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Technical report:
Cost optimal and nZEB energy performance
levels for buildings
May 30, 2011
Jarek Kurnitski
Sitra, the Finnish Innovation Fund
Arto Saari
Aalto University
Mika Vuolle
Equa Simulation Finland Oy
Finland
KENA cost optimal and nZEB def 2
Contents
1 Introduction ............................................................................................................................................................................ 3
2 Methods ................................................................................................................................................................................... 3
3 Technical definition for net zero energy buildings ................................................................................................. 4
4 Results ....................................................................................................................................................................................... 9
4.1 Net present value calculation ................................................................................................................................. 9
4.2 Results for detached houses ................................................................................................................................ 10
4.3 Results for apartment building ........................................................................................................................... 19
4.4 Results for office building ..................................................................................................................................... 23
5 Conclusions .......................................................................................................................................................................... 29
KENA cost optimal and nZEB def 3
1 Introduction
Energy performance of buildings is regulated in Estonia with the government act on minimum
requirements for energy performance. This act includes primary energy requirements for all common
building types, mandatory input data for energy calculation (standard use of the buildings) as well as
calculation rules and guidelines, and requirements for calculation tools. The framework and procedure
is one of the most generic and flexible in EU and can be used as is for net zero energy building energy
performance calculations. Energy certificate values for new buildings are also calculated with this act.
EPBD recast launched cost optimal principle of the minimum requirements and the roadmap to nearly
zero energy buildings in 2019-2021. Demanding construction clients need already today a common
definition for low energy and nearly zero energy buildings that would be more ambitious than existing
minimum requirements. These new top categories are to be implemented into energy performance
certificate in the future revision.
This report proposes definitions for cost optimal low-energy buildings and nearly zero-energy
buildings of selected building types. The cost optimal principle of EPBD recast with net present value
calculation is used to derive the low energy performance level. Nearly zero energy performance level is
derived as the lowest possible primary energy use technically reasonable achievable.
2 Methods
The definitions for low energy and nearly zero energy buildings were prepared for the following
building types:
• detached house
• apartment building
• nursing home
• day-care centre
• school building
• office building
Cost optimal primary energy use was calculated for each building type based on solutions leading to
minimum net present value with 30 years period. The net present value calculation included both
investment and operation cost discounted with common real interest rate of 3%. To show the
sensitivity to the interest rate, the escalation of the energy price was varied between 1 and 4%. For
initial energy prices the current price data was used.
The calculation procedure was started with selection of reference buildings. Energy and cost
simulations were conducted for two most feasible, currently known low energy concepts for each
building type. For each low energy concept a series of energy simulations were conducted with several
building envelope thermal insulation, heat recovery and other parameter values. After that, the net
present value calculation showed cost optimal levels for each building type. Energy simulations were
carried out for all building types studied (6), but economical calculations only for every second
building type due to similarity of technical solutions in non-residential buildings.
KENA cost optimal and nZEB def 4
All low energy building concepts studied were equipped with effective heat recovery. For detached
houses, the main concepts with ground source heat pump, air to water heat pump and passive house
with electrical heating and solar collector were studied. All other building types were calculated with
district heating as a main heat source. Two alternative construction concepts were formulated mainly
through daylight control and demand controlled ventilation options.
Energy simulations were conducted with input data and calculation rules of the Estonian act of
minimum requirements for energy performance. These include indoor climate, Estonian TRY and
standard use of the building (occupancy and other internal gains and DHW data). This calculation
procedure provides delivered energy use from which primary energy rating ET-value was calculated
with energy carrier factors.
Derived nearly zero energy and low energy performance levels are proposed to be implemented in the
energy performance certificate scale, so that category A will correspond to nearly zero energy and
category B or C to cost optimal low energy building energy performance level, depending on the
difference between the nearly zero and cost optimal, Figure 1.
Figure 1. A proposal for the implementation of nearly zero energy and cost optimal low
energy buildings energy performance levels into the energy performance certificate scale.
3 Technical definition for net zero energy buildings
The following general definition format proposed by REHVA Task Force “Nearly Zero Energy Buildings”
(REHVA Journal 3/2011) was used as a framework for nearly zero energy building energy performance
calculations. EPBD recast requires nearly nZEB buildings, but since it does not give minimum or
maximum harmonized requirements as well as details of energy performance calculation framework, it
will be up to the Member States to define what these for them exactly constitute.
Nearly net zero energy building definition shall be based on delivered and exported energy according
to EPBD recast and EN 15603:2008. The net delivered energy, which is delivered minus exported
energy per energy carrier, is shown in Figure 2 and described with detailed system boundary definition
in Figure 3. This system boundary definition is a general form modified from the one of EN
15603:2008. Suggesting the inclusion of energy use of appliances (households and outlets), the system
boundary proposes that all energy used in buildings will be accounted in net delivered energy as well
as in nearly net zero energy building definition, in accordance with Estonian act on minimum
Type of buildingEnergy performance value, kWh/(m2·a)
ANearly-
zero
BLow-
energy
C DNew
buildings
EReconst-ruction
F G
Detached house
Nursing home
Day-care centre
Apartment building
Office building
Schools
KENA cost optimal and nZEB def 5
requirements for energy performance. This means that energy use in the buildings includes inter alia,
energy used for heating, cooling, ventilation, hot water, lighting and appliances.
Figure 2. System boundary for nearly net zero energy building definition, connecting a
building to energy networks. Net delivered energy is delivered Edel,i minus exported energy
Eexp,i accounted separately for each energy carrier i. Primary energy E is calculated with
primary energy factors fi. (Adopted from REHVA Task Force “Nearly Zero Energy
Buildings”)
Net zero energy requirement has exact performance level of 0 kWh/(m2 a) primary energy. The
performance level of “nearly” net zero energy use is a subject of national decision taking into account:
• cost optimal and technically reasonably achievable level of primary energy use
• how many % of the primary energy is covered by renewable sources
• ambition level of the definition
The following definitions, proposed by REHVA Task Force “Nearly Zero Energy Buildings” (REHVA
Journal 3/2011) were used:
net zero energy building (nZEB)
energy use of 0 kWh/(m2 a) primary energy
NOTE 1 A nZEB is typically a grid connected building with very high energy performance. nZEB
balances its primary energy use so that the primary energy feed-in to the grid or other energy network
equals to the primary energy delivered to nZEB from energy networks. Annual balance of 0 kWh/(m2
a) primary energy use typically leads to the situation where significant amount of the on-site energy
generation will be exchanged with the grid. Therefore a nZEB produces energy when conditions are
suitable, and uses delivered energy during rest of the time.
nearly net zero energy building (nnZEB)
national cost optimal energy use of > 0 kWh/(m2 a) primary energy
DELIVERED ENERGY
EXPORTED ENERGY( )∑ −=
i
iiidel fEEE exp,,
KENA cost optimal and nZEB def 6
NOTE 1 The Commission shall establish by 30 June 2011 a comparative methodology framework for
calculation of cost-optimal levels (EPBD recast).
NOTE 2 Not all renewable energy technologies needed for nearly zero energy building have to be
cost-effective, if appropriate financial incentives are not available.
For the detailed energy boundary specification, the guidance is provided in EN 15603:2008. Inside the
boundary the system losses are to be taken into account explicitly, outside they are taken into account
in the conversion factor (=primary energy factor). Technical building systems located partly outside of
the building envelope are considered to be inside the system boundary. It is also clearly stated that the
assessment can be made for a group of buildings serviced by the same technical systems.
EN 15603:2008 states that for active solar and wind systems only the energy delivered by the
generation devices and auxiliary energy are taken into account in the energy balance (i.e. kinetic
energy of wind is not).
The detailed energy boundary is modified from EN 15603:2008 and as stated in EPBD recast,
renewable energy produced on site is not considered as part of delivered energy, i.e. the positive
influence of it is taken into account, Figure 3.
Figure 3. Energy boundary of net delivered energy and how it forms from energy need,
energy use of technical building systems, on site renewable energy production, delivered
energy and exported energy. The box of “Energy need” refers to rooms in a building and
both system boundary lines may be interpreted as the building site boundary. (Adopted from
REHVA Task Force “Nearly Zero Energy Buildings”)
Energy need represents energy need in a building for heating, cooling, ventilation, domestic hot water,
lighting and appliances (if appliances are included in the system boundary as proposed). Energy need
for heating is caused by heat losses and is reduced by solar and internal heat gains. Net energy need is
ENERGY NEED
HeatingCoolingVentilation
DHWLighting
Appliances
System boundary of net delivered energy
Net
deli
vere
d e
nerg
y
(ele
ctr
icity,
dis
tric
t h
ea
t, d
istr
ict
co
olin
g, f
ue
ls)
System boundary of delivered energy
heating energy
cooling energy
electricity for lighting
fuels
BUILDING
TECHNICAL SYSTEMS
Energy use and production
System losses and conversions
electricity
cooling energy
On site renewable
energy w/o fuels
district heat
district cooling
electricity
heating energy
Solar and internal
heat gains/loads
Heat exchange
through the building envelope
NET ENERGY
NEED
DELIVERED
ENERGY
EXPORTED ENERGY
(renewable and non-renewable)
electricity for
appliances
KENA cost optimal and nZEB def 7
the energy need minus heat gains, i.e. thermal energy without any system losses needed to maintain
indoor climate conditions. For the lighting and appliances electrical energy is needed.
Building technical systems supply the amount of net energy needs of heating, cooling and electrical
energy. To supply these net energy needs, building technical systems use energy and have typically
some system losses and energy conversion in some systems (i.e. heat pumps, fuel cells). The energy
used by the building technical systems is from delivered energy to the building or from on site
renewable energy (without fuels).
Delivered energy to the building is grid electricity, district heat and cooling, renewable and non-
renewable fuels. On site renewable energy without fuels is energy produced from active solar and wind
(and from hydro if available). Renewable fuels are not included in this term, because they are treated
as delivered energy to the building, i.e. off-site renewables. Energy from heat sources of heat pumps
(air, ground, water) is also renewable energy, but this information is not needed for heat pump system
and delivered energy calculations which are based on COP data of heat pumps. (However, energy
taken from heat sources of heat pumps is needed for calculation of the share of renewable energy,
which is additional information).
On site renewable energy production systems may supply other technical building systems, thus
reducing the need for the delivered energy to building, or may be directly exported to energy
networks. This is taken into account in the net delivered energy balance. Net delivered energy is
delivered minus exported energy, both expressed per energy carrier.
Primary energy use is calculated from net delivered energy, per energy carrier, as product of primary
energy factor and net delivered energy of that energy carrier.
An example of energy flow calculation (adopted from REHVA Task Force “Nearly Zero Energy
Buildings”)
Consider an nnZEB office building located in Paris with following annual net energy needs (all values
are specific values in kWh/(m2 a)):
• 3.8 kWh/(m2 a) net energy need for heating (including ventilation and DHW)
• 11.9 kWh/(m2 a) net energy need for cooling
• 21.5 kWh/(m2 a) electricity for appliances
• 10.0 kWh/(m2 a) electricity for lighting
Breakdown of the net energy need is shown in Figure 4.
The building has a gas boiler for heating with seasonal efficiency of 90%. For the cooling, free cooling
from boreholes (about 1/3 of the need) is used and the rest is covered with mechanical cooling. For
borehole cooling, seasonal energy efficiency ratio of 10 is used and for mechanical cooling 3.5.
Ventilation system with specific fan power of 1.2 kW/(m3/s) will use 5.6 kWh/(m2 a) fan energy. There
is installed a solar PV system providing 15.0 kWh/(m2 a), from which 6.0 is utilized in the building and
9.0 is exported to the grid.
Energy calculation results are shown in Figure 4, in the building technical systems box. Gas boiler with
90% efficiency results in 4.2 kWh/(m2 a) fuel energy. Electricity use of the cooling system is calculated
with seasonal energy efficiency ratios 10 and 3.5 respectively. Electricity use of free cooling, mechanical
cooling, ventilation, lighting and appliances is 39.8 kWh/(m2 a). Solar electricity of 15.0 kWh/(m
2 a)
reduces the net delivered electricity to 24.8 kWh/(m2 a). Net delivered fuel energy (caloric value of
KENA cost optimal and nZEB def 8
delivered natural gas) is 4.2 kWh/(m2 a). From these two net delivered energy flows, primary energy is
calculated with the result of 66 kWh/(m2 a).
Figure 4. Calculation example of the energy flows in nnZEB office building. (Adopted from
REHVA Task Force “Nearly Zero Energy Buildings”)
System boundary of delivered energy
3.8 heating
11.9 cooling
10.0 lighting
BUILDING TECHNICAL
SYSTEMS
15.0 PV electricity,from which 6.0 used
in the building and
9.0 exported
Fuel 4.2
Electricity 33.8
Solar and internal
heat gains/loads
Heat exchange
through the
building envelope
NET ENERGY NEED
(47.2 kWh/(m2 a))
DELIVERED ENERGYBoiler
3.8/0.9 = 4.2
Free cooling
4.0/10 = 0.4 Compressor cooling
7.9/3.5 = 2.3
Lighting 10.0
Ventilation 5.6
Appliances 21.5
Primary energy:
4.2*1.0 + (33.8-9.0)*2.5 = 66 kWh/(m2 a)
EXPORTED ENERGY
System boundary of net delivered energy
Ne
t d
eli
ve
red
en
erg
y
Electricity 9.0
21.5 appliances
(Sum of electricity 39.8)
21,5
10
3,2
0,61,1
10,8
NET ENERGY NEED (47.2 kWh/(m2 a))
Appliances
(users')
Lighting
Space
heating
Heating of
air in AHU
Cooling in
room units
Cooling of
air in AHU
KENA cost optimal and nZEB def 9
4 Results
4.1 Net present value calculation
Economic calculations included construction cost calculations and discounted energy cost calculation
for 30 years. Construction cost was calculated not as a total construction costs, but only construction
works and components related to energy performance were included in the cost (energy performance
related construction cost). Such construction works and components were:
• thermal insulation
• windows
• air handling units (without ductwork)
• heat supply solutions (boilers, heat pumps etc.)
In all calculated cases an under floor heating system and a hot water boiler was considered, but these
were not included in the energy performance related construction cost.
Labour costs, material costs, overheads, the share of project management and design costs, and VAT
were included in the energy performance related construction cost.
Global energy performance related cost was calculated as a sum of the energy performance related
construction cost and discounted energy costs for 30 years, including all electrical and heating energy
use.
For the energy prices, the current price levels were used as follows:
• Electricity 0.0983 €/kWh + VAT (20%)
• Natural gas 0.0395 €/kWh + VAT (20%) (consumption over 750 m3/year)
• Pellet 0.033 €/kWh + VAT (20%)
• Heating oil 0.0717 €/kWh + VAT (20%)
• District heating 0.0569 €/kWh + VAT (20%) (Tallinn, natural gas boiler)
Connection fees for electricity and heating were taken into account as follows:
• Electricity 111.85 € + VAT (20%) per 1 A
• Gas 2046 € + VAT (20%)
• District heating 2500 € VAT (20%)
For electricity connection, 20 A was considered in most of cases. In two less insulated electrically
heated cases, DH 0.76 and DH 0.96, 25 A was used.
Global energy performance related costs were calculated in the basic case with discounting interest
rate of 1% which corresponds to real interest rate of 3% and escalation of energy prices of 2%,
according to the Commission’s draft cost optimal document. This discounting interest rate, that was
used in the discounting of energy cost, is the difference between the real interest rate and the
escalation of the energy price. For example, discounting interest rate of 1.5% may correspond to real
interest rate of 3% and escalation of 1.5%, or real interest rate of 2% and escalation of 0.5%.
In order to include some safety margin into the discounting interest rate (making sure that the cost
optimal building will not underinvested in the construction phase), discounting interest rate of 0 %
corresponding to real interest rate of 3% and escalation of 3% was calculated. To show the sensitivity
to the escalation rate, discounting interest rate of 2% was used (corresponding to escalation of 1%
with the real interest rate of 3%).
KENA cost optimal and nZEB def 10
As such, the global energy performance related cost has a little meaning, because the basic
construction cost is not included. The difference in the global energy performance related cost can be
used for ranking of calculation cases. The case with the lowest global energy performance related cost
represents the cost optimal for 30 year period studied. The cost difference can be calculated also
relative to business as usual (BAU) construction, if BAU is well established. If so, a global additional
energy performance related cost can be either negative if BAU is not cost optimal, or positive if the
case studied leads to higher global cost than BAU.
4.2 Results for detached houses
The results are calculated for the reference detached house with heated net floor area of 178.6 m2,
Figure 5 and 6. The garage is considered as not heated and it is not included in the heated net floor
area. The reference house has 3 bedrooms and is intended for 4 persons.
Figure 5. Plans of 2-storey reference detached house with heated net floor area of 178.6 m2.
KENA cost optimal and nZEB def 11
Figure 6. IDA-ICE building simulation model of the reference detached house.
Analyses were conducted for four construction concepts, where the building envelope and ventilation
system energy performance levels were varied as shown in Table 1. These construction concepts are
marked with the specific heat loss value, i.e. DH 0.42 means the reference detached house with the
specific heat loss of 0.42 W/(m2 K). The specific heat loss includes transmission losses through the
building envelope and infiltration losses, and is calculated per heated net floor area. DH 0.42
construction concept represents the best available technology which may be associated with nearly
zero energy buildings. DH 0.96 represents business as usual construction.
KENA cost optimal and nZEB def 12
Table 1. Construction concepts for the reference detached house of 178.6 m2.
DH 0.42
“Nearly zero”
DH 0.58
“Low”
DH 0.76
DH 0.96
“BAU”
Specific heat
loss coefficient
H/A, W/m2K
0.42 0.58 0.76 0.96
External wall
170 m2
20cm LECA block, plaster
+ 35cm EPS-insulation
U 0.1 W/m2K
20cm LECA block, plaster
+ 25cm EPS-insulation
U 0.14 W/m2K
20cm LECA block, plaster
+ 20cm EPS-insulation
U 0.17 W/m2K
20cm LECA block, plaster
+ 15cm EPS-insulation
U 0.23 W/m2K
Roof
93 m2
Wooden beams, metal
sheet, 80cm min.wool
insulation, concrete slab
U 0.06 W/m2K
Wooden beams, metal
sheet, 50cm min.wool
insulation, concrete slab
U 0.09 W/m2K
Wooden beams, metal
sheet, 32cm min.wool
insulation, concrete slab
U 0.14 W/m2K
Wooden beams, metal
sheet, 25cm min.wool
insulation, concrete slab
U 0.18 W/m2K
Ground floor
93 m2
Concrete slab on ground,
70cm EPS insulation
U 0.06 W/m2K
Concrete slab on ground,
45cm EPS insulation
U 0.09 W/m2K
Concrete slab on ground,
25cm EPS insulation
U 0.14 W/m2K
Concrete slab on ground,
18cm EPS insulation
U 0.18 W/m2K
q50, m3/(hm
2) 0.6 1.0 1.5 3.0
Windows
48 m2
glazing/frame/total
4mm-16mmAr-SN4mm-
16mmAr-SN4mm
Insulated frame
0.6/0.7 W/m2K
0.7 W/m2K
4mm-16mmAr-4mm-
16mmAr-SN4mm
Insulated frame
0.8/0.8 W/m2K
0.8 W/m2K
4mm-16mm-4mm-
16mmAr-SN4mm
1.0/1.3 W/m2K
1.1 W/m2K
4mm-16mmAr-
SN4mm
Common frame
1,1/1,4 W/m2K
1,2 W/m2K
g-value 0.46 0.5 0.55 0.63
Ext. door, 6 m2 U 0.7 W/m
2K U 0.7 W/m
2K U 0.7 W/m
2K U 0.7 W/m
2K
Ventilation l/s, SFP, AHU HR
80 l/s, SFP 1.5
kW/(m3/s), AHU HR
85%
80 l/s, SFP 2.0
kW/(m3/s), AHU HR
80%
80 l/s, SFP 2.0
kW/(m3/s), AHU HR
80%
70 l/s, SFP 2.0
kW/(m3/s), AHU HR
80%
Heating
capacity, kW 5 6 8 9
Cooling
capacity, kW 5 5 5 8
Net energy need kWh/(m2 a)
Space heating 21.3 35.3 52.8 68.5
Ventilation
heating 3.9 5.5 5.5 5.5
Domestic hot
water 28.1 28.1 28.1 28.1
Cooling 13 10.6 8.8 14.4
Fans and
pumps 7.6 8.4 9.6 9.6
Lighting 7 7 7 7
Appliances 18 18 18 18
Total net
energy need 98.9 112.9 129.8 151.1
Table 1 shows also simulated net energy needs. Depending on heating and cooling systems, delivered
and primary energy use can be calculated. For each construction concept, the following heating
systems were considered:
• ground source heat pump
KENA cost optimal and nZEB def 13
• air to water heat pump
• district heating
• electrical heating
• gas boiler
• oil boiler
• pellet boiler
Delivered energy use was calculated for these heating systems, by dividing net energy needs with
relevant system efficiency. System efficiency values (combined efficiency of the generation and
distribution) are shown in Table 2. To calculate the combined efficiency, floor heating distribution is
considered according to Estonian method with average distribution efficiency of 0.9, which is included
in the combined efficiency values in Table 2.
Table 2. System efficiencies for delivered energy calculation.
Heat source (under floor heating)
Generation and distribution combined efficiency, -
Space heating/cooling Domestic hot water Gas/oil condensing boiler 0.86 0.83
Pellet boiler 0.77 0.77
Air to water heat pump 1.98 1.62
Electrical heating 0.90 0.90
Ground source heat pump 3.15 2.43
District heating 0.90 0.90
Cooling (electricity) 3.0
Highly insulated DH 0.42 and 0.58 cases are calculated with solar collectors with the size of 6 m2,
providing an half of domestic hot water. Other cases are calculated without solar collectors. Primary
energy values (ET-values), calculated with Estonian primary energy factors for all construction concepts
and heating systems, are shown in Figure 7.
Figure 7. Primary energy values (ET-values)in the reference detached house for all
combination of construction concepts and heating systems.
0
50
100
150
200
250
Gas/oil Pellet AWHP GSHP Electricity Distr. h.
Pri
ma
ry e
ne
rgy
, E
T-v
alu
e,
kW
h/
(m2
a)
DH 0.42
DH 0.58
DH 0.76
DH 0.96
min. req. 180
KENA cost optimal and nZEB def 14
Global energy performance related costs (explained in Ch. 4.1) are shown in Figure 8 and 9 for
discounted interest rate of 1 % that corresponds to real interest rate of 3% and escalation of 2%. The
global cost is presented as an additional cost compared to the business as usual construction concept
DH 0.96 with gas boiler, just complying the minimum requirement of 180 kWh/(m2 a) primary energy.
According to the results, the cheaper energy sources, district heating and gas achieve the lowest NPV
of the global additional cost with relatively high primary energy use. The lowest NPV defines the cost
optimal performance level which is achieved for district heating DH 0.76 construction concept with
primary energy of about 140 kWh/(m2 a). The global cost is marginally higher for gas heating which
achieves the lowest NPV value for DH 0.76 and DH 0.96 cases with primary energy of about 160–180
kWh/(m2 a).
If district heating or gas supply are not available, the cost optimal primary energy use will significantly
dropped down in both heat pump and oil heating cases. Therefore, for building sites without district
heat or gas supply, the cost optimal is achieved with ground source heat pump DH 0.96 construction
concept with primary energy of about 120 kWh/(m2 a). At that primary energy level the cost curve of
ground source heat pump is lower than that of gas heating, but still 18 €/m2 NPV higher compared to
district heating.
Figure 8. Global energy performance related costs in the reference detached house calculated
with discounting interest rate of 1% (the real interest rate of 3% and the escalation 2%) and
30 years time period. (AWHP – air to water heat pump, GSHP – ground source heat pump,
DH – district heating.) For each heating system, from left to right DH 0.42, 0.58, 0.76 and
0.96 cases are shown. Two last points of the electrical heating are out of the range of the
chart, being (200 kWh/(m2 a);153 €/m
2) and (226 kWh/(m
2 a);188 €/m
2).
-50
0
50
100
150
50 100 150 200
Glo
ba
l ad
dit
ion
al e
ne
rgy
pe
rfo
rma
nce
co
st (
NP
V),
€/m
2
Primary energy, ET-value, kWh/(m2 a)
Gas
Pellet
AWHP
GSHP
Electric
Oil
DH
KENA cost optimal and nZEB def 15
The results show that for cheaper energy sources the cost optimal of 140 and 160 kWh/(m2 a) was
achieved without solar collectors, (as well as without solar PV which is not cost efficient without feed in
tariff or investment support) and in relatively low thermal insulation level. Thus, this cost optimal
energy performance is still quite far from nearly zero energy performance level. Improved thermal
insulation level and solar collectors became cost effective for air to water heat pump, oil heating and
electrical heating.
It is important to notice that the additional global cost is less than additional investment cost, because
of reduced cost of energy use. The breakdown of global cost components is shown in Figure 9. It can
be seen that an additional investment cost of improved thermal insulation is about 13 000 € and air
handling unit 2 000 € from DH 0.96 to DH 0.42 construction concept. This 15 000 € investment drops
primary energy to about 75 kWh/(m2 a) in the ground source heat pump case and corresponds to
7 100 increase in NPV. For nearly zero energy building, this has to be supported with relevant solar PV
installation. If 5 kW solar PV installation with about 25 000 € investment cost is considered, this will
result in about 4500 kWh/a electricity generation (about 25 kWh/(m2 a)) corresponding to 1.5*25=37,5
kWh/(m2 a) primary energy reduction, leading to the performance level of nearly zero energy building
of about 40 kWh/(m2 a) primary energy and extra investment cost of about 40 000 € (224 €/m
2).
The results are sensitive to the interest rate. Figure 10 shows the effect of higher energy cost escalation
– discounting interest rate of 0% (real interest rate of 3% and escalation 3%). This interest rate will shift
the cost optimal to the left to the lower primary energy use. Negative NPV values compared to BAU
mean that the better construction standard can save some global cost. The cost optimal curves have
become more flat especially for gas, district heating and ground source heat pump. For gas and district
heating, the global cost is within 5 €/m2 for the primary energy range of 120–160 kWh/(m
2 a). Thus this
interest rate would allow to suggest 120 kWh/(m2 a) primary energy as cost optimal performance level.
For discounting interest rate of 2% (the real interest rate of 3% and the escalation 1%), the cost
optimal is generally shifted to the right, i.e. the cases with lower investment cost and higher primary
energy use became cost optimal, Figure 11.
KENA cost optimal and nZEB def 16
Figure 9. Breakdown of the global energy performance related costs for three heating system.
Interest rate of 1 % (the real interest rate of 3% and escalation2%) and 30 years time period.
First four categories from left are construction cost components and two last categories NPV
of energy cost.
30602
26245
21167
17611
5474
3445
3445
3445
9373
9373
9373
9373
4479
4479
0
0
8766
12205
19273
22735
20920
20920
21260
21260
0 20000 40000 60000 80000 100000
DH 0.42
DH 0.58
DH 0.76
DH 0.96
NPV, €
Gas
Building envelope
Ventilation units
Gas boiler
Solar collectors 6m2
Energy cost for heating
Energy cost for electricity
30602
26245
21167
17611
5474
3445
3445
3445
15542
15542
15542
15542
4479
4479
0
0
7806
10612
17033
19857
20920
20920
21260
21260
0 20000 40000 60000 80000 100000
DH 0.42
DH 0.58
DH 0.76
DH 0.96
NPV, €
Ground source heat pump
Building envelope
Ventilation units
Ground source heat pump
Solar collectors 6m2
Energy cost for heating
Energy cost for electricity
30602
26245
21167
17611
5474
3445
3445
3445
7215
7215
7215
7215
4479
4479
0
0
8688
12141
19124
22599
20920
20920
21260
21260
0 20000 40000 60000 80000 100000
DH 0.42
DH 0.58
DH 0.76
DH 0.96
NPV, €
District heat
Building envelope
Ventilation units
District heating substation
Solar collectors 6m2
Energy cost for heating
Energy cost for electricity
KENA cost optimal and nZEB def 17
Figure 10. The same results as in Figure 8, but with the interest rate of 0% (the real interest
rate of 3% and escalation of 3%).
Figure 11. The same results as in Figure 8, but with the interest rate of 2% (the real interest
rate of 3% and escalation of 1%).
-50
0
50
100
150
50 100 150 200
Glo
ba
l ad
dit
ion
al e
ne
rgy
pe
rfo
rma
nce
co
st (
NP
V),
€/m
2
Primary energy, ET-value, kWh/(m2 a)
Gas
Pellet
AWHP
GSHP
Electric
Oil
DH
-50
0
50
100
150
50 100 150 200
Glo
ba
l ad
dit
ion
al e
ne
rgy
pe
rfo
rma
nce
co
st (
NP
V),
€/m
2
Primary energy, ET-value, kWh/(m2 a)
Gas
Pellet
AWHP
GSHP
Electric
Oil
DH
KENA cost optimal and nZEB def 18
As a conclusion, primary energy ET= 120 kWh/(m2 a) can be proposed for the cost optimal energy
performance level for the reference detached house, if a small safety marginal of 5 €/m2 global cost
and escalation of 3% are used (Figure 10). ET= 120 kWh/(m2 a) is achievable with any heating system
studied. Compared to business as usual construction according to minimum requirement of ET= 180
kWh/(m2 a), this will lead to marginal 2.5 €/m
2 global cost increase.
Because of still far from zero energy, these cost optimal levels may be proposed most likely for low
energy building category C. Nearly zero energy performance level is not cost optimal with current
prices and may be defined through technically reasonable achievable level with current best practices
and renewable on site energy production. The lowest ET value, achieved with DH 0.42 with ground
source heat pump was about ET=75 kWh/(m2 a), which can be reduced with PV electricity production.
Therefore ET=40 kWh/(m2 a) was achievable for the reference detached house. The distance from
cost optimal to nearly zero energy performance level was about 224 €/m2 upfront investment cost
that corresponded to about 20% extra construction cost.
Given values (120 and 40) apply for the reference detached house and do not include any building size
and architecture related (compactness, No of floors, glazing size, etc.) safety margin which has to be
included in the final values. A factor of 1.15 may be proposed for final values.
KENA cost optimal and nZEB def 19
4.3 Results for apartment building
The results are calculated for the reference apartment building with heated net floor area of 1796 m2,
Figure 12–14. The building consists of 22 apartments intended for 62 persons.
Figure 12. Plans of the reference apartment building for ground floor and 4th
floor.
Figure 13. Plans of the reference apartment building for 2nd
floor and 3rd
floor.
KENA cost optimal and nZEB def 20
Figure 14. IDA-ICE building simulation model of the reference apartment building.
Analyses were conducted for four construction concepts, where the building envelope and ventilation
system energy performance levels were varied as shown in Table 3. These construction concepts are
marked with the specific heat loss value, i.e. AB 0.23 means the reference apartment building with the
specific heat loss of 0.42 W/(m2K), including transmission losses through building envelope and
infiltration and calculated per heated net floor area. AB 0.23 construction concept represents the best
available technology which may be associated with nearly zero energy buildings. AB 0.96 represents
business as usual construction.
KENA cost optimal and nZEB def 21
Table 3. Construction concepts for the reference apartment building of 1796 m2.
AB 0.23
“Nearly zero”
AB 0.32
“Low”
AB 0.43
AB 0.52
“BAU“
Specific heat
loss coefficient
H/A, W/m2K
0.231 0.315 0.431 0.521
External wall
591 m2
20cm LECA block, plaster
+ 35cm EPS-insulation
U 0.1 W/m2K
20cm LECA block, plaster
+ 25cm EPS-insulation
U 0.14 W/m2K
20cm LECA block, plaster
+ 20cm EPS-insulation
U 0.17 W/m2K
20cm LECA block, plaster
+ 15cm EPS-insulation
U 0.23 W/m2K
Roof
449 m2
Wooden beams, metal
sheet, 80cm min.wool
insulation, concrete slab
U 0.06 W/m2K
Wooden beams, metal
sheet, 50cm min.wool
insulation, concrete slab
U 0.09 W/m2K
Wooden beams, metal
sheet, 32cm min.wool
insulation, concrete slab
U 0.14 W/m2K
Wooden beams, metal
sheet, 25cm min.wool
insulation, concrete slab
U 0.18 W/m2K
Floor
449 m2
Concrete slab on ground,
70cm EPS insulation
U 0.06 W/m2K
Concrete slab on ground,
45cm EPS insulation
U 0.09 W/m2K
Concrete slab on ground,
25cm EPS insulation
U 0.14 W/m2K
Concrete slab on ground,
18cm EPS insulation
U 0.18 W/m2K
q50, m3/(hm
2) 0.6 1.0 2.0 3.0
Windows
433 m2
glazing/frame/total
U, W/m2K
4mm-16mmAr-SN4mm-
16mmAr-SN4mm
Insulated frame
0.6/0.7 W/m2K
0.7 W/m2K
4mm-16mmAr-4mm-
16mmAr-SN4mm
Insulated frame
0.8/0.8 W/m2K
0.8 W/m2K
4mm-16mm-4mm-
16mmAr-SN4mm
1.0/1.3 W/m2K
1.1 W/m2K
4mm-16mmAr-
SN4mm
Common frame
1,1/1,4 W/m2K
1,2 W/m2K
g-value 0.46 0.5 0.55 0.63
Ventilation l/s, SFP, AHU HR
1114 l/s*1), SFP 1.5
kW/(m3/s), AHU HR
85%
1114 l/s*1), SFP 1.7
kW/(m3/s), AHU HR
80%
1114 l/s*1), SFP 2.0
kW/(m3/s), AHU HR
80%
1114 l/s*1), SFP 2.0
kW/(m3/s), AHU HR
70%
Heating capa-
city, kW (te -21oC)
46 52 59 65
Cooling capacity,
kW 48 50 51 70
Net energy need kWh/(m2 a)
Space heating 7.1 13.0 21.9 28.4
Ventilation
heating 4.7 6.6 6.9 7.0
Domestic hot
water 35.6 35.6 35.6 35.6
Cooling 11.3 9.9 8.6 14.5
Fans and
pumps 8.9 9.9 11.6 11.6
Lighting 7.0 7.0 7.0 7.0
Appliances 22.3 22.3 22.3 22.3
Total net
energy need 96.9 104.3 113.9 126.4
Table 3 shows also simulated net energy needs. Depending on heating and cooling systems, delivered
and primary energy use can be calculated. For each construction concept, the following heating
systems were considered:
• district heating
• gas boiler
KENA cost optimal and nZEB def 22
• oil boiler
• pellet boiler
• ground source heat pump
• air to water heat pump
• electrical heating
Delivered energy use was calculated for these heating systems, by dividing net energy needs with
relevant system efficiency. System efficiency values (combined efficiency of the generation and
distribution) are shown in Table 4. To calculate the combined efficiency, radiator distribution is
considered according to Estonian method with average distribution efficiency of 0.97, which is
included in the combined efficiency values in Table 4.
Table 4. System efficiencies for delivered energy calculation.
Heat source (radiator heating)
Generation and distribution combined efficiency, -
Space heating/cooling Domestic hot water District heat 0.97 0.97
Gas/oil condensing boiler 0.92 0.89
Pellet boiler 0.82 0.82
Air to water heat pump 2.13 1.75
Electrical heating 0.97 0.97
Ground source heat pump 3.40 2.62
Cooling (electricity) 3.0
KENA cost optimal and nZEB def 23
4.4 Results for office building
The results are calculated for the reference four storey office building with modeled heated net floor
area of 2750 m2, Figure 15–17.
Figure 15. Plans of the reference office building for 2nd
floor.
Figure 16. Plans of the reference office building for 4th
floor.
KENA cost optimal and nZEB def 24
Figure 17. IDA-ICE building simulation model of the reference office building.
Analyses were conducted for four construction concepts, where the building envelope and ventilation
system energy performance levels were varied as shown in Table 5. These construction concepts are
marked with the specific heat loss value, i.e. AB 0.23 means the reference apartment building with the
specific heat loss of 0.42 W/(m2K), including transmission losses through building envelope and
infiltration and calculated per heated net floor area. AB 0.23 construction concept represents the best
available technology which may be associated with nearly zero energy buildings. AB 0.96 represents
business as usual construction.
KENA cost optimal and nZEB def 25
Table 5. Construction concepts for the reference office building of 2750 m2.
OB 0.25
“Nearly zero”
OB 0.33
“Low”
OB 0.45
OB 0.55
“BAU“
Specific heat
loss coefficient
H/A, W/m2K
0.245 0.334 0.454 0.548
External wall
1098 m2
20cm LECA block, plaster
+ 35cm EPS-insulation
U 0.1 W/m2K
20cm LECA block, plaster
+ 25cm EPS-insulation
U 0.14 W/m2K
20cm LECA block, plaster
+ 20cm EPS-insulation
U 0.17 W/m2K
20cm LECA block, plaster
+ 15cm EPS-insulation
U 0.23 W/m2K
Roof
621 m2
Wooden beams, metal
sheet, 80cm min.wool
insulation, concrete slab
U 0.06 W/m2K
Wooden beams, metal
sheet, 50cm min.wool
insulation, concrete slab
U 0.09 W/m2K
Wooden beams, metal
sheet, 32cm min.wool
insulation, concrete slab
U 0.14 W/m2K
Wooden beams, metal
sheet, 25cm min.wool
insulation, concrete slab
U 0.18 W/m2K
Floor
606 m2
Concrete slab on ground,
70cm EPS insulation
U 0.06 W/m2K
Concrete slab on ground,
45cm EPS insulation
U 0.09 W/m2K
Concrete slab on ground,
25cm EPS insulation
U 0.14 W/m2K
Concrete slab on ground,
18cm EPS insulation
U 0.18 W/m2K
q50, m3/(h m
2) 0.6 1.0 2.0 3.0
Windows
715 m2
glazing/frame/total
U, W/m2K
4mm-16mmAr-SN4mm-
16mmAr-SN4mm
Insulated frame
0.6/0.7 W/m2K
0.7 W/m2K
4mm-16mmAr-4mm-
16mmAr-SN4mm
Insulated frame
0.8/0.8 W/m2K
0.8 W/m2K
4mm-16mm-4mm-
16mmAr-SN4mm
1.0/1.3 W/m2K
1.1 W/m2K
4mm-16mmAr-
SN4mm
Common frame
1,1/1,4 W/m2K
1,2 W/m2K
g-value 0.46 0.5 0.55 0.63
Ventilation m
3/s, SFP, AHU HR
4.6 m3/s, SFP 1.5
kW/(m3/s), AHU HR
80%
4.6 m3/s, SFP 1.7
kW/(m3/s), AHU HR
75%
4.6 m3/s, SFP 2.0
kW/(m3/s), AHU HR
75%
4.6 m3/s, SFP 2.0
kW/(m3/s), AHU HR
75%
Heating capa-
city, kW (te -21oC)
151 160 172 181
Cooling capacity,
kW 155 156 160 193
Net energy need kWh/(m2 a)
Space heating 5.8 11.4 21.9 29.0
Ventilation
heating 2.8 4.1 6.2 6.4
Domestic hot
water 7.4 7.4 7.4 7.4
Cooling 32.9 30.9 28.9 37.8
Fans and
pumps 7.3 7.9 10.9 10.9
Lighting 18.9 18.9 18.9 18.9
Appliances 23.7 23.7 23.7 23.7
Total net
energy need 98.8 104.3 117.9 134.1
Table 5 shows also simulated net energy needs. Depending on heating and cooling systems, delivered
and primary energy use can be calculated. For each construction concept, the following technical
systems were considered:
• district heating
KENA cost optimal and nZEB def 26
• gas boiler
• oil boiler
• pellet boiler
• ground source heat pump
• air to water heat pump
• electrical heating
Delivered energy use was calculated for these heating systems, by dividing net energy needs with
relevant system efficiency. System efficiency values (combined efficiency of the generation and
distribution) are shown in Table 6. To calculate the combined efficiency, radiator distribution is
considered according to Estonian method with average distribution efficiency of 0.97, which is
included in the combined efficiency values in Table 6.
Table 6. System efficiencies for delivered energy calculation.
Heat source (radiator heating)
Generation and distribution combined efficiency, -
Space heating/cooling Domestic hot water District heat 0.97 0.97
Gas/oil condensing boiler 0.92 0.89
Pellet boiler 0.82 0.82
Air to water heat pump 2.13 1.75
Electrical heating 0.97 0.97
Ground source heat pump 3.40 2.62
Cooling (electricity) 3.0
Primary energy values (ET-values), calculated with Estonian primary energy factors for all construction
concepts and heating systems, are shown in Figure 18. All values are much lower compared to the
minimum requirement of 220 kWh/(m2 a) showing that business as usual construction with reasonable
massing and glazing leads to significantly improved energy performance.
Figure 18. Primary energy values (ET-values)in the reference office building for all
combination of construction concepts and technical systems.
0
20
40
60
80
100
120
140
160
180
200
220
Gas/oil Pellet AWHP GSHP Electricity Distr. h.
Pri
ma
ry e
ne
rgy
, E
T-v
alu
e,
kW
h/
(m2
a)
OB 0.25
OB 0.33
OB 0.45
OB 0.55
min. req. 220
KENA cost optimal and nZEB def 27
Cost optimal results shown in Figure 19 for discounting interest rate of 1% (real interest rate of 3% and
escalation of 2%) suggest that business as usual thermal insulation of building envelope leads to cost
optimal with district heating at around 140 kWh/(m2 a) primary energy. Global cost differences are
generally smaller between the most of cases compared to residential buildings. The breakdown of
global cost components is shown in Figure 20.
Figure 19. Global energy performance related costs in the reference office building
calculated with discounting interest rate of 1% (the real interest rate of 3% and the escalation
2%) and 30 years time period. (AWHP – air to water heat pump, GSHP – ground source heat
pump, DH – district heating.) For each technical system, from left to right OB 0.25, 0.33, 0.45
and 0.55 cases are shown.
-30
0
30
60
90
90 110 130 150 170
Glo
ba
l ad
dit
ion
al e
ne
rgy
pe
rfo
rma
nce
co
st (
NP
V),
€/m
2
Primary energy, ET-value, kWh/(m2 a)
Gas
Pellet
AWHP
GSHP
Electric
Oil
DH
KENA cost optimal and nZEB def 28
Figure 20. Breakdown of the global energy performance related costs for most typical technical
systems. Interest rate of 1 % (the real interest rate of 3% and escalation2%) and 30 years time period.
First four categories from left are construction cost components and two last categories NPV of energy
cost.
For nZEB performance level, on site renewable energy production has to be added for cases with
highest energy performance. Results calculated with solar PV show that primary energy of about 105
kWh/(m2 a) is achievable with most technical solutions studied. As solar PV can produce in office
buildings at least 15 kWh/(m2 a) primary energy, nZEB performance level of about 90 kWh/(m
2 a)
primary energy can be proposed.
333195
289806
259851
222201
52152
52152
52152
52152
50318
50318
59722
59722
43138
43138
0
0
34622
49323
76170
91723
356422
356032
369695
387067
0 200000 400000 600000 800000 1000000
OB 0.25
OB 0.33
OB 0.45
OB 0.55
NPV, €
Gas
Building envelope
Ventilation units
Gas boiler
Solar collectors 6m2
Energy cost for heating
Energy cost for electricity
333195
289806
259851
222201
52152
52152
52152
52152
19000
19000
19000
19000
43138
43138
0
0
33970
48620
75372
90871
356422
356032
369695
387067
0 200000 400000 600000 800000 1000000
OB 0.25
OB 0.33
OB 0.45
OB 0.55
NPV, €
District heat
Building envelope
Ventilation units
District heating substation
Solar collectors 6m2
Energy cost for heating
Energy cost for electricity
KENA cost optimal and nZEB def 29
5 Conclusions
Global cost calculations for construction concepts from business as usual construction to passive
house building envelope level combined with all possible technical systems showed that cost optimal
in the reference detached house was between ET= 120-140 kWh/(m2 a) primary energy and in
reference office buildings about ET= 140 kWh/(m2 a) primary energy.
In the reference detached house, ET= 120 kWh/(m2 a) was possible to select for the cost optimal
energy performance level, when accepting a small increase of 5 €/m2 in global cost and escalation of
3% (Figure 10). ET= 120 kWh/(m2 a) was achievable with any heating system studied. Compared to
business as usual construction according to minimum requirement of ET= 180 kWh/(m2 a), this led to
marginal 2.5 €/m2 global cost increase. When compared with passive house standard, the cost optimal
value is exactly the same that is required for passive houses (120 kWh/(m2 a) primary energy) in
Central European climate, but was achieved with water based heating systems with most cost
efficiently.
In residential buildings, the cost optimal performance level is reasonably lower that the current
minimum requirement of 180, but in the office buildings the current requirement of 220 is much
higher compared to cost optimal value. Because of still far from zero energy, the cost optimal levels
may be proposed for low energy building category B or C in the energy performance certificate scale
Nearly zero energy performance level is not yet cost optimal with current prices and is suggested to be
defined through technically reasonable achievable level with current best practices and renewable on
site energy production. With reasonable amount of PV electricity production ET=40 and ET=90
kWh/(m2 a) were achieved for the reference detached house and office buildings respectively. In the
detached house, the distance from cost optimal to nearly zero energy performance level was about
224 €/m2 upfront investment cost that corresponded to about 20% extra construction cost. In offices,
the extra construction cost was smaller, estimated to be about 10%.
To determine the final cost optimal and nZEB performance levels a safety margin taking into account
the building compactness and other architecture and energy supply related factors has to be used.
With a safety margin factor of 1.15 the values shown in Table 7 may be proposed.
Table 7. Proposed cost optimal and nZEB energy performance levels.
nZEB Cost optimal Current req.
kWh/(m2 a) kWh/(m
2 a) kWh/(m
2 a)
primary energy primary energy primary energy
Detached house 50 140 180
Apartment building 70 130 150
Nursing home 130 200 300
Day care centre 140 200 300
School building 80 120 300
Office buildings 100 150 220