Nowadays the concept of Net ZEB is well-known and widespread in the scientific community. The European Union has set ambitious targets for 2020 and even more ambitious for 2050. In order to reduce the domestic GHG emissions by 80-95%, compared to 1990 levels- till 2050, the building sector has to do its part and to pass through a deep restructure. Therefore, it is grown the interest in design and technical solutions for achieving a zero or nearly zero energy building. This paper investigate several construction technologies and system of energy production that can be adopted to build an “enhanced saving” (parsimonious) building, which can strive for the objective of NetZEB. Moreover the economic analysis of the feasibility of the NZEB target has been developed.

1. 2015 6th International Renewable Energy Congress (IREC)
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Design solutions for reducing the energy needs of
residential buildings
A. Gagliano*, F. Nocera, M. Detommaso, F.Patania
Industrial Engineering Department (DII)
University of Catania, Italy
*agagliano@dii.unict.it
Abstract— Nowadays the concept of Net ZEB is well-known
and widespread in the scientific community. The European
Union has set ambitious targets for 2020 and even more
ambitious for 2050. In order to reduce the domestic GHG
emissions by 80-95%, compared to 1990 levels- till 2050, the
building sector has to do its part and to pass through a deep
restructure. Therefore, it is grown the interest in design and
technical solutions for achieving a zero or nearly zero energy
building. This paper investigate several construction technologies
and system of energy production that can be adopted to build an
“enhanced saving” (parsimonious) building, which can strive for
the objective of NetZEB. Moreover the economic analysis of the
feasibility of the NZEB target has been developed.
Keywords—Net ZEB, energy performance, energy saving
I. INTRODUCTION
The Energy Performance of Building Directive (EPBD) [1]
states that Member States shall ensure that all new buildings
are nearly zero-energy building (NZEB) by 31 December 2020.
Member States should will draw up national action plans for
increasing the number of NZEBs and defining this concept in
practice. Further, Member States have to elaborate national
definitions and including policies and measures to stimulate the
refurbishment of the existing building stock into NZEB.
Consequently, NZEB target, of new or existing building, has
become a high priority for architects and multi-disciplinary
researchers related to architectural engineering and building
physics [2]. The idea of NZEB refers to a building with a net
energy consumption of zero over a typical year. It consists in
buildings that can satisfy all their energy demands through
renewable sources, locally available, non-polluting, low-cost
[3]. The target of a ZEB is not only to minimize the energy
consumption of the building with passive techniques, but also
balances the energy requirements exploiting of on-site
generation system from RES ( photovoltaic, solar thermal,
micro wind turbine) [4], [5]. Therefore, Net ZEB are buildings
connected to any energy infrastructure with which they
exchange energy. The connection to an energy infrastructure
introduces the issue of the building/grid interaction [6], and the
issue of the balance between delivered and exported energy.
Figure 1 depicts the connections between building and energy
grids [7]. The Primary Energy (PE) is the metric used for
making the balance between energy uses and renewable energy
production.
Fig. 1. Connection between building and energy grids [5]
The energy uses that have to be considered in assessing the
energy performance of the building are those related to heating
(H), cooling (C), production of hot water (W), ventilation (V)
and lighting (L). Consequently, the following expression holds:
= ∑ ( + + + − ) (1)
The result of equation (1) shall be almost zero in order to
center the target of Net-ZEB buildings.
Another important issue is the economic feasibility of
nearly zero energy buildings. The final report of the EU
Commission indicate that in mild climates and abundant solar
irradiation make nearly zero energy buildings in Southern
Europe technologically feasible with global costs over 30 years
equal or lower than ordinary buildings built today [8]. This
paper describes some possible strategies that can be adopted to
build an “enhanced saving” (parsimonious) building that can
strive for the objective of NetZEB.
Two targeted actions were evaluated: one related with the
energy performance of the building envelope and the other
with the heating and cooling technical systems. Moreover, the
exploitation increasing, as much as possible, of renewable
energy use has been analyzed. Thereby, the effectiveness of
each proposed intervention were evaluated in order to asses
which of them is the most convenient in terms of economic
payback.

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II. DESIGN SOLUTIONS FOR IMPROVING ENERGY
PERFORMANCE OF BUILDINGS
Constructive techniques that are often used for improving
the building energy performance are external wall insulation
systems, reduction of thermal bridging, “low-E and reflective”
windows, ventilated façades, green roof and so on.
A. External and load-bearing thermal insulation elements
The thermal insulation placed on the external side of the
building envelope (ETICS) reduces the thermal heat losses
both of the façades and the structural thermal bridge.
Anyway, the grade of thermal insulation must be correctly
chosen since an excess of insulation could increase the risk of
overheating during the hot season. The thermal bridges of the
balconies and the roofs can be corrected using special thermal
insulating elements able to produce the thermal decoupling.
As regards balcony slabs and roof up stands, thermal bridges
are avoided by using special thermally efficient load-bearing
connectors [9]. These connectors consist of a pressure-bearing
module made of micro-fiber high performance concrete,
connected with stainless steel bars and insulated with
polystyrene hard foam; this system can form a thermal break
whilst transferring load and maintaining full integrity with the
reinforced concrete structure of the building.
B. Low-E and reflective windows
Glazed surfaces have control heat loss in the winter and
reduce the heat gain during the summer period. Moreover, they
must to guarantee a sufficient good illumination trough natural
lighting. Therefore, the first task is select the windows
dimensions, at each orientations, taking into account the energy
balance of the glazing (the energy required for heating, lighting
and cooling the room). Enhanced thermally insulating glass
with low-emissivity coating (low-E), applied on the internal
surface, reduce the thermal losses.
Reflective coating allows reducing the incoming solar
radiation in the range of near infrared without compromising
the light transmission (77%) Moreover, the glass surfaces can
be used for installing PV system [10]. Table I shows the
features of the windows that have been used in the proposed
“parsimonious” building.
TABLE I. LOW-E AND REFLECTIVE WINDOWS FEATURES
Double glass (s=28 mm)
two 6 mm glass and 16 mm airspace
Heat transfer coefficient for glazing Ug=1.30 W/m2
K
Heat conductivity coefficient of the frame Uf=2.89 W/m2
K
Heat conductivity of window UW=2-00 W/m2
K
Solar factor g=42%
Emissivity of glass ε=0.1
Reflectance 0.9
C. Ventilated facades
“Ventilated facade” is a conventional expression meaning a
facade solution that entails the fixing of panels on a sub-frame,
which is fixed to the structural facade. It generally hides a gap
sufficiently wide to interrupt continuity with the underlying
wall and enable circulation of air. Ventilated facade acts
mainly reducing the energy needs for cooling. This aim is
obtained by the combined action of the shading of the external
walls, from the incident solar radiation, and the natural
ventilation of the air channel by the effect of air buoyancy.
Literature studies indicate that a reduction of about 50% of the
peak cooling load can be achieved [11], [12].
Table II shows the characteristics of the ventilated façade.
TABLE II. VENTILATED FACADES FEATURES
Dimensions s= 57,50 cm
Thermal transmittance U=0.30 W/m2
K
Thermal Mass MS=262 kg/m2
Thermal Capacitance Cint=38.83 kJ/m2
K
Periodic Thermal Transmittance Yie=0.07 W/m2
K
D. Green roof
A green roof is a layered system comprising of a
waterproofing membrane, growing medium and the vegetation
layer. The planted roofs mitigate solar radiation by the shading
effect of plants on the soil layer and by their biological
functions, such as photosynthesis, respiration, transpiration and
evaporation from soil and vegetation. The green roof energy
balance takes in account of the following contributes:
- long-wave and short-wave radiative exchanges within
the plant canopy,
- plant canopy effects on convective heat transfer,
- evapotranspiration from the soil and plants, heat
conduction (and storage) in the soil layer,
- moisture dependent thermal properties.
Many studies which have investigated the energy
performance of cool and green roof pointing out their
effectiveness for reducing the peak load cooling and the
building cooling needs [12],[13],[14]. Table III shows the
characteristics of an extensive green roof.
TABLE III. CHARACTERISTICS OF THE GREEN ROOF .
Layers S
(cm)
λ
(W·m-1
·K-1
)
ρ
(kg·m-3
)
Cp
(J·kg-1
·K-1
)
Vegetation layer - - - -
Soil layer 10.0 0.98 1460 880
Filter layer 0.1 0.22 910 1800
Drainage layer 10.0 0.92 900 1000
Root barrier 0.5 0.19 1400 1200
Waterproof membrane 0.5 0.19 1400 1200
Thermal insulation 0÷8.0 0.033 100 710
Moisture barrier 0.5 0.23 1100 1050
Concrete slab 24.0 2.30 2300 1000
E. Energy generation system
Two of the most efficient systems for domestic heating are
Condensing Boiler (CB) and Heat Pump (HP).
The CBs are high efficiency boilers which have an extra
heat exchanger that recover the thermal energy of the hot
exhaust gases for pre-heating the water in the boiler system.
At peak efficiency, the water vapor produced during the
combustion process condenses back into liquid form releasing
the latent heat of vaporization. As well know the performance,

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couched in terms of operating thermal efficiency, of a
condensing boiler is a function both of boiler design as well as
boiler operation, the latter a function of the hydraulic system
design and controls. It is reasonable consider an average
seasonal efficiency (ηgms) equal to 98 %.
The HPs are an electrically powered refrigeration cycle
devices, which absorb heat from a cold source (outside air,
water and ground) and release it indoors.
The coefficient of performance COP of a heat pump is
closely related to the temperature lift, i.e. the difference
between the temperature of the heat source and the output
temperature of the heat pump (condensation - evaporation
temperature). Thus, beneficial conditions are high source
temperature and low temperature distribution system.
The principal metrics used to quantify the energy
efficiency of HPs are the Seasonal Energy Efficiency Ratio
(SEER) and heating Seasonal Performance Factor (SPF)
respectively.
The SPF is defined as the ratio of the heat delivered and
the total energy supplied over the season. It takes into account
the variable heat source and sink temperatures over the year,
and includes the energy demand for defrosting.
The EU Directive on the promotion of the use of energy
from renewable sources recognizes HPs as a technology that
exploit Renewable Energy Sources (RES) from air, water and
ground [15]. The amount of annual renewable energy ERES
delivered through an electric heat pump is calculated as
follows:
ERES = Qusable( 1-1/SPF) (2)
Qusable = total usable heat delivered by HPs for space heating
and Domestic Hot Water (DHW). Only HPs, which achieve
115% efficiency, might be taken into account. Consequently,
SPF must satisfy the follows condition:
SPF > 1,15/η (3)
η = 0.4 (efficiency of electricity production in the EU).
III. TEST BUILDING
The Net ZEB study case is a multi-storey residential
building with 39 apartments located in Sicily (Acireale; lat
15°9’, long 37°36') [16]. The internal design temperature is
20°C during the heating period (15 November 31 March) and
26°C during the cooling period (1th Marzo 14 November).
Fig. 2. 3D view of the Residential Building
TABLE IV. GEOMETRIC DATA AND CHARACTERISTIC PARAMETERS
Heated gross-volume V 12580,88 m3
surface/volume
coefficient
S/V 0.417 m-1
Net floor area Su 3189 m2
Ventilation rate - 0.30 [vol/h]
Figure 2 and table IV and show the building and the main
geometric data. The building energy performance, based on
primary energy consumption (kWh/m2/year), was calculated
using the software MasterClima [17] which is based on a
simplified steady-state model in accordance with EN 15316
and UNITS 11300-2, that is a technical specification for the
nationwide application of EN 15316. Thermal losses and solar
gains are calculated considering the monthly mean values for
both temperature and solar radiation.
A Energy needs for the “standard” building
As one, the aim of this study is the evaluation of the payback
of the extra cost necessary to achieve the target of NetZEB,
the energy needs of a “standard” building that complies the
Italian regulations for new constructions (standard envelope),
has been calculated. Table IV reports the thermal
transmittance (Uvalue) of the building envelope components.
TABLE V. BUILDING THERMAL FEATURES
Type U-value (W/m2
K)
External wall 0.43
Roof slab 0.41
Ground Floor 0.43
Glazing Surfaces 3.16
Thermal bridge 0.80 W/mK
This building configuration has the following energy needs:
QH = 155.55 MWh; QW=55.19 MWh; QC=89.86 MWh;
The thermal energy for heating space and DHW production
are both provided through centralized gas-fired system with an
efficiency of about 90%.
The global efficiency of the heating system results of 77.80% .
Thus the primary specific energy for space heating (PEH), for
DHW production (PEW), the global energy needs (PEHgl) and
the specific energy for cooling (PEC) were calculated:
PEH = QH/ηH·Su = 62.66 kWh/m2
y (4)
PEW = QW/ηW·Su = 22.22 kWh/m2
y (5)
PEHgl = PEH + PEW = 84.88 kWh/m2
y (6)
PEC = QC/ Su =28.17 kWh/m2
y (7)
The values of the above reported indexes allow assigning
the performance certification of this building as stated by the
Italian rule. This building is rated in G class (heating) and, in
class III (cooling).
III.2 Energy needs of the “parsimonious” building
The energy performance of the test building has been
increased applying all the constructive technologies previously
described. Table VI reports the values of the thermal
transmittance (Uvalue) of the component of the building
envelope.

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TABLE VI. OPAQUE ENVELOPE AND GLAZING THERMAL FEATURES
Type U-value
(W/m2
K)
Low-E and reflective Windows 2.00
Green Roof 0.20
Ventilated Facades 0.30
External Wall Insulation Systems 0.33
Thermal bridge ( ) 0.15 W/mK
Figure 3 shows the heat fluxes through the green roof, with
a surface of 1000 m2
, and the heat fluxes exchanged through
the traditional roof. The graph highlights that the green roof
has a greater positive energy balance than traditional roof
during all the year.
Fig. 3. Comparison of thermal fluxes between traditional and green roof
The only exception is in March with an incoming energy
flux higher than traditional roof of 176 kWh. During summer
months, it allows obtaining a highest reduction of the energy
needs; in July, there is a reduction of 1701 kWh. Globally, this
new configuration of the building envelope requires the
following energy needs:
QH = 47.31 MWh; QW=55.19 MWh; QC=58.79 MWh
Table VII summarizes the contribute of each proposed
intervention on the reduction of the energy needs.
As results the proposed solutions reduce of about 69% the
energy needs for space heating, (107.87 MWh), and of 34%
(30.88 MWh) during the cooling period.
This result is higher than the summation of the single
intervention thanks to a “multiplicative Energy Saving
Effects” (mESE).
TABLE VII. REDUCTION OF ENERGY NEEDS
INTERVENTION REDUCTION
of QH
(MWh)
REDUCTION of
QC (MWh)
PEH PEC
WALL
INSULATIONS
80.97 +34.12 22.91 +10.70
WINDOWS 4.27 28.35 61.03 19.23
VENTILATED
FACADE
4.22 10.10 59.83 25.10
GREEN ROOF 2.75 4.25 61.54 26.44
SUMMATION OF
INTEVENTION
107.87 30.88 14.83 18.43
It is quite evident that there are interventions most efficient
during the heating period and other during the cooling. The
upgrade of wall insulation with the ETICS solution bridges
makes the building very isolated. Thereby it possible to notice
a reduction of about 50% of the heat losses during the
heating period but, also the increase of the energy needs
during the cooling period especially in the springer and
autumn months.
Otherwise, the “efficient” windows allow getting excellent
performances during the cooling period. The reduction of
energy needs is mainly due to decrease of the solar gains.
The ventilated façades give the greatest reduction of
energy need in the summer period.
Since only the apartments of upper floor are directly
affected by the green roof , its contribution is not very high.
Then, the comparison between the proposed interventions (J)
is realized in terms of ratio between the monthly energy
saving (ESJ)I and the surface extension of each intervention
(ΣAJ), as follows defined.
ES(sq)J, I = (ESJ) I /ΣAJ (4)
ES(sq)J, I = energy saving per unit of surface obtained through
the intervention “J” at the “I” month
ΣA J = surface where the intervention J is applied .
Fig. 4. Normalized energy saving of building components
Under this point of view, it can be noted that:
- the low-e and reflective windows show the highest energy
saving both in summer and winter months
- ventilated facades give a contribution all year around, and
are most efficient during the summer months ( June, July,
August and September).
- green roof gives the lowest values in all the months. This
result depends, as previously reported, by the fact that only
the apartments of upper floor can directly take advantages
by its effects.

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Globally, the “parsimonious building” is characterized by
the following indexes of energy performance: PEHgl =PEH +
PEW = 41.28 kWh/m2
y; PEC = 18.43 kWh/m2
y
The building is rated in D class (heating) and in class II
(cooling) as stated by the Italian rule on the energy
performance certification of buildings.
However, the global request of energy remains high;
especially to satisfy the energy needs for DHW. Therefore, to
further reduce the energy needs it is necessary improve the
efficiency of the energy production system and foresee the
exploitation of RES
IV. SOLAR THERMAL PLANT
The amount of thermal energy request for DHW represents
a significant percentage of the total energy demand of the
building. Indeed, the primary energy for domestic hot water
PEW amounted to 22.22 kWh/m2
y that is more than half of the
value of the global primary energy PEgl in the case of
“parsimonious” building envelope.
So, for reducing the primary energy requirement becomes
fundamental the exploitation of renewable energy sources.
Thus, the installation of thermal solar plants on the roof of the
building was foreseen. A surface of 3.0 m2
of simple solar flat
panel, south oriented with 35° of tilt angle, is sufficient to
satisfy the energy demand for DHW of each apartments, as
results of the “f chart” method [18]. Overall, 120.0 m2
of solar
panel are necessary. Moreover, each solar plants is equipped
with a stratified storage tank with capacities of 250 liters.
In this way the request of energy for DHW is almost zero
and, consequently the global specific primary energy PEgl is
drastically reduced to 19.16 kWh/m2
y.
Therefore, the most insulated building configuration in
addition to the solar thermal plants allow reaching the B class
in accordance with the national energy classification.
V. ENERGY PRODUCTION SYSTEMS: CONDENSING BOILER
AND HEAT PUMPS
The energy performance of building can be further
increased through the introduction of generation systems
characterized most efficient. Two different options were
evaluated: Condensing Boiler and Heat Pump
A Condensing boiler
As well known a condensing boiler is characterized by
very high combustion efficiency as it exploits the energy of
condensation of the water vapor contained in the exhausted
gases. An increase of about 12 % of the efficiency of the
condensing boiler compared to the traditional one has been
evaluated. Thus, the specific primary energy for the winter
heating PEgl varies from 19.16 to 16.89 kWh/m2
y. This result
does not allow the improvement of the energy class as the
building remains in class B.
V.II Heat Pump
As previously highlighted, the EU directive recognizes
HPs as a technology that exploit RES from air, water and
ground. Therefore, it has been evaluated as the energy
performance of the building is improved if an Heat Pumps is
installed. More specifically, an vapor compressed HP with air
source and a SPF=3.45 was chosen. The specific primary
energy PEgl becomes 10.60 kWh/m2
y and, the building reach
the class A. Moreover, from equation (2) it is possible to
evaluate how many is the annual renewable energy ERES
delivered by the HP is 3.06 of about the 71% of the thermal
energy request. These results allow to totally satisfying the
request of Italian law (50%).
The use of heat pumps resets the fuel needs but involves a
demand of electric energy that should be satisfied by the
optimal management of various RES [19].
VI. ZERO ENERGY BUILDING
The target of a ZEB is not only to minimize all the energy
consumption, including energy requirements for lighting and
other use, but also balances its energy requirements with the
local production of energy through RES.
Considering that the electricity energy consumption of
each apartment is about 3.000 kWh per year [20], the surface
of PV solar collectors necessary for balancing the total
electrical energy demand, of about 120,000 kWh, has been
calculated. The installation of two different PV solar panels
has been foreseen. The first PV plant can be installed in the
south facade of the building (tilt angle b= 90°), that is a BIPV
system. The second PV plant, installed on the roof, is south
oriented and 15° inclined. The PvGis tool was used for
calculating the energy yields of the two PV plants. Table VII
gives the energy yields of the two PV plants
TABLE VIII. ENERGY YELDS OF PV PLANTS
PV plant Ppeak (kW) Energy yealds (kWh)
b= 90° 40.0 36,000.00
b= 15° 60.0 84,000.00
Considering that of about 120.00 m2
of solar thermal panel
are necessary for satisfying the DHW needs, the available
surface is not so sufficient. Therefore, it is mandatory to install
solar panels which allow the simultaneously production of
electric and thermal energy, the so-called PVT panel [21]. In
this way, it will be possible optimizing the spaces available for
the installation of the RES in the buildings.
VII. ECONOMIC ISSUES
The reduction of energy consumption and CO2 emissions
not is able to leave the analysis of the cost on intervention out
of consideration. Table IX reports the costs of materials and
systems considered in this study that derive from a survey
over Italian market. The cost analysis of the solutions
proposed indicates an extra cost 165.463,56 €, considering the
FIT (tax deductions) that will be really with a payback period
of 13 -14 years.

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TABLE IX. – COST OF MATERIALS
As shown in figure 5, the interventions on the generation
systems are more effective than which ones on the envelope.
Fig. 5. Normalized PE energy saving
The effectiveness of these interventions on systems is an
important issue for buildings that are located in temperate
climates, for which the interventions on the building envelope
are costly and not decisive in terms of energy savings.
Fig. 6. Reduction of CO2 of components
In addition, the interventions on systems are more incisive
in terms of reductions in CO2 emissions. It also highlights that,
the only building system that offers a significant contribution to
the reduction of energy consumption and CO2 emissions appear
to be the thermal bridges.
VIII. CONCLUSIONS
The study analyse some solution to achieve the target of
NZEB. Interventions on the building envelope and on the
energy generation system have been investigated calculating
the reduction the energy needs. The proposed solutions
contribute at the reduction of energy demand of the building,
but the interventions on the building envelope are not able to
nullify the energy needs of the building. Therefore it is
mandatory foresee the use of RES. The results of the study
show that to obtain a NZEB it is necessary to install of about a
PV plant of about 100 kWp and 120.00 m2
of solar thermal
panels. So, for reducing the request of space where host these
RES the exploitation of PVT can be warmly suggested. The
financial analysis show that not all intervention are cost-
effective and sustainable for residential buildings, especially in
mild climate, for which are more interesting the interventions
on systems rather than building envelope. Therefore, high
initial costs and long payback periods, financial barriers, are
the main obstacles for increase the spreading the NZEB.
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Material of system Feature Unit Cost
Low-E and reflective windows ε=0,1 g=42% €/m2
42,00
Ventilated facades insulation (s=5 cm) €/m2
150,00
Thermal coat insulation (s=5 cm) €/m2
10,00
Green roof extensive (sg=10 cm) €/m2
50,00
Thermal bridge insulation (s=5 cm) €/m 230,00
Heat Pump 200 kW €/unit 36.000,00
Condensing Boiler 200 kW €/unit 25.000,00
Solar thermal system (DHW) Flat collectors €/m2
500,00
Monocrystalline PV system ηSTC=14% €/m2
400,00