Refurbishment design through cost-optimal methodology: The case study of a social housing in the northern Italy

According to the EPDB [5], “cost-optimal level” means the energy performance level which leads to the lowest cost during the estimated economic lifecycle. The methodology provided by the European Regulation n.244/2012 [11] is addressed to Public Authorities and thus requires the individuation of representative buildings of the typical and average building stocks for which cost-optimal solutions suitable for the same buildings’ types are considered.

In addition to the macroeconomic analysis that this procedure carries on, it is possible to achieve financial analysis on a case study to which the same methodology can be applied, as well as in this research. The representative building assessed belongs to a widespread social housing typology of the period between the thirties and fifties of the XX century in Lombardy region and it can be individuated in all the northern Italy regions.

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Figure 1. Cost-optimal methodology steps

Once selected the building to be studied, the steps are the following (Fig.1):

1) identification of improvement measures for building envelope and energy systems (Energy Solutions ES);

2) energy consumption and energy performance calculation of the building for each ES;

3) global cost calculation for each ES in terms of net actual value during the considered period, according to the methodology of EN15459:2007 [17];

4) energy and economic comparison between the different ES;

5) choice of the most appropriate solution in terms of energy and economic level and identification of the optimal levels of energy performance requirements in relation to the global costs. 

2.1 Case study

The research is intended to test the methodology to find the best costs-benefits balance, mainly for the use by Public Authorities and, to this purpose, the methodology has been applied on an existing residential social building.

The construction presents a “L” floor plan and three stairwells protruding towards the inner court that distribute the 30 small size residential units for a total living net area equal to 1411.3 m² distributed over three floors (Figure 2).

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Figure 2. 3D-model of the case study building

The ground floor contains the cellars and the centralized thermal power station, where the traditional gas generator is located, delivering heating to all the flats through a not-insulated distribution network.

The elevations are marked by 130x150 cm openings (Figure 3), with the only exception of four French doors 130x240 cm.

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Figure 3. Plan and section of the case study

The building presents the constructive connotations of the Fifties social housing, with internal net heights equal to 3.35 m, external walls of 40 cm thickness of solid bricks, pitched roof with not insulated and not conditioned attic; in Table 1 the current thermal transmittances and thicknesses of the existing envelope are reported. The primary energy consumption of the existing construction is evaluated in order to compare the results achieved with the energy refurbishments from the initial situation (Table 2).

The construction is located in an area with similar buildings and the studied solutions can be replicated in the whole district.

Table 1. Thermal transmittance and thickness of the existing building envelope elements

Table 2. Primary energy and energy class of the existing building

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Figure 4. Picture of the case study

2.2 Energy Refurbishment Solutions (ERS)

According to the Guidelines n.244/2012 [11], the more packages and variations of the measures included in the assessed scenario are used, the more accurate the calculated optimum of the achievable performance will be, with a minimum of packages/variations set equal to 10. For this reason a series of refurbishment measures divided into envelope, plants and renewable plants were set and combined in order to establish 18 different scenarios to be assessed. The cost-optimal methodology recommends to combine measures into packages, since meaningful combinations can create synergy effects that lead to better results than single measures. Variants are defined as a “global result and description of a full set of measures/packages applied to a building that can be composed of a combination of measures on the building envelope, passive techniques, measures on building systems and/or measures based on renewable energy sources"[11]. For this reason, the variants in the case study have been divided into the three categories (envelope, plants and renewable sources), in order to create different combinations to be assessed.

The U-values, fixed for the envelope refurbishment, respect the limit values set by the Italian national decree 26/06/2015 [6] for the reference building. Three envelope configurations were defined: in ERS1 and ERS2 the limit values respectively for 2015 and 2019/2021 were considered, while ERS3 aims at the assessment of an energy efficiency measure overcoming the national limits (Table 3). In particular, the following actions for the envelope refurbishment were assessed:

  - insulation of the ceiling slab towards the unconditioned roofspace: the choice was not to change the existing roof with a performing one but to insulate on the internal side of the roof, for a consistent economic saving. The insulating material used was glass wool of 0.14 m, 0.18 m and 0.22 m of thickness for ERS1, ERS2 and ERS3 respectively.

  - Insulation of the floor above the unconditioned ground floor with EPS of 0.14 m, 0.15 m and 0.22 m of thickness for ERS1, ERS2 and ERS3 respectively.

  - Insulation of external brick walls with EPS of 0.14 m, 0.18 m, 0.24 m of thickness for ERS1, ERS2 and ERS3 respectively.

  - Replacement of windows with double-glazing (ERS1, ERS2) and triple-glazing (ERS3) windows, replacing the polystyrene-lined rolling shutter casing.

Table 3. Energy Refurbishment Solutions (ERS) for the building envelope

Since the limit values listed in the Table 3 are comprehensive of thermal bridges, the insulation thickness of the different elements (ceiling, floor, walls) was oversized in order to respect the decree [6].

The thermal plants combinations were determined choosing mostly or totally renewable heat generators with the double aim to reach a high energy efficiency class and to fulfill the renewable percentage that must be provided for heating, cooling and DHW, according to the Legislative Decree n. 28/2011 [18]. In case of significant renovations, the energy power plants must be designed to ensure, through the use of energy produced from plants powered by renewable sources, the covering of the 50% of the consumption planned for heating, cooling and hot water.

The adopted solutions for plants are:

  - ERS4: Air to water heat pump combined with condensing boiler;

  - ERS5: Geothermal heat-pump;

  - ERS6: District heating connection;

  - ERS7: Solar panels;

  - ERS8: Solar and photovoltaic panels;

  - ERS9: Solar and photovoltaic panels (33% photovoltaic panels reduction in comparison to ERS8).

The combination of the different energy refurbishment solutions thought for the envelope and the thermal plants brought to the formulation of 18 Energy Scenarios (ES, Table 4) to be assessed. In each scenario, radiant panels were chosen as emission system, since the operating temperature is low (30-35°C) and it is the optimal terminal device to be associated with heat pumps.

Table 4. Energy Scenarios (ES) and the corresponding Energy Refurbishment Solutions (ERS) considered

2.3 Energy calculation

The energy model of the residential building has been built setting as climatic conditions the Italian climate zone “E” (2,404 Heating Degree Days), adopting the temperature and humidity data extracted from the national standard UNI 10349:2016 [19]. For the calculation of the energy needs, the monthly method has been employed, through a commercial software approved by the National Thermo-technical Committee, CTI [20]; the software implements the standard method of UNI/TS 11300 [21], the Italian standard transposing the EN ISO 13790:2008 [22].

Analyzing the results of the energy calculations for all the 18 scenarios (Figure 5) it can be observed that ES3 and ES9 have a primary energy demand higher than the other solutions. It is due especially to the presence of deep geothermal water-to-water wells, whose electricity need is very high to move the fluid to the heat pump and therefore the absence of photovoltaic panels in these solutions affects significantly the performance results.

In fact, the electricity demand is cut down when the photovoltaic panels are applied, even if the primary demand for electricity in these solutions is high.

2.4 Economic evaluation

As defined in the Guidelines accompanying Commission delegated 244/2012 [11], in the economic evaluation, the measures that do not have an influence on the energy performances of the building and the costs that are the same for all the scenarios were not considered in the global cost calculation. For this case study, the cost optimal was calculated at a financial level and not a macroeconomic one, so the following conditions were assumed:

  - costs considered comprehensive of VAT and taxes;

  - incentives for Public Authorities calculated according to the Interministerial Decree of 16^th February 2016 [23] and then detracted from the initial investment costs;

  - actualization rate assumed equal to the interest market rate;

  - CO₂ emission costs neglected.

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Figure 5. Primary energy demand for each ES subdivided in heating, DHW and electricity

In accordance with the Standard EN 15459:2007 [17], Global Cost C_G (τ) was calculated considering the initial investment costs, the running costs (energy, operational and maintenance costs), the replacement costs, and the final value, as follows in Eq. 1:

C_{G}(\tau)=C_{I}+\sum_{j}\left[\sum_{i=1}^{\tau}\left(C_{a, i}(j) x R_{d}(i)\right)-V_{f, \tau}(j)\right]      (1)

where:

  - C_I, Initial Investment Costs, achieved from the price list for the execution of public works and maintenances of the City of Milan [24]. The missing price voices were based on market analysis, as requested by the Guidelines [11].

  - Ca,i(j),  the Annual Costs for the component j at the year i, calculated considering a calculation period of 30 years in accordance to the guidelines [11] for retrofit analysis. The Annual Costs involve both energy consumption and operational, maintenance and replacement costs of each envelope and system component. To obtain the Energy Costs, gas and electricity consumptions provided by the quasi-state simulation were multiplied with the tariff set by the Italian Regulatory Authority for Electricity Gas and Water (€ 0.16 kWh-1 for electricity, € 0.75 m-3 for natural gas) [25]. To consider the variation of the energy prices, an actualization factor of 2% was applied [17]. Maintenance and replacement costs of systems components are provided by the Annex A of the Standard EN 15459 [17]. When data are not available, market analyses were considered. The lifespan of the envelope components is equal to the calculation period.  

- Rd(i), the discount Rate at the year i calculated by using a Market Rate of 4% [26] and an Inflation Rate of 0,49% [27], calculated comparing the December 2016 CPI to the December 2015 CPI.

- Vf,t(j), the Final Value of the component j, calculated in function of the lifespan at the end of the calculation period and then subtracted at the last replacement cost.  

2.5 Results

After the analysis of energy and economic performances reached by each ES, a comparison between the solutions was developed in order to define the best scenarios.

In Figure 6 Global Costs are represented divided in each item of the Eq.1: with the plus sign Investment Costs, Energy Costs, Replacement e Maintenance Costs, with the minus sign Residual Value and Incentives. In the three envelope configurations (ERS1, ERS2, ERS3), the ES with geothermal heat pump (ES3, ES4, ES9, ES10, ES15, ES16) have an Investment Cost and an Energy Cost higher than the other plants configurations, with a further cost addiction when the PV panels are added to the solar ones.

With the same plants configuration, to the addition of the PV panels always corresponds an increasing of the Investment Costs and of the Replacement Cost but also a reduction of the Energy Costs. The reduction of the PV panels of 33% in ES13, ES15 and ES17 implicates a reduction of the Investment Costs and Replacement Costs but a higher energy consumption than ES14, ES16 e ES18.

Except for the ES15 that does not obtain the nZEB class and has a reduction of the incentives in accordance to the Decree [23], in the other ES the PV panel reduction does not mean a significant reduction of the Global Cost: with subsides, costs and energy consumptions are balanced.

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Figure 6. Subdivision of incentives and costs for each scenario

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Figure 7. Cost optimal solution

In Figure 7 the Cost Optimal range is represented with the correlation between Global Costs for square floor meter (€.m-2) and primary energy use (kWh.m-2.y). The ES17 has the lower Global Cost: 247,536.70 €, with an amount of 175.40 €.m-2 and with an EP of 21.04 kWh.m-2y. Then, the ERS3 combined with district heating and solar panels with the 33% PV panels reduction (ES17) achieves the best economic and energy performance balance. At the same time the ES12, with ERS2 combined with district heating, solar panels and the total amount of PV panels, is a little less efficient than the ES17 (ES12, Global Cost is 252,660.29 €, with an amount of 179.03 €.m-2 and with an EP of 22.0 kWh.m-2.y).

Comparing Figure 6 with Figure 7 the incentives gained with the nZEB class (65% for all the interventions) influence a lot the reduction of the global cost. If the incentives are not considered, ES5 (349.97 €.m-2) and ES11 (362.15 €.m-2), would be the best scenarios: both scenarios with solar panels without PV panels and, respectively, with ERS1 and ERS2. This result demonstrates that PV panels are indispensable to reach the nZEB class but without incentives are not economical affordable. Among the three best solutions in Figure 7, the ES18 foresees the ERS3 with district heating, solar panels and the total amount of PV panels: the result is a lower EP equal to 19.99 kWh.m-2y but a higher cost equal to 183.83 €.m-2. The difference in comparison to ES17 and ES12 is low and this is a confirmation that a higher initial investment not necessary implicates a lower global cost in a calculation period of 30 years.

ES3 and ES4 (ERS1 with geothermal plants) have the higher global cost: the PV panels (ES4) reduce the energy consumption but the Initial Investment Costs and the Replacement Costs are too high to be balanced with the energy performance.

Figure 8 underlies how the three best solutions (ES12, ES17 and ES18) have a Pay Back Period lower than 10 years while ES 3 and ES 4 have a PBP close to 30 years.

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Figure 8. Payback Period of different ES for the case study

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Figure 9. Renewable and not renewable primary energy-ES17

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Figure 10. Renewable and not renewable primary energy-ES12

A reduction of the envelope performance can be compensated by an increasing of the PV panels quantity, earning the same nZEB class. Otherwise, a reduction of the PV panels can be compensated with an increasing of the envelope performance. This is evident from the comparison between the monthly primary energy demand in the two scenario ES17 and ES12 (respectively Figure 9and Figure 10); in both the renewable sources cover the same percentage of primary energy (56% for heating and domestic hot water, 75% for domestic hot water).

Therefore in ES17 the energy demand is lower, since the envelope is more insulated and the windows are triple glazed, while in ES12 the energy demand is higher but the larger quantity of PV panels compensates the greater demand.