Energy codes for Mediterranean Climates:
comparing the energy efficiency of High
and Low Mass residential buildings in
California and Cyprus.
Maria Spastri, MBS
Joon-Ho Choi, PhD, LEED AP
[University of Southern California]
[email protected]
[University of Southern California]
ABSTRACT
Population growth, city sprawls, increase of households and overuse of resources, affect the
environment and impact greatly the energy use. About 40% of the energy demand in US and Europe
goes to the buildings, with residential exceeding the commercial. Building codes and energy standards
aim to reduce the consumption by setting minimum standards. “Architecture 2030” and “Europe 2020”
target a higher reduction. Energy simulations are the first step towards the goals. Energy code
compliance software evaluate energy and environmental performance of a building by approving or
rejecting it according to the estimated outcome. Variations of materials, building strategies and systems
affect the energy use and consequently its efficiency. How do two different compliance code software
programs, evaluate a same building performance in a Mediterranean climate? For this study, a
comparative analysis using two code compliance software: Energy Pro and iSBEM-CY has been
conducted. A two story simple family detached house was selected with two construction design options:
(a) high thermal mass and (b) low thermal mass in the sub-tropical Mediterranean climate. For this
selected climate condition, Los Angeles in California and, and Larnaca in Cyprus were chosen in the
study. Through this comparison the variations have been examined whether they meet both codes and
ultimately the most energy efficient design option for each region has been identified. Differences in the
inputs, outputs and parameters between the two software programs which are estimated to have
impacted the results have been identified and described.
INTRODUCTION
Building energy usage accounts for 40% of the total energy consumption in the U.S. (DOE, 2008).
Architecture 2030 and Europe 2020 target the reduction of the building energy sector by 50% to 100%
(i.e., net zero energy). Federal and State energy codes and requirements become more stringent to meet
the energy reduction target. Although the number of households increased in the U.S. from 1980-2009,
the average household energy consumption actually decreased (RECS, 2013). In contrast, household
energy consumption in the EU-27 increased by 7.5% (EEA, 2012) between 1990 and 2009.
Energy consumption in buildings has a major impact not only on the environment but also on
building occupants’ environmental comfort. Since 90% of the modern people spend their time indoors
(EPA, Report to Congress on indoor air quality: Volume 2. EPA/400/1-89/001C, 1989), thermal comfort
and indoor environmental quality have a great impact on people’s health and productivity. Therefore, the
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design of a building should consider the climate conditions. The Mediterranean climate in both cities
studied, Los Angeles as shown in Figure 1 and Larnaca as shown in Figure 2, is sub-tropical, with the
warm to hot summer and mild winter. Both climates are semi-arid.
Climate change, urban sprawl, and heat island effect have increased building energy demands.
Depending on the heating and cooling seasons, temperature swings between indoor and outdoor
environment vary. More specifically, and from a climatic perspective, the parameters that affect this
fluctuation are humidity, solar radiation, outdoor temperature, etc. For indoor conditions, the number of
occupants, their activities, lighting, and equipment all contribute to the energy consumption. High peak
loads, especially during the summer, can result in the use of bigger mechanical systems. Consequently,
the amount of energy use increases and operating costs follow accordingly. A major building component
which affects buildings energy usage and its efficiency is its envelope. Therefore it is necessary to study
the climate conditions of every site location and to identify the most appropriate design strategies for it.
Figure 1
Figure 2
Annual temperature range (°F) for California climate zone 8 from Climate Consultant.
Annual temperature range (°F) for Larnaca, Cyprus from Climate Consultant.
The annual average and monthly temperature ranges of the two locations are shown in Figures 1
and Figure 2. Mean temperatures in Los Angeles (Figure 2), located in California climate zone 8 are
within the comfort zone in July, August, and September. The rest are below the comfort zone and
heating is required. The average annual ΔΤ in Los Angeles is 21°F (6.1 C°) with the highest recorded
temperature at 98°F (36.6 C°) and the lowest at 34°F (1.1C°). In Cyprus, only May and October provide
mean temperatures within the comfort zone. From June to September there is a need for cooling. The
rest of the months show a heating demand. The average annual ΔT in Larnaca is 7°F (13.8 C°) with the
highest recorded temperature at 98°F (36.6 C°) and the lowest at 34°F (1.1 C°).
HIGH & LOW MASS BUILDING PERFORMANCE IN MEDITERRANEAN CLIMATES
Historically, buildings in Mediterranean climates were constructed with high mass materials.
Specifically in Cyprus, for the vernacular architecture, stone and adobe blocks were mainly used in the
structure. Nowadays, the majority of the residential buildings are constructed with concrete and brick.
Controversially, in California’s dwelling history, buildings tended to be of lightweight construction,
primarily wood. Until today, the tradition of lightweight buildings is still the common practice for
residences. Materials with high thermal capacity absorb solar radiation during the day and release it
during the night. This property of the materials has been used in architecture as a passive strategy to
achieve desired indoor temperature levels and comfort. By contrast, materials with low thermal capacity
have a limitation in storing heat. As a result, this thermal phenomenon can cause the shift of peak
temperatures between indoors and outdoors very quickly, i.e. thermal lag is reduced. This study
examined the properties and performance of the typical high and low mass envelopes in a residential
building for the Mediterranean climate of Los Angeles and Larnaca. Thermal mass is classified into (a)
exterior thermal mass- defined as the mass of the elements which are exposed to the exterior
environment, and (b) interior thermal mass- defined as the mass of the elements inside the envelope such
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as interior walls, floors, chimneys, ceilings, etc. (Chi-wai, 2003). In this study, thermal mass refers to the
constructed building elements, both interior and exterior, excluding any movable objects such as
furniture.
Figure 3
Internal temperature profiles of high and low levels of thermal mass.
High Mass - Cyprus
Residential buildings in Cyprus are primarily constructed out of high mass materials. The building
structure is made out of reinforced concrete and the non-bearing walls with brick. There are two main
variations for the wall assemblies in the iSBEM-CY code compliance software:
1. Double brick wall 1’(30cm) thickness with 2”(5cm) air gap in the middle, U-value 0.198Btu/
2°
2
ft F (SI: 1.29W/ m K).
2. Double brick wall 1’(30cm) thickness with 1”(2.5cm) air gap and 1”(2.5cm) extruded polystyrene
U-value 0.107Btu/ ft2°F (SI: 0.608 W/ m2K).
According to the Cypriot building code only the second material assembly (b), complies with the energy
rating standards. A wall with higher U-value than 0.149 Btu/ ft2°F, even if it has such a relatively low
insulation-performance, is allowed when the thermal mass is adopted as a passive strategy for heating and
cooling. These wall assemblies are both provided in the Cypriot code compliance software.
Low Mass – California
For a California residential building, the wall assembly was preselected from HEED, Scheme 1: the
auto generated code compliance energy model was adopted:
1. Stucco or Face brick on 2x4 Wood studs at 16” with Plaster board interior with the U-value 0.09
2
Btu/ ft2°F (SI: 0.511 W/ m K).
METHODS & APPROACH
2
For this study, a single family detached house of 1,600 sq.ft. (148.64m ) was initially designed in
HEED (Home Energy Efficient Design). HEED is an energy design tool primarily used for low rise
residential buildings. For the performance comparisons, two building energy models were used in each
location, Los Angeles and Larnaca: high and low mass .During the study it was observed that in HEED
the Larnaca climate was translated into California climate zone 8. The energy performance of the high
2
mass simulation showed an annual average EUI of 28.07 kbtu/sf/y (88.55 Kwh/m ), and an EUI of 34.81
2
kbtu/sf/y (109.81 Kwh/m ) was estimated for the low mass. From this comparison the first drawn
conclusion is that the high mass buildings are more efficient overall throughout the year than the low
mass for the Mediterranean climate. The next step was to use EnergyPro to identify whether the models
comply with the California energy code, Title 24. Similarly, the models were designed to the
corresponding Cypriot code compliance software iSBEM-CY. The generated outcomes from the two
code compliance software were compared. Software similarities and differences were found and
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described.
EnergyPro
EnergyPro is one of the California code compliance (Title 24) energy analysis program and one of
its potential is the energy verification for low-rise residential buildings and whether they comply with
the energy code. For the purpose of issuing a certificate based on the code, the software is originally
designed only for California climates, and the software adopts DOE-2 for a simulation engine. The
Larnaca climate files were not able to be used per the software notification: Larnaca does not have a
valid California Climate Zone for the California Title 24 calculations. In order to run the models the Los
Angeles weather file Climate 8 was used. All the building elements were checked and verified in the
software. The models were designed as single zone for more accurate calculations of the thermal mass
impact to their energy use intensity.
High and Low mass building in EnergyPro. EnergyPro assumes certain thermal mass
characteristics for the calculations. All residential buildings are considered to contain a pre-set amount of
“light” thermal mass. Heavy thermal mass is modeled based on the conditioned area of slab floor as 20%
exposed 80% of it as rug-covered slab and 5% of the non-slab area as exposed 2 inch thick concrete
(EnergySoft, 2011). Concrete floors that are covered by carpet are not considered exposed thermal mass.
For the calculations of the high mass building loads, all the required values were taken from the
Cypriot code and converted to IP units. The material properties selected in this study are listed in Tables
1 and 2. Walls, roof and floor had to be customized in order to generate the same U- values. The Uvalues were managed to be adjusted 95%. One of the of the software’s limitation is that customizing
high mass components for Heat Capacity (HC) is not possible. A default HC condition of the selected
wall type was adopted in this study, while some of the other elements were changed to “0” as shown in
Table 2.
Performance of the thermal mass in EnergyPro. Using Energy Pro, an experiment was made
before the residential building was modeled. The scope was to identify whether the software encountered
the thermal mass impact to the energy use intensity by changing the settings of the thermal mass for
roof, walls and floor. In the software two available options exist regarding the thermal mass and how it
affects the energy calculations, these are: None or Mass Type. Under Mass type these variations are
available: Adobe, Concrete heavyweight, Concrete lightweight, Masonry partial grout, Masonry solid
grout, Wood solid logs and Wood cavity wall. After selecting the Mass type the option of having it
exposed (as pre-mentioned above) or not is available. Furthermore, the thickness of the mass can be
imputed.
For the testing, a Masonry partial grout wall was used, and the heavyweight concrete mass was
selected for the roof and floor. Two variations were made. Figure 4 shows the performance of the same
envelope “with non exposed mass” and “with exposed mass”. The form of the line demonstrates the
effect of the thermal mass to the envelope’s efficiency. The smooth curve illustrates this transition and
decrease of the EUI as expected. During the experiment, none of the other settings in the model were
changed. The total ΔEUI between the 6” (0.15m) non-exposed to 30” (0.76m) thickness exposed is about
2
6kBTU/sq.ft./y (18.92 Kwh/m ).
The next step was to examine the effect of the thermal mass to the overall building’s performance.
In this case all the building systems were used as per the code requirements. The efficiency of the
systems is listed in Table 1. Similarly, two runs were made, with the mass non-exposed and exposed. As
shown in Figure 5, it is clear that the exposed thermal mass contributes to the reduction of the energy
loads and can be calculated in EnergyPro.
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Figure 4 Relation of EUI and thickness.
Figure 5 Thermal mass behavior in Energy Pro.
iSBEM-CY
For Cyprus, the corresponding code compliance software is iSBEM, which stands for interface of
Simplified Building Energy Model. The program is based to the British SBEM software and BRE rating
system. Since the law of certifying buildings’ performance launched in 2010, the iSBEM-CY is new and
consequently has some limitations. The rating system of certifying buildings has a range from A to F. A
building will be certified only if meets at least the “B” rating score of EUI: “A” < 15.99 kBtu/ ft2, “B”
from 16 kBtu/ft2 to 31.99 kBtu/ ft2, “C” from 32 kBtu/ ft2 to 46.99 kBtu/ ft2, “D” from 47 kBtu/ ft2 to
63.69 kBtu/ ft2, “E” from 63.7 kBtu/ ft2 to 94.99 kBtu/ ft2 and “F’ > 95 kBtu/ ft2.
High mass building in iSBEM-CY. For the design of the model that meets the code, the building
elements with the highest U-values were used. This was made for two reasons:
1. To test if the model will get a certification.First numbered item
2. To compare its energy use intensity with the model in EnergyPro Second numbered item
In iSBEM-CY as mentioned at 2.1 High mass – Cyprus, only two available wall assemblies exist:
the one with higher U-value than the code requirement and one that meets the requirements. In the user’s
manual there was no reference for the one that does not apply. The assumption for the first type is that
it’s being used for existing buildings. If none of the above choices is desired, alternatively someone can
input its own U-Value. Similarly, the “Cm” setting, which is the Heat Capacity of the element, can be
modified. There is no option for changing the thickness as in EnergyPro. In the same way all the
envelope elements such as roof, floors, doors and windows, can be adjusted. Regarding the effect of
thermal mass in the iSBEM-CY, it is not clear yet. More details are given at 3.3 Comparison of
performance and 3.4 Comparison of software.
Low mass building in iSBEM-CY. For the design of the low mass code compliance building from
California, all the values and units were converted from IP to SI. The U-values and heat transfer
coefficient were inputted. Therefore, walls, roofs, floors, doors, were only assigned by these properties
while glazing had additionally the Tvis (L-solar) and SHGC (T-solar). Table 2 shows all the values and
units required for the high and low mass code compliance residential buildings of Los Angeles and
Larnaca.
COMPARISON OF PERFORMANCE
Overall four runs were made in the two software, EnergyPro and iSBEM-CY: In EnergyPro: High
mass that meets the Cypriot code and, Low mass Title 24 code compliance. In iSBEM-CY: High mass
that meets the Cypriot code and Low mass Title 24 code compliance. For the highest possible accurate
results the same HVAC and domestic hot water (DHW) systems were used as shown in Table 1.
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Table 1. Building Systems and Efficiency
Type
Efficiency
Gas Boiler
59% Energy Factor
Gas Furnace/Boiler
78% AFUE
Split System
SEER 13, EER 13
System
DHW
Heating
Cooling
In EnergyPro, model (2) was the low mass Title 24 code compliance. The goal was to see if a
residential house which complies as “B” performance to the Cypriot code would comply to Title 24.
Indeed, the runs showed that the model did meet the California code requirements.Similarly, model (4)
was designed in iSBEM-CY, with the standards of the California code. In this case, the model met the
requirements of the Cypriot code and was classified as “B” in its performance.Table2 shows the inputs
used in the two software and the modifications that were made in order to abridge the models between
their values as much as possible. In the Building components “h/m” stands for the propertied of the high
mass energy model, and “l/m” for low mass.
Building
Compo
nent
Wall (h/m)
Wall (l/m)
Roof (h/m)
Roof (l/ m)
Floor (h/ m)
Floor (l/m)
Door (h/ m)
Door (l/ m)
Glass (h/m)
Glass (l/m)
Table 2. Building Component Properties
EnergyPro
iSBEM-CY
EnergyPro
U-value
U-value
HC
Btu/h
Btu/h
Btu/ ft²°F
ft2°F
ft2°F
0.148
16.3
0.149
0.095
0
0.095
0.131
0
0.132
0.028
0
0.028
0.36
0
0.35
0.034
0
0.034
0.60
0
0.669
0.50
0
0.50
0.67
0.669
Tsolar=SHGC=0.76
Lsolar=Tvis=0.80
Tsolar=SHGC=0.76
Lsolar=Tvis=0.80
0.669
0.40
iSBEM-CY
HC
Btu/h ft2°F
5.145
0.10
11.025
0.10
11.368
0.1
0.49
0.10
Tsolar=SHGC=0.76
Lsolar=Tvis=0.80
Tsolar=SHGC=0.40
Lsolar=Tvis=0.49
The energy use intensity of the models in the two software programs is shown in Figure 6. As
illustrated, the high mass residential building in EnergyPro is more efficient that the low mass. In
contrast, in the iSBEM-CY the low mass is shown to be more energy saving. The assumption for this
difference lies in two possible reasons:
1. The iSBEM-CY does not calculate in the same way thermal mass as the EnergyPro High mass
that meets the Cypriot code.
2. The lack of the elements thickness customization when changing the U-value and heat capacity
of the assembly might not reflect the performance respectively.
The difference between the two software results in performance is beyond 100% in EUI as shown in
Figure 6. The outocomes were expected vart but the disparity had not been anticipated to be so high.
The results were estimated to differ due to the factors shown in Table 3. The software engines for the
performance calculations differ. EnergyPlus has a higher resolution of inputs than iSBEM-CY, which is
a simplified building energy modeling tool taking many parameters as defaults. Therefore the algorithms
for heat transfer or the alculation of infiltration, radiation and conduction etc. are different; but they have
not been examined for the research. The weather files used in the software differ and their values impact
the performance calculations. Geometry in this study was the same. Finally the compatibility of the file
format for comparison would provide more answers but iSBEM-CY does not use or generate .gbXML
files or similar; and consequently more detailed comparison between the files could not be implemented.
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Figure 6
System
Engine
Weather files
Geometry
File type
Energy performance simulations.
Table 3. Software input comparisons
EnergyPro
iSBEM-CY
DOE 2
SBEM
TMY 3
Climate file from the software
Same geometry
Same geometry
.bld
.nct
COMPARISON OF THE SOFTWARE
A comparison among the two code compliance software has been performed. Similarities,
differences and their limitations have deen identified:
Similarities
Both EnergyPro and iSBEM-CY were used for code compliance verification of residential and nonresidential buildings.None of them showed a graphical representation of the building. None of the
software is a design tool.
Differences
The iSBEM-CY software does not use .xml files for the model design but .nct. As a result, it was
not possible to import and use the same project files between the software. Instead they had to be
designed separately in the iSBEM-CY. It generates though .xml files for the official submission to the
Register. The iSBEM-CY requires a SHW (Solar Hot Water) system since it is required by law. In
EnergyPro the SHW is optional. For the purpose of this study, SHW was used in both cases. In
EnergyPro it is not required to add an HVAC for cooling or heating. A fact which can give a better
understanding of effect the buildings elements have to its performance. The two software use different
units: EnergyPro uses the imperial system (IP) and the iSBEM-CY the metric system (SI). Regarding the
result outputs, EnergyPro according to the EUI shows % of savings compared to the Title 24. iSBEMCY generates the EUI and categorizes the building between the A to F range, where A the most efficient.
EnergyPro and iSBEM-CY do not have the same options of adjusting the elements and building systems.
Limitations
For the iSBEM-CY, there are several settings which need to be adjusted in the Control Panel before
running the program: for example, changing the “Regional and Language Settings” to United Kingdom,
and changing the “User account settings” to “Never Notify”. If these settings are not changed, the
software will not run properly, and will fail to make the calculations and generate the reports. iSBEMCY has a limited library of building elements and systems compared to EnergyPro. It is not currently
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possible to input the envelope’s thickness manually. Therefore, it was not possible in the iSBEM-CY to
evaluate how it calculates the effect of the thermal mass on the building’s performance.
CONCLUSIONS
One of the most important findings in this study is that high mass buildings are more energy
efficient in Los Angeles and in Cyprus the low mass. Taking into consideration the graphs as shown in
Figure 1 and Figure 2, we could identify that more months are heating dominant than cooling in both
locations. According to Climate Consultant, HEED and EnergyPro High mass residential buildings are
more efficient than the low mass buildings in this climate. The EnergyPro and HEED results confirmed
it. The Cypriot vernacular architecture and current construction materials applied also reinfornce the use
of high mass materials as more efficient. Therefore, building materials with high thermal mass are
suggested as a passive strategy for enhancing the building performance in this climate. The iSBEM-CY
results contradicted these conclusions and consequently further studies should be carried in regards to
iSBEM-CY to evaluate its outputs.
Futhermore, both the High and Low mass models passed the EnergyPro with 24% energy savings
compared to Title24. Similarly, both the High and Low mass models passed the iSBEM-CY and were
rated as “B” in their performance. EnergyPro counts thermal mass into its calculations. In contrast for
iSBEM is not clear at what percentage its being calculated since the high mass had greater energy
consumption compared to the other software. The attempt to simulate the models without an HVAC was
possible in EnergyPro. iSBEM-CY cannot proceed to the calculations without an HVAC system. The
assumption is that the developers of the software did not want to allow certification without an HVAC
since the climate it Cyprus makes a mechanical system mandatory. Finally, Title 24 is more stringent
than the Cypriot Energy code.
For Future works, a more extensive research with the iSBEM-CY software in order to obtain a
clearer understanding of how the thermal mass is being perceived and calculated. Finally a comparison
of the high and low mass models in other software could be used for validation of the results.
ACKNOWLEDGMENTS
We would like to acknowledge and thank professor Murray Milne for his guidance and supervision
along the process of this study. We would also like to thank Tighe Lanning and Praveen K. Sehrawat for
their technical support.
REFERENCES
CEC. (2008). Reference appendices, Regulations / Standards, Joint Appendices, Appendix JA1 –
Glossary, p.JA1-50). California Energy Commission.
Chi-wai, Y. (2003). Effect of internal thermal mass on building thermal performance. Hong Kong.
DOE. (2008). Energy consumption report. Department of Energy, USA.
EEA. (2012). Energy efficiency and energy consumption in the household sector (ENER 022) Assessment.
European
environment
Agency,
http://www.eea.europa.eu/data-andmaps/indicators/energy-efficiency-and-energy-consumption-5/assessment.
EnergySoft. (2011). EnergyPro Version5 User Manual. EnergySoft, LLC.
EPA. (1989). Report to Congress on indoor air quality: Volume 2. EPA/400/1-89/001C. U.S.
Environmental Protection Agency. 1989. Report to Congress on indoor air quality: Volume 2.
EPA/400/1-89/001C. Washington, DC. Washington, DC.: U.S. Environmental Protection Agency.
RECS.
(2013).
Residential
Energy
Consumption
Survey.
http://www.eia.gov/consumption/residential/reports/2009/consumption-down.cfm#fig-1.
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