Evaluation of Roofing Renovation Methods by an Energy-Based Life-Cycle Analysis

EVALUATION OF ROOFING RENOVATION METHODS
BY AN ENERGYBASED
LIFECYCLE
ANALYSIS
IVAN LEE; STEVEN MURRAY, PENG, PMP; ROBIN KOKE
MORRISON HERSHFIELD LIMITED
1005 Skyview Drive, Suite 175, Burlington, Ontario, L7P 5B1
Phone: 905-319-6668 • Fax: 905-391-5548 • E-mail: ilee@morrisonhershfield.com
2 7 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • MA R C H 1 5 2
0 , 2 0 1 2 L E E A N D MU R R A Y • 1 6 3
ABSTRACT
As one of the most critical parts of the building enclosure, the roofing system has a significant
impact on both the energy performance and the durability of the building. It is widely
known that roofs of greater insulation value will provide greater energy savings in operational
energy. However, very little consideration is given to the embodied energy of the roofing
materials as it relates to the total energy of the assembly. This presentation takes a
holistic approach in evaluating the energy performance of various conventional roofing
replacement options, taking into account the embodied energy cost, operational energy savings,
and expected service life of the roof construction to determine the actual life-cycle cost
on an energy basis.
SPEAKERS
IVAN LEE — MORRISON HERSHFIELD LIMITED BURLINGTON,
ON
IVAN LEE is the sustainable design coordinator at Morrison Hershfield’s Burlington, ON,
office. He has been involved in various sustainable-design and LEED projects as well as
durability reviews of building enclosures for both institutional and residential buildings.
Through his involvement in sustainable design, Lee has developed an expertise in energy
modeling, hygrothermal analysis, sustainable building materials, and building-enclosure
design. As a graduate student studying building science, he is familiar with good buildingenclosure
design principles and sustainability concepts. Having participated in sustainable
design competitions, such as the 2009 DOE Solar Decathlon, Lee is familiar with the energy
considerations of sustainable building design, particularly after collaborating with fellow
students on the evaluation of the carbon footprint of various design options to produce a
building that is comfortable, durable, and energy-efficient, resulting in a net-zero-energy
solar home.
STEVEN MURRAY, PENG, PMP — MORRISON HERSHFIELD LIMITED BURLINGTON,
ON
STEVEN MURRAY, PENG, PMP, a principal with Morrison Hershfield, is the director of
the Building Science groups in Ontario and Alberta and manages Morrison Hershfield’s
Burlington, ON, office. He has been practicing in the building science field for over 20 years
and has developed particular expertise in envelope rehabilitation, roofing design, and
infrared thermography through the investigation and repair of dozens of envelope failures.
Steve regularly lectures and presents on technical issues. He has been the roofing module
lead since the inception of OBEC’s Building Science Certificate program at the University of
Toronto School of Continuing Studies nearly ten years ago. Murray presented at ASHRAE 9
in Clearwater, Florida, and at BST 10 in Ottawa. Steve is a past board member of the RCI
Ontario Chapter and a past winner of the RCI Horowitz Award for his paper “Solving Roof
Leaks with Fans.”
1 6 4 • L E E A N D MU R R A Y 2 7 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • MA R C H 1 5 2
0 , 2 0 1 2
Background and Methodology: Embodied
Energy of Roofing Materials
EVALUATION OF ROOFING RENOVATION METHODS
USING AN ENERGYBASED
LIFECYCLE
ANALYSIS
INTRODUCTION
As one of the most critical elements of
the building enclosure, roofs have a significant
impact on building performance in
terms of both durability and energy performance
since they are subjected to severe
environmental loads. Like all parts of the
building enclosure, the materials used in
the roof assembly have a finite service life
that requires maintenance and, ultimately,
replacement. Traditionally, building and
energy codes and capital costs have driven
the design and approach to roof replacements
and the specification of roofing materials.
This approach has been slightly
altered by the recent sustainability movement,
particularly by the market transformation
influences of green building rating
systems such as LEED®. However, many of
these rating systems are subjective and are
single-criterion-orientated, choosing to
focus only on materials containing higher
recycled content levels rather than the
impact of the material over its lifespan.
Instead, a life-cycle approach is considered
a much more effective way of determining
the environmental impacts of building
materials and construction, since environmental
life-cycle assessments (LCA) provide
a unique method of evaluating the environmental
impact of a material or system. LCA
reports on the environmental burden associated
with the manufacture, transportation,
maintenance, and disposal of the
product throughout its life cycle.
Since LCA take into account the entire
lifespan of the material or system, additional
environmental benefits such as savings in
operational energy may also be taken into
account. When the environmental burden of
a material or system is expressed in financial
terms, LCA, in the form of life-cycle
costing (LCC), can fiscally capture the environmental
burden associated with the material
itself, as well as its impact on the performance
of the building on which it is used
in terms of dollars. The effects of the environmental
burden and potential benefits in
building performance can be expressed as a
payback period. For example, the use of
additional insulation materials will incur a
heavier environmental impact, but this will
be offset by a reduction in operational energy.
In this case, the payback period is simply
the time in which the operational environmental
benefit equals the embodied environmental
effect of the increased insulation.
This makes LCC a powerful tool in helping
building owners and designers make the
appropriate environmentally and fiscally
responsible decisions.
The paper takes an LCC approach to
determine the environmental burden associated
with various roofing retrofit designs
for a typical commercial/institutional building.
The embodied environmental burden
and operational energy benefits of these different
roofing types will be evaluated over
the expected lifespans as payback periods.
The analysis is based on a case study of a
typical commercial building located in
southern Ontario.
All building materials require energy for
production and transportation. The amount
of energy used during this process is
referred to as embodied energy. Embodied
energy plays a significant role in sustainability
because it is an important measure
of the overall environmental impact of the
product that goes beyond its service life. In
most cases, products that have higher
embodied energy typically have a much
more severe impact on the environment
since they typically require more energy to
extract, process, and transport the necessary
resources. Materials with higher
embodied energy typically also have a higher
carbon footprint. Therefore, materials
with lower embodied energy are typically
better for the environment.
Most materials used in a roof assembly
can be typically categorized into two categories
based on their function: thermal
insulation and vapor protection. Because of
the extreme conditions that most roofs are
exposed to, in order to provide the adequate
thermal and vapor resistance, most roof
assemblies use specialized materials, each
designed for a particular environmental
load. For most commercial roofs, thermal
insulation is typically provided by rigid
insulation such as polyisocyanurate, while
moisture protection is typically provided by
a bitumen membrane, such as a two-sheet
modified bitumen. The embodied energy of
these materials, as well as roofing materials
from the past, is provided in Figure 1. The
embodied energy was calculated using software
developed jointly by Morrison
Hershfield and the Athena Institute. The
Figure 1 – Embodied energy of roof materials.
2 7 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • MA R C H 1 5 2
0 , 2 0 1 2 L E E A N D MU R R A Y • 1 6 5
Product manufacturing:
On-site construction:
Maintenance and replacement:
Building end of
life :
Operational Energy of Roofing
Assemblies
Figure 2 – Embodied energy and insulating value of roofing materials.
software is called Athena Impact Estimator
for Buildings. It takes into account the full
life cycle of the materials. The life cycle
includes every step of the material’s life,
from resource extraction to disposal,
including manufacturing, transportation,
construction, and maintenance of the materials
as a life cycle inventory (Athena
Institute, 2010). Athena contains LCI profiles
of a variety of building materials,
including roofing materials. It covers a
building’s life-cycle stages from cradle to
grave or end of life, encompassing the following
stages:
• extraction
(from nature or the technosphere);
resource transportation; and manufacturing
of materials, products, or
building components.
• product
/component transportation
from point of
manufacture to the
building site and onsite
construction activities.
•
life cycle
maintenance and replacement
activities
associated with the
structure and enclosure
components
based on building
type, location, and
user-defined life for
the building.
• “
” simulates demolition energy
and final disposal of materials incorporated
in a building at the end of
the building’s life.
Athena’s building materials databases
are local to regions within North America,
representing average or typical manufacturing
technologies and appropriate transportation
modes and distances. The databases
are able to determine the embodied
energy of various materials over 12 geographic
regions (including Toronto, upon
which this study was based).
From Figure 1, it is clear that roof membranes
are the most energy-intensive material
used in the roof assembly. This is due to
the membrane’s energy-intensive manufacturing
process and the inability to effectively
recycle these materials. Conversely, the
Figure 3 – Roof plan.
insulation materials considered used only
1.6% to 2.6% of the roof membrane’s
embodied energy.
In the interest of life cycle energy performance,
it is important to look at not only
the embodied energy density but also the
thermal performance, since any energy
spent during the manufacturing process
may be recovered through operational energy
savings. Figure 2 shows the difference in
embodied energy and thermal resistance of
the roofing materials.
Since sustainability takes into account
both embodied energy and operational
energy, it is important to look at both of
these metrics when considering the overall
energy performance of roofing materials to
help choose which components provide the
greatest potential in achieving an embodied
and operational energy balance.
The operational energy of the various
roofing designs was determined using a relatively
simple method of heating degree
days (HDD) and cooling degree days (CDD)
based on the building code to which the
building was designed. The HDDs and
CDDs were based on a temperature of 18ºC.
The insulating value of the roof assemblies
was calculated, and the HDD and CDD were
applied to determine the heating and cooling
loads, which were then converted into
an annual operational cost through local
utility rates, similar to the rates used to
convert the embodied energy of the building
materials. The HDD and CDD were from
CWEC weather data provided by the U.S.
Department of Energy that is typically used
1 6 6 • L E E A N D MU R R A Y 2 7 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • MA R C H 1 5 2
0 , 2 0 1 2
Background
Alternative Roofing Options
Energy Performance of Alternative
Roofing Options
Option 1 Roof Replacement to Match
Existing Default)
Option 2 Complete Roof Replacement
Figure 4 – Existing roof design cross-section.
for building energy simulation
software such as
EnergyPlus and eQuest.
This method of analysis
only takes into account the
secondary energy associated
with the embodied energy
and operational energy of
the roofing systems. As a
result, it does not consider
the overall environmental
impact, such as expressing
the overall energy consumption
in terms of primary
energy would have. To
effectively convert from secondary
to primary energy is
difficult since such conversion is dependent
on power generation sources. This method
also does not take into account market
forces that will have a significant influence
on the overall payback period of these materials.
CASE STUDY
The Royal Botanical Gardens (RBG),
located at the west end of Lake Ontario in
Burlington, ON, contain multiple buildings,
32 kilometers of trails, and many outdoor
gardens.1 The buildings on site were constructed
at multiple time periods from the
late 1950s to the 1980s. One section of the
RBG Centre, an office block that was constructed
in 1957, required a roof replacement.
2 The roof was estimated to have been
constructed in the 1980s and was beyond
its 25-year service life. Portions of the roof
had leaked, and some sections of insulation
were known to be wet. A layout of the roof is
provided in Figure 3.
The existing roof was constructed of 25
mm of fiberglass insulation and 25 mm of
wood fiberboard that were protected from
moisture with a 4-ply roof membrane and a
layer of pea gravel ballast. This assembly
has a total R-value of R-6.5 (U=0.874
W/m2K). A cross section of the existing roof
is shown in Figure 4.
In order to bring the roof up to date with
new construction standards required by the
building code, the upgraded roof required
an insulating value of R-20 (U=0.289
W/m2K). It is noted that the local building
code standards for renovations only require
the roof’s original overall R-value to be
maintained as a minimum. Several alternatives
were proposed using the existing
building materials and newer polyisocyanurate
insulation and modified-bitumen roof
membrane. The roofing options are listed
below:
1. Complete replacement of the existing
roof; the new roof will be rebuilt
using the same types of materials as
the original to R-6.5.
2. Complete replacement of the existing
roof; the new roof will be built to
R-20.
3. Overlay new roofing materials over
the existing roof with enough material
to meet R-20.
4. Overlay new roofing materials over
the existing roof with the same quantity
of materials from option 2; this
will bring the roof assembly to R-26.
5. Overlay of new roofing materials
over the existing roof with an additional
insulation from option 4 to
provide a roof with an insulating
value of R-33.
These five options were considered in
the analysis to balance embodied energy
with operational energy savings over the
expected service life of the roof.
In order to determine the overall energy
performance of the roofing options, both the
embodied energy and operational energy
performance were determined.
The embodied energy of the alternative
designs were determined using the embodied
energy densities from the Athena Impact
Estimator for Buildings, while the operational
performance was determined by calculating
the overall thermal transmittance
(U-value) of the roof assembly with the HDD
and CDD for the Toronto, ON, climate. The
roof assembly surface characteristics were
assumed to be the same (i.e., emissions,
albedo, and reflectance) because the granule
surfaces of the modified-bitumen cap
sheets were similar to the gravel ballast.
–
(
This is the default option in which the
entire existing roof assembly will be
removed and replaced with new materials.
The new roof will use the same types of
materials as the existing roof and will have
an R-value of R-6.5 (U=0.87 W/m2K). Since
this option removes the embodied energy
built into the existing roof, it has the second-
highest embodied energy density at
15,594 MJ/m2 and also the highest annual
heating and cooling loads at 17.52 MJ/m2
and 308.79 MJ/m2, respectively.
–
The first option is a traditional roof
replacement where the existing roof assembly
is removed and is replaced with 76.2
mm (3 in.) of polyisocyanurate insulation
and two-sheet modified-bitumen roofing
membrane. This produces a roof assembly
with an R-value of 19.7 (U=0.29 W/m2K).
This option is the most embodied energy-
intensive option since it not only
removes the existing roofing material but
also adds a significant amount of new materials
to the roof. As a result, this option has
an embodied energy density of approximately
19,392 MJ/m2.
Since this option does not make use of
the existing roof insulation, it has one of the
lowest R-values. In the Toronto, ON, climate,
it is expected to use 5.79 MJ/m2 for
2 7 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • MA R C H 1 5 2
0 , 2 0 1 2 L E E A N D MU R R A Y • 1 6 7
Option 3 Roof Overlay to R-20
Option 4 Roof Overlay to R-26
Option 5 Roof Overlay to R-33
Life Cycle Cost Analysis: Capital Costs
Operation Costs
Roof Types Embodied Energy
[MJ/m2] Heating Load
[MJ/m2] Cooling Load
[MJ/m2] Total
[MJ/m2] Default
Option 1 7797.00 17.52 308.79 8,123.31
Option 2 19391.82 5.79 101.97 19499.58
Option 3 11539.66 5.80 102.31 11647.77
Option 4 11601.44 4.35 76.67 11682.46
Option 5 11663.22 3.48 61.31 11728.01
Table 1 – Energy performance of roof alternatives.
heating and 101.97 MJ/m2 for cooling
annually.
–
In this option, the new roofing materials
considered in Option 2 will be installed over
the existing roofing assembly, while only the
pea gravel ballast will be removed. However,
in a conscious effort to reduce the amount
of embodied energy density and materials,
the amount of insulation will be reduced by
25.4 mm (1 in.) to 50.8 mm (2 in). This will
give the roof assembly an overall R-value of
R-19.7 (U=0.29 W/m2K) and produce a roofing
assembly of R-19.7 to meet that of
Option 2, but with a 40% reduction in
embodied energy at 11536 MJ/m2. The
annual heating and cooling load is expected
to be 5.80 MJ/m2 and 102.31 MJ/m2,
respectively.
–
In this option, the same 76.2-mm polyisocyanurate
insulation and 2-ply modifiedbitumen
roof membrane will be installed
over the existing roof. Only the pea-gravel
ballast will be removed. Like Option 3, this
option makes use of the embodied energy
and thermal insulation already invested in
the existing roof, but this roof is more thermally
insulating. As a result, the roof has
an overall R-value of R-26 (U=0.22 W/m2K),
a significantly lower embodied energy density
at 11,601 MJ/m2 and has an annual
heating and cooling load of 4.35 MJ/m2 and
76.67 MJ/m2, respectively.
–
Similar to Options 3 and 4, Option 5 is
another roof overlay. However, in anticipation
of stricter energy codes, Option 5
increases the amount of insulation by 25.4
mm (1 in.), from 76.2 mm (3 in.) in Option
3 to 101.6 mm (4 in.). This is considered the
practical limit of insulation that can be
added to the roof. Adding any new materials
to the roof would increase roof thickness
and reduce curb heights, which would
require reconstruction of the roof curbs and
parapets. This would add an additional
installation cost not applicable to other
options in this analysis. This raises the Rvalue
to R-34 (U=0.174 W/m2K) and the
expected heating and cooling load is 3.48
MJ/m2 and 61.31 MJ/m2, respectively.
A summary of the embodied energy and
estimated heating and cooling loads is provided
in Table 1.
From Table 1, it is clear that Option 2 is
the most energy-intensive, with the highest
embodied energy density and operational
energy loads. This is effectively the industry
default option given the conservative
approach to dealing with potential retained
moisture in the assembly. However, at this
point it is difficult to determine which
option uses less energy over its expected
lifespan and which option offers the greatest
payback. In order to quantify these performance
metrics, a life cycle cost analysis
must be performed.
The capital costs of the options include
both construction and embodied energy
costs. The construction costs can be considered
as capital costs in a traditional roof
rehabilitation project and include demolition,
disposal, construction, and materials
costs. The costs used in this analysis were
based on costs from similar roof replacement
projects, including the RBG project,
where the existing roof was removed and
replaced. The comparison projects were
selected for their relevance to overlays versus
complete removal and where competitive
tender pricing was received for both
options. This ensures that the cost analysis
presented reflects that of current market
pricing.
For options in which only partial demolition
took place, such as the overlay
options, the disposal costs were reduced by
the proportion of the embodied material left
in place. The remaining demolition costs,
including mobilization, were constant
throughout all options.
The tender-pricing data was used to
determine the typical proportional costs for
demolition and disposal for options not
specifically tendered.
Similarly, the construction costs
remained constant for all options, except for
variations in the material cost of the insulation.
For simplicity, the material cost of the
insulation was assumed to be on a perarea,
per-thickness basis. This made it easier
to determine the costs of the polyisocyanurate
insulation for the various thicknesses
in this analysis.
A summary of the capital cost is provided
in Table 2.
The operation costs were based on the
heating and cooling loads of the roofing
Options Demolition Costs
($/m2)
Supply and
Install Costs
($/m2)
Total Cost
($/m2)
Option 1 $104.53 $201.58 $306.11
Option 2 $104.53 $211.58 $316.11
Option 3 $30.05 $201.58 $231.63
Option 4 $30.05 $211.58 $241.63
Option 5 $30.05 $282.10 $312.15
Table 2 – Construction costs of roofing options.
1 6 8 • L E E A N D MU R R A Y 2 7 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • MA R C H 1 5 2
0 , 2 0 1 2
Life-Cycle Costs
Life-Cycle Energy Balance
Fuel Type Rates
($/kWh)
Rates
($/MJ)
Electricity $0.05 $0.18
Natural Gas $0.05 $0.18
Table 3 – Utility rates.
options and average utility rates for electricity
and natural gas. Since this analysis does
not take into account the HVAC system of a
typical building, it was assumed that the
fuel type consumption is split evenly for
both types of fuels at 50%. The utility rates
used in this analysis are listed in Table 3.
In order to determine the full life-cycle
cost of the roofing options, taking into
account the capital and operations costs
and expected service life, the future value of
the heating and cooling costs were determined
for the entire service life of the roof.
This was determined using an assumed
inflation rate of 3%. Since these rehabilitation
techniques will likely have different
service lives, full replacement (Options 1
and 2) at 25 years, and overlays (Option 3
to 5) at 20 years, the operation costs must
be normalized such that these options can
be compared. This was done by calculating
the present value of the operation cost
using an estimated interest rate of 5%. The
resulting capital costs and present value
operation cost are presented in Figure 5.
Figure 5 shows that while there is some
variation in total cost, the capital costs
remain relatively consistent across all five
options. Much of the variation in this cost is
from the heating and cooling loads as the
options with increasing R-values have lower
operational costs. Figure 5 also suggests
that there seems to be a limit in operational
energy savings when compared to capital
costs. For the assumed utility rates and
southern Ontario climate, the optimal insulation
thickness for a roof overlay design is
approximately 76.2 mm (3 in.) or approximately
an additional R-20. Increasing the
insulation beyond this thickness will further
reduce the operation costs; however,
the savings will be less than the additional
capital costs of the added material. This is
expected, as the tendency of diminishing
returns in more insulating materials is well
understood.
This trend in diminished overall returns
in cost is also clear when comparing the
return on investment among
the four roof upgrade options,
Options 2 to 5. The capital
and operation cost savings
were calculated based on
Option 2 costs for the overlay
designs. Figure 6 shows a
comparison of the cost savings
for Options 3 to 5 when
compared to Option 2 costs. Since all three
of these design options include an overlay of
the existing roof, there are savings in construction
and demolition costs.
As in Figure 5, Option 4 showed the
greatest return on investment, yielding a total
savings of $140.43 and a good balance
between capital and operation savings. This
balance was not found in Options 3 and 5,
where much of the savings in Option 3 was
from capital costs, while the significant savings
in Option 5 were from operation costs.
It is interesting to note that the total savings
of Option 5 are actually lower than Option
3. This indicates that at current utility market
rates, a decrease in energy consumption
of 12 kWh/m2/yr. does not offset the added
capital cost from the extra 50.8 mm (2 in.)
of polyisocyanurate insulation.
The life-cycle cost analysis, however,
does not take into account the embodied
energy of the materials required and disposed.
Instead, a life-cycle energy balance
Figure 5 – Capital and operation cost of roof options.
Figure 6 – Return on investment of capital amongst roof overlay options.
2 7 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • MA R C H 1 5 2
0 , 2 0 1 2 L E E A N D MU R R A Y • 1 6 9
Sensitivity Analysis
Figure 7 – Life-cycle energy balance of roof options.
was conducted to determine the economic
and environmental impact of the different
roof rehabilitation options.
In order to effectively take the embodied
energy into account in an economic way,
the embodied energy of the materials referenced
from Athena were converted into a
capital energy cost, using the same utility
costs listed in Table 3. Although the actual
utility costs associated with manufacturing
and transportation will vary, using the
same rates for both the embodied and operational
energy keeps
the associated energy
costs consistent
between both types of
energy. The embodied
energy costs were
then added to the present
value of the operation
energy costs to
provide an energy balance.
Figure 7 shows
the energy balance
expressed as a cost for
all roofing options.
From Figure 7, it is
clear that, among all
roof designs, the embodied
energy significantly
dwarfs that of
the operation energy.
This is especially the
case for Option 2,
which is considered
the typical roofing
rehabilitation design
that replaces the
existing roof with new materials. The roof
overlay designs, by comparison, use significantly
less energy by reusing the existing
roofing material. The savings are significant
despite having a shorter service life of 20
years as opposed to 25.
While the operation costs are relatively
small compared to the embodied energy
costs, the delicate balance in operation
energy and embodied energy from using
various thicknesses of insulation does affect
the total energy cost. Since the additional
embodied energy cost from the additional
insulation is relatively small compared to
the embodied energy of the roofing membranes,
the operational energy saved from
the insulation is significant. From Figure 7,
the optimal design in terms of energy balance
is Option 5 as opposed to Option 4 due
to the operational cost savings. However,
the difference in the overall energy balance
is very small.
In addition to the base analysis, a sensitivity
analysis of some of the variables was
also taken to determine how the results
would be altered. Among the variables considered
were the following:
1. A shorter service life of 15 and 20
years for the overlay and replacement
designs, respectively
2. Equal service lives of 20 years for
overlay and replacement designs
3. Increased fuel cost, up to double the
cost for both electricity and natural
gas
4. Higher interest and inflation rates
from 5% and 3%, to 15% and 13%,
respectively
The results of these factors on the overall
trends of the capital and operation costs
are presented in Figure 8. This figure shows
that altering the service lives, fuel cost,
Figure 8 – Sensitivity analysis of capital and operation cost of roof options.
1 7 0 • L E E A N D MU R R A Y 2 7 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • MA R C H 1 5 2
0 , 2 0 1 2
Figure 9 – Sensitivity analysis of life-cycle energy balance of roof options.
interest, and inflation
rates does not significantly
alter the trends
observed from the
base case. The capital
and operational cost
trends did not significantly
change for service
lives between 15
and 25 years, suggesting
that service lives
within this range have
very little effect on the
life-cycle costs of the
project. Doubling the
fuel costs did increase
overall life-cycle cost
by raising the operational
cost. It also
resulted in greater
savings for heavily
insulated designs
such as Options 4 and
5. The cost difference
between these designs
significantly decreased.
However, the
increase in fuel costs was not significant
enough to change the optimum design from
Option 4 to 5. If fuel costs were to continue
to climb, the more insulating design would
be favored. Conversely, increasing both the
interest and inflation rates significantly
decreased the operation cost to favor the
less-expensive construction designs, such
as Options 3 and 4. However, even at
extremely high interest and inflation rates
of 15% and 13%, respectively, Option 4 is
still the most cost-effective design over a 20-
year lifespan.
Similar trends were also noted on the
life-cycle energy balance as shown in Figure
9 as Option 5 has the least energy cost and
is still the optimum design for all scenarios.
The service life showed very little effect on
the overall energy balance, while higher fuel
costs tended to favor the heavily insulated
designs, and higher interest and inflation
rates favored the least capitally expensive
designs.
CONCLUSIONS
From the results of this life-cycle cost
analysis, it is clear that roof overlay designs
for major roof rehabilitation projects provide
an environmentally sustainable and
cost-effective method of repairing roofs.
This design approach makes use of the
existing roofing materials by prolonging the
roof’s service life. This reduces the demand
for virgin or recycled process materials and
waste materials sent to the landfill, resulting
in significant savings in embodied energy.
With the assumed market utility rates
and a typical service life of 20 years, a roof
overlay project with approximately 76.2 mm
(3 in.) of insulation provides the optimal
savings in both capital and operations cost.
Providing less insulation will result in
greater heating and cooling costs that will
offset the savings in material cost.
Similarly, providing more insulation will
yield operations savings that are less than
the additional materials cost. However, from
an energy-balance perspective, the optimal
design is to provide an additional inch of
insulation to the roof overlay. This results
in energy savings that are significantly
greater than the additional embodied energy
used in the insulation. From these
results, it is likely that the optimal design in
terms of both life-cycle cost and energy balance
is between 76.2 mm and 101.6 mm (3
and 4 in.).
Similarly, a sensitivity analysis revealed
that altering the service life from 15 to 25
years showed very little change in the overall
cost and energy balance, while increasing
the fuel cost favors the more-insulating
roof designs, and increased interest and
inflation rates favor the less-capital-cost
designs. However, doubling the fuel cost
and raising the interest and inflation rates
by 10% did not shift the optimum points
from Options 4 and 5 for overall costs and
energy balance, respectively.
LESSONS LEARNED
Owners and designers often reject the
option of an overlay system due to perceived
risks such as these:
• Water retention within the original
assembly,
• Structural concerns due to added
dead load,
• Adequacy of securement of the original
assembly, and
• Reduced life expectancy compared
to a complete replacement option.
In addition, there is often a perception
that pursuing more environmentally sustainable
options incurs extra costs.
The authors have termed the efforts
required to address these risks as the
“repair and prepare” provisions; i.e., repair
the existing assembly, remove wet materials,
and prepare the surface of the assembly
for the overlay application. The case
study design included repair and prepare
provisions such as these:
• Potential areas of retained water
2 7 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • MA R C H 1 5 2
0 , 2 0 1 2 L E E A N D MU R R A Y • 1 7 1
were locally removed.
• The dead load of the new assembly
was offset by the removal of the original
gravel ballast.
• A small perimeter strip of the original
assembly was removed and
replaced to effect system securement.
(see Figures 3 and 4).
The financial analysis used actual construction
costs, procured through competitive
tendering, to capture the costs of these
“repair-and-prepare” provisions, and the
analysis assumed that the overlay would
have a shorter lifespan than the complete
replacement.
The authors believe that this case study
has demonstrated that there are actually
financial benefits possible from the intelligent
integration of environmental benefits.
The option selected and installed at the
subject facility achieved capital cost savings
and operational cost savings. This is an
attractive proposition for building owners
and managers normally faced with decisions
based on trade-offs of capital versus
operational costs.
The embodied energy of the materials in
the original assembly can continue to
reduce operational energy usage through
another life cycle. This effectively amortizes
the embodied energy investment over twice
the time frame and extends the time frame
to receive the dividends of the investment.
Extending this logic further, it is possible to
fully offset the embodied energy investment
if a long enough service life is achieved.
The concepts and design options presented
in this paper are relatively simple;
however, they provide a unique framework
by which to assess roof renewal projects.
The authors hope that owners and designers
may make use of the framework to more
critically evaluate roofing renovation
options by utilizing energy-based life-cycle
analysis.
REFERENCES
Athena Institute. “The Athena Institute
– Impact Estimator for Buildings,”
retrieved from athenasmi.org, 2011.
R. Koke, Comparing the Energy Efficiency
of Alternative Roofing Designs,
Morrison Hershfield Limited,
Burlington, Ontario, 2010.
M. Lucuik, “A Comparison of the Embodied
and Operational Environmental
Impacts of Insulation in
Office Buildings,” 10th Canadian
Conference on Building Science and
Technology, Ottawa, Ontario, 2005.
Royal Botanical Gardens, 2010.
J. Straube, E. Burnett, Building Science
for Building Enclosures, Building
Science Press, Westford MA, 2005.
U.S. Department of Energy, retrieved
from energy.gov, 2011.
M. Vervoorn, “RBG – Condition Letter
Report,” Letter to Mel Gedruj, director
of RBG, 2009.
FOOTNOTES
1. Royal Botanical Gardens, 2010.
Retrieved 2010 from www.rbg.ca.
2. M. Vervoorn, “RBG – Condition
Letter Report,” Letter to Mel Gedruj,
director of RBG, 2009.
1 7 2 • L E E A N D MU R R A Y 2 7 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • MA R C H 1 5 2
0 , 2 0 1 2