National energy savings potential in HUD-code housing from thermal envelope and HVAC equipment improvements.
Manufactured homes are built and installed to the U.S. Department of Housing and Urban Development's (HUD) Manufactured Home Construction and Safety Standards (MHCSS). The standards address structural, fire safety, and energy-efficiency issues and require adequate ventilation. The MHCSS (HUD 1994) supersedes local and state building codes and is the current minimum standard that all HUD-code homes are required to meet. The National Fire Protection Association (NFPA) periodically updates NFPA 501, Standard on Manufactured Housing (NFPA 2005). NFPA 501 is the standard currently approved by industry and other stakeholders but has yet to be adopted by HUD. The NFPA does not have authority over the MHCSS but rather provides recommendations to HUD. Research conducted in 2004 by U.S. Department of Energy (DOE) authors has contributed to NFPA 501's improved stringency of thermal efficiency [U.sub.o] (overall building thermal transmittance, Btu/h*[ft.sup.2]*[degrees]F) factors (Conner et al. 2004). NFPA 501 has incorporated improvements over the current HUD-code based on the experiences of energy-efficient manufactured home programs such as ENERGY STAR[R] and the DOE Building America Industrialized Housing Project, which can significantly improve the energy and indoor air quality performance of manufactured homes. These NFPA 501 improvements include: (1) ductwork air leakage testing guidelines; (2) an increase in crossover duct insulation from R-4 to R-8; (3) requirements for mastic systems to seal ductwork; (4) quality assurance protocols and materials that systematically address air leakage of the building envelope and ductwork; (5) de-pressurizing limits to reduce fireplace back drafting and potential problems from moisture condensation; (6) quiet, durable, and energy-efficient whole-house ventilation fans; (7) lower thermal transmittance heat loss; (8) window, roof color, and overhang/shading approaches that lower solar heat gains in hot climates; and (9) use of T-8 lighting when linear fluorescent light fixtures are used (NFPA 2005).
The International Energy Conservation Code (IECC 2006) and its predecessors are the predominant codes used for site-built housing in more than half of the states in the US. Although the IECC does not apply to manufactured housing, it is interesting to compare this code to the MHCSS because these two codes are by far the most important national residential energy efficiency codes. The IECC has a different structure and climate zones compared to the MHCSS, but these codes can readily be compared for any given home design.
The ENERGY STAR Manufactured Home Program is a voluntary program with guidelines that seek to substantially improve energy efficiency over minimum HUD-code by focusing on improved insulation and HVAC systems and requiring quality assurance performance testing protocols for factories and field installations. ENERGY STAR manufactured homes built in 2006-2007 may qualify for a $1000 federal energy tax credit (IRS 2006). There are four climate zone regions for ENERGY STAR manufactured homes, and the building options vary with fuel type, climate zone, use of set-back thermostats, domestic hot water energy factors, duct leakage rates, etc. For analysis simplification, and because some manufacturers do not offer ENERGY STAR with heat pumps or electric heat in certain climate zones, the ENERGY STAR requirements for natural gas heating with an 80% annual fuel utilization efficiency (AFUE) were selected to represent the ENERGY STAR program thermal efficiency package in all cases. This has the effect of underestimating per house and national "fuel and production weighted" energy savings associated with ENERGY STAR because the heat pump and electric heat packages have lower building envelope thermal transmittance ([U.sub.o]) values than the gas package.
The Best Practice case represents insulation levels, duct and envelope leakage rates typical of over a hundred thousand ENERGY STAR/Building America HUD-code homes built in the Pacific Northwest over the past 15 years. Best Practice uses the current ENERGY STAR guidelines as developed by the Environmental Protection Agency (EPA) and a stakeholder consortium of utilities, manufacturers, and state energy offices in the Pacific Northwest. The Best Practice package is fuel blind and is believed to represent the tightest duct and envelope leakage rates of HUD-code homes currently built. The Best Practice analysis assumes practices are adopted nationally and may be overkill in some milder climate zones.
The analysis approach evaluates a matrix of climates, efficiency levels, and HVAC system fuel types and efficiencies. There are five levels of envelope and HVAC distribution system thermal efficiency: (1) HUD-code (1994), (2) NFPA 501 (NFPA 2005), (3) IECC (2006), (4) ENERGY STAR (EPA 2004), and (5) Best Practice (BAIHP 2005; NEEM 2004). Three climates (Houston, TX; Raleigh, NC; and Chicago, IL) were selected to cover the three zones in the MHCSS and to represent hot, mixed, and cold climates, respectively. Six HVAC equipment packages were evaluated for electric and gas furnaces, heat pumps, and air-conditioning that include minimum National Appliance Energy Conservation Act (NAECA) and ENERGY STAR efficiency levels. This analysis matrix includes a total of 90 cases. The analysis assumes that the MHCSS-required whole-house ventilation systems are operated continuously by the occupants. This assumption represents significant energy use, which may not represent the real world, and results in significant periods where the homes (especially the HUD 1994 homes) are overventilated. Previous research suggests that significant energy savings potential exists in HUD-code manufactured homes from improved ventilation controls that reduce periods of overventilation (Lubliner et al. 2005; Persily 2000; Stevens et al. 1997). Future sensitivity analysis is needed to evaluate energy impacts related to occupant ventilation and control issues over a range of climate types and duct and envelope leakage rates.
The analysis was conducted using a DOE-2 (LBNL 1981) hourly simulation residential energy analysis software program called EnergyGauge[R] USA, version 2.5 (FSEC 2006). The EnergyGauge analysis assumptions are provided in Table 1. Duct insulation values are all R-8 except for HUD (R-4) and ENERGY STAR (R-6).
Table 1. EnergyGauge USA Analysis--Thermal Input Assumptions City and [U.sub.o] Analyzed Floor/Ceiling/Wall Thermal (Btu/h*[ft.sub.2]*[degrees]F) R-Value Efficiency Level Houston HUD (1994) 0.116 11/30/11 NFPA 0.098 11/28/11 (2005) IECC 0.097 13/30/13 (2006) ESTAR 0.087 11/30/11 (2004) Best 0.056 33/38/21 Practice Raleigh HUD (1994) 0.095 11/30/11 NFPA 0.089 14/28/11 (2005) IECC 0.067 19/38/13 (2006) ESTAR 0.084 11/33/13 (2004) Best 0.056 33/38/21 Practice Chicago HUD (1994) 0.078 22/30/11 NFPA 0.073 22/33/13 (2005) IECC 0.062 25/38/19 (2006) ESTAR 0.059 33/36/19 (2004) Best 0.056 33/38/21 Practice City and Fenestration Glazing Air Duct Leakage Thermal U-Factor SHGC Exchange Rate (25 Efficiency (1) Rate (ach PA/[ft.sub.2]) Level (2) at 50 PA) Houston HUD (1994) 1.10 0.70 9.0 Qn = 12% NFPA 0.52 0.60 7.0 Qn = 7% (2005) IECC 0.75 0.40 7.0 Qn = 9% (2006) ESTAR 0.38 0.40 7.0 Qn = 5% (2004) Best 0.34 0.40 4.0 Qn = 3% Practice Raleigh HUD (1994) 0.52 0.60 9.0 Qn = 12% NFPA 0.52 0.60 7.0 Qn = 7% (2005) IECC 0.40 0.40 7.0 Qn = 9% (2006) ESTAR 0.38 0.40 7.0 Qn = 5% (2004) Best 0.34 0.40 4.0 Qn = 3% Practice Chicago HUD (1994) 0.52 0.60 9.0 Qn = 12% NFPA 0.52 0.60 7.0 Qn = 7% (2005) IECC 0.35 0.55 7.0 Qn = 9% (2006) ESTAR 0.38 0.40 7.0 Qn = 5% (2004) Best 0.34 0.40 4.0 Qn = 3% Practice (1) Conversations and e-mail correspondence with R. Garcia, Fleetwood Housing Division, Riverside, CA, 2006. (2) ach = air changes per hour.
A typical 56 ft double-section three-bedroom manufactured home prototype with 12% glass-to-floor area was used in this study. Previous HUD-code related research efforts have used this same prototype, which is generally accepted as representative of the majority of HUD-code homes (Conner et al. 1992; Conner et al. 2004). In 2005, double-section homes represented roughly 80% of the market share.(1) The vented roof has typical dark asphalt shingles and is built using flat 2 x 2 ft roof trusses 24 in. on center. Insulation is assumed to be blown and tapered at baffled eave vents. The 2 x 6 framed, 24 in.-on-center floor is located over a vented crawlspace with blanket/batt floor insulation located in the "belly" and compressed at the I-beams. The walls are assumed to be 16 in. on center and 2 x 4 for the R-13 and R-11 batt insulation cases and 2 x 6 for the R-19 and R-21 batt insulation cases. The doors and windows are industry representative and available models, with the exception of the IECC case, which assumes the prescriptive U-factor requirements of 0.75, 0.4, and 0.35 Btu/h*[ft.sup.2]*[degrees]F for the three cities examined. Electric domestic water heating with 50-gallon tanks located in the conditioned space with an energy factor of 0.90 are assumed for all cases. Table 1 provides the prototype assumptions used in the analysis.
Overall Thermal Transmission
The analysis approach defined insulation R-values and associated [U.sub.o] overall heat loss transmission (MHCSS) for the HUD, NFPA 501, and ENERGY STAR cases. For the IECC and Best Practice cases, [U.sub.o] is determined based on the prescriptive R-values. The U-factors used were taken from previous HUD-code research and [U.sub.o] calculated as follows:
[U.sub.o] = ([U.sub.celling]x[A.sub.celling] + [U.sub.wall]x[A.sub.wall] + [U.sub.floor]x[A.sub.floor])/([A.sub.celling] + [A.sub.wall] + [A.sub.floor])
U = thermal transmittance of the envelope component, Btu/h*[ft.sup.2]*[degrees]F
A = area of the envelope component, [ft.sup.2]
In the development of these cases, it is assumed that the manufacturer first improves windows from single pane aluminum to double pane vinyl; then additional insulation is added to ceiling, floors, and walls; then upgrades to windows are again made. All assumptions used for each of these cases are provided in Table 1, including the [U.sub.o] analyzed based on the insulation and windows.
Six HVAC system packages evaluate fuel type and minimum efficiency and ENERGY STAR-level efficiency heat pumps, air-conditioning systems, and gas furnaces, as well as electric furnaces. Table 2 provides a description of the fuel type and assumed seasonal energy efficiency ratio (SEER), heating seasonal performance factor (HSPF), and/or AFUE equipment efficiency levels. The HVAC system options provide a way to evaluate the energy usage and saving impacts from interactions between equipment efficiency and home thermal efficiency cases. The thermostat setting was assumed to be 68[degrees]F for heating and 78[degrees]F for cooling. It should be noted that the Best Practice and ENERGY STAR cases typically require a set-back thermostat, which means that the energy use for these cases may be higher than presented in this analysis.
Table 2. Heating and Cooling System Assumptions Heating Cooling Efficiency Efficiency (SEER) Standard Improved Standard Improved Level Level Level Level Electric Furnace 1.00 1.00 13 14 Gas Furnace 78% AFUE 90% AFUE 13 14 Heat Pump 7.7 HSPF 8.5 HSPF 13 14
Thermal Distribution System
A portion of supply duct system outside the conditioned space is in the crawlspace for all climate zones. A minority of homes in Climate Zone 1 have supply ductwork located in the attic. These homes are believed to have lower thermal distribution system efficiencies and, if analyzed, would result in greater energy savings because the energy efficiency of the buildings is improved compared to what is presented in this paper. This ductwork is assumed to be flexible duct with R-values of R-4, R-6 and R-8, as shown in Table 1, and a surface area of 64 [ft.sup.2]. The return ductwork and air handler location is within the home. The duct leakage rates (cfm per [ft.sup.2] of floor area leakage to outside at a test pressure of 25 Pa) are assumed to be 3%, 5%, 7%, 9%, and 12% and are believed to be representative of typical practices associated with program guidelines and standards (BAIHP 2005; NEEM 2004; EPA 2004; Davis 2003; Lubliner et al. 2003). Duct leakage tests and EnergyGauge analysis were conducted in accordance with procedures provided in ANSI/ASHRAE Standard 152-2004, Method of Test for Determining the Design and Seasonal Efficiencies of Residential Thermal Distribution Systems (ASHRAE 2004).
Infiltration and Ventilation
Air infiltration rates used in the analysis are: four air changes per hour (ach) at 50 Pa pressure, 7 ach at 50 Pa, and 9 ach at 50 Pa and are again believed to be representative of typical practices based on blower door testing from published research and guidelines (Lubliner et al. 2003; Persily et al. 2003; Stevens et al. 1997; Palmiter et al. 1992). HUD MHCSS requires that whole-house ventilation systems be installed. For this analysis, the ventilation system is assumed to be a continuously operated 55 cfm whole-house exhaust fan system to comply with the MHCSS requirements of 0.035 cfm/[ft.sup.2] of floor area (HUD 1994). All whole-house fans are assumed to be 50 W, except for the Best Practice case, where the fan energy is 25 W. The assumption of continuous whole-house ventilation system operation has a significant impact on energy use and savings from this analysis. It should be noted that occupants, not engineers, generally decide to how much to operate the whole-house mechanical ventilation system.
HUD Climate Zones 1, 2, and 3 are evaluated using representative cities selected to approximate the heating degree days (HDD) of the three HUD-code zones, as determined by average weighted placements of new manufactured homes in 2004:
Houston: 1599 HDD vs. Zone 1 average of 1678 HDD
Raleigh: 3457 HDD vs. Zone 2 average of 3267 HDD
Chicago: 6176 HDD vs. Zone 3 average of 5974 HDD
Each of the three cities are close to the zone averages and, therefore, are appropriate representatives for the HUD climate zones.
The national residential average electricity price of 9.80 cents/kWh for July/August 2005 and 9.25 cents/kWh for December 2005 (DOE 2006a) was assumed for cooling and heating costs, respectively. Because natural gas prices have varied greatly over the past few heating seasons, the DOE projection of average future residential prices over the next five years of $11 per million Btu (DOE 2006b) was assumed. Fuel prices will vary by location and future prices cannot be known with any accuracy. Therefore, these national average prices are only intended to represent typical estimated prices.
The heating, cooling, and HVAC system fan energy annual energy costs per home are provided in Figures 1 through 4 for Houston, Raleigh, Chicago, and the national average, respectively. These figures are for the low heating and cooling efficiency levels. The figures contain the results by climate zone for the five energy efficiency levels and three heating system types. The bars show heating and cooling energy use, with fan energy broken out, while the clusters represent the three heating system types. Aggregation to national averages is based on manufactured housing placements by state using 2004 data. HUD Zones 1, 2, and 3 have 31%, 35%, and 34% of the national total of 128,840 placements, respectively (MHI 2004).
[FIGURE 1 OMITTED]
We were not able to obtain detailed data on heating system types by climate, but it is likely that electric resistance and heat pumps are common in southern locations while natural gas (or propane) is more common in colder locations. As expected, the highest energy cost, almost $1500 per year (mostly heating), shown in Figure 3, is the current HUD-code home in Chicago (HUD Climate Zone 3) with electric resistance heating. The lowest energy cost of about $350 per year (slightly more heating than cooling), shown in Figure 2, is the Best Practice home in Raleigh (HUD Climate Zone 2) with a heat pump. With the fuel prices and system efficiencies assumed in this analysis, heat pumps have a lower energy cost than natural gas furnaces.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Table 3 reproduces the national average results shown in Figure 4. This table provides the energy cost savings over the worst case, the current HUD-code. The savings are shown both with and without the improved heating and cooling equipment efficiencies. The first three columns of results are at the standard, or low, efficiency level from Table 2. The final column accounts for both the improved codes/programs and the improvement in HVAC efficiency.
[FIGURE 4 OMITTED]
Table 3. National Average Energy Costs by Heating System Type Heating, Cooling, Total, Savings $ $ $ Over HUD, $ Without With Improved Improved HVAC HVAC Electric HUD 814 229 1043 -- 25 resistance NFPA 702 213 915 128 152 IECC 636 187 823 220 241 ENERGY 638 179 817 226 246 STAR Best 474 150 624 419 435 Practice Natural HUD 466 234 700 -- 75 gas NFPA 402 221 623 77 143 IECC 368 198 566 134 194 ENERGY 366 191 557 143 203 STAR Best 270 165 435 265 312 Practice Heat pump HUD 428 228 656 -- 61 NFPA 366 212 578 78 132 IECC 337 187 524 132 181 ENERGY 335 178 513 143 190 STAR Best 252 149 401 255 293 Practice
Table 4 shows the same results but using source energy, not energy cost. Source energy takes into account the impact of power plant and distribution system efficiency by multiplying the energy used by 3.2 for electric and by 1.02 for gas (DOE 2006c; DOE 1995).
Table 4. National Average Source Energy Use by Heating System Type Heating, Cooling, Total, Savings MBtu MBtu MBtu Over HUD, MBtu Without With Improved Improved HVAC HVAC Electric HUD 96 25 121 -- 2 resistance NFPA 83 24 107 14 17 IECC 75 21 96 25 27 ENERGY 75 20 95 26 28 STAR Best 56 17 73 48 50 Practice Natural HUD 44 26 70 -- 8 gas NFPA 38 25 63 7 14 IECC 35 22 57 13 19 ENERGY 34 21 55 15 20 STAR Best 25 18 43 27 32 Practice Heat pump HUD 51 25 76 -- 7 NFPA 43 24 67 9 15 IECC 40 21 61 15 21 ENERGY 40 20 60 16 22 STAR Best 30 17 47 29 34 Practice
Figures 5 through 7 show the savings from improving heating and cooling system efficiencies, as described in Table 2. Electric resistance furnaces are 100% efficient and therefore cannot be improved (other than by the use of a heat pump).
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Comparisons of energy-efficient and minimum HUD-code homes suggest significant improvements in energy efficiency and HVAC performance are achievable. Adoption of any of these improvement scenarios would result in hundreds of millions of dollars of ongoing utility savings to new HUD-code homebuyers, reduce national residential energy consumption, and reduce power plant greenhouse gas emissions, while improving occupant comfort and control of the indoor environment.
ENERGY STAR manufactured homes with high-efficiency equipment save from $190 to $246 a year in average energy costs over the minimum HUD-code, or 24% to 29% of total heating and cooling costs. This improvement in energy efficiency adds up to $25 million to $32 million of energy savings for each year of new construction (assuming the 2004 number of new home placements) or $750 million to $960 million over 30 years (undiscounted). There would also be $128 million of income tax credits available per year.
The HUD-code lags well behind its counterpart code for site-built housing, the IECC. Even if the HUD-code is updated to the specifications in NFPA Standard 501, it will still fall short of the IECC, particularly in colder climates--HUD Zones 2 and 3. Even the ENERGY STAR levels for manufactured homes barely exceed the IECC code requirements.
The savings presented do not consider the fact that many HUD-code homes are built to more efficient thermal standards than minimum code assumptions used in this analysis. The current HUD-code is sufficiently lenient so that a market evaluation of HUD-code minimum versus actual practice is required to quantify these savings.
The impact of the continuously operated 55 cfm whole-house exhaust fan system on the annual energy cost was assessed. The presence of mechanical ventilation doesn't have a clear effect on savings from improving the code. The ventilation typically increases the total heating and cooling cost by 10% to 15% depending on the efficiency level and city. Again, this is assuming the ventilation is operated 24 hours a day. Further evaluation and research related to occupant ventilation as well as other occupant behavioral issues such as thermostat setpoint is suggested.
Large potential national savings suggest the need for HUD and DOE to conduct further cost-benefit analyses that evaluate life-cycle costs, increased mortgage "purchase power," increased resale value, federal energy tax credits, and evaluation of environmental benefits.
Funding for this work was provided by the Department of Energy's (DOE) Building Technology Division. The authors would also like to acknowledge the contributions from the Manufactured Housing Research Alliance, DOE Building America Industrialized Housing Program, EPA ENERGY STAR Home Program, Ecotope Inc., and Lee Link of the Washington State University Extension Energy Program.
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(1.) Conversations and email correspondence with R. Garcia, Fleetwood Housing Division, Riverside, CA, 2006.
Robert Garcia, PE
Robert Lucas is a senior research engineer with Pacific Northwest National Laboratory in Richland, Washington. Philip Fairey is deputy director of the Florida Solar Energy Center in Cocoa, Florida. Robert Garcia is a senior engineer with Fleetwood Enterprises Housing Group in Riverside, California. Michael Lubliner is a building science specialist at the Washington State University Extension Energy Program in Olympia, Washington.
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|Title Annotation:||Housing and Urban Development; heating, ventilation, and air conditioning|
|Author:||Lucas, Robert; Fairey, Philip; Garcia, Robert; Lubliner, Michael|
|Article Type:||Technical report|
|Date:||Jul 1, 2007|
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