National energy savings potential in HUD-code housing from thermal envelope and HVAC equipment improvements

ASHRAE Transactions, July, 2007 by Robert Lucas, Philip Fairey, Robert Garcia, Michael Lubliner

INTRODUCTION

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.

ANALYSIS APPROACH

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.