performance and natural infiltration rates. Long-term testing included gathering data in various configurations, such as with interior doors open and closed, with window blinds in various positions, and with transfer fans on and off. Sensible gains (heat from hypothetical occupants, appliances, and other miscellaneous energy uses in the home) were simulated by the use of electric heaters timed to operate according to a specified schedule.
House C outperformed House B and House A in every category, but testing revealed some surprises.
Energy consumption. During the hot months, House C used half as much cooling energy as the other houses. House B used slightly less energy than House A for cooling. House B and House C used about the same amount of heating energy, which was about one-third of the heating energy used by House A.
Comfort. All three homes operated within indoor comfort guidelines specified by the Air Conditioning Contractors Association. When interior doors in the home were open, room-to-room temperature variation was less than ±3°F from the temperature set at the thermostat during the cooling period. House C used one-third of the heating energy used by House A, with superior comfort results. Some bedrooms in House A and House B had difficulty maintaining temperatures within ±2°F of the set heating point, when the doors were shut.
Peak-load performance. Peak-demand energy use for HVAC was significantly lower in House C than it was in the other two houses throughout the year. House B had somewhat lower peaks than did House A during most months. House A and House B had similar peaks during the winter months, suggesting that House B’s peak occurred when the heat pump was not operating and the house was relying solely on electric- resistance backup. On average, during peak hours for the Tennessee Valley Authority system, House B had an 18% lower peak demand than House A, and House C had a 69% lower peak demand than House A.
Real-world performance of heat pumps. The heating coefficient of performance (COP) was calculated for both heat pumps using two independent methods: (1) airflow and temperature measurements, and (2) coheat testing measurements. The coheat testing method was deemed more reliable. Using this method, the COP of both the traditional split-system heat pump and the ductless mini-split averaged approximately 2.5. This was well below what would be expected for the mini-split based on manufacturer data. However, when the mini-split air-handling unit fan was forced to run on high speed, its COP increased to 4.11, indicating that low airflow could have been a cause of poor heat pump performance with this unit.
Propensity for moisture problems. Moisture meters installed in the walls showed that the wall cavity wood moisture content was slightly higher in House C than in House B, but it was still well within safe limits. This was because of the addition of exterior foam insulation to the walls in House C. (The expanded polystyrene foam insulation reduces the overall vapor permeability of the walls.) However, compared to House B, the use of foam sheathing on House C also resulted in an average higher temperature (5.5°F) on the interior face of the sheathing during the heating season, reducing the risk of condensation. Note that latent loads were not simulated.
Estimate of costs and benefits. The researchers estimated that it would cost the manufacturer $2,060 more (for the energy efficiency measures) to build House C than to build House A, and $1,166 more to build House C than to build House B. They estimated that it would cost the customer $6,600 more to buy House C than to buy House A, and $4,350 more to buy House C than to buy House B, assuming that the houses were in regular production. In 2014 the average price for a 1,200 ft2 manufactured home was about $57,500, including installation costs.
For more photos and information, visit DOE Zero Energy Ready Home Tour of Zero.
A full DOE research report describing this project is available on the DOE Building America website.
The estimated energy savings for House C compared to House A and House B ranged from 5.3 to 16.0 MMBtu per year when the houses are located in (IECC) Mixed-Humid Climate Zones 3 and 4. In terms of utility bill savings for these all-electric homes, owners of House A could expect to pay $1,656 per year in total energy costs at $0.1059/kWh, while owners of House B would pay $1,263, and owners of House C would pay $1,055 (see Table 2). If the increased first costs to the homeowner are added into the typical fifteen-year mortgage for a $57,500 manufactured home at 4.5% interest with 10% down, home buyers could expect to pay about $36 more per month for House B than for House A and $55 more per month for House C than for House A. However, utility bill savings are about $33 per month for House B compared to House A and about $50 per month for House C compared to House A. So for about the cost of a cup of coffee per month, home buyers can get a far superior product in terms of energy performance. And if energy costs should go up, buyers of the DOE Zero Energy Ready home will see even greater relative savings.
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