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Cool Roof Calculator
Vishal Garg, Shikher Somal, Rathish Arumugam1, Aviruch Bhatia
International Institute of Information Technology, Hyderabad (India)
ABSTRACT
In tropical countries such as India, increasing the roof albedo helps in reducing the heat
ingress through roof. This reduces the HVAC energy consumption in conditioned buildings and
to increases comfort in un-conditioned buildings. The benefits of high albedo roofs are depended
on various parameters such as the outdoor climatic conditions, operational schedules, internal
loads, envelope gains, efficiency of the air-conditioning system (in a conditioned space) etc. In
order to help users determine the benefits of high albedo roofs under varying conditions, a simple
calculator has been developed. Parameters such as location, building type, roof area, surface
properties of the roof are taken as inputs. Annual EnergyPlus simulations are performed for the
given parameters and the results are displayed in both graphical and tabular formats. It also
calculate the simple payback by comparing a given base case roof albedo with the proposed roof
albedo. The calculator can also perform comfort simulations for un-conditioned buildings.
The calculator also simulates measures such as radiant barrier system and under deck
roof insulation. The calculator also runs a parametric simulation between insulation thickness
and roof albedo to find an optimum roof insulation thickness based on Incremental Internal Rate
of Return. Each user can choose to have a user account in which the simulations performed by
the user will be stored and displayed. These results are cached to provide faster results. This
paper presents the features of the cool roof calculator and the type of analysis that could be
performed using the cached results.
1. Introduction
A roof that reflects and emits the solar radiation back to the sky rather than transferring it into the
building is termed as cool roof (Cool Roof Rating Council, 2013). Cool roof helps in reducing
air-conditioning energy consumption in a conditioned building and improves thermal comfort in
an un-conditioned building. The energy savings achieved in a building from cool roofs is
dependent on various parameters like the location, orientation of the building, local shading by
trees or other buildings, construction type, insulation type, plenum ventilation, equipment load,
occupancy and operational schedules (Konopacki&Akbari, 2001).
There have been several studies ((Parker, Sherwin, Sonne, &Barkazi, 1996)(Parker, Sonne, Sherwin, &
Moyer, 2002),(Konopacki& Akbar, 2000).(Konopacki&Akbari, 2001)(Akbari et al.,2011)(Bozonnet, Doya,
& Allard, 2012))to demonstrate the benefits of cool roofs. Most of them have been experimental
studies. Considering the advantage, and flexibility in evaluating the effect of cool roof through
simulations and models, research exists in the development of various simulation models and
tools calculating the effect of cool roof.
1
Corresponding Author: [[email protected]], [phone], [Center for IT in Building Science,
IIIT, Hyderabad, India PIN 500032].
There exists two internet based calculators for understanding the benefits of cool roofs. Both
these tools are developed for US. Both these tools use DOE 2.1 platform for simulations. The
features of both the calculators are described below:
 DOE Cool Roof Calculator &DOE steep slope calculator
 Roof Savings Calculator (RSC) (http://www.http://rsc.ornl.gov/),
DOE Cool Roof Calculator calculates the annual energy savings in cooling and heating for
a given roof solar reflectivity (SR) in comparison with a black roof, for low slope 79 roofs.
Alternatively, the tool also calculates the necessary roof insulation R-value to achieve the same
savings as achieved by thegiven solar reflectivity. This tool has mainly been developed for the
US weather data. There are 243 different locations that are available in the pull-down list of the
calculator, with 235 of them belonging to US, Pacific territories and Puerto Rico, and the rest 8
to Canada. The inputs those are to be provided by the user in this calculator are: R value, solar
reflectivity, infrared emissivity (IE), electricity costs and HVAC equipment efficiency. The
output includes: cooling and heating energy savings and the required R value of the roof to
achieve a same savings that has been achieved by the given solar reflectivity value. Further, this
calculator also estimates the energy and peak demand savings for a given solar reflectivity in
comparison with a black roof. The input fields are the same; however, the outputs include the
heat load reduction values during the cooling season. Also, the DOE steep slope calculator
estimates cooling and heating savings for residential roof with non black surfaces, for steep
roofs.
Roof Savings Calculator (RSC) developed by ORNL and LBNL together, has been very
recently updated, and is now running in its beta release. Earlier the calculator use to simulate a
typical kind of building without any mention of the building type. Now, there exist two
categories: Residential and commercial, and two modes: simple and advanced. The calculator
performs whole building energy simulations are performed using DOE 3.1e engine for fast
energy simulation and integrates AtticSim for advanced modelling of modern attic and cool
roofing technologies. The calculator runs an hourly simulation for the provided building
properties such as: conditioned floor area, number of floors, window details, HVAC details, and
the existing and proposed roof details, for the selected location. Annual energy savings reported
are based upon heating and cooling loads and thus this calculator is only relevant to building
with a heating and/or cooling unit. Joshua New et al. (2012) has discussed interface and UI of the
calculator.Chand Jones et al. (2012) built a visual analytics tool to assist in the explorationand
identification of features in the datafor Roof Savings Calculator Ensembles.
A tool that has been developed here works on EnergyPlus and does online simulations for
Indian cities for which weather data files are available. Compared to the othertools, the Cool
Roof Calculator provides more input details about the building and also adifferent set of results.
One other major difference between the Cool Roof Calculator and theother two is that, the
former has a capability of simulating an un-conditioned building, forwhich the comfort achieved
through the use of cool roof will be reported. Therefore, frommany others, some of the major
differences of the Cool Roof Calculator compared to othercalculators can be the features as listed
below:


Performs online simulation
Works on EnergyPlus engine




Capable of modellinga un-conditioned space
Generates thermal comfort results
Does payback analysis
Works for all major Indian cities
2. Description of models
There are four kind of building types considered in the calculator - Office, Institute, Retail and
Residential. All these models are square in shape with flat roof. There are five zone in each
model – one core and four perimeter. Plan of the model is shown in Figure 1. The base model is
generated
from
EnergyPlus
example
file
generator
(http://apps1.eere.energy.gov/buildings/energyplus/cfm/inputs/) based ASHRAE 90.1-2007
specifications. The envelope details, lighting power density, Equipment power density, and
Occupancy density of the models used in the tool are summarized in Table 1. The base model
was modified as per the user inputs at the run time and commonly used schedule in such
buildings in India. Schedule profile used in the model is shown in Figure 2.
Figure 1 Top view of the model
Table 1: Input parameters for all models for simple calculator
S.
No.
1.
2.
3.
4.
5.
Residential
Parameter
Exterior Wall
(U-value W/SqmK)
Roof (U-value
W/Sqm-K)
Window
(U-value W/SqmK, SC)
Lighting power
density (W/Sqm)
Equipment power
density (W/Sqm)
Office
Institute
Retail
Type - A
Type – B
0.70
0.70
0.70
0.70
0.70
0.94
0.94
0.94
0.94
0.94
6.8, 0.29
6.8, 0.29
6.8, 0.29
6.8, 0.29
6.8, 0.29
10.76
12.91
16.14
12.91
12.91
16.14
5.38
2.69
5.39
5.39
Parameter
Occupancy density
(Sqm/ Person)
Operation
Office
Institute
Retail
Residential
9.29
2.7
9.29
Mon-Fri
9:00 – 18:00
100%
occupancy
Mon-Sat
8:00 – 18:00
100%
occupancy
Mon-Sun
8:00 – 18:00
100%
occupancy
13.94
13.94
Mon-Sun
Mon-Sun
0:00 – 09:00 0:00 – 09:00
&
&
18:00 – 24:00 18:00 – 24:00
100%
100%
occupancy
occupancy
09:00 – 18:00
50%
occupancy
7
A. Uses pattern for Office model
C. Uses
pattern
for Retail
Residential
Schedule
(Night
time) model
Legends
Occupancy
Equipment
23-24
22-23
21-22
20-21
19-20
18-19
17-18
16-17
15-16
14-15
13-14
12-13
11-12
10-11
9-10
8-9
7-8
6-7
5-6
4-5
3-4
2-3
Lighting
1-2
E. Uses pattern for residential model (Night time operation)
Figure 2 Schedule profile for all type of models
22-23
21-22
20-21
19-20
18-19
17-18
16-17
15-16
14-15
13-14
23-24
23-24
22-23
21-22
20-21
19-20
18-19
17-18
16-17
15-16
14-15
13-14
12-13
11-12
D. Uses pattern for residential model (24 Hours operation)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0-1
12-13
11-12
10-11
8-9
9-10
9-10
8-9
7-8
6-7
5-6
4-5
23-24
22-23
21-22
20-21
19-20
18-19
17-18
16-17
15-16
14-15
13-14
12-13
11-12
10-11
9-10
8-9
7-8
6-7
5-6
4-5
3-4
2-3
0%
3-4
20%
2-3
40%
1-2
60%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0-1
80%
1-2
7-8
B. Uses pattern for Institute model
100%
0-1
6-7
5-6
4-5
3-4
2-3
0-1
23-24
22-23
21-22
20-21
19-20
18-19
17-18
16-17
15-16
14-15
13-14
12-13
11-12
10-11
8-9
9-10
7-8
0%
6-7
0%
5-6
20%
4-5
40%
20%
3-4
60%
40%
2-3
80%
60%
1-2
80%
0-1
100%
1-2
Institute Schedule (Modified)
100%
10-11
S.
No.
6.
3. User Interface and Backend
The tool is developed on Apache/2.2.16 (UNIX). Web pages are developed in PHP and use Ajax
Technologies. At the back end, socket programming has been implemented, the code for which
is written in C. The inputs submitted from the User Interface (UI) are posted to a PHP script for
simple or detailed case which generates the corresponding EnergyPlus Input Data File (IDF) for
both normal and cool roof simulation. The IDF name and the weather file name are then sent to
an executable waiting for a socket connection (to open). The executable then calls a bash script
which performs the simulations while the front end shows the simulation progress. Once the
simulations are done, the UI is redirected to the results page which shows the results.
The calculator_simple.html and calculator_detailed.html pages consist of a form that the user has
to fill in order to submit the parameters for the simulation. It consists of various fields such as
floor area, window percentage, window, roof, wall, floor types, etc. On submission of the
calculator_simple.html form, either of idfgenerator_simple.php or idfgenerator_detailed.php is
called depending on which form is submitted- simple or detailed. Architecture of the calculator is
shown in Figure 3.
Figure 3 Architecture of calculator
As PHP runs under Apache server, so C program is used that makes the system call. Thus, the
callserver.php page opens a socket connection with idfserver.c and passes the weather file and
the count value (IDF no). Once the socket connection is established, callserver.php redirects
itself to checkfile.php. The job of this file is to check if the simulation is completed. The file
refreshes every 5 seconds to check if the simulation has finished. It also makes the environment
more interactive, for it can cancel a running simulation, while it is being simulated. It also shows
a waiting bar and the system time in order to ensure that the simulation is going on. On the
backend, idfserver.c (the object file of it) passes the count value (IDF no) and the weather file
name to executeidf.sh. This script file makes the system calls to perform EnergyPlus simulations.
Once the simulations are done, it signals checkfile.php to redirect itself to the results page i.e.
results.php. The file results.php is called with the count value (IDF no), so it knows which results
are to be parsed. It parses the results file to get the energy savings for the model simulated.
When compared to the existing cool roof calculators, the calculator developed here is more
detailed in terms of the inputs that are to be provided by the user. Broadly the inputs can be put
under the headings of building envelope, internal loads, HVAC, building geometry, and the
location. View of the calculator is shown in Figure 4.
Figure 4 View of the Calculator
4. Description of HVAC Systems
In calculator four different type of Heating Ventilation and Air-conditioning (HVAC) systems
are available for selection –Packaged Terminal Heat Pump (PTHP), Packaged Single Zone
(PSZ)-HP, Central Air conditioner with water cooled chiller and Central Air conditioner with air
cooled chiller. Each building type can be selected with any one of the available HVAC option.
Unconditioned spaces can be simulated by selecting option - No system.(All described in table)
Table 2 Input parameters for different HVAC systems
S.
No.
Parameter
PTHP
1.
Cooling Type
Direct
Direct
Expansion Expansion
coil
coil
2.
Heating Type
3.
Fan Control
4.
5.
6.
7.
8.
PSZ-HP
Central Air
conditioner
Water
cooled
system
Central Air
conditioner
parameters
with air
cooled
chiller
system
Water
cooled
screw
chiller
Air cooled
chiller
None
Electric
Resistance
Electric
Resistance
None
Variable
Air Volume
Variable Air
Volume
None
Unconditioned
Space: No
system
Electric
Heat
Pumps
Constant
Volume
Electric
Heat
Pumps
Constant
Volume
Coefficient of
performance
2.8
3.0
5.5
3.1
Not Applicable
HSPF
1.91
1.91
Not
Applicable
Not
Applicable
Not Applicable
None
None
None
None
None
None
Cooling :
24°C and
Heating
20°C
None
Cooling :
24°C and
Heating
20°C
None
Cooling :
24°C and
Heating
20°C
None
Cooling :
24°C and
Heating
20°C
None
Air-side
economiser
Heat Recovery
Zone air set
point
Not Applicable
5. Simulation Results
The calculator can model several combinations of the building type, location, material type,
HVAC type etc. leading to millions of combinations. It is practically not possible to generate
model for all these combination and check them for accuracy. One simple modelis simulated
with all HVAC system type and weather data is selected for New Delhi climate. Area of the
model is considered 1600 sqm.Roof reflectivity for white roof and dark roof was considered as
0.8 and 0.3 respectively.
A. Office type
A preliminary analysis has been done for office type building simulated with all HVAC
system types. Results for the simulation have been presented in Figure 5.
Figure 5 shows annual energy consumption for all end uses. In each pair of bar first bar is for
dark roof and second is for cool roof. By application of cool roof there is reduction in energy
consumption between 7 to 16 kWh/Sqm/year.
Figure 5Simulation result for simple office model
B. Institute type
A preliminary analysis has been done for Institute type building simulated with all
HVAC system types. Input parameters are as per simple model only. Results for the simulation
have been presented in Figure 6, shows annual energy consumption for all end uses. In each pair
of bar first bar is for dark roof and second is for cool roof. By application of cool roof there is
reduction in energy consumption between 11 to 21 kWh/Sqm/year.
Figure 6Simulation result for simple institute model
C. Retail type
A preliminary analysis has been done for retail type building simulated with all HVAC
system types. Input parameters are as per simple model only. Results for the simulation have
been presented in Figure 7, shows annual energy consumption for all end uses. In each pair of
bar first bar is for dark roof and second is for cool roof. By application of cool roof there is
reduction in energy consumption between 10 to 23 kWh/Sqm/year.
Figure 7 Simulation result for simple retail model
D. Unconditioned Residential type
A simulation has been done for unconditioned residential type building. Results for the
simulation have been presented in Figure 8 and 9. There is no apparent effect on energy savings
by cool roof for unconditioned buildings but cool roof is capable of increasing the comfort
condition inside buildings. So to demonstrate to affect monthly Mean Air Temperature for cool
roof and dark roof is shown for all months.
Figure 8 shows change in mean air temperature of the zone with cool roof. Maximum
reduction in temperature observed is 3.9°C in month of May. Figure 9 shows change in
FangerPMV with application of cool roof. Reduction in FangerPMV in the range of 0.6 to 1.6
has been observed.
Figure 8Mean air temperature for unconditioned space
Figure 9FangerPMV for unconditioned space
6. Parametric simulation
The calculator provides parametric simulation results for the different insulation
thickness with and without cool roof. User can select insulation thickness from the curve. The
calculator also provide graph between roof insulation and energy savings from cool roof over
normal roof as shown in Figure 10.
Figure 10 Results from parametric simulation
7. Stability and compatibility
Current version has been tested for stability and compatibility. Tool is able to handle the
load and no crash has been observed. System has feature to report for any network or server
failure through email to administrator. For this external service is used which tells if the cool
roof server is not responding to pings.Tool has been tested on different popular browsers like
Mozilla Firefox, Internet Explorer, Google Chrome and Opera.
8. Conclusions
A simple calculator has been developed to help users to understand the benefits of high
albedo roofs. Sample results have been provided for New Delhi climate, shows upto
23kWh/sqm-year electricity can be saved with application of high albedo roof. Also for
unconditioned buildingsmaximum reduction in temperature observed is 3.9°C in month of May
and reduction in Fanger PMV in the range of 0.6 to 1.6 has been observed.
9. Acknowledgments
This work was supported by Joint Clean Energy Research and Development Center
(JCERDC) for buildings called Center for Building Energy Research and Development
(CBERD) funded by the Indian Ministry of Science & Technology, and U.S. Department of
Energy and administered by Indo-US Science and Technology Forum in India.
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