**5.3 Optimization of multi-family building's energy retrofitting**

Retrofitting of multifamily buildings in Ljubljana includes improvement of building's envelope. Building with a useful area 1950 m<sup>2</sup> (**Figure 22**) is heated by a district heating system. The existing building with the brick wall without thermal insulation (Uwall = 0.986 W/m<sup>2</sup> K) and double paned glass windows with wooden frames

**Figure 22.** *Multifamily building in Ljubljana analyzed in the case study.*

(Uw = 3.0 W/m<sup>2</sup> K) was taken as a reference project. The ceiling toward the unheated attic was already insulated. The energy needs for heating Q'NH of the reference project is 147.7 kWh/m2 a.

Optimization was made according to the specific costs of investment (including all façade layers and labor costs) and energy carriers over the 30-years period. It was found that maximum cost savings of 52 € per m<sup>2</sup> of the useful area can be achieved with thermal insulation's thickness of 25 cm. Such measure will result in decreasing primary energy demand from 251.6 to 196.8 kWh per m<sup>2</sup> of useful area, taking into account all installed building service systems. The cost of eqCO2 emissions decreases up to the much larger thickness of thermal insulation and no optimum value can be determined. This means that according to the macro-economic cost of eqCO2 emissions there is no need to limit subsidies based on thermal insulation thickness or U-value of building structures (**Figure 23**).

**Figure 23.** *Thermal insulation thickness optimization based on the criteria of cost-effectiveness in the life-cycle.*

#### **Figure 24.**

*Windows replacement optimization based on decreasing primary energy demand and the criteria of cost-effectiveness in the life cycle.*

*Evaluation of Energy Efficiency of Buildings Based on LCA and LCC Assessment… DOI: http://dx.doi.org/10.5772/intechopen.101820*

The impact of the replacement of windows was evaluated based on heat losses and solar gains. The life-cycle cost assessment showed that double glazed windows provide savings of 33 € per m<sup>2</sup> of useful area, while triple glazed windows are not cost-efficient and besides that such windows do not decrease primary energy demand more than double glazed windows (**Figure 24**). The macroeconomic costs of eqCO2 emissions also give priority to double-glazed windows.
