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    Structural Insulated Panels in a Manufactured Home Roof System: Engineering Guidelines
Manufactured Housing Research Alliance, New York, 2000

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Structural insulated panels, or SIPs, are a building component that has been in commercial use for over 20 years. Typically consisting of a rigid insulating material sandwiched between inner and outer membranes (often oriented strand board or plywood), the SIPs behave structurally like an I-beam and exhibit superior thermal properties when compared to standard frame construction with low-density batt or blown insulation. SIPS also have a higher first cost than conventional framing methods, relegating their use to niche markets. However, their superior structural and thermal properties, ability to be manufactured to high dimensional tolerances, and potential to speed home assembly make SIPs a compelling technology for HUD-code homes.

A few HUD-code home manufacturers are experimenting with SIPs but none have homes in commercial production using the technology. The major barriers to SIPs were identified by the project committee and include the following:

  • Compliance with the HUD Standards. Do the HUD standards in any way restrict the use of SIPs?

  • SIPs Vendor Support. In what ways and to what extent are SIPs vendors willing to assist in obtaining HUD third-party approvals and provide general engineering and design support?

  • Engineering Analysis. What engineering analyses have been performed regarding roof loading and shear stresses?

  • Span. Can panels clear span the 14 to 18 feet from marriage line to sidewall? Up to what roof load?

  • Economics. What are the home manufacturer's costs of SIPs versus wood frame truss construction (product first cost and net costs including changes in manufacturing and production)?

  • Transport. What are the critical structural points during home transport to the retailer and site? Is special transport reinforcing advised?

  • Inside Finish. Can panels be constructed with an interior finish panel, such as gypsum board? What are the design limitations of such panels?

  • Wiring. What are the options for installing wiring in a roof panel? Do all wires need to be snaked through chases? How fast can a SIP ceiling be wired? How is the electrical junction between ceiling and wall handled?

  • Fire. What are the toxic byproducts of EPS, XPS, urethane or other panel insulating materials when they burn? How do the panels perform in fires?

  • Exterior Finish. What materials are suitable for exterior roof finish? Are there special venting requirements to reduce surface temperatures?

  • Wind Zone Design. Will using SIPs in HUD wind Zone II and III pose a problem due to the necessity to attach the roof with brackets or strapping to sidewall framing members?

Over the course of several months, the SIPs project committee debated the following two points:

  1. What applications will maximize the potential benefits associated with SIPs?

  2. What first steps should the committee take in assessing the value of SIPs?
Details of Roof Openings

SIPs are used in site building for roofs, exterior walls and floors and a case can be made for all three applications. The home manufacturers on the committee, however, discounted using SIPs for floor and wall construction. Either application would engender a relatively high up-front cost to change manufacturing processes and might significantly slow production. Walls in particular would require unique panel sizes and configurations that complicate the erection process and place a heavier burden on coordinating panel production and home assembly. Using SIPs for roofs had fewer disadvantages and some distinct advantages.

Among the most appealing aspects of SIP roofs for manufactured homes are the following:

  1. The high thermal value of SIP construction is most beneficial for roof construction, the building component that is subject to severe dimensional constraints, highest indoor/outdoor temperature differences, and most difficult to insulate using standard frame insulating methods.

  2. Roofs have few penetrations and represent a fairly continuous skin that can be constructed of identical, usually factory-fabricated, panels. The virtual elimination of specialty panels (the roof is comprised of one or two panel sizes) simplifies and can speed the assembly process.

  3. The slim profile of the SIP when compared to standard cathedral designs creates a steeper interior roof pitch and more design appeal. With interior space at a premium, this feature has a good deal of marketing value.

In responding to the second question - what is the scope of the current effort - the committee recognized that there are many issues that need to be addressed before any manufacturer commits to sell homes made with SIPs. Some of the technical questions discussed earlier engender a rigorous program of research, analysis, testing and prototyping. Further, consideration needs to be given to the reliability of SIP supply/manufacture and the impact of the product on plant procedures and worker training. These important matters were beyond the scope of the current effort, but are clearly key topics for subsequent phases. Rather, the committee agreed that the decision to take the next steps rested on the following three factors:

  1. Is it feasible to build SIP roofs under the federal Manufactured Home Construction and Safety Standards?

  2. What is the cost of replacing standard roof framing construction with SIPs?

  3. What is the value of SIP roofs from a marketing standpoint?
Mating Line Connection

To respond to all the questions, the committee decided to develop a generic Design Approval Primary Inspection Agency (DAPIA) package. The package, contained within the technical report, includes construction sketches of SIP designs and a review by a third party familiar with the DAPIA certification process and the HUD standards. The cost and marketing issues are far more complex and manufacturer specific. Therefore, the committee agreed that the DAPIA package would help individual manufacturers answer these latter two questions for themselves and not attempt to provide industry average figures.

A team of experts was assembled to provide advice to the project committee and assist in the preparation of the DAPIA package. The project committee, at several key points during the development process, reviewed the work of the team and made mid-course corrections and refinements. The results of these efforts are documented in the technical report.

The manual provides typical details that may apply to a large variety of manufactured home models and structural systems. The typical details should be modified and incorporated into a home manufacturer's DAPIA approved engineering details. The details presented in the manual have been prepared in accordance with accepted engineering principles, sound judgment and the Manufactured Home Construction and Safety Standards. However, the details must be incorporated into the manufacturer's overall engineering package and submitted to its DAPIA for review and approval.