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Influence of material selection on energy demand in residential houses
Agya Utama∗
The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi,
Bangkok, Thailand
Abstract
Utilizing local materials for improving the energy demand in the single landed houses in Semarang is
very promising, since it also entails less cost during production and transportation. Many possible scenarios have
been developed to meet these criteria, such as; improving craftsmanship (including air leakage prevention by
adding rubber or sealant in all possible gaps, using double walls with cavity, double walls with bamboo in between
and introducing less transparent glass The study also revealed the Break Event Point (BEP) in energy and cost, the
energy BEP by calculating the initial embodied energy up to construction with the reduction of the electricity
consumption and cost BEP by calculating the initial investment with the monthly electricity saving. Both BEP is
calculated based on the BASELINE scenarios.
Keywords: Residential building; Embodied energy; cost BEP; energy BEP; Building envelopes material

Corresponding author.
E-mail address: agya@kgsee.kmutt.ac.th
1

2. 1. Introduction
The electricity consumption in Indonesia is 40% generated for residential houses [1] as the
figure in the world that 30-40% primary energy used in buildings[2], in the US 35% electricity demand
is generated for buildings [3]. In terms of cost, building envelopes contribute the major percentage of
material used in the residential building, walls contributing 46% and roofs 16% [4], and are the second
heaviest with 14-17% of total building weight (after 80% for foundation and structure Error: Reference
source not found[3]). For the overall energy usage during the life cycle of the building, the operation
phase constitutes more than 80% of the demand. Thus, the energy use during the occupation phase
needs to be analyzed. Heat gains through building envelopes contribute approximately 50-60 percent of
the total heat gain in the building [4].
Economic development and population growth have created a high demand for housing
compounds (some big cities develop high-rise apartments as an alternative to cope with expansion to
the rural areas) and traffic problems. Some cities will not rapidly cope with traffic and land availability
problems, however sooner or later they will face problems similar to metropolitan cities. The single
landed houses are still far more attractive compared to the high-rise apartments in these cities.
Semarang is a good example of this kind of city, the rapid population growth as well as urbanization
creates problems for the government in providing adequate shelter for its citizens. For decades, it is the
private sector that has come to provide alternative residential compounds.
Semarang (SMG) with a total population of about 1.35 million [5] has different characteristics
in terms of urban residential buildings compared to Jakarta (JKT) which is a metropolitan city. The
trend of residential buildings in JKT is more towards high-rise houses (apartments, condominiums,
etc.) rather than landed houses. The typical houses in Semarang are single houses owned and developed
by private companies, followed by government owned single housing (Perumnas). Based on local
municipality facts and figures, presently there are 114 single housing compounds owned and developed
by private companies or government, with the number of households varying from 25 units up to 5000
units per compound.
Previous studies have shown that buildings last longer compared to building materials and
equipment; the data required for analysis of life cycle energy and cost are numerous and analysis would
be tedious and time consuming. Therefore it is not practical for a designer to predict the effect of a
certain design decision over its lifetime [6][7]. Hence, it is important to assess the parts of the building
2

3. which have the biggest influence to its life cycle and give various design options and strategies for
typical buildings based on performance over their entire lifetime.
There is only a limited amount of research literature on energy efficiency in residential
buildings (landed and high-rise apartment) in hot and humid climates: most of the literature has focused
on the comfort conditions for occupants while the majority of the studies are done in apartments rather
than single landed houses [8]. Also, most of the studies focus on particular envelope components
towards energy effects and human comfort without considering the embodied energy, for example,
energy–efficient envelope [8], wall insulation thickness and positions (Bojic et al in [8]), ventilation
strategies [9], or heating and cooling energy by the utilization of passive design [10]. This study
focuses on single landed houses (commonly used in developing countries) and considers not only
human comfort and energy impact of the selected materials but also considers embodied energy during
raw material extraction, transportation, production and construction and especially utilization of
materials available in the local market. It also compares cost efficiency and is an easy guide for
building stakeholders in selecting appropriate materials for enclosure and lower impact to the
environment, low embodied energy and also better influence in terms of reducing the electricity
consumption on cooling loads.
The purpose of this article is to assess the best scenarios based on Indonesian climate and
occupant behaviour for building envelopes by utilizing local materials available in the market, in
relation to the embodied energy and energy demand during use phase also including cost implications
for single landed houses in Semarang. The comparison will focus on scenarios for possible reduction of
heat gain through the building envelope using local materials available in the market. Since some
possible scenarios for reducing energy during use phase are already being implemented in some
countries, such as adding low U-value insulation to the walls, additional photovoltaic for facades, and
passive cooling systems etc., it seems unlikely that modern design solving point of view above can be
implemented easily due to lack of availability of materials, high cost, lack of know-how, or technical
difficulties (low quality, unskilled labour, etc). The model scenarios will be compared to the typical
houses used in Indonesian middle class residential compounds which are used as baseline (this will be
further explained in 2.2).
The methodology developed will evaluate the possible scenarios vis-à-vis the efficiency of
electricity consumption (by AC) by adjusting the material envelopes (excluding the building form and
3

4. shape) using local materials available in the market, which have not been assessed in the studies so far
in Indonesia. It could be a useful tool for designers who may wish to consider more design options.
Therefore the designer can evaluate a design based on embodied energy, reduction of AC electricity
consumption, and cost comparison.
Possible scenarios include improving the building craftsmanship during construction and
introducing additional air barriers therefore reducing leakage between zones and the external
environment to the conditioned zones, double layer walls by means of bricks, external layers and
gypsum board for inner layer (with air gap in between), applying locally abundant raw material
(bagasse) as main constituent of non-firing bricks, reducing windows to walls ratio (WWR) from
regular WWR (within the acceptable natural lighting), replacing regular window glass with less
transparent glass and adding the bamboo sheet in double layer walls. These scenarios will be simulated
based on the room behaviour of Semarang single landed houses for AC utilization, and will also
calculate the infiltration between zones (three zones) and influence of the external zones. However the
simulation will ignore the influence of internal sensible and latent heat (such as from appliances and
human heat).
This study will focus on the cooling load associated with its enclosure influence to the
perimeter zones as most of the air conditioned zones in single landed houses are located in this zone.
The perimeter zone, or perimeter load, is the component of the sensible heat load, which is calculated
under the condition that the internal heat generation is zero [11].
2. Methodology
Figure 1 Stages diagram on stakeholder influences on the study
Simulations
Scenarios by utilizing
available local material for
building envelopes and its
associate available in the
market
Designer
Land developer
Contractors
Users’ behaviour
Houses
4

5. 2.1. Embodied energy
Embodied energy in a product comprises the energy used to extract, transport and refine the
raw materials and then to manufacture the components and assemble the product Error: Reference
source not found. The energy consumed directly at each phase of material production is clearly
definable and measurable; this paper assesses the direct energy consumed by residential enclosure
building materials. Process analysis has been used to assess the energy consumed during raw material
extraction, and material production up to the construction process, including energy consumed during
its transportation.
Though the study site for the buildings is the city of Semarang, the data collection for the
enclosure materials has been conducted throughout Java as not all the material is manufactured locally.
Most of the data used for embodied energy assessment in this study is obtained from the authors'
previous study [12] and some from other sources such as bamboo from [13] and bagasse bricks from
[14][15][16].
The materials selected for scenarios in this study are based on the typical materials available
in the local market (clay bricks, gypsum board, bamboo sheet and clear glass), and materials which are
not yet available on the market but potentially available as the natural resources and recycled material
are very abundant (for instance bagasse). Local materials will be the strong focus for selecting
scenarios in this paper, as imported material and inter regional material consume more energy than
local material. The embodied energy and initial cost will be considered only for the selected zone
(room) which the air conditioned are utilized. So the other zone materials remain the same as the
baseline case.
2.2. Case study (baseline)
Middle class residential houses were used in this study as baseline; the typical materials that
are currently used have been chosen along with typical floor area and occupancy rate. The studied
houses are predominantly made from clay based material, by means of having external walls that use
clay bricks as the main material, and roof enclosure made from clay tiles. Other studied houses use
cement base as the core material, such as concrete blocks for walls and concrete roof. Both types of
houses have the average air conditioning type and power (General brand from Japan with 7000 Btu/h
5

6. capacity and coefficient of performance 3.2) and similar occupant behaviour (4 people: 2 adults and 2
children).
The data was gathered in Semarang using open interview and questionnaire from 35
households (mainly brick material) with a floor area ranging from 50-65 square meters. The households
have two bed rooms (one of which is air-conditioned) and are considered as middle class homes with a
monthly average family income between five to ten million Rupiah (USD 550-1100).
6

7. 7

8. 9.50
6.00
2.80 3.80
2.80 2.80AIR CONDITIONED AREA
0.68
3.10
8.82
LIVING ROOM
Figure 2 Typical floor plan and perspective for typical single landed house (54 m2
floor areas)
The typical house in this study (see figure 2) has an air-conditioned area of approximately 10
m2
from a total of 54.5 m2
, and one combined living room/kitchen. The wall is mainly clay brick
covered by 10 mm plasterboard on both sides. The ceiling is made from local 3mm gypsum, with roof
and window/door frames made from wood.
The house used as baseline for this case study (clay based house) is chosen as the result of a
previous study revealing that a clay based house has better performance in its life cycle (40 years)
compared to other common type (cement based); hence improvement options are sought for this type.
The baseline will be compared with the possible scenarios of reducing electricity demand associated
with cooling load. The baseline also takes into consideration some standard materials, such as structure
column (made from concrete and steel reinforcement), roof frame (made from wood frame
8

9. classification two), gypsum ceiling, concrete slabs with ceramics tiles on ground, windows and doors
wooden frame, occupant behaviour and weather conditions.
The data collected in the selected house and utilization behaviour will be used as input in
ECOTECT, a dynamic building analysis software used to simulate the cooling load. ECOTECT will
simulate based on data such as average temperature setting, occupants, material for enclosure,
humidity, AC efficiency, AC utilization behaviour, zone infiltration, external infiltration, air speed, and
internal heat (appliances and human) [17][18]. The simulation will be carried out on an annual basis as
most of the months are hot and humid and a few months (4-5 months) wet and rainy. ECOTECT
provides information of heat gain/loss through opaque portions of the enclosure, and natural ventilation
in 3D [19].
2.3. Baseline and scenarios
Figure 3 shows the typical brick configurations used for walls of single landed houses in
Indonesia.
Figure 3 Typical bricks layer arrangements for masonry
These typical brick layer assemblies will be used in the scenarios, for example, Double Walls with
cavity (DWg) uses half pair bricks for external and gypsum board in the inner layer, and Double Walls
with bamboo sheet (DWb) uses pair bricks on the bottom followed by quarter bricks built along
vertically to the top, etc.
Pair bricks half pair bricks quarter pair bricks
9

10. OUTSIDE
INSIDE
Figure 4 Cross section of layers from typical Indonesian single landed walls
Figure 4 shows the typical cross-section of walls in single landed houses in Indonesia. The layers
consist of 250Lx110Wx50T mm bricks covered by mortar plaster on both sides. Brick density in the
model is 950 kg/m3
. Mortar plaster thickness is 10mm; therefore the total thickness is 130 mm.
In this study the building enclosure material available in the local market have been used to
obtain a low embodied energy as well as the most feasible cost associated with reasonable energy
reduction during use phase in typical single landed household behaviour under local weather and
climate conditions. The scenarios will be categorized into three points of view regarding
responsibilities (land developer/contractors, designer, and a point of view combining the previous two).
10
plaster ½ pair bricks plaster

11. Table 1 Thermal properties of enclosure materials for baseline case and selected scenarios
Type of model and material
U-value
[W/m2
.K]
Time lag
[hours]
Width
[mm]
Density
[kg/m3
]
Conduc-
tivity
[W/m.K]
Specific
heat
[J/kg.K]
Trans-
parency
[0-1]
[overall] [overall]
WALLS
Baseline 1.58 3
lightweight bricks 110 950 0.27 840 none
Double walls gypsum (DWg) 1.12 4
lightweight bricks 110 950 0.27 840 none
air gap 50 1.3 5.56 1004 none
gypsum 10 1100 0.65 840 none
Double walls bamboo (DWb) 0.77 3.5
lightweight bricks 110 950 0.27 840 none
bamboo sheets 20 950 0.031 2095 none
Bagasse bricks (WBb) 2.72 2 110 1600 0.765 1000 none
Plaster 10 1250 0.431 1088
Window GLASS
Clear glass standard (92%
transparency)
5.44 6 2300 1.046 836.8 0.92
Glass 70% transparency 5.44 6 2300 1.046 836.8 0.7
Double glazing 2.71 6 2300 1.046 836.8 0.92
2.3.1. Land developer/contractors' influence
Scenario Crf; Using the baseline case to improve the craftsmanship between gaps, therefore
minimizing air-flow across the gaps. Air movement occurs from the air leakage due primarily to the
pressure difference of air across the opening [20]. Often low quality craftsmanship occurs in
Indonesian housing; the use of low quality materials, below standard brick-laying, low quality
finishing, and so on. Therefore some cases occur such as leaks between joints, uneven window/door
frames, and big gaps between windows/doors and the frame, sometimes resulting in the
windows/doors' inability to be properly closed.
11

12. Typical such openings in Thailand, as studied by Chirarattananon [20], include leakage
between frame and wall 2.2×10-4
m2
, between door and frame 9×10-4
m2
, between window frame and
wall 1.34×10-4
m2
, and between window and frame 4.2×10-4
m2
. There can also be leakage via ceiling
8.6×10-3
m2
(on ceiling and wall fixtures, and between ceilings). The typical leakage area is more or
less the same as in the Indonesian case. The baseline airflow caused by leakage is assumed to be 0.45
L.s/m2
or 0.5 ACH (Air Changes per Hour), which is higher than typical houses in Sweden (0.35 l.s/m2
or 0.4 ACH) [21].
Reducing the possible bigger gaps will reduce air movement (especially to external
environment and lower pressure zones), however better craftsmanship is not enough to completely
eliminate leakage. Further reduction of the flow can be achieved by introducing additional air barriers
for windows, doors, and gaps between wood, sheet, gypsum, panels etc. Air barriers are also capable of
resisting wind loads, positive pressure and suction [5]. However gaps will still continue to occur
through differential movement, degradation of components, material shrinkage or expansion (due to
weather) as the building ages, and therefore good quality material should be introduced.
Moreover the air barrier material should also be easily available in the market. Some water
barrier materials such as airtight drywall approach (ADA), spray polyethylene foam, or any other
advance air barrier materials are available in the global market, but not in the local market. Some
options available locally are sealants, rubber tapes and asphalt.
By introducing high quality craftsmanship, and introducing air barriers, the possible gaps and
openings (especially during the hot season) will be reduced by as much as 50% of the ACH. The
baseline ACH was 0.5 for the typical Indonesian landed house. If the craftsmanship is improved and air
barriers introduced, the ACH can be reduced to 0.25 ACH (~0.2 L.s/m2
), and from 0.25 ACH for
ambient air infiltration to 0.1 ACH (0.1 L.s/m2
). This will entail some additional cost due to the
purchasing of wind barrier materials, as well as additional expenses for labour as working hours will
increase.
2.3.2. Designers' influence
Scenario DWg; Using double layer walls with cavity and gypsum, half pair brick layers (pasangan
setengah bata), a common practice for constructing bricks masonry for houses) as external layers and
gypsum plaster board at the inner layer, and an air gap with wooden frame (to which the gypsum is
attached) in between. The cross section of the wall, as seen in Figure 5 below, shows the structure of
12

13. the wall. The material properties, as seen in , show the total U-value to be lower than a typical wall
structure. Moreover an initial cost for gypsum will be necessary, as well as the cost of the wood frame
as a supporting structure for the gypsum. Since the construction process is done by human labour,
embodied energy during assembly as well as initial cost for construction machinery will not be
included. However the cost for the labour will be taken into account.
OUTSIDE
INSIDE
Figure 5 Cross section of double layer walls with cavity
As seen in the time lag for this scenario is longer (4 hours) compared to 3 hours for the
typical wall (baseline), due to the introduction of gypsum in the inner layer. This may cause heat gains
at 11.00 to reach the conditioned zones whereas for the typical wall when the delay is 3 hours, only
heat gains at 12.00 will influence the cooling load at the conditioned zone. However the overall U-
value of this scenario is lower than the typical wall, therefore the cooling load requirements should be
lower.
Scenario DWb; Using double layer brick wall filled with bamboo, and a combination between one pair
brick layers (pasangan satu bata) and quarter pair bricks (see Figure 3). The pair bricks are used for
structural/strength purposes, and the quarter bricks are used for creating the ‘double wall’ within which
the bamboo sheets can be placed (as seen in the Figure 6 below).
13
plaster ½ pair bricks air gaps gypsum

14. Figure 6 Cross section of double wall with bamboo sheet in the middle
Scenario WBb; Using bricks made from bagasse cross section as seen in Figure 7. Bagasse is
abundantly available as residue from sugar mills in Asian countries [14][15][16] use of which reduces
emissions since most of this material will go to landfill or be burnt. Bagasse/sugar cane ash bricks have
thermal properties such as conductivity of about 0.7651 W/m.K at density 1600 kg/m3
[14][16] [15].
The specific heat is higher than regular bricks [14], suggesting potential to replace regular clay based
bricks. This material also has a minimum tensile strength of 40-50 kg/cm3
which is sufficient for
structural walls (the average Indonesian bricks are 30 kg/cm3
) [14][15].
Another interesting characteristic of this material is its low embodied energy (1597 MJ/m3
)
[14] in comparison to the regular bricks (5614 MJ/m3
), as it does not require a firing process because
the main material consists of 40-50% clay and <10% cement and the remaining 40% is bagasse or
sugar cane ash. The energy outlay during transportation should also be taken into account. This
information is not available in the reference papers, however to reduce energy during transportation the
production of bagasse/sugar cane ash bricks should be located near the sugar mill factory.
However choosing building materials based only on low embodied energy levels would not be
sufficient since building materials will also have a significant impact on the cooling load during the use
phase. These effects are included in the calculation.
OUTSIDE
INSIDE
plaster ¼ bricks bamboo sheets ¼ pair bricks plaster
14

15. OUTSIDE
INSIDE
Figure 7 Cross section of wall from bagasse bricks
Scenario G70; Using single tint glass (70% transparency) is an option to replace standard clear glass,
as it has the same thermal properties except a lower transparency which will reduce heat transfer to the
conditioned zone. Moreover this type of glass is available abundantly in the local market (commonly 6
mm and 8 mm). The price is around 195,000 Rupiah/m2
(21.43 USD/m2
), an increase of 65,000
Rupiah/m2
(7.14 USD/m2
) compared to regular clear glass. The scenario will use 6 mm thick less
transparent glass and simulate the energy reduction on an average monthly basis. Moreover the
embodied energy of tint glass is 1.86 MJ/kg higher than clear glass.
Scenario DG; Using double glazing. This is similar to scenario G70, with the addition of a second layer
of glass to the base case. Both sheets of glass are 6 mm thick with an air gap in between (non-
vacuumed). However the additional glass layer inside may cause condensation, therefore a special
design needs to be introduced, such as creating an opening to the inner layer glass allowing the gap to
be cleaned, and also allowing air circulation in the gap (reducing the likelihood of trapped humidity).
The material costs as well as embodied energy would be double that of the baseline.
The option of double low E-value vacuumed glass in Indonesia is unlikely to be used as it is
difficult to find in the market and, when available, only at a relatively high cost.
Scenario WWR; Reducing Window to Wall Ratio; the typical single landed houses in Semarang have a
WWR of 0.45 by using two large glass windows. Removing one window will then lower the WWR to
0.22, and has potential also to reduce the heat gain. The change in embodied energy reduction
compared to the baseline resulting from the removal of one window frame and its glass and its
15
plaster ½ pair bagasse bricks plaster

16. replacement by wall area (more bricks and mortar will be used) is also considered in this scenario. The
wood frame will reduced by 5 meter (as the frame dimension is 0.05 by 0.07 meter) therefore 0.82 MJ
will be reduced. The embodied energy from window glass will reduce by 258 MJ (1.5 m2
less windows
area). However the brick wall area will increase as the bricks and mortar will replace the missing
window area (an additional 926 MJ).
2.3.3. Combined option
Scenario COMB; Using a combination of the most energy conserving scenarios (either contractor or
designer influence). It is necessary to compare whether a combination of scenarios is viable in terms of
installation costs, or whether initial costs are too high thus making the option impossible for the
owner/developer to implement. The combination would be that of double walls with cavity and
gypsum, lowering WWR (from 0.45 to 0.22), installing double glass with 70% transparency and
introducing air barrier and improving craftsmanship.
All the scenarios above will meet the criteria of human comfort outlined in the human comfort
diagram [21], the Relative Humidity (RH) for the comfort zone is between 40-60% with a temperature
range of 18-26°C. This comfort zone will be used as input in the simulations. The baseline room
temperature thermostat, provided by the AC system, is set at 24°C and 60% RH.
3. Results and Discussion
3.1 Embodied energy, cost saving and energy saving
The baseline model is based on real measurements as shown by normal energy demand, initial
cost and embodied energy in the typical clay brick single landed houses in Indonesia. The typical
energy demand on the typical enclosure materials for the baseline case is represented as zero (X=0,
16

17. Y=0) in Figure 8 and Figure 9.
Crf
DWg
DWb
G70
DG
WWR
COMB
WBb
BASELINE
-10
-5
0
5
10
15
20
25
30
35
40
45
50
(3000.00) (2000.00) (1000.00) 0.00 1000.00 2000.00 3000.00 4000.00 5000.00
additional embodied energy [MJ]
averageenergysavingpermonth[kWh]
Crf DWg DWb G70 DG WWR COMB BASELINE WBb
Figure 8 Embodied Energy Gradient Diagram
17

18. The gradient diagram above (Figure 8) shows the average energy saving per month for the
scenarios on the Y axis and additional embodied energy for each scenario (added or reduced) including
embodied energy during construction (X axis). The diagram is divided into four areas. The first zone is
the most energy saving per month as well as the least initial embodied energy. The second zone shows
high energy saving but with high embodied energy. The high embodied energy is caused by high
energy consumption during the production phase and initial energy consumed during construction and
transportation. The third zone shows the least energy reduction and also the least embodied energy. The
fourth zone shows the worst alternative scenarios with no significant reduction of energy use and the
initial embodied energy is also large.
The Glass 70% transparent (G70) has little additional embodied energy compared to clear
glass windows (approximately 29 MJ/m2
). These figures are similar for double walls with bamboo
since no additional brick layer is introduced (see Figure 3); in fact the embodied energy is less as each
square meter of brick masonry only consists of 48 bricks instead of the 60 of regular bricks. Reducing
Windows to Walls Ratio (WWR) consumes zero additional embodied energy for glass and frame, since
the required quantity of glass and wooden frame is reduced. However the initial embodied energy for
WWR still shows an increase as the glass will be replaced by bricks, for which the initial embodied
energy is, approximately an additional 667 MJ. Another consequence of reducing the WWR is less
natural daylight. This may increase the electricity used for lighting, but because of its negligible
influence, this impact is not included in the study.
Double Glazing (DG) seems to have less energy reduction and noticeably higher embodied
energy (in comparison to the baseline). DG is placed at gradient 3 for this alternative scenario.
However it is close to gradient 4 because the embodied energy is double that of the baseline and the
potential energy reduction is not significant (8.47 kWh/month). For some reason these glass scenarios
can be avoided as an alternative, since the initial embodied energy is significant enough for the whole
life cycle of the building, except the possible to reduce its transparency (< 70%) and increase its
thickness. For instance, the G70 and DG can be the 8mm or even thicker instead of 6mm. This will
show a reduction of 0.2 kWh/month for 8mm thick G70, and 0.1 kWh/month for 8mm thick DG.
DWc (Double wall with cavity) has high energy reduction but also high embodied energy.
This is because of the introduction of a gypsum layer for the whole inner wall 2,477 MJ additionally
only for adding gypsum board and 24.7 MJ for its reinforced frame. Double walls filled with bamboo
18

19. sheet (DWb) will reduce electricity consumption by 8.54 kWh/month compared to the baseline.
Moreover, the initial embodied energy of the bricks is not significant because the number of the bricks
does not increase significantly (by introducing a combination of one pair and quarter pair brick layers
to cover the gap without adding any additional brick layers). The additional initial embodied energy
only comes from bamboo: 500 MJ is needed to produce 1 m3
of bamboo in developing countries [13].
Crf
DWg
DWb
G70
DG
WWR
COMB
WBb
BASELINE
-10
-5
0
5
10
15
20
25
30
35
40
45
50
(60.00) (40.00) (20.00) 0.00 20.00 40.00 60.00 80.00 100.00 120.00
additional extra initial cost [USD]
averageenergysavingpermonth[kWh]
Crf DWg DWb G70 DG WWR COMB BASELINE WBb
Figure 9 Initial Cost gradient diagram
The gradient diagram in Figure 9 shows the energy reduction during the operation phase
versus the additional cost (material as well as construction cost) invested for each scenario. The first
zone is the most energy saving per month as well as the least initial investment cost. The second zone
shows high energy saving but with higher initial investment. The third zone shows the least energy
reduction (even no reduction or more electricity consumption) and also the least initial cost. Most of
the alternative scenarios are located in this gradient as a consequence of little effort producing minimal
results. The fourth zone shows the worst alternative scenarios with no significant reduction of energy
use (similar with third zone), and the initial investments are huge.
19

20. The craftsmanship (including additional rubber/sealant insulation) and reductions of WWR
are both on the same gradient (3). This means these scenarios have less significance in terms of
reducing electricity during the use phase (less than 10 kWh/month) and have more initial investment
cost (more than 10 USD). Changing from clear glass to 70% transparency glass increases the initial
investment to almost 60 USD and only reduces electricity consumption by 0.34 kWh/month. Moreover
double glazing is similar but has better electricity reduction per month (8.47 kWh). The best electricity
reduction is shown by DWg or double walls with cavity and gypsum (30.36 kWh/month). However this
alternative has an effect on the price as the initial cost reaches 76.10 USD (gradient 2).
The least investment cost, while achieving quite significant electricity reduction (> 20
kWh/month reduction from that of the baseline), is that of double walls with bamboo (DWb).
Introducing bamboo sheets in the middle of Double walls results in a reduction of 20.50 kWh/month: a
good example of reducing electricity use while keeping a low initial investment cost (21.76 USD).
3.2 Break even point (BEP)
Compared to the baseline, most of the model scenarios are located in gradient three for
embodied energy and initial cost, or in other words most of the possible model scenarios will not
deviate much from the baseline (0,0). Moreover some scenarios have high energy saving together with
a high initial investment for example the Double wall with cavity and gypsum -DWg- and combined
scenario -COMB-.
To have a better idea of the relative gain in energy, the study also compares the possible
energy break even point of the various scenarios. This ratio shows the energy payback or time taken for
the initially increased embodied energy to be recovered by the electricity savings from reduced cooling.
A similar calculation has also been done in terms of cost and the results for both are presented in Table
2, the table represents the cost BEP scenarios for the initial cost of the material divided by the potential
reduction of energy per month. The energy saving (kWh/month) is multiplied by the average electricity
cost for a medium house with a 2200 W electricity capacity installed (Rp 390/Kwh or 0.043
USD/kWh). The Cost BEP for G70 scenario shows that more than 75 years are required to reach the
break even point, due to very low energy saving 0.34 kWh/month (0.01 USD/month) and relatively
high initial cost for investment (13.01 USD). This option in terms of cost is not economically feasible,
therefore these can be neglected by some designers. The best BEP in terms of introducing additional
material is DWb (24.7 months or 2 years, as seen in the figure above) since it has a very low initial
20

21. investment cost and large reduction of electricity consumption (21.76 USD and 0.88 USD/month
respectively). WWR in this case shows an initial cost that is 10.3 USD cheaper, a monthly saving of
around 0.31 USD/month compared to the baseline, meaning this option is already saving money even
before it is used
Table 2 Break even point (BEP) for selected scenarios compared to the baseline
Scenarios Initial
BEP [years]
Energy Cost
Craftsmanship and air barrier Crf 1.26 3.78
Double Wall cavity & gypsum DWg 0.70 4.87
Double Wall bamboo DWb 0.13 2.06
Bagasse bricks WBb 1.73 7.84
Glass 70% transparency G70 1.91 75.49
Double glass DG 1.03 11.49
WWR reduction WWR 0.77 -2.73
Combined option COMB 0.67 4.19
As seen in the gradient graphs (Figure 8 and Figure 9) the DWg has a low embodied energy as
well as high monthly energy saving. However, the energy BEP shows that DWg has longer BEP than
DWb and combined option. The low energy BEP for DWb is due to the low embodied energy installed
316.80 MJ and the effect on the energy reduction (20.50 kWh/month) is significant compared to DWg
(2501.64 MJ and 30.36 kWh/month respectively). The figures also occur due to the lower embodied
energy for bamboo compared to gypsum: 316 MJ to 2477 MJ respectively. G70 has the largest energy
and cost BEP due to high embodied energy consumed as well as very small monthly electricity
reduction. The WWR moreover shows that the initial embodied energy is 667.13 MJ higher than the
baseline, including the energy consumed during construction.
Bagasse bricks (WBb) is a special case where the embodied energy from the substitution is
lower than the base case and so is the cost. However, the energy consumption during the operation
phase is higher than the base case and hence, so is the cost. Thus, in this case, the there is an initial gain
of embodied energy and cost which is offset by the increased energy use and cost during operation
phase. This case is especially interesting because it is currently being promoted in Indonesia due to the
low initial cost consideration. However, the Table 2 shows that the initial cost benefit is already offset
in 8 years after which a higher price is paid for electricity.
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22. 4. Conclusion and discussion
The use of local material to reduce the energy expenditure during the use phase is a wise
decision since it will also reduce the initial embodied energy as well as cost, especially transportation
cost. Low energy housing is not always about using high tech material or technology, but knowing the
local material and its characteristics, which can lead us to a possible life cycle energy saving for the
house through reducing cooling loads.
From all scenarios, reducing the window to wall ratio has the lowest embodied energy and is
reasonable enough in terms of energy reduction compared to the baseline. Moreover the figures for
both Energy and Cost Break Even Point show that low WWR (but still maintaining the potential of
daylighting) will reduce electricity consumption caused by cooling loads quite significantly and have
negative BEP, as it does not need any additional material to implement (rather is has a lower material
requirement resulting in reduced cost and reduced embodied energy). However some households seem
to neglect the option to have low electricity consumption, as the electricity price is considerably low
(~390 Rupiah/kWh or 0.043 USD/kWh) because of the subsidy from the government Error: Reference
source not found. To achieve public awareness of low electricity consumption by adjusting or
retrofitting its envelopes, full support of the government is needed (either by push factors or pull
factors), such as increasing the price of electricity in certain areas to avoid an impact on lower income
families. For instance, the energy consumption for households with a 2200 W capacity installed
(mostly used by middle class families) could have a lower subsidy compared to those who have 900 W
installed (push factor). Another way is developing building code which encourages only material with
thermal property values that are beneficial to the hot and humid climate, therefore reducing heat gains.
If the government increased the electricity price per kWh by 25% (0.01 USD) the cost BEP figures
would show better results (as seen in the table below). The cost BEP would be on average 20% faster if
the electricity bill was increased by 25%.
22

23. Table 3 Scenarios as if the electricity bill is increased
Scenarios
Cost BEP [years]
increasing
current 10% 25%
Crf 3.78 3.44 3.03
DWg 4.87 4.43 3.90
DWb 2.06 1.88 1.65
Wb 7.84 7.13 6.28
G70 75.49 68.63 60.39
DG 11.49 10.45 9.19
WWR (2.73) (2.48) (2.19)
COMB 4.19 3.81 3.35
Double wall with bamboo is promising as it has low embodied energy as well as low cost. In
Indonesia bamboo (as a native plant) is abundant and easy to grow, therefore the price is reasonably
low, and with no complicated production process reducing the initial embodied energy (except for
transportation). In the case of Semarang, the city is surrounded by rural areas which are a source of
bamboo. However the cost of bamboo can increase in line with high demand. Since bamboo is not bulk
farmed, but harvested from its natural habitat, future price increases are inevitable. Therefore it is
necessary for the government and private sector to industrialize the bamboo plantations in the near
future. Based on the strong resistance of bamboo to shear load and pressure [13], walls filled with
bamboo can also survive earthquakes which are common in Indonesia due to its location between the
Eurasian and Australian plates.
Double walls with cavity are being used in case studies in many countries, and can be
implemented in Semarang. However, gypsum is not relatively cheap and is therefore located in the
second area of the diagram gradient. Even so the energy saving per month is considerably low, even in
comparison to Double walls with bamboo. The initial embodied energy is high compared to other
alternative scenarios. This is because the gypsum production process is energy intensive.
By utilizing the bagasse, or sugar mill waste such as ash, as building materials will increase
the eco-friendly value of the material, and definitely reduce its embodied energy if the process has
lower energy content (such as firing). The use of waste material, in this case bricks made from bagasse
and/or sugar mill ash, that avoids the firing process not only reduces embodied energy (see Figure 8)
but also reduces potential degradation of the natural resources, in this case the good quality of clay soil
which is also commonly used for paddy fields. Avoiding the firing process also reduces fuel
consumption: the traditional firing process for clay bricks mainly uses wood chip, rice husk and wood
23

24. as these materials are very limited and in some cases are expensive. However low energy and
environmentally friendly material will not always be the best option for the life cycle of the building.
As seen in Table 2 the energy BEP of the model lasts more than 20 months meaning the low embodied
energy will be paid off by the higher electricity consumption during its utilization, and after that this
material will not gain any benefits. Therefore this option should be neglected.
The most rational way to choose the best model (by additional materials, and not reducing
WWR) is to compare which gradient they are and then compare their BEP (energy and cost). By doing
this, it is found that DWg, DWb and COMB are the three options located in gradient or zone 1 and 2
which have reasonably low embodied energy and a high potential of electricity reduction compared to
the BASELINE. Moreover, if we look at the BEP table it shows that DWb (Double wall with bamboo
sheets) has about 1.6 months for Energy BEP and need around 25 months for the Cost BEP. These
values are relatively low compared to DWg which has an Energy BEP more than five times higher, and
is more than double in terms of Cost BEP.
By locating the scenarios on the gradient diagram we can find the best possible enclosure
design in terms of choosing material at a point where costs are not too high and the initial embodied
energy is also relatively low. The possible scenarios will not only consider the local material available
in the market, but the designer can choose any materials available in the global market and locate on
the gradient and expand its X and Y axis to any number depending on the maximum and minimum
scenarios values. The idea is that the designers, by locating their options into this gradient and then
comparing the Energy BEP and Cost BEP, will get a better overview of the option they have chosen
without calculating the whole life cycle energy or life cycle cost. This should be done as a first
screening of the building envelope materials.
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