Do Historic Buildings Have Poor Energy Performance, and Will Energy Optimization Compromise Their Historic Values? A Study of Danish Apartment Buildings

Abstract

Historic buildings are often assumed to have poor energy performance, and energy optimization of the buildings is perceived as threatening their cultural values. This study tests these assumptions. First, it examines the energy performance of Danish historic apartment buildings (buildings constructed before 1950 with a high preservation value, according to the national SAVE system (Survey of Architectural Values in the Built Environment)). Second, it assesses the extent to which the energy improvements in the historic buildings conflict with their historic value. An analysis of energy performance certificates (EPC) in 13,000 Danish historic apartment buildings reveals that they perform no differently than apartment buildings with a low preservation value, with 46% of historic apartment buildings achieving an EPC rating of “C”. Nevertheless, significant potential for further energy improvements is identified. Expert interviews and three case studies indicate that typical interventions for enhancing buildings’ energy performance rarely interfere with its historic values. This is partly due to structural conditions where shoulder-by-shoulder location, high building compactness, and supply with district heating gives a beneficial foundation for a high energy performance. Potential conflicts between energy improvements and historic values exist but are often resolved through dialogue between local authorities and owners about the interventions.

1. Introduction

There is substantial potential for energy optimization of historic buildings, as 35% of European Union (EU) buildings are more than 50 years old [1], and the building stock is characterized by low energy performance and high energy consumption [1,2]. Historic buildings face threats, partly due to decay and partly due to convenient renovations instead of careful restoration [1]. There is widespread concern that historic buildings cannot meet new energy requirements unless radical energy improvement initiatives are implemented, which might ultimately reduce the historic value of these buildings [3].

Current national and international policies emphasize the urgency of reducing energy consumption across the entire existing building stock, including historic buildings, while simultaneously highlighting the dilemma of maintaining the architectural and cultural values of historic buildings. The challenge of energy optimizing historic buildings without compromising their architectural values is a shared problem across European countries and cities. Despite potential challenges related to energy optimization of historic buildings, including architectural and historical restrictions [4], several studies have found that significant energy reductions are possible without harming buildings’ historic values [4,5,6] and that cost-efficient energy solutions are achievable if suitable materials and solutions are adopted [7]. However, energy optimization of historic buildings presents numerous challenges, including addressing potential problems with indoor climate and moisture, matching old and new materials, and managing economic considerations, decision-making processes, and collaboration between different stakeholders in decision-making and renovation processes [8,9,10]. The recent literature describes this as a balancing act “in which energy consumption and conservation principles are balanced against one another in order to achieve continued, long-term use of the building” [4]. Although valuable guidelines and best practices have been developed [4,9,11] to address these challenges, there remains a need for knowledge dissemination [3,10,11] to decision makers, and it remains uncertain to what extent results from scientific studies are being transferred to practice. This uncertainty exists partly because systematic research and knowledge on energy optimization of historic buildings beyond individual case studies have been rare [12]. Some studies [3,13,14] have used register-based analysis to combine data on energy performance certificates (EPC) and preservation values, creating overviews of specific stocks of historic buildings, albeit in limited geographical areas. The results from these studies vary; a Swedish study [3] found only a limited correlation between heritage classification and energy performance but highlighted the substantial heterogeneity in the stock of historic buildings [3]. A Dutch study [14] found that the energy performance of historic dwellings (built before 1970) is lower compared to modern dwellings (built after 1970), but the conclusions on whether historic values limit the utilization of more energy-efficient solutions are not definitive. These two studies [3,14] highlight that different definitions are used for “historic buildings”, “listed buildings”, and “heritage values”, necessitating caution when interpreting results. For instance, in the Dutch context, historic buildings are defined as those built before 1970, representing 46% of the Dutch building stock, including explicitly “listed” buildings. The Swedish study used heritage values of the local building stock on a scale from A to C, defined by different regional bodies in Sweden [3].

The distinction between “historic” and “listed” buildings is important. “Listed” or “protected” buildings exist as a category that typically allows very few changes to the buildings. This category typically contains a relatively small number of buildings and is often reserved for outstanding buildings of particular interest, typically designated by national authorities. “Historic” or “heritage” buildings” are recognized as buildings with certain preservation values [15,16] and are typically included in heritage registers or inventories maintained by local, regional, or national authorities but often with no or limited legal force. This makes it easier to implement changes, improvements, renewal, and energy optimization of the buildings but also makes it challenging to preserve their historic qualities. There are two reasons to be concerned about these buildings compared to listed or protected buildings: (1) Decisions about changes are made by building owners, and it is uncertain to which extent they use consultants and skilled workforce and engage in dialogue with the local authorities about the historic value of the building, meaning that decisions are under less “control” compared to listed buildings. (2) The group of historic buildings is much larger than the group of listed buildings [15,16] and thus has a relatively high environmental and architectural impact at city and national levels.

It is problematic that there is limited knowledge about the actual energy performance of historic buildings and about which types of interventions and changes are necessary to achieve an “acceptable” EPC rating. This often leads to assumptions that historic buildings all over Europe will suffer from demands on energy optimization [17] and need to be “wrapped in external insulation” to increase their energy performance. Moreover, in planning and politics, “historic” and “listed” buildings are often confused, or the terms are used interchangeably, leading to assumptions about which types of buildings are included. It is often argued that historic buildings (or buildings “worthy of preservation”) should be exempted from energy regulation, as it is assumed that these buildings represent a very small share of the building stock. This is true for listed buildings but not for historic buildings. Historic buildings should therefore not be ignored in urban climate policies and when setting goals for building energy performance in general [4,9].

What remains uncertain, however, is how historic buildings are being maintained and upgraded in practice, and whether such suitable solutions are implemented. As owners of “everyday” historic buildings are often a mix of professionals and laypeople, and restrictions on changes to the buildings are often limited, there are many uncertainties about (1) whether the buildings architectural values are respected and maintained, (2) whether the energy performance of the buildings lags behind as the buildings are typically older that the remaining building stock and whether owners are often more reluctant to make changes to the buildings, and (3) whether there are ways in which both preservation value and energy efficiency can be increased.

This has led to the following research questions: What is the energy performance of historic apartment buildings compared to apartment buildings with no preservation values? And will energy optimization of historic apartment buildings lead to a reduction in their preservation values?

2. Methodology

An analysis of Danish historic apartment buildings built before 1950 is used as a case to answer the research questions. In Denmark, there are many historic apartment buildings and registers with data about their preservation value, energy performance, and other building data, which allow for comprehensive register-based analysis. The following methodological approaches were used in the study:

• To examine the energy performance of historic buildings, the SAVE register (preservation values) and the EPC register (energy performance) were combined. In the SAVE register, the highest-rated apartment buildings (SAVE values from 1 to 4) were used to represent “historic apartment buildings”, whereas buildings with SAVE values from 5 to 9 were used as representatives of apartment buildings with limited preservation value (see detailed explanation under Data Sources, Section 3). From this combined register, data were aggregated and analyzed according to SAVE value and EPC rating;
• To examine the extent to which energy optimization of historic apartment buildings has conflicted with their historic and architectural values, different steps were used: (a) Historic apartment buildings with a high EPC rating (“A”–“C”) were selected, and typical energy-improving initiatives were identified. (b) Approximately 30 EPC reports of historic apartment building with a EPC rating of “C” or “B” were studied, combined with external visual inspection of approximately 20 buildings;
• The potential for further energy optimization of historic apartment buildings was assessed by (a) using the potential energy label in EPC reports for all historic buildings (SAVE values 1–4) and (b) data from the EPC register on the degree of implementation of the typical physical interventions to achieve a high EPC rating (as identified under 2a);
• Interviews: To learn about the practical implementation of energy-conserving measures in historic buildings, their potential, and possible conflicts with historic value, qualitative interviews were conducted with practitioners. The interviews focused on (a) the quality of EPC rating, (b) typical energy-saving measures in retrofit of historic buildings, and (c) municipalities’ approaches to preventing changes in the historic value of buildings. Three interviews were conducted with owners of energy consultancy firms, specializing in the renovation of historic buildings. Three interviews were conducted with civil servants from three municipalities (Frederiksberg, Aarhus, and Sønderborg) about their policies and efforts in identifying, preserving, and optimizing historic apartment buildings. All interviews were recorded and transcribed, and subsequently, the main observations from the interview were summarized;
• Case studies were performed of three selected historic buildings with a high EPC rating. The case studies were mainly based on interviews with owners and consultants, visual inspections of the buildings, and EPC reports and other written material about the building. Moreover, knowledge and information from interviews with EPC consultants and municipalities were included in the case studies. The three cases were chosen because they represent typical examples in larger cities (Copenhagen, Frederiksberg, and Aarhus), as the majority (76%) of historic apartment buildings with EPC rating of “A”, “B”, or “C” are located here (55% in Copenhagen, 12% in Frederiksberg, and 9% in Aarhus). Another criterion for choosing the three cases is that they represent different approaches for energy optimization: a step-by-step approach with several minor changes over time, a larger renovation, and a transformation, respectively. Finally, the two latter cases represent ambitious renovations where the local municipality was actively involved in the decision-making process. This gave insight into how local authorities can guide and support building owners regarding balancing energy optimization and historic values.

3. Data Sources

3.1. The Danish SAVE Register

In Denmark, historic buildings are registered by using the so-called SAVE methodology (Survey of Architectural Values in the Built Environment), which assesses the preservation value of existing buildings based on conservation values ranging from 1 to 9, with 1 being the highest [18]. The grades are assigned within categories of architectural value, cultural-historical value, environmental value, originality, and construction technical condition. This results in an overall SAVE value. Typically, municipalities group SAVE values in the following way [18]:

• SAVE 1: Buildings with highest preservation value, often overlapping with listed buildings;
• SAVE 2–4: Historic buildings with high preservation value;
• SAVE 5–6: Even but nice buildings, where typically unsuitable replacements and conversions detract from their character;
• SAVE 7–9: Buildings without architectural expression or historic importance or buildings that have undergone changes to such a degree that they have lost their originality.

The 98 Danish municipalities are responsible for registering historic buildings in their municipality; however, this is carried out with some variation [19]. All SAVE registrations are stored in the Danish Agency for Culture and Palaces’ SAVE database for listed and historic buildings. This database contains information on 9000 listed buildings and 350,000 buildings that have had their conservation value assessed, including 72,000 apartment buildings. This corresponds to 73% of all the country’s approximately 100,000 apartment buildings 1. Of these, 19,734 have a SAVE value between 1 and 4 and can thus be regarded as historic buildings. The distribution of SAVE values in Danish apartment buildings erected before 1950 is shown in Figure 1.

Among the 19,734 historic apartment buildings (SAVE values between 1 and 4), the majority has SAVE values of “4” (33%) or “3” (16%). Only a small share (2%) has the highest SAVE value of 1. There are 1898 listed apartment buildings in Denmark, but they are not included in Figure 1.

3.2. The Danish EPC Register

In line with EU legislation [20], Denmark has had an act on Energy Savings in Buildings since 1997. EPC rating is mandatory upon sale for residential buildings. The certificate is based on an inspection conducted by a trained energy consultant. The inspection is mandatory before sale or rent for both new and existing buildings. Based on the inspection, the energy consultant issues an EPC report that concludes with an energy rating on an “A to G” scale. This rating is based on a standard calculation scheme (BE 18 that calculates the primary energy needed to heat the building) that encompasses the heat loss of each part of the building envelope, a standard value for hot water consumption, and energy loss by heat production. After the primary energy need is calculated, it is adjusted according to the heat supply: 0.85 for buildings with district heating (based on combined heat and power (CHP)), 1.0 for buildings supplied with gas or oil, and 1.9 for building heated with electricity. The adjusted energy need is then compared to the values below, which determines the buildings’ EPC rating. See

Table 1. Energy classes based on the Danish EPC calculation scheme for dwellings. “A” (right column) is the heated area of the building.

Energy Class	Limit Value [kWh/m2]
A2020	27.0
A2015	<30.0 + 1000/A
A2010	<52.5 + 1650/A
B	<70.0 + 2200/A
C	<110 + 3200/A
D	<150 + 4200/A
E	<190 + 5200/A
F	<240 + 6500/A
G	>240 + 6500/A

.

All information recorded by consultants is compiled in the EPC database, resulting in its continuous growth. As of autumn 2023, the database contained nearly 1 million energy certificates, of which approximately 65,000 were for apartment buildings. For each building, the EPC register contains data on current energy performance, potential energy performance (achievable through affordable energy retrofits), actual (measured) energy consumption, building ownership, year of construction, size, and municipality affiliation. This includes all types of buildings, including historic and listed buildings. EPC reports must be prepared by certified EPC consultants, based on physical inspection of the buildings [21]. Only EPC consultants in companies certified according to the current standard on quality management systems can carry out EPC assessment.

The quality of EPC ratings has been questioned in some studies [21,22]. For instance, [21] estimated an error rate between 36% and 62% in the British EPC reports. According to interviews with Danish EPC consultants [23], the quality of EPC assessment and EPC-reports has improved considerably in recent years, and in general, EPCs after 2010 are regarded as reliable. Earlier (in the 2000s), some EPC labels were prepared superficially and with a low quality, which made the general quality of the EPC labels less reliable. Additionally, EPCs are now monitored by a national agency that checks for outliers and unusual values in the EPC reports, which has the right to recall an EPC report if it finds assessments with flaws or dubious results. Other factors that have contributed to raising the quality include that EPC labels are increasingly being used in municipal policies, e.g., as indicator for status and progress in the energy performance of the local building stock or as a basis for urban renewal subsidies, and building owners increasingly view an EPC report as a tool to implement energy-saving measures, and they are therefore willing to pay for a well-documented EPC report [23]. Out of the sample for this study (approximately 13,000 apartment buildings with SAVE values from 1 to 4), 76% of the EPC ratings were completed after 2010. Furthermore, 52% have an EPC rating valid up to 2022, while 48% have an EPC label valid from 2022 onwards. Based on this, the data were found reliable for this study.

By combining the EPC database and the SAVE register, a comprehensive register was constructed with detailed information for each historic apartment building in Denmark. The total number of entries is shown in

Table 2. Overview of data sources and number of buildings from the SAVE register and the EPC register (2023). The analysis of SAVE and EPC values is based on the data with hatching in the table.

	Apartment Buildings in Denmark	Apartment Buildings in the SAVE Register with SAVE Value 1–9 *	Historic Apartment Buildings in the SAVE Register with SAVE Value 1–4	Historic Apartment Buildings with SAVE Value 1	Listed Apartment Buildings
Total number of buildings	100,229	37,292	19,734	588	1898
Buildings with EPC	Approximately 65,000	no data	12,572	77	no data

* There are 72,079 apartment buildings in the SAVE register, but 34,787 do not have a SAVE value.

below.

4. Results

4.1. Energy Performance of Historic Apartment Buildings Based on EPC Labels

Figure 2 shows the distribution of EPC labels for all 13,000 historic apartment buildings (with SAVE values 1–4) compared to EPC ratings for all apartment buildings constructed before 1950 and EPC ratings for apartment buildings with the highest preservation value (SAVE 1).

The analysis shows that 46% of historic buildings have an EPC rating of “C”, which is higher than buildings from the same age in general (36%) and for buildings with the highest preservation value (SAVE 1, 33%). For EPC label “D”, the distribution is similar across the three groups of buildings (36%). For EPC labels “E”, “F”, and “G”, the share of historic buildings is lower than for buildings in general and buildings with the highest preservation values. Overall, the share of buildings with EPC labels “F” and “G” is quite low, less than 10% for all buildings, and even lower for buildings with higher SAVE values.

The SAVE 1 buildings have a higher share of EPC “E” ratings and a lower share of EPC “C” ratings, which supports the assumption that high preservation value makes it more difficult to achieve a high EPC label. However, the sample of SAVE 1 buildings is limited (77 buildings), so the result for this group is less consistent.

Overall, the numbers indicate that the preservation values of buildings have not hindered their energy optimization. For buildings with SAVE 1 value, energy performance is approximately at the same level as older buildings in general.

4.2. Factors Influencing the Energy Performance and the Potential for Further Improvements

According to interviews with EPC consultants [23], interventions to improve the energy performance of historic apartment buildings to EPC ratings of “C” or “B” typically include insulation of the roof (250 mm), insulation of parapet under windows, changing to low-emission windows, use of opto-glass bay windows, and in some cases external insulation of rear facades. More controversial—but rarely used—solutions include external insulation of front facades with foam elements, where cornices and other façade decorations can be re-established [23]. External re-insulation is often performed in conjunction with the installation of balconies, which in this way achieves full depth even after insulation [23].

To identify which physical and technical interventions had been made in buildings with high energy performance, approximately 30 EPC reports for historic buildings with an EPC label of “B” or “C” were reviewed. The reviews were based on the brief historic summary of renovations and changes that have been made over time, which are typically included in EPC reports. The typical interventions and improvements are listed in

Table 3. Types of interventions that are frequently used to increase the energy performance of historic apartment buildings (with SAVE values 1–4). The interventions are on one hand distributed across building components and on the other the prevalence of different insulation levels. Interventions marked with * represent a risk of threatening the building’s historic value. The table is based on EPC reports for Danish historical residential properties with EPC ratings of “B” or “C”, a SAVE value between 1 and 4, and data from the Danish EPC register.

Building Components	1. Typical Interventions According to EPC Reports for Historic Apartment Buildings with EPC “B” or “C”	2. Prevalence According to EPC Register: Share of Buildings Where Interventions Are Registered
Roof and ceilings	External insulation of flat roof (“Copenhagen roof”) Internal insulation of sloping walls Internal insulation of non-insulated attic space Insulation of vertical skunk walls	26% roofs < 100 mm 72% roofs 100–300 mm 2% roofs > 300 mm
Floor separation	Insulation of separation against unheated basement	72% floors < 50 mm 26% floors 50–200 mm 2% floors > 200 mm
Windows	* New window with 2- or 3-layer energy glass 1-layer glass replaced by 2-layer energy glass Internal front frame fitted with energy glass Internal frameless glazing made of tempered glass Insulation of window niches	60% windows with 1-layer glass or older 2-layer thermal windows 36% windows with 2-layer energy glass or internal window with energy glass 5% windows with 3-layer glass
Internal walls	Insulation of walls against unheated stairways	No data
External walls	* Re-insulation of the facade facing the street * Re-insulation of the facade facing the yard * External re-insulation of gables	59% un-insulated external walls 35% external walls < 75 mm 6% external walls with 75 mm or more
Pipes	Insulation of heating pipes Insulation of hot water pipes	No data
Technical installations	Replacing single-strand system with double-strand Energy management with night and peak setbacks Energy management with climate correction Improvements of heating system Mechanical ventilation with heat recovery	93% supplied with district heating 5% supplied with natural gas 3% supplied with other sources (oil, electricity, or wood pellets) 18% of buildings (4% with heat recovery)
Solar cells (PV)	* Solar cell installation (PV)	0.4% of buildings

below (middle row). Moreover, from the EPC register, data on how often these interventions have been used in historic buildings were extracted. These data are included in Table 3 (right column) and provide an indication of the potential for further energy optimization in existing historic apartment buildings.

The review of EPC reports for historic apartment buildings that had achieved an EPC rating of “B” or “C” supports EPC consultants’ experiences, i.e., that high energy performance is mainly achieved with relatively simple interventions that can be implemented without compromising the building’s historic values.

However, it is not only the physical interventions in the building that determine energy performance; other factors such as the types of heat supply and location next to other buildings are important. Danish apartment buildings in city centers are usually five or six stories high and with a considerable building depth. This gives them a small surface area compared to their total heated floor area. One consultant stated in the interviews that old apartment buildings in large cities are “born energy-efficient” simply because they are tall and deep and stand shoulder to shoulder, and in this way, the building envelope helps to “retain the heat” [23]. Therefore, relatively few interventions are necessary to raise the EPC rating to “C” [23]. To test the effect of the “shoulder-by-shoulder” effect on EPC rating, data on the average ratio between external walls (i.e., not including shared walls with neighbor buildings) and floor area for different EPC ratings were retrieved from the EPC register. This revealed that buildings with EPC ratings of “B”, “C”, or “D” have an average ratio between external walls and floor area of around 0.6, while buildings with EPC rating “F”–“G” have an average ratio between external walls and floor area from 0.7 to 0.95 (see Figure 3). These rations are included in the calculations of the EPC rating and thus give buildings with a low ratio between external walls and floor area an advantage compared to buildings with a higher ratio. However, the differences between “B”, “C”, and “D” labels are caused by other circumstances, e.g., the energy optimization interventions in the buildings.

4.3. Potential Improvements

Table 3 indicates substantial potential for further improvements of historic apartment buildings. This includes virtually all types of insulation (roofs, windows with 3-layer glass, internal insulation of floors, and external insulation of walls). A further estimate of potentials was made from recommendations for cost-effective energy optimization interventions in the EPC reports (interventions with a payback time of less than 10 years) [24]. In EPC reports, each recommendation has information about cost saving, CO₂ reduction, required investment, time for accomplishment, and the potential EPC rating if recommended improvements are implemented. In

Table 4. Potential EPC labels that existing historic apartment buildings could achieve, distributed according to their existing EPC label. Source: EPC reports for 12,591 historic apartment buildings.

Potential EPC
Present EPC	A2020	A2015	A2010	B	C	D	E	F	G	Total
A2020	7									7
A2015	0	5								5
A2010	2	4	29							35
B	1	0	80	186						267
C	11	12	455	1575	3791					5844
D	23	14	170	839	2596	930				4572
E	13	10	39	210	558	359	81			1270
F	4	3	15	76	164	99	40	13		414
G	1	0	9	26	70	30	18	12	11	177
Total	62	48	797	2912	7179	1418	139	25	11	12,591

below, the potential EPC ratings for historic apartment buildings are summarized.

The table shows that the majority of historic buildings could achieve a better EPC rating:

• The number of buildings with an EPC “C” rating could increase from 5844 to 7179;
• The number of buildings with an EPC “B” rating could increase from 267 to 2912;
• More than half of buildings with an EPC “D2” rating could improve to an EPC “C” label (2596 buildings out of 4572 buildings, i.e., 57%);
• The 591 buildings with EPC ratings of “F” and “G” could virtually disappear (only 36 of the present “F” and “G” buildings (6%) are estimated to have no potential for improving the energy rating within “cost-effective” investments).

The potential energy savings were estimated using data for energy use in typical apartment buildings with various EPC ratings [25] (see

Table 5. Data for measured energy consumption in various apartment buildings, according to their EPC rating [25]. The measured heat consumption (by meter, kWh/m²) includes hot water use in 239 apartment buildings. The measured consumption was degree-day adjusted.

EPC Rating	A2020	A2015	A2010	B	C	D	E	F	G
Measured energy use [kWh/m2]	51	62	70	91	109	125	139	142	n.d.

).

According to Table 5, improving an apartment building’s EPC rating from “E” to “C” would on average result in 22% energy savings (from 139 kWh/m² to 109 kWh/m²). Improving from a “D” to a “C” on average results in 13% energy savings (from 125 kWh/m² to 109 kWh/m²). As argued above, improvements up to a “C” in historic apartment building could for most buildings be implemented without interfering with the buildings’ historic values. Raising the EPC rating from a “C” to a “B” would, on average, lead to 17% energy savings (from 109 kWh/m² to 91 kWh/m²). This, however, would typically require more radical interventions, e.g., external insulation of back facades or installation of PVs.

Although the estimates of potential EPC ratings are uncertain and should be taken with some reservation, they indicate that there is a substantial potential for further energy improvements. Better windows and better insulation of roofs have particularly large potential for improving the energy performance of historic buildings.

4.4. Conflicts Between Energy Efficiency and Preservation Values and How to Overcome Them

Although many energy optimization interventions can be implemented without compromising buildings historic value, municipalities experience various conflicts and ongoing discussions with owners about more controversial solutions, including the following:

• Window types (e.g., preserving original windows and providing them with a bay window versus installing new low-emission windows with 3-layer glass and alu-cover on the outside to reduce maintenance);
• PV panels on roofs (difficult to fit appropriately on historic buildings);
• External gable insulation and in some cases external façade insulation.

For municipalities, it is important to have dialogue with the owner prior to renovation to prevent problematic changes to the historic values while also improving the energy efficiency of the building [26]. Through dialogue, the owner can be guided to choose better solutions and often receive help (competencies and subsidies) from the municipality. However, it is often challenging for municipalities to contact the owner before renovation. Therefore, many municipalities use urban renewal subsidies as “bait” to contact owners for historic building renovation. Requiring building permits, which are mandatory for larger renovations and transformations, is also an “entry point” for dialogue with the owner [26]. However, smaller changes (such as changing windows) do not require a building permit, making it difficult for municipalities to keep track of these changes. In cases of so-called step-by-step renovations, which might take place over longer periods, with smaller changes made over years, it is mostly up to the owner and craftsmen (as well as consultants, e.g., an architect, although architects are rarely used for smaller changes) to ensure that the historic values of buildings are respected.

According to municipalities and consultants, it is not possible to identify interventions that are generally unacceptable, as this depends on how interventions are designed and implemented [23,26]. Some energy improvements might damage historic value if carried out traditionally but can be made acceptable through more careful design and by slightly altering traditional construction principles. One example is extra roof insulation that might lead to the lifting of eaves to make space for extra insulation. This can be avoided if insulation is made thinner close to the façade and thicker at the top of the roof; this will provide the same amount of insulation but will not require changes to the eaves [23]. Additionally, external insulation of rear facades and gables needs to be carefully implemented to respect the origins of building and surrounding buildings. However, this requires that the owner, craftsmen, and consultants have the necessary skills and competencies.

4.5. Case Studies of Energy Optimization in Three Historic Buildings

To illustrate how energy optimization of historic apartment buildings can take place, three case studies are presented: a step-by step renovation (Gothersgade 137), a larger renovation (Tårnborgvej 14), and a transformation (Mejlgade 74).

Gothersgade 137: A step-by step renovation. This apartment building is located in the inner city of Copenhagen. Built in 1888, it has five floors and a basement. The building has a SAVE value of 4 and an EPC label of “B” (2020). See Figure 4.

According to the EPC report, the building has not undergone any changes since a major renovation in 1957. The 2020 EPC report states that the cold attic facing the living area was insulated with 125 mm of mineral wool, as were the sloping ceilings. Flat roofs facing heated rooms are estimated to be insulated with 150 mm of mineral wool. All exterior walls consist of 36–72 cm brick walls without cavities and insulation. At a later stage, the property was fitted with new windows with triple-pane energy glass. Additionally, parapets under windows were reinsulated, and all heating pipes received 40 mm of insulation. The building is large and compact, which means the limited surface area exposed to the outside contributes substantially to its energy performance. Over time, the owner and administrator made decisions on upgrades without consulting the municipality. Nonetheless, the owner was surprised to find that the building had achieved an EPC label of “B”, as none of the upgrades were specifically intended to improve energy efficiency. This example shows that under certain conditions, e.g., compactness, district heating (based on combined heat and power production (CHP)), etc.—ordinary maintenance and replacing old windows with modern, low-energy windows can be sufficient to achieve high energy performance. This means that the building, with the limited interventions made, can be further improved in terms of energy performance and potentially reach an EPC label of “A” without jeopardizing its conservation value of 4 on the SAVE scale.

Tårnborgvej 14: A larger renovation. This apartment building is located in the municipality of Frederiksberg, part of Greater Copenhagen. It is a multi-story property with five floors, built in 1902 and owned by a private fund. The building has a SAVE value of 4 and an EPC label of “C” (2020). Prior to renovation, the EPC label was “E”. See Figure 5.

The building was renovated in 2018 with urban renewal support from the Frederiksberg municipality. The renovation included restoring the facade to its original appearance, thus improving the architectural value of the building. At the same time, external insulation was added to the rear facade facing the yard. The gable to the north was retrofitted in the same way. The renovation also included insulation of the roofs and floor separations as well as the window niches behind the radiators facing the facade. An old one-pipe heating system was replaced with a two-pipe system. Mechanical ventilation with heat recovery was installed during the kitchen and bathroom renovations. The windows from 1980 were retained, although they differ from the original design. The municipality played an active role in the renovation process, collaborating with the owner and an energy consultant. The insulation of the rear facade, in particular, required many considerations and discussions on how to design it to be less visible and fit with the existing neighboring buildings.

Mejlgade 74: A transformation. This apartment building, located in Aarhus, has undergone both renovation and transformation. Three additional floors in a modern style have been added on top of the old building. Before the transformation, the building had a SAVE value 2. (No information is available about the original EPC rating, but the energy consumption was “high”, according to the owner.) After the transformation, the building achieved an EPC rating of “C”. The heat consumption is, according to the owner, 98 kWh/m². See Figure 6.

The ground floor, which until the renovation was a shop front, has been restored to its original appearance and converted into an office, and the entire building has been harmonized with the rest of the streetscape. It lies in the middle of a row of historic buildings. Originally built in 1891, the apartment building formerly had three floors and now has five floors with a used attic. Today, it is subdivided into owner-occupied flats. The superstructure, which differs significantly in appearance from the rest of the building, was added in 2019. With the superstructure, three new floors have been added, complying with building regulations for insulation. The extension and rear facade are clad in tombak (a brass alloy). The rear facade has been re-insulated on the outside with facade panels of the same type as the new superstructure. The new windows in the facade are fitted with low-energy glazing. The municipality was active in the renovation process and called in their architecture panel, consisting of internal and external experts, to discuss the design of the renovation. This example of transformative renovation shows that even major interventions in a property’s exterior architecture can be carried out if done with great architectural care. The present energy consumption is 98 kWh/m².

The three cases of historic apartment building renovations have led to improved energy performance and considerable energy savings without compromising the historic value of the buildings. Moreover, the renovation of Tårnborgvej and the transformation of Mejlgade have improved and reverted the historic values and qualities of the buildings. The three cases are not necessarily representative of the way historic apartment buildings are energy-optimized in general but demonstrate that it is possible to make physical interventions in historical apartment buildings that improve the energy performance without compromising the cultural and historical value.

5. Discussion

Our findings support various case studies of historic buildings from existing research that show energy optimization is possible and feasible without compromising historic value [4,5,6,7]. Compared to other systemic studies of energy performance in historic buildings, which have covered selected cities and regions as well as a mix of building types [3,13,14], this study covers an entire national context with one specific type of historic building (Danish apartment buildings). This adds to a more systemic overview of the energy performance of historic apartment buildings as well as a discussion of the extent to which energy optimization collides with the preservation of historic building values.

The high share (46%) of EPC rating “C” in historic apartment buildings in Denmark aligns with [14], which found that the share of “C” ratings among historic buildings in some Dutch cities can exceed 50%, although this was based on a smaller sample and a different national context. The Dutch study also found that replacing glazing and heating systems was common, whereas external insulation and renewable energy sources were rare [14], similar to the Danish cases. However, while [14] suggested that replacing glazing and heating systems has a limited effect on the EPC rating, our study found that it has a relatively large effect on the EPC rating, in combination with other simple improvements. In contrast, ref. [14] found that external insulation could provide a more radical improvement of the EPC ratings and potentially save many buildings from demolition. (Many Dutch post-war buildings have been demolished due to poor energy performance.) It should also be noted that [14] included a variety of Dutch building types, whereas the Danish study focused on one specific building type (apartment buildings).

We found that Danish historic buildings play an important role in CO₂ reductions in the entire building stock, as they represent approximately 20% of all apartment buildings, which is in line with other studies [4]. The analysis also found a large potential for further energy improvements in historic buildings. The methodology did not, however, include modeling or statistical analysis of which factors influence the energy performance of historic buildings. For instance, the shoulder-by-shoulder effect is reported in interviews with EPC consultants but was not modeled or tested in our analysis of EPC ratings. Also, other factors, such year of construction, building size, and construction type, could in theory have a systemic influence on the comparison of historic buildings and “ordinary” apartment buildings. This possibility could be examined in other further research projects.

It should be noted that the existing research shows significant differences in the energy performance of historic buildings across different cities and districts, such as urban and rural settings, and therefore, the potential for further energy improvements differs [3,14]. We found similar geographic variations in energy performance amongst historic buildings across Danish municipalities, where municipalities outside growth areas in general have a lower share of EPC ratings of “C” in their historic buildings. This can be due to a less compact building structure, a lower share of district heating, and fewer resources devoted to building renovation in general because of lower property prices compared to the larger cities. Also, we found differences in local owners’ appreciation of historic buildings across geographic areas [23,26]. All in all, the structural and local conditions for formulating policies on historic apartment buildings is very different across geographical contexts, even within the borders of Denmark—and undoubtedly larger when comparing across national contexts.

When it comes to “balancing” energy performance and historic values, we found that most interventions to improve the EPC rating of historic apartment buildings can be implemented without compromising historic value. This is based on expert interviews, reviews of EPC reports for historic buildings with high energy performance, and studies of selected buildings. In this material, we did not find examples of buildings that have lost their historic value or were destroyed or compromised due to energy optimization. This does not, however, mean that such examples do not exist but only that we did not come across them—and methodologically, it would be challenging to examine on a larger scale to which extent this is the case. It can also be difficult to decide whether a careless building renovation without respect for the historic value is due to an energy optimization or from an “ordinary” renovation of the building.

The three examples of building renovations of historic buildings do not serve as a “general” approval of such interventions, as definitions of “historic buildings” vary across national contexts as well as perceptions of “acceptable” building interventions, and thus, these examples might seem controversial from a preservation point of view or from another national context. The main point is that the decisions were made in dialogue with experienced architects and approved by the parties involved.

However, important conditions for the energy performance of these locally “approved” examples are the building’s structure, location, and energy supply (large volume, shoulder-by-shoulder, and district heating based on CHP production), which means that they only need a few further energy improvements to achieve a high EPC rating—as well as the process and design of the renovation involving a high level of competencies, resources, and awareness about the historic qualities amongst the involved actors (building owner, consultants, architects, municipality, users, etc.). In particular, the local municipality is a central actor, both as a local authority (identifying and regulating historic buildings) and as a provider of knowledge, competencies, and subsidies.

In cases where these factors are not in place—e.g., free-standing buildings heated with electricity or oil boilers, where the historic quality has not been mapped, identified, or protected, with low knowledge and awareness amongst owner and craftsmen as well as no involvement of the municipality—there is a higher risk that energy optimization of the building will harm its historic value. Thus, the increasing focus on energy optimization of the existing building stock calls for awareness of resources and competencies in municipalities and amongst building owners for building preservation. On the other hand, information must be disseminated to political decision makers that energy optimization of historic building is possible without harming the historic value—when carried out with the appropriate competencies. There is large potential for further energy savings in historic apartment buildings, even without “wrapping the buildings in external insulation”.

For future research, we recommend focusing on historic buildings where conditions for energy performance upgrades are suboptimal due to constraints such as location, economic factors, infrastructure limitations, local housing market conditions, and inadequate knowledge or resources. Studies should examine how these buildings might address energy performance requirements despite such constraints. Priority should be given to buildings with preservation value that remain unregistered by local authorities, as these structures face heightened risk of deterioration and loss of historic significance. Additionally, there is a critical need for research into best practices for proactive municipal engagement in balancing energy efficiency goals with historic preservation objectives.

6. Conclusions

We defined two research questions for this study: For the first question, “What is the energy performance of historic apartment buildings compared to apartment buildings with no preservation values?”, we conclude that the historic apartment buildings in Denmark are performing at least as well as the general stock of apartment buildings in Denmark. This is based on data from approximately 13,000 Danish historic apartment buildings constructed before 1950, where we found the following:

• 46% of historic apartment buildings have an EPC rating of “C”, which is at least as good as general apartment buildings constructed before 1950;
• The EPC rating of “C” is often achieved with relatively simple improvements. Additionally, these buildings benefit from large volumes, side-by-side locations, and district heating, which all contribute to a better EPC rating;
• We also found that there is significant potential for further energy improvements in historic buildings. For instance, the share of buildings with an EPC “C” rating could be substantially increased with relatively simple improvements.

Regarding the second research question, “Will energy optimization of historic apartment buildings lead to a reduction in their preservation values?”, our findings provide strong indications that most energy optimization interventions of apartment buildings, to achieve an EPC rating “C”, can be executed without compromising the historic integrity of buildings. However, the preservation of historic value is contingent upon several critical factors: the specific type of interventions selected, the expertise and commitment of property owners, and the technical competency of the craftsmen involved in the implementation. Furthermore, local authorities play a crucial role in this process by identifying and protecting historic buildings while providing essential support and guidance to property owners.