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Benefits and Trade-Offs of Low-Slope Roofing System Insulation and Reflectance

May 15, 2009

INTRODUCTION
Increasingly, roof systems are being
required to supply more benefits than basic
protection from weather. In addition to the
aesthetic aspects of residential roofing
material design, both residential and commercial
roofs are now seen as significant
contributors to the energy usage of buildings.
According to the U.S. Department of
Energy, one-third of all energy consumption
in the U.S. is used to condition and light
buildings.
Roof systems make two main contributions
to building energy use by virtue of
their reflective and insulative properties.
Reflectance is important to reduce heat gain
and thereby lower building cooling costs.
Conversely, insulation has traditionally
been viewed as reducing heat loss, with
greater insulation prescribed in colder
zones. However, insulation also acts to
reduce heat flow into buildings, leading to
the question: When is it appropriate to
focus on improving reflectivity and when to
focus on insulation?
This article examines the benefits and
trade-offs associated with improved roofing
membrane reflectivity versus improved
insulation for low-slope buildings. In addition,
the effects of climate zones and geographical
location within the U.S. are examined.
Clearly, any discussion of reflectivity
and insulation must take into account any
associated building codes, and these are
also reviewed.
ROOF MEMBRANE REFLECTANCE
By designing membranes with higher
reflectance, it is possible to lower the heat
gain experienced by the total roof system.
Such reflective systems are referred to as
cool roofs and potentially provide for lowered
cooling demand for building interiors.
In simple terms, a cool roof is highly
reflective and can return a substantial portion
of the sun’s energy back to the atmosphere.
Therefore, the surface stays cooler
and transfers less heat into the building. An
additional feature of a cool roof is that it can
more easily emit the small amounts of heat
that are absorbed. By combining high
reflectivity and high emittance, it is possible
to substantially reduce the heat load on the
system.
The highest reflectivities are being
achieved by single-ply thermoplastic membranes
and by various coatings. Single-ply
systems typically achieve high reflectivity by
incorporating titanium dioxide as a pigment.
This mineral reflects light and heat
across a broad portion of the sun’s spectrum
and is extremely white in appearance.
Solar energy consists of 5% ultraviolet
light, 43% visible light, and 52% near
infrared. The different colors perceived by
the human eye are dependent on which
wavelengths of visible light are reflected by
objects. Over 50% of solar energy is near
infrared light, which contributes to heat
buildup if absorbed by roofing materials.
With the reduced temperatures provided
by reflective roofing materials, there are not
only environmental benefits such as energy
savings and reduction of the heat island
effect, but also potential improvements to
the longevity of these materials. It is recognized
that since heat accelerates the degradation
of polymeric membranes, technology
that lowers temperatures should result in
better retention of their physical properties.
The reduced thermal expansion and contraction
due to the reduced temperatures
with reflective pigments also help to extend
the life of roofing materials.
COOL ROOFING STANDARDS AND CODES
Cool roofing standards are based on
both solar reflectance and emissivity.
The solar reflectance (SR) is intended to
indicate how much of the sun’s ultraviolet,
visible, and infrared energy (i.e., solar flux)
is reflected. SR is defined as the fraction of
solar flux reflected by a surface, expressed
as a percent or within the range of 0.00 to
1.00. Measurement is done using the methods
described by the American Society for
Testing and Materials’ (ASTM) Standard
E903, ASTM C1549, or ASTM E1918.
ASTM E903 measures solar reflectance
over the wavelength range of 250 to 2500
nm using integrating spheres. Such integrating
spheres ensure that specular,
direct, and scattered reflections are measured.
By using a commercial portable solar
reflectometer, which is calibrated using
specimens of known SR, ASTM C1549
determines SR from measurements at four
wavelengths in the solar spectrum: 380 nm,
16 • I N T E R FA C E DE C E M B E R 2009
500 nm, 650 nm, and 1220 nm. ASTM
C1549 shows the comparison results
between C1549 and E903 methods. The SR
results at air mass 1.5 measured per ASTM
C1549 are generally 1.9% greater than
those obtained with ASTM E903.
ASTM discontinued E903 in August
2005 in accordance with section 10.5.3.1 of
the Regulations Governing ASTM Technical
Committees, which requires that standards
be updated by the end of the eighth year
since the last approval date. ASTM E1918
covers the SR measurement for various horizontal
and low-sloped surfaces and materials
in the field, using a pyranometer. The
test method is intended for use when the
angle of the sun to the surface is less than
45 degrees. The SR data reviewed in this
paper is measured per ASTM C1549.
While SR defines the percentage of all
solar radiation that is immediately reflected
from a material surface, any energy that is
not reflected from a surface is absorbed by
the material. The emissivity, or thermal
emittance (TE), is a measure of how much
absorbed energy is radiated from a material.
TE is defined as the ratio of the radiant
heat flux emitted by a sample to that emitted
by a black-body radiator at the same
temperature. Measure ment is done using
ASTM C1371 or ASTM E408. ASTM C1371
is the standard test method for determination
of emittance of materials near room
temperature using portable emissometers.
ASTM E408 is the standard test method for
total normal emittance of surfaces using
inspection-meter techniques. The TE data
reviewed in this paper is measured per
ASTM C1371.
The solar reflectance index (SRI) is a
commonly used value that incorporates
both solar reflectance and emittance into a
single value. SRI measures the relative Ts
(steady-state surface temperature) of a surface
under the sun with respect to the standard
white (SRI=100, for a sample with
reflectivity of 0.80 and emissivity of 0.90)
and standard black (SRI=0, for a sample
with reflectivity of 0.05 and emissivity of
0.90) under the standard solar and ambient
conditions. SRI is calculated using equations
based on previously measured values
of solar reflectance and emittance as laid
out in ASTM E1980.
Due to the accumulation of surface particles
like dew, dust, and air pollutants on
roof materials, as well as the material’s
aging, the SR tends to decrease over time.
Also, the roof slope affects aged SR, with
steeper slopes accumulating less dirt and
particles through dislodging.
For a material to be qualified as an
ENERGYSTAR® product, in addition to the initial
SR, the SR after aging for a minimum of
three years must be measured by following
the guidelines of ENERGYSTAR® Version 2.0
Program Requirements for Roof Products.
There are national and local programs
or policies related to cool roofing, each with
its own unique criteria and definitions.
• The ENERGYSTAR® Roof Products
Program was introduced by the
Environmental Protection Agency
(EPA) in 1998. The EPA estimates
that an ENERGYSTAR®-labeled roof
can reduce a roof temperature by as
much as 100°F.
• Established in 1998 as a nonprofit
organization, the Cool Roof Rating
Council (CRRC) is now recognized by
the California Energy Commission
(CEC) as the sole entity responsible
for labeling roofing products that are
allowed in the California Energy
Code Title 24. To be Title 24-compliant,
the roofing material has to meet
prescriptive requirements of 0.70 SR
and 0.75 TE and be a CRRC-listed
product.
• Started in 2000, the U.S. Green
Building Council’s Leadership in
Environmental and Energy Design
(LEED) program has gained in popularity
in the construction industry.
LEED is a whole-building design
program that encourages an integrated
design and construction
process whereby points are awarded
for the use of sustainable products
or building practices.
Table 1 lists some of the programs, criteria,
and related test methods.
LOW-SLOPE THERMAL INSULATION
Unlike residential roofing, the vast
majority of low-slope insulation is rigid.
Premanufactured rigid insulation is formed
into boards from either fibrous materials or
plastic foams. Alternatively, in the case of
foam insulation, it is sometimes formed in
place, using special equipment to meter,
mix, and spray reactive chemicals onto the
deck.
Heat transfer from a hot to a cold surface
involves a combination of conduction,
convection, and radiation. Thermal insulation
functions by reducing conductive and
radiative transfer and eliminating convection.
It should be noted that convection is
PROGRAM POLICY MIN. INITIAL MIN. AGED MIN. TEST
REQUIREMENTS SOLAR SOLAR EMISSIVITY METHODS
REFLECTANCE REFLECTANCE
Title 24 Required 0.70 None 0.75 SR: ASTM E903,
(CA Energy Code) (ASTM C1371) E1918, C1549, CRRC
Test Method #1
TE: ASTM C1371
Energy Star SR: ASTM E903,
Low Slope (<2:12) N/A 0.65 0.50 None C1549, CRRC
Steep Slope (>2:12) N/A 0.25 0.15 None Test Method #1
LEED Credit 0.90 for 75% of SRI: ASTM E1980
the roof surface TE: E408
Low Slope (<2:12) SRI: 78 min (ASTM E408)
Steep Slope (>2:12) SRI: 29 min
Table 1
“Required” refers to policies where cool roofs are not mandatory, but an energy penalty is given if one is not used.
“Credit” refers to policies where cool roofs are not mandatory, but an energy credit is earned if one is used.
DE C E M B E R 2009 I N T E R FA C E • 1 7
only eliminated if all air spaces
between the hot and cold surfaces
are filled with insulation. If there
are gaps, such as between insulation
boards, then convection
occurs, and a thermal short circuit
exists.
An important characteristic of
insulating materials is the thermal
resistance or R-value. The value is
calculated from the material thermal
conductivity (k), units (Btuin)/(
hr•ft2•˚F), and product thickness
(L), using the following equation:
= L
k
This is often expressed as R per inch to
enable a comparison of the effectiveness of
various materials to be compared. Plastic or
polymer-type insulation includes polystyrene,
polyurethane, and polyisocyanurate.
Polyisocyanurate or “iso” provides the
best insulating value per inch, typically R-
5.8 to 6.5, with the most commercially
available product being at R-6/in.
ENERGY SAVINGS MODELS
Several tools exist to calculate estimates
of energy savings due to different roofing
systems. A calculator published by Oak
Ridge National Laboratories (ORNL), shown
online at http://www.ornl.gov/sci/
roofs+walls/facts/CoolCalcEnergy.htm,
considers SR and emissivity for the membrane,
insulation value, and energy costs.
Another version has been made available by
the ENERGYSTAR® organization, at
http://roofcalc.cadmusdev.com/RoofCalc
BuildingInput.aspx. This calculator is similar
to the ORNL version but does not consider
membrane emissivity.
For this study,
preliminary work
showed some differences
between
the two calculators,
which depended
largely on the geographic
location of
buildings. This is to
be expected given
that the ENERGY –
STAR® model does
not include emissivity.
In practice,
low emissivity can
be a benefit in
northern latitudes
during cooler
months, while high emissivity is required
for southern regions, where building cooling
is more important. Based on the preliminary
work, the ORNL model was used for
the work described here.
For the ORNL calculator, the air conditioner
efficiency (coefficient of performance)
was set as 2.0, and the heating system efficiency
was set as 0.7 (i.e., both were
assumed to be average). Heating was
assumed to be gas-based. For any energy
savings calculation, it is critical to have
valid energy costs. For this study, commercial
2008 natural gas and electricity cost
data were obtained from the Energy
Information Administration (EIA) data published
at http://www.eia.doe.gov/. For
the gas costs, a conversion factor of 100
cubic feet = 1 Therm, was used. Yearly average
energy costs were used throughout.
ROOF SYSTEM COMPARISONS
The impacts of the three factors – insulation
(R-value), solar reflectance (SR), and
thermal emittance (TE) – on energy savings
of low-slope systems were evaluated with
ORNL’s energy-saving model. Unless otherwise
noted, the annual energy saving displayed
herein is based on calculation compared
to a black surface.
INSULATION (R-VALUE) WITH HIGH REFLECTANCE
MEMBRANES
The roofing insulation R-values used
are listed in Table 2.
To study the impact of roofing insulation
R-value on energy savings for different
areas, a typical commercially available,
white, single-ply membrane with 0.78 initial
SR and 0.90 initial emissivity, commonly
used in low-slope systems, was selected,
and its annual energy saving was calculated
(Figure 1).
For the calculations shown in Figure 1,
an average power cost was used. However, it
should be noted that there can be significant
differences in terms of both electricity
and gas costs across the U.S.
As shown in Figure 1, the annual energy
savings are realized most when a white
membrane is used in combination with 5 to
25 R-value roofing insulation in Florida,
California, and Tex as. For roofing insulation
with an R-value over 25, the energy
R-Value Example Configuration to Achieve R-Value
(hr•ft²•°F)/Btu
5 1.5-in-thick wood fiberboard insulation on a 0.75-in-thick plywood deck
13 2-in-thick aged polyisocyanurate insulation on a 20-gauge Type B metal deck
25 4-in-thick aged polyisocyanurate insulation on a 20-gauge Type B metal deck
32 5-in-thick aged polyisocyanurate insulation on a 20-gauge Type B metal deck
Figure 1 – Total annual energy savings vs. R-value of insulation with a reflective membrane (TSR = 0.78, E = 0.90)
compared to a black roof. Assumes electricity cost of $0.1028/kWhr and gas cost of $12.91/1000 cf.
18 • I N T E R FA C E DE C E M B E R 2009
Table 2 – R-values of roofing insulation, quoted from ORNL paper “Effect of Solar Radiation Control
on Energy Costs – A Radiation Control Fact Sheet for Low-Slope Roofs,” by T.W. Petrie et al.
R
saving with white
membrane is re –
duced but may still
be significant. The
large differences
be tween the savings
achieved in
some states versus
others are best un –
derstood by considering
summer
versus winter energy
usage as a function
of climate.
Figure 2 shows the
climate zones de –
lineated by DOE
/In ter national En –
er gy Conservation
Code (IECC) for the
U.S.
In hot, sunny
regions such as
Zones 1 through 3, cooling loads dominate
annualized energy usage, and therefore,
reflective membranes provide for significant
energy savings. However, in colder regions
such as Zones 6 and 7, heating loads dominate
and membranes that absorb solar
heat can actually slightly lower energy
usage.
This can be illustrated by the cooling
versus heating data that were combined for
Figure 1. Figure 3 shows the individual contributions
for each city, assuming an R-13
insulation value. Note that positive numbers
reflect a savings, while negative numbers
represent a cost increase.
These data clearly show that cities with
the greatest savings achieve those savings
through a reduction in cooling costs and are
DE C E M B E R 2009 I N T E R FA C E • 1 9
Figure 2 – 2004 International Energy Conservation Code (IECC) U.S. climate zones.
situated in hotter zones. In contrast, in
Chicago and Newark, where heating costs
dominate, the cooling savings are almost
canceled out by the increase in heating
costs. In the case of Zones 5 through 7
(Figure 2), the analysis would need to be
done on a case-by-case basis. For an existing
building, that analysis would examine
previous years’ heating and cooling ex –
penses. If cooling is the larger expense, then
a highly reflective roof membrane would be
appropriate. For new buildings, the analysis
would have to take into account wall-to-roof
areas, insulation levels used in walls,
intended building use, and local energy
costs.
It is important to note that all the savings
data shown here are based on the
ORNL model using
average heating
and cooling system
efficiencies. This
may be appropriate
for making reroofing
choices be –
tween reflective
ver sus absorptive
roof membranes.
However, for new
buildings, conditioning
system efficiencies
are most
likely much great –
er.
The data shown
in Figure 3 suggest
that white, highly
reflective membranes
cause an increase in heating costs
relative to dark membranes, depending on
location. This is clearly the case for Chicago
and Newark. However, it must be noted that
cooling costs dominate commercial building
energy costs. Consequently, arguments that
darker membranes have a role in controlling
energy costs in northern climates cannot
be justified by the data.
Clearly, however, as insulation value
increases, the influence of membrane temperature
on internal building temperature
is minimized. This indicates that there are
diminishing returns to the cost of a highly
reflective roof; or, put another way, the payback
period will be longer. Nevertheless,
there are savings to be achieved – especially
in sunnier regions, regardless of the insulation
value.
As energy costs escalate, then clearly
the value of the savings will also increase.
This is shown in Figure 4, with a projected
future cost of gas being $25/1000 cf and
electricity being $17/kWhr.
However, in those northerly zones where
the heating increases and cooling savings
balance out, there is no change. Again, it
should be noted that the data assume average
energy costs across the U.S.
CONCLUSIONS
1. In every case examined, the use of
highly reflective roof membranes
results in energy savings for commercial
buildings, regardless of the
insulation level used.
2. Energy costs incurred by commercial
buildings are dominated by
cooling costs. Therefore, the contribution
of less-re flective roofs in low-
Figure 3 – Annual heating and cooling saving vs. location for a reflective membrane (TSR = 0.78, E = 0.90)
compared to a black roof over R-13 insulation.
20 • I N T E R FA C E DE C E M B E R 2009
ering heating
costs in northern
climes is
not significant
to overall energy-
cost reduction.
In other
words, the val –
ue of black
roofs in lowering
heating
costs is outweighed
by
low er cooling
costs given by
highly reflective
membranes.
3. Not unexpectedly,
the modeling
shows
that the value
of highly reflective
roofing is
greatest in U.S. Climate Zones 1
through 3. Within these three zones,
further calculation may not be
required, and white roofing may be
assumed to always provide substantial
savings.
4. For U.S. Climate Zones 4 through 7,
while white reflective roofing has cost
savings, these may be very small,
depending on individual building
design. It is strongly recommended
that in these zones, an analysis be
done on a case-by-case basis. Such
an analysis would involve estimating
likely annual heating and cooling
costs. In situations where cooling
costs are projected to dominate, then
white reflective roofing will offer savings.
DE C E M B E R 2009 I N T E R FA C E • 2 1
Figure 4 – Annual total energy savings vs. energy costs for a reflective membrane (TSR = 0.78, E = 0.90)
compared to a black roof, for five different locations: 1. Miami, FL; 2. Fort Worth, TX; 3. Fresno, CA; 4, Newark,
NJ; 5, Chicago, IL.
Thomas J. Taylor is the director of low-slope research and
development for GAF Materials Corporation. This position
involves new product development as well as marketing and
manufacturing support. Tom has over 18 years of experience
in the building products industry, all working for manufacturing
organizations. He received his PhD in chemistry from
the University of Salford, England, and holds approximately
30 patents.
Thomas J. Taylor
Linlin Xing is a principal scientist of the research and development
department for GAF Materials Corporation. She has
over 11 years of industrial experience. She received her PhD
from the Department of Polymers and Coatings at North
Dakota State University. Xing is a recipient of the 1998 Roon
Foundation Award from the Federation of Societies for
Coatings Technology (FSCT). She has published over 20 technical
articles and holds eight U.S. patents and one Chinese
patent. She may be reached at lxing@gaf.com.
Linlin Xing
CLARIFICATION
The research presented by authors Bas Baskaran and Suda Molleti in the article, “Air Intrusion vs. Air Leakage:
The Dilemnma For Low-Sloped, Mechanically Attached Membrane Roofs,” published in the November 2009 issue of
Interface, was sponsored by the Special Interest Group for Dynamic Evaluation of Roofing Systems (SIGDERS).
SIGDERS is a consortium of partners interested in roofing design. The partners are: Atlas Roofing Corporation;
Canadian General-Tower Ltd.; Canadian Roofing Contractors’ Association; Carlisle SynTec Incorporated; Dow
Roofing Systems; Duro-Last® Roofing, Inc.; Firestone Building Products Company; GAF-Elk Materials Corporation;
IKO Industries Ltd.; Johns Manville Inc.; National Roofing Contractors Association; OMG Roofing Products; Public
Works and Government Services Canada; RCI, Inc.; Sika Sarnafil; Soprema Canada Inc.; Tremco Inc.; and Trufast
Corporation.