Improving the Energy Efficiency of Manufactured (Mobile) Homes
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Introduction
Manufactured housing (including mobile homes) accounts for around 25% of annual new, residential construction and home sales in the United States. Millions of Americans live in manufactured homes, which represent more than 10% of the total housing stock. In many rural areas, the percentage is much higher. The sales prices for a new manufactured home range from under $30,000 for a single-section home with basic features to over $100,000 for a deluxe multisection home. Low purchase cost, with cost savings of 20%-40% for manufactured housing over that of site-built housing, is the major attraction of manufactured homes. The cost of heating and cooling some older manufactured homes, however, can be twice as much as site-built houses of comparable size and age.
HUD Code Requirements
Unlike all other forms of factory-built housing, which must meet state and local codes, mobile homes must conform to the U.S. Department of Housing and Urban Development (HUD) 1976 Federal Manufactured Home Construction and Safety Standards Acts, commonly known as the "HUD Code." Although "manufactured housing" is a term broadly applied to any type of factory-built housing, the 1980 amendment to the original 1976 HUD Code, defines "manufactured homes" as mobile homes. This information brief uses the HUD definition and focuses on mobile homes.
The Energy Policy Act of 1992 required HUD to revise its standards. New rules for manufacturers took effect in October 1994. Manufactured homes built after that date must have higher insulation levels (see table below), double-pane windows in all zones, ventilation fans in kitchens and bathrooms, and a whole-house ventilation rate of 0.10 air changes per hour. When added to the average natural ventilation rate for new manufactured homes of 0.25 air changes per hour, this meets the 0.35 air changes per hour rate recommended by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) for residences. In addition, the HUD climate zone maps were also changed. Previously, only homes manufactured for Alaska were required to meet the strict Zone 3 standards. The new map places most of the Northern United States into Zone 3 as well. Only the states bordering the Gulf of Mexico and South Carolina remain in Zone 1.
Old versus New Insulation Standards
| 1976 Standards | 1994 Standards | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Single-Wide | Double-Wide | Single-Wide | Double-Wide | |||||||||
| Zone # | 1 | 2 | 3 | 1 | 2 | 3 | 1 | 2 | 3 | 1 | 2 | 3 |
| Ceiling | R-11 | R-11 | R-19 | R-11 | R-11 | R-19 | R-14 | R-19 | R-19 | R-14 | R-22 | R-22 |
| Walls | R-7 | R-7 | R-13 | R-7 | R-11 | R-11 | R-11 | R-13 | R-19 | R-11 | R-13 | R-19 |
| Floor | R-7 | R-11 | R-14 | R-7 | R-11 | R-19 | R-11 | R-19 | R-19 | R-14 | R-19 | R-22 |
While the HUD minimum standards are helping to reduce energy costs for manufactured home buyers, still there is room for improvement. Several manufactures are building homes that exceed the minimum HUD insulation standards, and that have advanced, energy-efficient ventilation systems to maintain healthy indoor air quality even with very tight construction. Such homes use 30-50% less energy for space heating than homes built to the minimum HUD standards. Several manufacturers are partners in the Energy Star® and Building America Programs (see referrals below).
Effective Retrofit Measures
There are many differences between manufactured homes built before the HUD Code took effect and those built afterward. Many units made before 1976 are likely to have ventilated (leaky) walls, little or no insulation, no vapor retarder in the roof cavity, uninsulated heating system ducts, and uninsulated doors. Sealed-combustion furnaces (the combustion air for the burner comes from the outdoors) have been standard since the 1970s for safety reasons. Because of the tremendous opportunity for energy savings in older manufactured homes, utilities and state and local agencies have focused their attention on energy conservation retrofits. The Solar Energy Research Institute (SERI), which became the National Renewable Energy Laboratory (NREL) in 1991, tested pre-1976 mobile homes from 1988-1991. From these experiments, NREL developed retrofit methods that were better suited to manufactured housing than standard retrofit methods. The following reductions in heat loss were noted for different measures:
13% Roof Blown-In Insulation
11% Interior Storm Windows
11% Belly Blown-In Insulation
10% General Repairs (Caulking, ducts, etc.)
9% Wall Insulation
7% Insulated Skirting
6% Belly Wrap
5% Roof Cap
A survey of 36 mobile homes by the Colorado Division of Housing found that using the NREL-developed retrofits resulted in a 31% reduction in heating fuel usage.
Often, the simplest and most cost effective energy conservation strategy is to reduce air infiltration by weather stripping doors and windows, caulking window and door frames, and sealing the openings around plumbing and ducts. Sealing the ducts themselves is more difficult. It is often necessary to cut through the outside metal to reach them. The effects of duct leaks are exaggerated in manufactured housing, due to the small zones and single returns located at the central heating, ventilation and air conditioning (HVAC) unit. This sometimes creates high negative pressures that can suck in outside air through leaky supply ducts. Duct connections, especially those below the floor, often pull loose or are crushed when the mobile home is moved. A study in the Pacific Northwest involving both new and old mobile homes showed savings as high as 32% from duct repair and sealing.
Blowing insulation into an existing manufactured home is difficult because of the narrowness of the wall and roof cavities. Pre-1976 homes often had only 2" × 2" studs (5.08 cm × 5.08 cm) (new manufactured houses are required to have at least 2" × 4" [5.08 cm × 10.16 cm]). Trusses also hinder adding more roof insulation. Rolling back the roof to add insulation can lead to realignment problems and leaks. Furthermore, if the rollback method is used, adding some type of mechanical ventilation system to alleviate moisture condensation problems in the roof cavity may be necessary.
Many installers prefer roof caps for insulating, rather than rolling back the roof, in spite of their inferior performance. Roof caps come in kit form and consist of insulation boards, usually of dense fiberglass, with a synthetic rubber or metal covering. Roof caps can insulate the roof to R-19 without disturbing the existing roof. If all leaks in the old roof covering are sealed, the old roof acts as a vapor retarder, eliminating condensation problems and the need for ventilation.
Solar Applications
Adding a sunspace to increase the south-facing glazing area, and a thermal storage mass (rocks or water) to store excess heat for release at night, is an effective passive solar heating measure for manufactured homes. Site orientation, landscaping, and shading devices are other important elements to consider for reducing energy consumption.
Active solar applications for manufactured homes are much more limited than for site-built houses. The lightweight construction of manufactured homes limits roof support for heavy solar collectors. You can use lightweight roof-mounted air collectors for home and water heating. Heavier liquid-type solar collectors may have to be installed on the ground. Providing adequate thermal mass may be difficult for some systems because of transport and space limitations. Creating thermal storage space may require excavation. Consequently, active solar applications are best suited for manufactured homes that are seldom moved. Active solar energy systems can also be quite expensive to install.
Another potential solar application is a solar electric, or photovoltaic (PV) system. PV cells in modules or panels convert solar radiation directly into electricity. An electrochemical battery stores the power, and an inverter converts the stored power from direct current (DC) to alternating current (AC). Most common household appliances run on AC electricity. Unlike solar heating systems, PV systems are much lighter and require less construction work. Although the initial cost of PV systems is high, installing such a system may be a viable option for remote areas far from electric utility lines. In 1994, one manufacturer began to build PV-integrated modular homes.
Sources of Weatherization Information and Assistance
State energy offices may offer information and assistance for improving manufactured home energy efficiency. Each state has a Weatherization Assistance Program (WAP) that will provide assistance for low-income households. The U.S. Department of Energy (DOE) provides funding for these programs.
Local non-profit organizations such as Community Action Agencies and Human Resource Development Councils, and others, may have experience in manufactured home weatherization, and may offer assistance with caulking, weather-stripping, skirting, adding storm windows, and minor repairs. More extensive measures, such as adding insulation, have normally not been provided by these organizations.
