## 3 Typology of the Building Stock, Market Segments and Economics of Refurbishing
### The Economics of Refurbishing
#### 3.1.3. Energy Refurbishing and Building Maintenance  
The Danish building stock includes 2.52 million buildings (of which 1.4 million detached houses and summer houses) with a total floor area of 716 million m\({}^{2}\). The building topology has been mapped out and estimates made of potential energy savings in different sectors [33; 34; 35].  
The building stock built before 1930 accounts for 22% of the total heated floor space, and is expected to be renovated towards the end of the normal lifecycle of building components. Buildings built during 1961-1972 represent 21% of heated floor space, a segment generally representing poor energy standards [32] (Figure 6, p. 19).  
Previous estimates of total costs of energy refurbishing by [PERSON] (2009) [24] amounted to 150-900 dkk/sqms (2009 prices) by use of optimal technical solutions, equivalent to between 21.000 and 126.000 dkk for a standard house of 140 sqm. These estimates are below realized costs of extensive renovations, but are here assumed to cover replacement of certain building components and works such as insulation of the attic. New research [32] focuses on net-energy savings potential and investments required for energy refurbishing the building stock gradually during regular maintenance cycles over the years until 2050, up to valid construction standards at the time of renovation.  
Scenarios show the different effects of performing maintenance of the building envelope as usual following the future standards set by the Danish building and construction regulations (BR), Energistyrelsen (2014) [14], or further upgrading of the energy standards. The energy savings by 2050 relative to the 2011 level range from 29.2% (business as usual) to 41.1% [32] (Tabel 3, p. 10).  
Table 7 shows that interventions beyond the normal renovation and lifecycle investments with a baseline savings potential of 60.845 TJ/year in 2050 are delivering between 4% and 40% further reductions in expected energy savings, the latter only in case of tightened regulations on ventilation and heat recycling (A.4). Scenario A.1 + A.2 + A.3 is delivering an estimated 11.7% added annual reduction by 2050 at the accumulated additional cost of 16.7 billion dkk. The highest potential in increasing energy savings beyond the regular standards could be generated by installing mechanical air-conditioners with heat recycling (Table 7, option A.4), but improvements come at the cost of a higher up-front investment and affect electricity consumption. Thus, the study shows limited marginal benefits of applying higher energy standards than the standards already defined, at high marginal costs. The higher standards will be successively implemented as buildings are renovated anyway according to [32] (p. 7), with the exception of some 20% of the buildings for a variety of reasons.  
[PERSON] [20] analysed the cost optimality of the energy requirements in the Danish Building Regulations 2010 in respect to new buildings and to existing buildings undergoing major renovation. The costs and macroeconomic effects of upgrading to different energy standards were calculated under a range of defined assumptions on energy price development, interest rates, _etc._ While some energy standards turned out to be economically justifiable on the given assumptions, other energy standards were not so. The law of diminishing returns on investments also applies in the case of energy standards in Denmark.  
Below the renovation and refurbishing costs are further discussed in relation to specific building segments and cases.