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
- Engineering Analysis. What engineering
analyses have been performed regarding roof loading and shear
- Span. Can panels clear span the
14 to 18 feet from marriage line to sidewall? Up to what roof
- 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
Over the course of several months, the SIPs
project committee debated the following two points:
- What applications will maximize the potential benefits associated
- 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
Among the most appealing aspects of SIP roofs for manufactured homes
are the following:
- 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
- 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.
- 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
- Is it feasible to build SIP roofs under the federal Manufactured
Home Construction and Safety Standards?
- What is the cost of replacing standard roof framing construction
- 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.