Manufactured Housing: Proving Ground for SIPs?

HUD-code manufactured housing is one of the fastest growing types of housing and one of the most affordable. Recent studies suggest that introducing structural insulated panels (SIPs) into the manufactured housing industry may be an effective way to improve energy efficiency, improve construction quality, reduce dependence on diminishing and increasingly costly lumber supplies, and bring down the cost of SIPs.

Manufactured homes, often referred to as mobile homes, are constructed in a factory under conditions that permit systematic cost and quality control. For the past five years, the U.S. Department of Energy (DOE) and Pacific Northwest National Laboratory (PNNL) have been investigating the feasibility of building manufactured homes using SIPs.

HUD-code manufactured homes are transported structures built on a permanent chassis and regulated by the U.S. Department of Housing and Urban Development (HUD). The HUD building code is preemptive, so HUD-code housing is not subject to local or state codes. The market share of manufactured homes has grown rapidly in the past five years; today they constitute about one-third of all new single-family homes in this country. In 1996, about 360,000 manufactured homes were produced (see Heat Pumps and Manufactured Homes: Making the Marriage Work, HE Nov/Dec '96, p. 14).

Energy efficiency improvements in manufactured homes have not kept pace with the advances in site-built homes in many locations. Bonneville Power Administration, utilities, state energy offices, and their industry partners in the Pacific Northwest have conducted voluntary programs to address this need. These programs have highlighted the need to investigate other construction materials that might improve energy efficiency. The volatility and general upward trend of lumber prices has been another force pushing the industry to investigate other construction materials.

SIPs offer several advantages that might make them attractive to the manufactured housing industry, yet important questions remain. The SIP industry sees manufactured housing as an attractive and potentially very large market.

SIP producers tell researchers that SIPs are a good match with manufactured housing because both rely on assembly line production techniques. SIP producers also note that a major advantage is the possibility of keeping panel production rates at high levels year-round. Underutilization of production facilities appears to be one of the major contributors to SIP costs.

When SIP producers identify potential disadvantages of using SIPs in manufactured homes, one major concern is the SIP industry's lack of familiarity with HUD-code housing. Panel manufacturers say that HUD-code producers are probably resistant to change. SIP producers also acknowledge that panels are probably not currently cost-competitive with conventional construction.

In 1992-93, PNNL researchers surveyed six regional HUD-code manufactured home producers. They presented foam panel technical specifications, and then administered a questionnaire. They wondered what initial impressions manufacturers had about how using foam panels in manufactured homes would affect production and design. Table 1 summarizes the perceived advantages and disadvantages from the survey and from subsequent industry input.

Manufacturers identified several potential disadvantages to using SIPs as well. Their biggest concern was that SIPs would probably cost more to use, initially. Many of their comments reflected a lack of familiarity with the product, including how wiring would be done. They identified the floor system as the component requiring the most redesign. Some felt that SIPs would reduce design flexibility. They all raised the issue of obtaining approval under the HUD code--and specifically, what engineering and testing would be required. They were also uncertain how transportation would affect a manufactured home constructed of SIPs.

At this point, there is no way of knowing what it would cost to use SIPs in manufactured homes. Very likely, the costs will be higher initially, but volume production might lead to significant cost reductions in the long run.

This demonstration will show the feasibility of using SIPs in a home that complies with the HUD code. This will necessitate going through the usual review and inspection process and demonstrating that the homes comply with HUD's requirements, including performance during transportation. A secondary objective will be to document the design and construction process, including labor requirements and costs. Data for the first units built of SIPs will not reflect conditions when SIPs are fully integrated into the production process, but the information will be useful to suggest where improvements can and should be made.

Allen D. Lee, formerly of PNNL, now works with XENERGY Incorporated in Portland, Oregon. George James is program manager of the Industrialized Housing Program at U.S. Department of Energy.

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