The lowdown on structural insulated panels used in building our modular homes
Last summer's sharp spike in energy costs has caused an
increased interest in the potential energy savings for homeowners
and labor savings for homebuilders offered by structural
insulated panel (SIP) technology (see "SIPs Face the
Skeptics," HE Mar/Apr '98, p. 13). Now new test results
from Oak Ridge National Laboratory (ORNL) confirm that SIPs
can provide a higher whole-wall R-value than a comparable
stick-built house.
Whole-wall measurements take into consideration heat loss
due to seams and thermal bridging through wall studs, and
are therefore more accurate than testing only the insulation
material when measuring the R-values of buildings (see "Calculating
Whole-Wall R-Values on the Net," HE Nov/Dec '99, p.
22).The 4-inch SIP scored R-14 on the whole wall tests,
compared to R-9.8 for a 2 x 4 wood frame wall. The results
of whole-wall tests of 6-inch SIPs compared to 2 x 6 wood
stud walls were similar. The SIP wall scored R-21.6, while
the wood stud wall scored a whole-wall R-value of 13.7 (see
Figure 1).
[FIGURE 1 OMITTED]
These results are not that surprising, since SIP-built
houses have fewer seams and therefore tend to be more airtight
than stick-built houses. Also, since the insulation exists
between two load-bearing panels, there is less framing needed
in SIP building and therefore less thermal bridging through
wall studs.
Jan Kosny, the author of the ORNL study, emphasizes that
the SIPs must be installed correctly to avoid air and moisture
infiltration. Laboratory conditions involve the best possible
installation materials and techniques. Real practices, if
they are to achieve similar results, must be done just as
well. Kosny adds that, with SIP building, there should be
an emphasis on mechanical ventilation. If the SIPs are installed
correctly, you end up with a pretty tight house. There has
to be a way for fresh air to enter the living spaces. Without
sufficient ventilation, high indoor humidity can lead to
moisture buildup in occupied SIP homes during the winter
in cold climates. In one Alaskan development, 37 out of
38 homes built with SIP roofs are experiencing major moisture
problems, most likely due to poor installation.
In the field, Habitat for Humanity homes built with SIPs
were tested for overall energy efficiency. The testing was
spearheaded by the Florida Solar Energy Center, with support
from Sumter County Habitat for Humanity, the Structural
Insulated Panel Association, (SIPA), and the U.S. Department
of Energy. The testing was performed to provide comparison
numbers for both airtightness and energy use for two houses
built with SIPs and one built conventionally, with stick
framing and batt insulation. All three houses were built
on neighboring lots in Plains, Georgia, and were built to
have a similar calculated energy performance, or HERS score.
Test results show remarkable differences in both categories.
All three houses had HERS scores of 83%. The SIP house achieved
a whole-house leakage rate of 1.8 ACH at 50 Pa compared
to 3.9 ACH for the frame house. Monitoring equipment installed
to measure the total energy use and heating energy use collected
data for December 1998 and January 1999. The SIP houses
used 25% less heating energy than the conventional house.
Some of this difference was due to the fact that the stick-built
house had ducts outside the conditioned space and the SIP
houses had ducts within the conditioned space. But in a
similar comparison study done in Louisville, Kentucky, where
the stick-built and SIP houses both had ducts inside the
conditioned space, the SIP house used 15% less heating energy
than the stick-built house.
Builders Weigh In
Do these test results translate into actual energy savings
in the real world? John Hensler, a builder with RJT Homes,
has become an SIP convert, as the energy savings his home
buyers see have helped his sales flourish. In 1995 in Alpine,
California, Hensler built Crown Hills, one of the first
major developments to incorporate SIPs. According to data
provided by San Diego Gas & Electric, the average cost
of heating and cooling a 1,600 [ft.sup2] home in Crown Hills
was $48.41 per month compared to $125 per month for a similar-size
stick-built home. Homeowners living in Crown Hills reported
that not only were their utility bills half of what other
Southern Californians were paying, but they lived in a much
quieter house as well.
This feedback, combined with the speed with which the subdivision
sold new homes, convinced RJT to use SIPs in their next
development. The new development, Palmilla, located in La
Quita, California, will boast 150 homes, all made with SIPs.
SIPs are proving their worth to many other builders because
they save labor. Home builders are faced with not just
a labor shortage but more specifically, a shortage of qualified
labor. An SIP structure can be skinned with panels in less
time than it takes to skin with conventional framing. With
value-added features like complete precutting, SIP packages
lessen the need for highly skilled carpenters. Full-service
SIP suppliers routinely offer design and preengineering
with complete precut packages. These packages significantly
decrease the rime needed to get a structure dried in. Decreased
labor time can mean labor cost savings that will offset
increased material costs. Mic Carmichael, a veteran panel
installer from Nevada City, California, claims that "when
comparing 2 x 6 stick-built walls with 3 1/2-inch core panels,
the difference in cost is zero."
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