Architect's Knowledge ResourceDocuments
Performance and Manufactured Homes
By Linda Reeder, AIA, LEED AP
Manufactured homes are those built entirely in a factory and transported and installed on a building site. They include modular homes, panelized homes, and pre-cut homes. “Mobile homes” is a term the U.S. Department of Housing and Urban Development (HUD) reserves for factory-produced homes produced before the HUD Code instituting minimum energy efficiency standards for manufactured housing went into effect in 1976. A January 2006 HUD report stated there were eight million units of manufactured housing in the United States.
While manufactured homes are factory-produced, architects have been taking a greater interest in their design of late. Michelle Kaufmann’s Glidehouse™ (showcased in a 2006 National Building Museum exhibit) and Michael Berk’s GreenMobile (100 of which are being produced for Mississippi with a FEMA grant for affordable post-Katrina housing) are two of the better known examples. Both incorporate sustainable design practices, and improving building performance is a growing concern for the industry as a whole.
Factory production is an opportunity to optimize integrated building design, building systems and assemblies. But while manufacturing in a controlled environment can improve some aspects of quality control, factory-built homes often move through many hands—including the transporter and installer—during which they may be exposed to damage. Durability of materials and systems is an important component of manufactured home design.
Although typically constructed of light-framed wood, designing with structurally insulated panel (SIP) walls, ceilings, and floors can increase the strength of manufactured homes and dramatically improve energy efficiency over stick-framing. However, SIPs also increase first costs, a problem for this housing type grounded in the affordable market.
Owing to low first-costs and availability in rural areas, the heat in the vast majority of manufactured homes is generated by electricity. Most are heated and cooled with ducted forced air. Since 23 percent of manufactured homeowners’ housing budgets go toward energy (compared to 17 percent of all homeowners), energy savings are key to improving the affordability of manufactured homes. The Manufactured Housing Research Alliance (MHRA) and others are researching cost effective ways to achieve this. Future energy savings may need to be considered in financing, to offset higher first costs.
The MHRA, in a study funded by HUD, found that conditioned crawl spaces offer energy and moisture mitigation advantages over the more common technique of securing manufactured home to piers on surface-mounted footings. They recommend frost-protected shallow foundations (FPSFs).
FPSFs raise the frost depth around a building’s perimeter by using heat from the earth and the building to raise the sub-slab soil temperature. (Interior temperatures must be kept at least 64 degrees year-round.) FPSFs are typically placed 12 to 16 inches below grade, a depth that can often be achieved with a trencher instead of a backhoe, contributing to excavation savings. The savings over traditional foundations are greater in colder climates where greater depths are required; generally this foundation system is most cost effective where the frost depth is at least 30 inches. Rigid insulation encapsulates the outside of the FPSF, and, as with rigid insulation at any foundation wall, must be protected from mechanical and ultra-violet damage and, in some regions, termites. Good foundation drainage is also key in this system.
For warmer climates with shallower frost depths (and almost 60 percent of all manufactured homes were located in the South, the American Housing Survey found in 2003) the FPSF system may not be economically feasible. Well-insulated floors with an appropriate vapor barrier are necessary in pier-mounted manufactured housing. For example, the GreenMobile is an affordable, ecological factory-built home well-suited to the southern climate. Its foundations are helical steel piers which can be installed quickly with portable hydraulic equipment and easily removed if the home is relocated.
Indoor Environmental Quality
Moisture from condensation and resulting mold has historically been a problem with manufactured homes, particularly in hot humid climates. Accounting for this and the movement that may occur during shipping in the design can help mitigate this problem, as can providing regional variations of the same design (adjusting, for example, the location of the vapor barrier depending on climate). A ventilation system integrated with the building system is obviously important to indoor air quality, as is specifying low-emitting materials.
In light-framed manufactured homes, noise outside and within the home can be a problem. Major external sources include airports and busy roads. Inside, the mechanical room, often located next to the living area, is often a source. As blower speed has been increased to improve efficiency, noise has increased as well. Insulating and sealing the building envelope for energy efficiency will also improve the sound separation. Increasing the STC rating of the mechanical room with insulation or extra layers of sheetrock will reduce indoor noise.
Keywords: Building performance, Sustainability, Sustainable design knowledge, Prefabrication, Prefabricated buildings, Manufactured homes, Mobile homes, Article