Brock University study quantifies
superior thermal performance
of SIPs
 |
Dr. Tony Shaw of Brock University compared
the thermal efficiency of two units in these nearly
identical semi-detached homes. The house on the left
was built with SIPs, while the other was framed with
studs and batt insulation. |
The thermal
qualities of Structural Insulated Panels (SIPs) have
long been argued and are generally accepted, but true
comparison to traditional stud wall systems often gets
bogged down by misleading R-value ratings. Furthermore,
many field studies are partially flawed because they compare
different structures in different environments.
That’s why a recent study by Dr. Tony Shaw of Brock
University was a refreshing change from much of the existing
research on the thermal performance of SIPs. Dr. Shaw’s
work involved a side-by-side evaluation of nearly identical
residential buildings – one constructed with SIP
exterior walls and one conventionally framed with studs
and batt insulation.
The detailed study, which was supported by the National
Research Council of Canada (NRC), provides tremendous insight
into the energy efficiency properties of SIPs. But before
getting into the findings, a bit of background is warranted.
Thermodynamics
101 and the limitations
of R-Values
When two bodies with different temperatures are brought
into contact with one another, heat always transfers from
the hotter object to the colder one. This is fundamental
to our discussion: minimizing heat transfer within a wall
system is the key to energy efficiency.
There are three different types of heat transfer: conduction,
convection and thermal radiation. Conduction is where heat
transfers between two bodies through actual physical contact.
For example, heat from a stove element is conducted to
the frying pan. Convection involves the transfer of heat
through the movement of a fluid (e.g. air), which is easy
to comprehend when you sit to close to a campfire. Finally,
radiation involves energy radiated from hot surfaces through
electromagnetic waves, similar to a light bulb emitting
light and heat.
When
we’re talking about the energy efficiency of
a wall system, it’s conduction and convection that
matter most. Conduction of heat occurs through sheathing,
studs and insulation. Convection occurs through cracks,
gaps and openings within the wall, as well as air cells
in batt insulation.
The
problem with using R-values to gauge the energy efficiency
of
a home is that insulation is typically rated in a laboratory
under controlled conditions. But in an actual stick and
batt wall, heat conducts not just through insulation, but
more significantly through studs, reducing the overall
efficiency of the system. And gaps in the wall – sill
plates, top plates, electrical outlets, window jambs and
even nail holes – further reduce the true R-rating
of the wall because of convective heat transfer.
A
SIP wall’s
ability to perform closer to its rated R-value is a result
of its tightness as a system, which
minimizes convective heat loss. The rigid EPS insulation
of SIPs eliminates air circulation and moisture that is
often prevalent in stud walls.
Furthermore,
the structural high-density EPS insulation of a SIP ensures
less surface area for conductive heat
transfer than conventional walls, which require studs every
16" or 24" for structural support – prime
vehicles for conductive heat loss.
The Brock University study:
comparing
identical buildings
When it comes to quantifying actual heat loss in different
wall systems, the Brock University study provided an excellent
opportunity for accurate comparison between SIP and stick
construction in the real world.
The two structures involved in the study were rental housing
units, located immediately adjacent to one another. Both
buildings were identical and had similar east-west orientations,
ensuring the same exposure to outdoor temperature and wind
conditions. Except for brief periods both houses were occupied
throughout the course of the study, which took place over
a 12-month period from February 2000 to January 2001. Both
units were heated with a natural gas / forced air system.
One
unit was constructed with 4.5" SIPs, while the
other used 2x6 studs with batt insulation. Both houses
were constructed according to the Ontario Building Code
(OBC). The units were built by the same crews, with no
one being aware that scientific tests would be conducted
afterwards.
The study incorporated several test methods to analyze
different determinants of energy efficiency: thermographic
imaging, hourly temperature readings and air leakage measurement.
 |
Figure
1a:
Thermal photography of stud and batt wall
This thermal photograph of a stud wall reveals multiple
points where heat can escape – primarily along
studs themselves. |
 |
Figure 1b: Thermal
photography of SIP wall
The SIP wall allows for minimal heat loss along the
wall surface. The only heat loss evidenced here occurs
in the corner area. |
Thermographic Analysis
The
deceiving nature of R-values was illustrated by infrared
imaging
on the two structures on a day in early March.
Energy loss measured at the conventionally framed building,
which used insulation rated at R-20, performed at an R-4
equivalent. By comparison the SIP home, performed at a
true R-17 level. Thermographic analysis, at an outdoor
temperature of -10.5 ºC (13.1 ºF), also demonstrated
that the stud home consumed nearly four times as many BTUs
as the SIP home.
Furthermore, thermographic photographs provided visual
confirmation of areas of thermal weakness in the 2x6 wall,
where thermal bridging (i.e. conduction) is visible around
each stud, along with pockets of air leakage (see figure
1).
Temperature Trends
This
imaging evidence was supported by temperature data recorded
hourly
by a series of sensors located within the
walls of each building (see figure 2). Temperatures recorded
in the middle wall (T3) and inside the exterior wall surface
(T2) of the stud construction showed the greatest fluctuation,
corresponding closely to the variation in outdoor ambient
temperatures, especially during the cold months of December,
January and February. In comparison, the SIP wall sensors
recorded significantly higher and more stable temperatures
at those locations. The temperature of the middle wall
sensor (T3) averaged 1.95 ºC (35.5 ºF) for the
stud wall, while the SIP wall averaged 15.61 ºC (60.1 ºF)
in the month of January.
 |
Figure 2: Sensor locations
This cross-section shows the positioning of the temperature
sensors used in the Brock University study, comparing
the thermal performance of stud and SIP wall systems. |
These variances are key because, once again, heat will
always move from the hotter body to the cooler one. The
higher temperature at the T3 sensor demonstrates that the
SIP wall experienced less heat loss than the stud wall,
and consequently, is more energy efficient.
Also
of notable significance are the temperature differentials
recorded
between the inside interior wall surface (T4)
and the inside exterior wall surface (T2). Over the course
of the year, lower differentials were recorded for the
SIP wall (an average of 6.51 ºC (43.7 ºF) as
compared to 12.31 ºC (54.2 ºF) for the stud wall),
further demonstrating its reduced susceptability to heat
loss. Figure 3 shows the overall daily thermal performance
of the two walls in the cold month of January. The T3 measurement
for the stud wall was consistently close to the actual
exterior wall surface temperature while the SIP wall demonstrated
a steady and sizeable gap.
 |
Figure 3: Thermal performance
of stud and SIP wall systems
Data from the temperature sensors in the stud and SIP
walls demonstrates the relative energy efficiency of
the two systems. This graph is based on measurements
throughout January 2001. Temperatures at the middle
wall sensor for the stud construction are very close
to the exterior temperature. In contrast, data shows
how the SIP wall maintained much higher temperature
at the same sensor locations – an indication
of superior energy efficiency. |
Air tightness comparisons
In addition to the thermal performance and thermography
components of the Brock study, air leakage tests were conducted
to compare the tightness of the two units. This analysis
shows the relative convective properties of each, a key
determinant of overall energy efficiency.
The results of the air leakage tests showed the SIP house
to be much tighter than the stud house. The SIP house had
1.55 ACH (air changes per hour) at a pressure differential
of 50 Pa, while the framed wall house had 2.60 ACH at 50
Pa, or a 68% more leakage. This means that, all other factors
being equal, the SIP house would use less energy for heating,
would be more comfortable, have better heat retention and
be less drafty.
Conclusion
Based on the heat loss
data collected in the Brock University study, a natural-gas
heated, 2,000 sq. ft. SIP house would save $88 on a
monthly heating bill in an average winter month. |
The
U.S.-based Oak Ridge National Laboratories 1998 study
under laboratory
conditions stands out among the most authoritative
work on the subject, and Habitat for Humanity has provided
several opportunities to compare different wall systems
under similar conditions. Likewise, Dr. Shaw’s research
is a very insightful analysis on the thermal properties
of SIP and stud construction. Studies such as Brock University’s
SIP/stud comparison are relatively uncommon, but they are
generating tremendous interest by government, industry
and consumers alike.
As awareness builds surrounding the environmental impact
of buildings on greenhouse gas emissions and urban air
quality, the construction industry will be under increasing
pressure to adopt new standards and practices to reduce
energy consumption. Regardless of the Kyoto Protocol,
where signatory governments agree to take concrete measures
to reduce greenhouse emissions – inevitably rewarding
environmentally friendly technologies at the expense
of less efficient ones – the economics of energy
costs and natural resources availability will make non-traditional
building materials such as Structural Insulated Panels
more and more attractive.
|
A new study by the Oak
Ridge National Labs (ORNL) proves that a 4-inch SIP wall outperforms 2"x4" stick
and batt construction, and even edges out 2"x6" construction in terms of thermal
performance. Because SIPs are the structural elements, there are no studs or
braces to cause breaks in the insulative action. The end result is a more
comfortable, energy efficient structure that performs up to spec in real-world
conditions. Unlike stick and batt construction, which can be subject to poorly
installed - even missing - insulation, the nature of SIPs is such that the
structural and insulative elements are joined as one. There are no hidden gaps,
because a solid layer of foam insulation is integral to panel construction. 