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Brock University study quantifies superior thermal performance of
SIPs
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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
orporated several test methods to analyze different determinants
of energy efficiency: thermographic imaging, hourly temperature
readings and air leakage measurement.
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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.
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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.
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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.
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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
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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 reasing 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.
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A new study by
the Oak Ridge National Labs (ORNL) proves that a 4-h 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. 
