heritage

Article
Evaluating the Implementation of Energy Retrofits in Historic
Buildings: A Demonstration of the Energy Conservation
Potential and Lessons Learned for Upscaling
Daniel Herrera-Avellanosa 1, * , Jørgen Rose 2 , Kirsten Engelund Thomsen 2 , Franziska Haas 3 ,
Gustaf Leijonhufvud 4 , Tor Brostrom 4 and Alexandra Troi 1

1 Eurac Research, 39100 Bolzano, Italy; alexandra.troi@eurac.edu

2 Department of the Built Environment, Aalborg University, 9220 Aalborg, Denmark; ket@build.aau.dk (K.E.T.)
3 Department Citizens—Heritage—Society, Bavarian State Office for Heritage Preservation,
80076 Munich, Germany
4 Department of Art History, Uppsala University, 753 10 Uppsala, Sweden;
gustaf.leijonhufvud@konstvet.uu.se (G.L.); tor.brostrom@konstvet.uu.se (T.B.)
* Correspondence: daniel.herrera@eurac.edu

Abstract: This study presents an in-depth analysis of 69 case studies focusing on the energy retrofit
of historic buildings, uncovering challenges, best practices, and lessons learned to balance energy
efficiency improvements with heritage preservation. The findings highlight several challenges en-
countered during renovations, such as complex heritage evaluations, restrictions on alterations,
coordination issues with authorities, technical limitations, higher investment costs, and knowledge
gaps. On the other hand, identifying factors promoting renovation, including demonstrating en-
ergy savings while respecting heritage, early collaboration between planners and authorities, and
quantifying investments, could incentivize owners and authorities. The limitations of a still-limited
sample size, occasional incomplete data, and potential sample bias call for cautious interpretation of
the presented analysis. Despite these, the study provides valuable insights into successful projects,
emphasizing the need for scalability, knowledge transfer from innovative policies, and targeted
Citation: Herrera-Avellanosa, D.;
policy-making for successful replication. The study concludes with a call for further development
Rose, J.; Thomsen, K.E.; Haas, F.;
of the HiBERatlas (Historic Building Energy Retrofit atlas), an extensive resource for historic build-
Leijonhufvud, G.; Brostrom, T.; Troi, A.
Evaluating the Implementation of
ing renovation, expanding its database, collaborating with agencies, and tailoring guidance for
Energy Retrofits in Historic Buildings: stakeholders to foster energy retrofits in heritage buildings.
A Demonstration of the Energy
Conservation Potential and Lessons Keywords: historic buildings; energy retrofit; best practice; case study; database
Learned for Upscaling. Heritage 2024,
7, 997–1013. https://doi.org/
10.3390/heritage7020048
1. Introduction (or Why Documenting Best Practices Is a Good Idea)
Academic Editor: Kristian Fabbri
According to the European Commission, buildings in the EU are still responsible for
Received: 5 January 2024 40% of total energy consumption and 36% of greenhouse gas emissions, and 75% of all
Revised: 8 February 2024
buildings within the EU are still energy inefficient [1]. To reduce energy consumption and
Accepted: 9 February 2024
thereby mitigate climate change, there is a need for significantly increasing the energy effi-
Published: 15 February 2024
ciency of the existing building stock. However, a significant portion of existing buildings,
i.e., more than 25% (source?), predates the 1950s, and many of these buildings hold pro-
found cultural, architectural, social, and heritage significance, rendering their preservation
Copyright: © 2024 by the authors.
a matter of shared importance. To unlock the potential of these buildings [2], careful renova-
Licensee MDPI, Basel, Switzerland. tion and restoration are needed, and increasing the energy efficiency can be complex since
This article is an open access article consideration for heritage values can often significantly reduce possibilities and sometimes
distributed under the terms and even rule out certain standard improvements completely [3–6]. Demonstration projects and
conditions of the Creative Commons best practice advice are regarded as effective strategies to overcome barriers to the uptake of
Attribution (CC BY) license (https:// energy-efficient technology in buildings [7,8]. The need for knowledge-sharing regarding
creativecommons.org/licenses/by/ best-practice approaches specifically for heritage buildings was clearly underlined in a
4.0/). review by Lidelöw et al. [9].

Heritage 2024, 7, 997–1013. https://doi.org/10.3390/heritage7020048 https://www.mdpi.com/journal/heritage

Heritage 2024, 7 998

Research has already been performed in relation to the difficult task of improving
the energy efficiency of heritage buildings. In their work, Buda et al. [10] presented
an approach that supports decision makers in selecting relevant retrofit solutions for
exterior walls, windows, Heating, Ventilation, and Air Conditioning (HVAC) systems, solar
thermal, and Photovoltaic (PV) technologies. Their research points towards a need for
specifically tailored energy retrofits, i.e., meaning that no one-size-fits-all is feasible. Rieser
et al. [11] provided a systematic approach for pairing ventilation solutions with heritage
buildings based on assessment criteria in terms of heritage significance and building
physics according to methodologies given in the standard EN 16883:2017 [12]. Their paper
provided a review and an overview of the interrelationships between heritage conservation
and the need for ventilation in energy-efficient buildings.
Similarly, previous studies have developed methodologies aimed at facilitating the
renovation of historic buildings with a focus on the trade-offs between efficiency and
conservation (as in [13,14]), the quantification of scenarios (as in [15]), environmental
aspects (as in [16,17]), the use of digital technologies like BIM (as in [18]), or specific to
certain typologies (like educational buildings in [19]). However, assessment of actual
implementations of energy retrofits still remains rare, and the analysis of several cases
combined from which to draw lessons that can inform research and policy to an foster
efficient built heritage is particularly needed.

2. Materials and Methods

Despite the many issues related to improving the energy efficiency of heritage build-
ings, many renovations have been carried out by engaged owners and frontrunner practi-
tioners and it is vital to support decision makers by promoting some of the best-practice
examples so that they can serve as inspiration and help accelerate the green transition.
In pursuit of this objective, the IEA SHC Task 59 project (Renovating Historic Buildings
Towards Zero Energy) [20] and the Interreg Alpine Space ATLAS [21] have collaboratively
developed a tool for disseminating these examples, known as the HiBERatlas (Histori-
cal Building Energy Retrofit Atlas, www.hiberatlas.com accessed on 19 December 2023,
available online since October 2019) [22]. The database presents best-practice examples of
how a historic building can be renovated to achieve high levels of energy efficiency while
respecting and protecting its heritage significance.
This study analyses the first sample of cases collected by these two research projects
with a quantitative approach and the goal to draw lessons that support the upscaling of such
renovations. The database considers best practice as any example where: (i) the renovation
of the whole building has been considered; (ii) the project has been implemented; (iii) the
intervention followed the results of a thorough heritage value assessment; (iv) a significant
energy demand reduction was achieved; and (v) a detailed documentation of the decision
process, technical solutions, and evaluation results was made available. Establishing a
single quantitative criterion or threshold to measure the degree of success of an intervention
exclusively as a function of the energy saving (i.e., kWh/m2 ) would not be compatible with
the definition of “lowest possible energy demand” proposed in [23], as it goes against the
principle that every building must be considered individually.
Ensuring the quality of the best practices displayed in the database is crucial to help
eradicate any concern about professionals’ expertise. The implementation of a review
process inspired by the academic peer review process can assess the validity of the projects,
and, most importantly, the way they are documented becomes crucial. The primary
objective of the review process was not to dismiss submitted examples, but rather to
enhance their robustness and presentation.
Most projects were rated as “Recommended with limitations”, what meant the projects
seemed suitable for the database, but the documentation had to be at least partially com-
plemented. Only one project was “Not recommended” in a first review from a Heritage
expert. However, after a second review, this project was also “Recommended with lim-
itations” and was asked to provide additional information. Only one of the evaluated
Heritage 2024, 7 999

projects was “Recommended as role model” in both aspects, heritage and technical, after
the first review. It is worth keeping in mind that all projects included in this report were
brought forward by researchers who were involved in the projects (Task59 and ATLAS)
and therefore knowledgeable in the field.
The contributions from partners of IEA SHC Task 59 and Interreg AlpineSpace ATLAS
had reached 69 case studies as of July 2022. Such a repository is expected to attract the
interest of decision-makers and additionally create the momentum needed to attract new
examples from architects, owners, or new research projects. The analysis of the case studies
follows the same structure as the projects that were documented: general information about
the location type and size of the building, description of the renovation process as well
as the state of repair of the building prior to the intervention and heritage significance,
documentation of the technological solutions implemented (thermal envelop, Heating,
Ventilation, and Air Conditioning -HVAC- and Renewable Energy Systems -RES-), and
final evaluation of the performance.

Limitations
Due to the limited sample size and available information, any analysis presented here
must be taken carefully. These results will gain robustness when the number of projects
documented form a more statistically significant sample that allows for the consideration
of the effect of the different variables listed above. In general, subset samples are in many
cases still limited, making the analysis difficult. The sample included here is not a perfect
representation of the built stock; thus, it is heavily influenced by the partners in the research
project consortium. For instance, South Tyrolean (IT) farmhouses are dominant among
buildings built before 1600, whereas Austrian farmhouses are the most common type
among the cases built between 1600 and 1800. The main difficulties with the documentation
of the case studies rely on the management of incomplete or inconsistent metrics, as
case studies used different methods and metrics for energy use, savings, etc., making
comparison difficult.

3. Results
A first analysis categorized the sample according to regional distribution, size, type of
building, level of protection, age, and year of renovation, as summarised in Table 1. The
case studies were collected in 12 different countries and are located in Austria, Belgium,
Denmark, France, Germany, Italy, Slovenia, Spain, Switzerland, Turkey, the United King-
dom, and the USA. Buildings were categorized as small (<800 m2 ) or large (>800 m2 ), with
rural residential buildings predominantly small, while urban residential buildings showed
more diversity. Overall, 55 case studies were small buildings and 14 were large.
Heritage protection and conservation area status were considered, revealing 36 listed
buildings, 24 in protected areas, and 9 without protection. Nineteen of the cases were
both listed and in conservation areas. Case studies were also classified according to their
construction period to explore any relation between age and state of repair or construction
technologies. Buildings ranged from before 1600 (14 case studies), 1600–1700 (7 case
studies), 1700–1800 (9 case studies), 1800–1849 (5 case studies), 1850–1899 (19 case studies),
1900–1944 (12 cases studies), 1945–1959 (2 case studies), to 1980–present (1 case study).
Buildings included in the period from 1850 to 1899 are primarily located in urban areas,
whereas the rest of the case studies are more evenly spread between urban and rural areas.
Most of the renovations (56 case studies) took place since 2013.
Changes in building use were analysed, identifying 18 extensions and 32 transforma-
tions among the 69 case studies. Extensions included attic space additions and additional
living space, while transformations often involved converting agricultural buildings to
residential or changes in the type of ownership.
Heritage 2024, 7 1000

Table 1. Overview of analysed case studies. Cases listed or in a conservation area are indicated with
an X, whereas Extensions and Transformations are indicated with a E and T respectively.

No Country Size Conserv. Building Last Extension

Case Study Building Use Listed Area Period Renovation /Transformation
1 Kaiserstrasse Austria Large Residential (urban) X - 1850–99 2013 E
2 Trins Austria Small Residential (rural) - - 1600–99 2019 E/T
3 Hof 6 Austria Small Residential (rural) X X 1600–99 2013 E
4 Kelchalm Austria Small Hotel/Comm - - 1800–49 2013 -
5 Mariahilferstrasse Austria Large Residential (urban) - - 1850–99 2018 E
6 House Maurer Austria Small Residential (rural) - - 1800–49 2015 -
7 House Breuer Austria Small Residential (rural) - X 1900–44 2015 T
8 Music school Austria Small Educational - - 1900–44 2015 T
Velden
9 Hof Neuhäusl Austria Small Residential (rural) X - 1700–99 2017 T
10 Zwischenwasser Austria Small Cultural - 1900–44 2015 -
11 Freihof Sulz Austria Large Multipurpose - - 1700–99 2006 T
12 Josef Weiss Austria Small Residential/Comm X - 1850–99 2016 -
13 Giatla Haus Austria Small Hotel/Comm X - 1600–99 2015 T
14 House Irgang Austria Small Residential (rural) - 1850–99 2008 -
15 Baur Residence Austria Small Residential (rural) - - 1850–99 2011 E/T
16 Kasperhof Austria Small Residential (rural) - - 1600–99 2019 T
17 Maison Rubens Belgium Small Residential (urban) - - 1850–99 2008 -
18 Alken Belgium Small Residential (rural) - - 1600–99 2016 -
19 Doragno Castle Switzerland Small Residential (rural) X X <1600 2017 T
20 Solar silo Switzerland Small Office - - 1850–99 2014 T
21 Feldbergstrasse Switzerland Small Residential (rural) - - 1850–99 2009 -
22 Magnusstrasse Switzerland Large Residential (rural) - - 1850–99 2007 E
23 St. Franziskus Switzerland Large Religious X X >1980 2018 -
Kindergarten
24 Switzerland Small Educational - X 1900–44 2016 T
Chur
25 Bern Switzerland Small Residential (urban) - X 1850–99 2015 -
26 Gstaad Switzerland Small Residential (rural) X - 1700–99 2018 E/T
27 Glaserhaus Switzerland Large Residential (rural) X - 1700–99 2015 -
28 PalaCinema Switzerland Large Cultural X X 1850–99 2017 T
29 Casa Rossa Germany Small Residential (urban) - X 1900–44 2018 -
30 Bergrheinfeld Germany Small Town Hall - - 1600–99 2018 E
31 Farmhouse Germany Small Residential (rural) X X 1700–99 2018 T
Straub
32 Sep Ruf Germany Small Residential (rural) - X 1900–44 2014 -
33 Ackerbürgerhäus Germany Small Residential (rural) X - <1600 2015 -
34 Ritterhof Germany Small Residential (rural) X 1850–99 2016 T
35 Burgkunstadt Germany Small Townhall - - <1600 2009 -
36 Osramhuset Denmark Large Commercial X - 1945–59 2009 T
37 Klitgaarden Denmark Small Residential (rural) X X 1850–99 2016 -
38 Ryesgade 30 Denmark Large Residential (urban) - - 1850–99 2011 -
39 Alsace France Small Residential (rural) X X 1700–99 2015 -
40 School Mulhouse France Large Educational - - 1700–99 2015 E
41 Timber-framed France Small Residential (rural) X - 1700–99 2016 T
barn
42 Rainhof Italy Small Residential (rural) - - <1600 2016 E/T
43 Villa Castelli Italy Small Residential (rural) X - 1850–99 2013 -
44 Ansitz Kofler Italy Small Residential (urban) X - 1700–99 2008 -
45 Collemaggio Italy Large Religious X - <1600 2017 -
46 House Pernter Italy Small Residential (rural) X - 1900–44 2017 T
47 Kohlerhaus Italy Small Residential (urban) - X <1600 2011 T
48 Ruckenzaunerhof Italy Small Residential (rural) - X <1600 2015 E
49 Aussergrubhof Italy Small Residential (rural) X X 1600–99 2014 E/T
50 Oberbergerhof Italy Small Residential (rural) - - <1600 2016 -
51 Platzbonhof Italy Small Residential (rural) X X <1600 2016 T
52 Mairhof Italy Small Residential (rural) - - <1600 2018 E
53 Obergasserhof Italy Small Residential (rural) X - <1600 2013 E/T
54 Rebecco Farm Italy Small Hotel/Comm X - <1600 2017 T
55 Villa Italy Small Residential (urban) - - <1600 2017 T
Capodivacca
56 House Moroder Italy Small Residential (urban) X X 1900–44 2015 -
57 Huberhof Italy Small Residential (rural) - - <1600 2008 T
58 Notarjeva vila Slovenia Small Residential (urban) X X 1900–44 2015 T
59 Rožna ulica 15 Slovenia Small Residential (urban) X X 1850–99 2018 E
60 Hiša trentarskih Slovenia Small Multipurpose X - 1900–44 2012 E/T
61 Idrija mercury Slovenia Large Educational - - 1945–59 2017 E/T
62 Mercado del Val Spain Large Commercial X - 1850–99 2016 E
63 Correria 119 Spain Small Residential (urban) X X 1850–99 2020 -
64 Ahmet Aga Turkey Small Offices - X 1800–49 2020 T
65 Nwcip Pasa Turkey Small Library X X 1800–49 2017 -
66 Downie’s Cottage UK Small Residential (rural) X X 1800–49 2016 T
67 Hollyrood Lodge UK Small Commercial X X 1850–99 2017 T
68 Annat Road UK Small Residential (urban) X X 1900–44 2014 -
69 Aspinall USA Large Offices - X 1900–44 2013 -
Heritage 2024, 7 1001

3.1. Planning Process

Most assessed buildings are officially protected, and understanding how the heritage
value was assessed becomes even more important. However, in most cases, the information
collected in the two sections foreseen (“Heritage value assessment” and “Elements worthy
of preservation”) intermix. There is often information about the elements worthy of
preservation, but given the limited text length requested from the database (100 words), this
information is generally limited to some examples or are described in very general terms.
A few projects present how the heritage value assessment was carried out. Interestingly, in
a number of cases, it was mentioned that the building owner complemented the heritage
assessment and identified further elements worthy of preservation (like Ansitz Kofler
in [24]).
Even in the cases where the building was lacking official recognition, building owners
expressed an interest in preserving the “character” of the building. The examples gathered
in the database illustrate how a lack of a detailed heritage value assessment might hamper
the process (e.g., Villa Castelli in [25]) and the importance of an open dialogue with the
heritage authorities (e.g., Rainhof in [26]). The case study Correria 119 [27] in Spain is an
interesting example of a participatory process where renovation processes and heritage
value assessment were defined in two workshops.
The driver for refurbishing varied greatly among the cases studied, from improved
comfort and functionality, to change of use (e.g., former industrial buildings turned into
offices), or restoration of damaged structures. All projects had ambitious targets for energy
performance, but only a few had explicitly quantified a target. Most of the projects instead
had a “lowest possible” approach [23]. Some reported an explicit aim to reduce carbon
emissions over the whole life cycle, and a few had unusually ambitious targets that included
microgeneration (e.g., St. Franziskus Church [28], Kindergarten Chur [29]). It is important
to keep in mind that case studies were documented based on what owners communicated
post intervention, and that a certain participant bias might exist.
Among the 69 case studies, a large number of different tools used to support the design
phase was identified. This is an indication of the wide range of tools that already exist in
the market and may lead to difficulty for practitioners in identifying the most suitable one.

3.2. Types of Measures

Of the 69 cases, only six examples did not carry out any thermal improvement of
the exterior walls. A total of 67 different wall concepts were identified (some buildings
have several different wall solutions documented) with 41 featuring internal insulation,
22 using external insulation, and one implementing cavity insulation. Internal insulation
can increase the thermal resistance of the wall while preserving historic exterior finishings.
In these cases, driving rain protection might be compromised [30], and the use of diffusion-
open capillary active internal insulation systems was prioritised. Twenty-eight out of the
forty-one solutions with internal insulation were executed with capillary-active materials
or a diffusion-open structure [31,32]. Insulating plasters were used to a large extent with a
thickness of around 60 to 80 mm and achieved U-values between 0.39 and 1.23 W/m2 K.
Internal insulation systems with vapour retardant layers were used mainly in cases of
solid wood walls and mostly using OSB boards as vapour retarders and an airtight layer.
Vapour retarding systems usually relied on soft panels or blown-in insulation and a trend
towards the use of ecological materials (e.g., hemp, wood, or cellulose) could be seen [17,33].
External insulation is generally perceived as a much safer solution (in relation to moisture,
condensation risk etc.) [34] and as such, higher levels of performance (six cases with
U-values below 0.15 W/m2 K) are achieved using this approach.
Roof refurbishments were documented in 45 cases, predominantly maintaining the
original shape, with pitched or hipped roofs being the most common. Thermal upgrades
involved insulation between rafters using materials like mineral wool, glass wool, cellulose,
and wood fiber. U-values reduced significantly from 0.26–5.07 W/m2 K to 0.09–0.43 W/m2 K.
In 22 cases, the roof truss was entirely renewed, reproducing the original shape and materi-
Heritage 2024, 7 1002

als. Ground floor refurbishment was documented in 45 of the 69 examples, differentiating

between existing concrete slabs (10 cases) and new ones (20 cases). Insulation measures,
either above or below the slab, achieved notable thermal resistance improvements, ranging
from 0.10 to 1.10 W/m2 K.
Sixty-seven window solutions were documented, varying from careful restoration to
the addition of new low-energy windows [35]. Solar transmittance (rarely documented)
ranged from g = 0.45–0.50 for triple-layer to g = 0.55–0.65 for double-layer windows,
although specific values were rarely documented. Figure 1 shows a comparison of the
approximate total window U-values before and after the renovation. Among the analyzed
solutions, 45 involved replacing existing windows. In eight cases, no restrictions were ap-
plied to the new windows, allowing off-the-shelf solutions. In 24 cases, new windows were
Heritage 2024, 7, FOR PEER REVIEW handcrafted to closely resemble the originals, often with frames adjusted for energy-efficient
8
glazing. Buildings where window replacement was not feasible prioritized restoring and
renovating existing windows, with 11 cases also enhancing their energy efficiency.

Heritage 2024, 7, FOR PEER REVIEW 8

Figure 1.
Figure Window glazing
1. Window glazing U-values
U-values before
before and
and after
after renovation.
renovation.

In 62 cases, a change in the main heating system was documented. More than one
third changed to a biomass boiler system (pellets, wood chip or wooden stoves), seven
buildings were connected to district heating (with four of those fuelled with biomass),
18 buildings are now equipped with heat pumps, nine buildings had gas heating systems,
and just one used solar thermal as the main heating system. The tendency towards biomass
and heat pump systems is especially pronounced in rural residential buildings, as Figure 2
shows, whereas gas condensing boilers have been chosen mainly in urban residential
buildings. Other systems that include the district heating systems are most often used in
Figure 1. Windowbuildings.
non-residential glazing U-values before and after renovation.

Figure 2. New heating system by use and system type.

Based on the information provided on the replaced heating system (for the 39 cases
where this information is available), Figure 3 shows a clear shift from non-renewable to
renewable systems [36]. Solar thermal systems come mainly into use as secondary
systems, together with biomass systems [37].
Ventilation must be considered in every refurbishment project for comfort and health
reasons, energy saving and climate change mitigation, but especially because it can
support the long-term preservation of the building fabric by decreasing the interstitial
condensation
Figure
Figure2.2.New risk. However,
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ventilation system in a listedwhen it comes
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 Heritage 2024,
Heritage 2024, 77, FOR PEER REVIEW 10039

Figure 3. Change of heating system. Reddish colours for non-renewable and yellowish colours for
renewable energy
renewable energy systems
systems (district
(district heating
heating and
and heat
heat pump
pump remain
remain grey
grey because
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Heritage 2024, 7, FOR PEER REVIEW Ventilation must be considered in every refurbishment project for comfort and health 9
reasons, energy saving and climate change mitigation, but especially because it can support
the long-term preservation of the building fabric by decreasing the interstitial condensation
risk. However, when it comes to heritage conservation, ventilation systems should be as
unintrusive and invisible as possible [38]. Of the 41 cases with explicit information on
the ventilation, four rely exclusively on natural ventilation, two have exhaust ventilation
systems, and the remaining 35 buildings (i.e., 50%) are equipped with mechanical ven-
tilation systems with heat recovery (MVHR) (1 room-by-room system, 5 decentralized,
26 centralized, 2 combinations of central and decentralized, and 1 case coupled central
MVHR with natural ventilation).
Looking at the conservation status (Figure 4), the number of cases with MVHR suggest
Figure 4. Ventilation with heat recovery with consideration of the conservation status.
that it might be slightly, but not considerably, more difficult to integrate a ventilation system
in a listed building or a building in a conservation area. Ventilation systems have more
Sixty-five percent of the analysed cases included renewable energy solutions, and in
often been considered in retrofits of urban than rural residential buildings. They are more
many cases, different types of solutions were combined (e.g., Villa Castelli [13]): 22
commonly found in Belgium (100%), Austria (69%), Denmark (67%), and France (67%).
projects rely on one technology, 15 projects combine two, 7 combine three, and 1 project
There is no correlation in the data between the building age nor year of retrofit and the
(Single family house in Bern [39]) combines four different renewable energy technologies.
implementation of a MVHR system. Documentation of ventilation systems proved to be
There3.isChange
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buildings in each category that utilizes the different types of renewable energy sources. If
we compare the setting of the residential buildings (16 urban/30 rural) and the use of
renewable energy sources, it is clear that in particular, photovoltaics and biomass are more
often favoured in a rural setting with 33% of rural residential buildings having installed
PV and 47% using biomass, compared to 19% of urban residential buildings with PV and
31% with biomass.

Figure
Figure4.4.Ventilation
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specific combinations
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that are favoured
rural buildings.
more often than others. Figure 5 shows the total number of buildings and the number ofin
than others. Figure 5 shows the total number of buildings and the number of buildings
buildings in each category that utilizes the different types of renewable energy sources. If
we compare the setting of the residential buildings (16 urban/30 rural) and the use of
renewable energy sources, it is clear that in particular, photovoltaics and biomass are more
often favoured in a rural setting with 33% of rural residential buildings having installed
PV and 47% using biomass, compared to 19% of urban residential buildings with PV and
 Sixty-five percent of the analysed cases included renewable energy solutions, and in
many cases, different types of solutions were combined (e.g., Villa Castelli [13]): 22
projects rely on one technology, 15 projects combine two, 7 combine three, and 1 project
(Single family house in Bern [39]) combines four different renewable energy technologies.
Heritage 2024, 7 There is no specific combinations of technologies or single technologies that are favoured 1004
more often than others. Figure 5 shows the total number of buildings and the number of
buildings in each category that utilizes the different types of renewable energy sources. If
we compare
each categorythe setting
that of the
utilizes thedifferent
residential buildings
types (16 urban/30
of renewable rural) and
energy sources. thecompare
If we use of
renewable energy sources, it is clear that in particular, photovoltaics and biomass
the setting of the residential buildings (16 urban/30 rural) and the use of renewable energy are more
often favoured in a rural setting with 33% of rural residential buildings having
sources, it is clear that in particular, photovoltaics and biomass are more often favoured ininstalled
PV and setting
a rural 47% usingwithbiomass, compared
33% of rural to 19%buildings
residential of urbanhaving
residential buildings
installed with
PV and 47%PVusing
and
31% with biomass.
biomass, compared to 19% of urban residential buildings with PV and 31% with biomass.

Distributionof
Figure5.5.Distribution
Figure ofrenewable
renewableenergy
energysystems
systemsin
inurban
urbanand
andrural
ruralbuildings.
buildings.

Using biomass (e.g., pellet boilers) as a primary heating source is a solution typically
adopted in areas where there is no possibility for connecting to e.g., district heating, gas or
similar. As secondary heating (e.g., using wood stoves), biomass is used in living rooms
where people spend most of the time. Some modern (low energy) homes also have wood
stoves, but then it is usually for the sake of comfort or creating a certain atmosphere in the
house. Twenty-five of the analysed cases utilise biomass as a renewable energy source.

4. Evaluation of Outcomes
The last part in the documentation of case studies foresaw an evaluation section
subdivided in four parts: Energy efficiency, with a summary of the building’s post-retrofit
energy performance; Internal climate, with considerations on the effect on users’ comfort,
users’ energy behaviour and artefact conservation; Costs, with detailed information of the
financial aspects of the retrofit; Environment, as an overview of the environmental aspects
of the intervention.
Of the 69 cases studies included in this paper, 51 (74% of cases) have an Energy Perfor-
mance Certificate (EPC) [40], 16 have some sort of voluntary certification (23%) (such as
LEED, BREEAM, etc. [41,42]), mostly regional or national standards. EPCs methodologies,
as well as the thresholds between classes, change between countries, and a consequent
comparison based on the energy classes could be misleading. Information about energy use
was gathered to differentiate between energy demand for heating (plus Domestic Hot Water
if relevant) and total primary energy. It is important to highlight that just 14 cases were fully
documented, but also that no information about energy was reported in 4 cases. In general,
the information varied notably between cases, making the comparison across the sample
challenging. The average heating energy demand was around 215 kWh/m2 y (Standard
Deviation 109) before the intervention and 68 kWh/m2 y (SD 68) afterwards. Although
these data should be treated carefully, since it includes different methodologies, climatic
conditions, and very different building typologies, a first analysis indicates an average
total energy reduction of around 70%. In addition to this information, the calculation
method used for the energy performance assessment was also documented. Of the 35 cases
documented, the majority (21) used steady-state simulations (e.g., PHPP [43] or national
tools), while only two cases use dynamic simulation. In two cases, the energy use was
measured on site, whereas in one case, it was derived from energy bills.
The results of energy use for space heating before and after the retrofits (Figure 6)
show a significant reduction (around 70%), but also a much more concentrated distribution
of results.
 (e.g., PHPP [43] or national tools), while only two cases use dynamic simulation. In two
cases, the energy use was measured on site, whereas in one case, it was derived from
energy bills.
The results of energy use for space heating before and after the retrofits (Figure 6)
Heritage 2024, 7 show a significant reduction (around 70%), but also a much more concentrated 1005
distribution of results.

Energy use
use for
for space
space heating
heating in
in kWh/m 2 before and after retrofit (n nafter
Figure 6.
Figure 6. Energy kWh/m2yybefore and after retrofit (nbefore = 26,
before= 26, nafter = 62).
= 62).
Heritage 2024, 7, FOR PEER REVIEW 11
Since these results are heavily influenced by the climatic conditions, a detailed analysis
Heritage 2024, 7, FOR PEER REVIEW Since these results are heavily influenced by the climatic conditions, a detailed 11
of a subsample with homogeneous climatic conditions was performed (Figure 7). Koppen’s
analysis of a subsample with homogeneous climatic conditions was performed (Figure 7).
Cfb (Temperate oceanic) climate [44] is by far the most representative (38 out of 69). The
sample, with
Koppen’s a great majority
Cfb (Temperate of cases
oceanic) above
climate [44] is100
by kWh/m 2y before the
far the most renovation
representative and
(38 out of
distribution of results in the2 Cfb climate is almost identical to that of the entire sample, with
between
69). The
sample, 25 and 75
distribution
with a greatkWh/m
of y
majorityafterwards.
results in the Cfb climate is almost identical to that of the entire
aboveof 100cases abovey before
100 kWh/m y before the
and renovation
between 25and
2 2
a great majority of cases kWh/m the renovation and
between 25 and
2 75 kWh/m
75 kWh/m y afterwards. 2y afterwards.

Figure 7. Energy use for space heating in kWh/m2y before and after retrofit in climatic zone Cfb.
Energyuse
Figure7.7.Energy usefor
forspace
spaceheating
heatingin
inkWh/m
kWh/m 2 before and after retrofit in climatic zone Cfb.
2y y
Figure before and after retrofit in climatic zone Cfb.
The plots in Figure 8 show the energy savings achieved (in %) as a function of the
building
The construction
Theplots
plotsininFigure period,
Figure 88show Heating
show Degree
theenergy
the energy Daysachieved
savings
savings (HDD), and
achieved (in%)
(in Net
%)asFloor
as Area (NFA),
aafunction
function ofthe
of the
building
respectively.
building construction
In general,
construction period, Heating
the correlation
period, Degree
betweenDays
Heating Degree Days (HDD),
the energy
(HDD), and
savings
and NetNet Floor
andFloor
all threeArea
Area (NFA),
variables
(NFA),
respectively.
studied is In general,
weak. the correlation
Surprisingly, there between
is a the energy
positive savingsbetween
relationship and all three variables
savings and
respectively. In general, the correlation between the energy savings and all three variables
studied isconstruction
building weak. Surprisingly,
periods, there is a positive
suggesting that relationship
more modern between savings
buildings andmore
profit building
(in
studied is weak. Surprisingly, there is a positive relationship between savings and
construction
relative terms)periods,
from suggesting
the energy that more
retrofits. modern
When it buildings
comes to HDD profit
and more
NFA, (in
therelative terms)
relationship
building construction periods, suggesting that more modern buildings profit more (in
from
is the energy
slightly negative retrofits. Whenrespectively,
it comes to HDD and NFA, relationship is slightly
relative terms) from and positive,
the energy retrofits. When itsuggesting
comes to HDDthat the
andcolder
NFA, thetherelationship
climate and
negative
the and positive,
biggernegative
the building, respectively,
the higher suggesting
is the energy that the colder
saving achieved the climate and the bigger
is slightly and positive, respectively, suggesting that theincolder
relation
thetoclimate
the energy
and
the building,
demand before the higher
the is the energy saving achieved in relation to the energy demand
intervention.
the bigger the building, the higher is the energy saving achieved in relation to the energy
before the intervention.
demand before the intervention.

(a) (b) (c)

(a) Figure 8. Energy
Energysavings
savings(in (b)in terms of space heating demand (n = 25)
(in%)%) (c) as a function of (a)
Figure 8. in terms of space heating demand (n = 25) as a function of (a) building
building age (R = 0.05), (b) Heating Degree Days
2 (R = 0.02), and (c) Net Floor Area
2 (R2 = 0.01).
age (R28.= Energy
Figure 0.05), (b)savings
Heating(inDegree
%) in Days 2 = 0.02),
terms(Rof and (c) Net
space heating Floor(n
demand Area (R2as= 0.01).
= 25) a function of (a)
building age (R2 = 0.05), (b) Heating Degree Days (R2 = 0.02), and (c) Net Floor Area (R2 = 0.01).
The energy use (in kWh/m2y) and savings (in %) was also studied according to the
use of
Thetheenergy
building
use(Figure 9) and
(in kWh/m 2y)allowing for a comparison
and savings of subsets
(in %) was also even
studied with different
according to the
sample sizes. Most of the cases lie within 25 and 75 kWh/m 2y and savings above 60%,
use of the building (Figure 9) and allowing for a comparison of subsets even with different
although
sample withMost
sizes. someof small
the differences between
cases lie within the uses.
25 and In urban
75 kWh/m residential
2y and savingsbuilding, the
above 60%,
initial energy demand was the lowest, whereas the energy savings achieved
although with some small differences between the uses. In urban residential building, the lie mostly
Heritage 2024, 7 1006

The energy use (in kWh/m2 y) and savings (in %) was also studied according to
the use of the building (Figure 9) and allowing for a comparison of subsets even with
different sample sizes. Most of the cases lie within 25 and 75 kWh/m2 y and savings
above 60%, although with some small differences between the uses. In urban residential
building, the initial energy demand was the lowest, whereas the energy savings achieved
Heritage 2024, 7, FOR PEER REVIEWlie mostly above 80%. Rural residential buildings and non-residential building have a more
12
comparable behaviour, with most cases distributed evenly between 25 and 75 kWh/m y 2

and an energy demand reduction of 60–80%.

(a) (b) (c)

(d) (e) (f)

Figure 9.
Figure 9. Energy
Energy use
use for
for space
space heating
heating(in
(inkWh/m
kWh/m22y)
y) (a–c)
(a–c) and
and energy
energy savings
savings (in
(in %)
%) (d–f)
(d–f) according
according
to building use (residential rural, residential urban, and non-residential).
to building use (residential rural, residential urban, and non-residential).

To understand
To understandthe thelimitations
limitationsimposed
imposedbybythe thelegal
legal status
status of of
thethe building
building (in(in terms
terms of
of heritage
heritage protection),
protection), thethe same
same analysis
analysis wasperformed
was performedby bylooking
lookingatatlisted
listedand
and unlisted
unlisted
buildings (Figure
buildings (Figure 10).
10). Energy
Energy use
use after
after the retrofit
retrofit is slightly
slightly higher
higher in listed buildings (top
left). In contrast, the energy savings histograms show verydifferent
left). In contrast, the energy savings histograms show very differentprofiles.
profiles.Whereas
Whereas in
thethe
in case of unlisted
case building,
of unlisted the the
building, cases are are
cases spread
spreadacross the different
across ranges,
the different in the
ranges, incase
the
of listed
case buildings,
of listed most most
buildings, of theof
cases
the are concentrated
cases in the range
are concentrated in thebetween 60–80%60–80%
range between energy
reduction.
energy reduction.

(a) (b)
 of heritage protection), the same analysis was performed by looking at listed and unlisted
buildings (Figure 10). Energy use after the retrofit is slightly higher in listed buildings (top
left). In contrast, the energy savings histograms show very different profiles. Whereas in
the case of unlisted building, the cases are spread across the different ranges, in the case
Heritage 2024, 7 of listed buildings, most of the cases are concentrated in the range between 60–80% energy
1007
reduction.

Heritage 2024, 7, FOR PEER REVIEW 13

Heritage 2024, 7, FOR PEER REVIEW 13

(a) (b)

(c) (d)
(c) (d)
Figure 10. (a,b) Energy use for space heating in kWh/m22y2 and (c,d) energy savings in % for listed
Figure
Figure 10. (a,b)
(a,b) Energy use for space heating in kWh/m
kWh/m yyand
and(c,d)
(c,d)energy
energy savings
savings in
in %
% for listed
and not listed buildings.
and not listed buildings.

To
To understand
understand the the relationship
relationship between
between energy
energy consumption
consumption and and other
other parameters
parameters
of the building or site (building(building construction period, Heating Degree Days, and Net
construction period, Heating Degree Days, and Floor
of the building or site (building Net Floor
Area), the energy use in kWh/m 2 y is again
againplotted
plottedas asaaafunction
functionofofthese
thesethree
threeparameters
Area), the energy use in kWh/m22yyisisagain
in kWh/m plotted as function of these three parameters
parameters
in Figure 11. It is
Figure 11. ItIt is important
isimportant
important to to notice
tonotice how
notice how
howinin all
inall three
allthree plots,
threeplots, a similar
plots,aasimilar trend
similartrend can
canbe
trendcan beobserved.
observed.
in Figure be observed.
The dependency
dependencyof all three variables decreases noticeably after after
the retrofit. That is,Thatbefore
The dependency ofofallall three
three variables
variables decreases
decreases noticeably
noticeably the retrofit.
after the retrofit. That is,
is, before
the energy
before the intervention,
energy the
intervention, energy
the demand
energy demand for space
for heating
space heating (per m
(per m
2) 2 is inversely
) is inversely
the energy intervention, the energy demand for space heating (per m ) is inversely 2
proportional
proportionalto tothe
thebuilding
buildingage ageandandnetnetfloor
floorarea,
area,while
whilethethecorrelation
correlationbetween
betweenenergy energy
proportional to the building age and net floor area, while the correlation between energy
use
use and
and HDD
HDD is positive.
is positive. After
Afterthe
theretrofit,
retrofit,the
thesignsign
is is maintained
maintained (negative
(negative for for building
building age
use and HDD is positive. After the retrofit, the sign is maintained (negative for building
age and
and and area,
area, area, and
and positive positive for
for HDD), HDD), but
but the but the
dependency dependency is
is much weakermuch weaker and the
age and positive for HDD), the dependency is muchand the variability
weaker and the
variability
between casesbetween cases significantly
significantly lower. lower.
variability between cases significantly lower.

(a) (b) (c)

(a) (b) (c)
Figure
Figure 11.
11. Energy
Energyuse use(in
(in kWh/m
kWh/m2y)
2 2 before and after retrofit as a function of (a) building’s age
Figure 11. Energy use (in kWh/m y)y)before
beforeand
andafter
afterretrofit
retrofitas
as aa function
function of
of (a)
(a) building’s
building’s age
age
(R222before = 0.11,
before == 0.11, R
0.11, R
2after = 0.03), (b) Heating Degree Days (R2before = 0.05, R2after = 0.01), and (c) Net
2
R2after
after==0.03),
0.03),(b)
(b)Heating Degree Days (R22before == 0.05,
Heating Degree 0.05, R22after
after ==0.01),
0.01), and
and (c)
(c) Net
Net
(R before
Floor Area (R222before = 0.16, R22after = 0.04).
Floor Area
Area (R before 0.16,RR2after
before ==0.16, after==0.04).
0.04).

The effect of different parameters of the intervention on the energy use after the ret-
The effect of different parameters of the intervention on the energy use after the
rofit was studied in more detail (Figure 12). In general, no great differences were observed
retrofit was studied in more detail (Figure 12). In general, no great differences were
when looking at the effect of a single variable, perhaps with the exception of MVHR.
observed when looking at the effect of a single variable, perhaps with the exception of
Buildings without heat recovery present a much higher variability and overall higher en-
MVHR. Buildings without heat recovery present a much higher variability and overall
ergy use than those with MVHR. Surprisingly, when looking at the effect of wall insula-
higher energy use than those with MVHR. Surprisingly, when looking at the effect of wall
tion, the cases with no intervention present better results. However, it is worth noticing
 Heritage 2024, 7 1008

The effect of different parameters of the intervention on the energy use after the retrofit
was studied in more detail (Figure 12). In general, no great differences were observed when
looking at the effect of a single variable, perhaps with the exception of MVHR. Buildings
without heat recovery present a much higher variability and overall higher energy use than
those with MVHR. Surprisingly, when looking at the effect of wall insulation, the cases
with no intervention present better results. However, it is worth noticing than only six
Heritage 2024, 7, FOR PEER REVIEW 14
cases with no intervention in the external walls are included in the sample; therefore, the
significance of the results might be limited.

(a) (b) (c)

Figure
Figure12.
12.Energy
Energyuse
use(in
(inkWh/m
kWh/my)2 y)
2 after retrofit
after according
retrofit accordingtotoimplementation
implementationofof(a)
(a)wall
wallinsulation
insulation
and (b) MVHR (sample size from left to right n= 6, 56, 30, 32) and (c) total primary energy use (in
and (b)2 MVHR (sample size from left to right n= 6, 56, 30, 32) and (c) total primary energy use (in
kWh/m y)2 in cases with and without PV systems (n= 34, 12).
kWh/m y) in cases with and without PV systems (n= 34, 12).

Lastly,
Lastly,the theeffect
effectofofrenewable
renewableenergy energysystemssystemsisispresented
presentedininFigure Figure12c.12c.InInthis
thiscase,
case,
and
and to take into consideration the contribution to the total energy performanceofofthe
to take into consideration the contribution to the total energy performance the
building,
building,the theprimary
primaryenergyenergyuse useof ofthethe case
case studies
studies after
after the the retrofit
retrofit (in
(in kWh/m
kWh/my)
2 2 is
y) is
studied
studiedinincases
caseswith
withandandwithout
withoutSolar Solarthermal
thermal(left)(left)and
andPV PVsystems
systems(right).
(right).TheThepositive
positive
effect
effectofofsolar
solarenergy
energyisisevident
evidentininthe thecase
caseofofPV, PV,where
wherethe thetotal
totalprimary
primaryenergy energyuse useisis
reduced
reducedfrom from113.6
113.6toto56.1
56.1kWh/m
kWh/m 2y.
2 y.
The
Thesection
sectiondedicated
dedicatedtotothe theinternal
internalclimate
climategathered
gatheredthe theinsights
insightsofofoccupants
occupantsand and
users
users of the buildings documented. That is, the information gathered in this sectionisis
of the buildings documented. That is, the information gathered in this section
mostly
mostlyqualitative
qualitative data, some some sort sortofofstructured
structuredPost-Occupancy
Post-Occupancy Evaluation
Evaluation (POE)
(POE) [45][45]
was
was conducted
conducted in only
in only sevenseven cases.
cases.
Occupants’
Occupants’opinionopinionon onthethethermal
thermalcomfort
comfortwas wasdocumented
documentedinin4444(64%) (64%)ofofthe thecases.
cases.
This
Thisisisaatopic
topicthat
thatisisclearly
clearlycomprehensible
comprehensibleand andappealing
appealingtotothe theoccupants
occupantsofofa abuilding;
building;
therefore,
therefore,gathering
gathering information
information was relativelyrelatively easy easy when
whencompared
comparedtotoother othercategories.
categories.In
Ingeneral,
general, the perception
the perception of thermal comfort comfort
of thermal after the intervention improved considerably
after the intervention improved
considerably when compared to the “before” situation. Similarly, improvement ofAir
when compared to the “before” situation. Similarly, improvement of Indoor Qual-
Indoor
ityQuality
Air (IAQ) and (IAQ) access to natural
and access light was
to natural lightdocumented
was documented in 41in(59%) andand
41 (59%) 34 (49%)
34 (49%)of theof
cases,
the respectively.
cases, respectively. IAQ IAQ is an easily
is an easilycomprehensible
comprehensible and andappealing
appealing topic
topicto users,
to users,and andthe
collection
the collection of of
information
informationwas wasrelatively
relativelysuccessful,
successful,whereas
whereas the information
information on onlight
lightisis
rathersuperficial.
rather superficial.
The evaluation
The evaluation of the theacoustic
acousticperformance
performance of of
thetheintervention
interventionproved to beto
proved signif-
be
icantly more difficult to report, and only 38% of the documented
significantly more difficult to report, and only 38% of the documented projects include projects include some
information
some information about it; it it;
about remains
it remainsunclear the reasons
unclear the reasons for the lacklack
for the of consideration
of consideration of thisof
topic
this during
topic during andand
afterafter
the the
renovation.
renovation. In line withwith
In line acoustic
acousticcomfort, onlyonly
comfort, 15% 15%of theofcases
the
reported
cases information
reported informationthat considered
that considered the impact
the impactof theof renovation on theon
the renovation artefact conser-
the artefact
vation. Arguably, this can be due to the fact that most of
conservation. Arguably, this can be due to the fact that most of the documented buildings the documented buildings are
residential and do not have any historically significant artefacts
are residential and do not have any historically significant artefacts that need especial that need especial attention.
For the sake of comparison, all financial information is reported in Euros and a fixed
attention.
conversion
For the sakerate of
was used (as for
comparison, all18 May 2021).
financial Forty-eight
information (69.5%)inof
is reported the documented
Euros and a fixed
conversion rate was used (as for 18 May 2021). Forty-eight (69.5%) of the the
cases included some considerations about the financial aspects of the project; extent and
documented
depth
cases of this information
included varies greatly
some considerations aboutacross the cases.
the financial It would
aspects be project;
of the important thetoextent
know
whether the lack of response is due to difficulties in accessing
and depth of this information varies greatly across the cases. It would be important to the information or because
know whether the lack of response is due to difficulties in accessing the information or
because cost was not an issue considered explicitly during the retrofit. Only three case
studies (4%) have included a Life Cycle Cost (LCC) assessment of the intervention.
Information regarding the investment (total or energy related) costs was included in
33 cases (48%), but both considerations are included in only a third of them (11 cases). The
Heritage 2024, 7 1009

cost was not an issue considered explicitly during the retrofit. Only three case studies (4%)
have included a Life Cycle Cost (LCC) assessment of the intervention.
Information regarding the investment (total or energy related) costs was included
in 33 cases (48%), but both considerations are included in only a third of them (11 cases).
The average investment of the interventions sums up to 3350 €/m2 , whereas the purely
energy-related costs are less than 600 €/m2 . That would represent that the energy efficiency
improvement of a building represents only the 18% of the renovation cost. However, these
results are to be considered carefully and due to the great variability between countries, a
larger sample would be needed for a further analysis of the investment costs. Unfortunately,
the information about the running costs of the retrofitted buildings is even more scarce; the
information was reported in only 17 (25%) of the cases. The average total running cost for
the retrofitted project was 10,876 € (11.6 €/m2 y). When looking at different energy bills
separately, it was observed that the average yearly cost of heating was 4022 € (5.0 €/m2 y),
whereas the electricity bills account for 9571 € (4.5 €/m2 y) per year on average.
The last section was dedicated to the environmental performance of the building.
Unfortunately, the cases documented so far did not include a great deal of information;
therefore, the analysis of the results mainly focuses on whether the different sections had
been compiled or not. Greenhouse gas emissions have been calculated in 14 of the cases
(20%), and as part of the EPCs in many cases. Some sort of life cycle assessment (LCA) has
been conducted in four (less than 5%) of the cases included in this study. Considerations
about the water management were included in just eight cases (11.6% of the projects) and
considerations about transport and mobility were included in just five cases (7%).

5. The Planning Process: Lessons Learned

The results gathered are presented below in relation to the planning process, the
evaluation of renovation performance, and ultimately the framework conditions that
favour or hinder energy renovations.
The case studies documented in this research provide a series of examples that illus-
trate the most crucial aspects of the overall planning process. Complex projects require
an ambitious planning team and an open-minded client, as well as a continuous process
of coordination between planners and heritage authorities. Informal cooperation with
heritage authorities in the early stage pays back later in the project and ensures a close coop-
eration within the planning team and with the users is essential for the success of complex
projects. Planning authorities that decline initial standard solutions but engage in an early
and iterative dialogue leads to a process where innovative solutions can be developed.
The building owner is a driving force in renovation projects; without their persistence
it is not possible to realise them. Creating a positive mindset among all involved project
partners early in the project can be a success factor when solving complex problems that
require cooperation. Ideally, future facility managers of the building would also be involved
during the planning phase. Private owners are often reluctant to test new methods, but
acceptable solutions can be found through dialogue with the owner, planning team, and
heritage authorities. Costs for innovative materials should be calculated early in the project
to avoid unnecessary planning efforts, as it can take time and effort to make new and
innovative building components to work in practice.
Looking back, “ex-post” [46] helps future implementations both by re-assuring that
targets were reached with qualitative and quantitative assessments and pointing out
possible weak points. However, the number of projects in this study that included detailed
information about the evaluation of the intervention is limited. Of the four sections that
structure the “Evaluation” chapter, energy efficiency is by far the best documented part.
This is most likely due to policies implemented at a regional or national level that made
these calculations compulsory. On the other hand, the on-site monitoring of buildings’
performance is still very rare and seems to be limited mostly to research related projects.
Although financial aspects are often highlighted as both triggers (reducing running
costs) and limitations (excessive investment costs) for carrying out an energy renovation,
Heritage 2024, 7 1010

the projects documented so far have included very limited information. An effort should
be made in documenting this because it could play a crucial role in fostering renovation,
especially among private owners.
In many of the examples, we can observe the interaction and overlapping of several
favourable framework conditions that made the project happen. In addition to the classic
financial incentive models (tax reductions, subsidies, soft loans), there are also interesting
alternative approaches such as sponsoring by private companies and private investors,
which have made it possible in one case to transform a former industrial site into a multi-
functional area. The public sector can play an important role as an owner when it comes to
exemplary redevelopment and the development of innovative projects that are also open to
the public.
To ensure long-term economic viability, a building change of use can sometimes make
an important contribution to ensure its preservation. For instance, a change of ownership
(sometimes generational) is a key moment in the life-cycle of Alpine farmhouses.

6. Conclusions
From the analysis of the case studies presented here, a number of challenges that might
emerge when renovating historic buildings have been identified: lack of detailed heritage
evaluations; restrictions on alterations due to the official status as heritage building; need
for customized technical solutions; difficult coordination with public authorities; physical
limitations of the existing building and associated technical risks; higher investment costs;
occupants’ concerns; knowledge gaps (from lack of skills to lack of monitored data).
However, it is worth noticing the high reduction in energy use (up to 70%) achieved
in these renovations, independently of the type of use or level of protection of the building.
A detailed analysis has showed that the energy demand in historic buildings is much
more dependent on factors like climate, building age, typology, or size before than after
the retrofit. That is, retrofitted historic buildings can achieve a good energy performance
independently of the starting conditions. Additionally, ventilation with heat recovery
noticeably reduced variability in energy use compared to natural ventilation. Regarding
the documentation of the internal climate, comments from users on the achieved thermal
comfort are recorded in most cases and often underline the improvement.
Analysing good examples helps reveal not only challenges but also factors that can
promote and foster the renovation of historic buildings that can be drawn. Firstly, demon-
strating the feasibility of achieving major energy savings while respecting heritage can
increase confidence among owners, planners, and authorities. It is crucial to highlight the
importance of early collaboration between planners and authorities in these cases because it
streamlines the process and prevents conflicts. While quantifying investments and payback
periods would incentivise private and public owners, describing non-energy benefits (e.g.,
occupant feedback) emphasizes added value beyond efficiency. There is still the need to
exploit and scale-up the results from front-runners. Transferring insights from innova-
tive policies like research funding and public-private partnerships (by means of capacity
building through examples like HiBERatlas) can enable replication, and with that, faster
achievement of economies of scale. Celebrating successful projects will inspire others, and
eventually, understanding key decision points will help target successful policy-making.
However, the effort needed to develop and populate such a database is worth noting. This
included the definition of selection criteria and minimum information agreed among the
large number of experts involved in the research projects, the development and balancing
of an online tool that includes numerous information and elaborated features while trying
the maintain a simple interface for users to document new cases autonomously, or the
complex and time-consuming process of collecting all necessary consents for processing
intellectual property and information privacy.
Heritage 2024, 7 1011

Further Development of HiBERatlas and the Promotion of Historic Building Renovation

Overall, the HiBERatlas has collected a considerable amount of case studies to serve as
inspiration for the renovation of historic buildings towards zero energy. The database will
continue growing to include more case studies, and to cover more building types, locations,
solutions, etc. As the database grows, new filters will facilitate finding relevant cases, and
eventually calculation tools based on the documented cases could provide estimations of
potential savings. Another important aspect is the increasing focus on life-cycle assessment
and CO2 -emissions, which is largely missing in the database. In the future, this focus will
only increase, but there is still scant information about these calculations in most countries.
Denmark can be mentioned as an example, where there are now specific requirements for
CO2 equivalents for new buildings in the Building Regulations [47]. Within a few years,
these requirements will most likely also include renovations, and requirements for LCA
will be widespread in all countries. This must also be reflected in the database; therefore,
LCA should be included as a very relevant indicator.
The growth of the database can be strengthened through the collaboration with agen-
cies and associations that can integrate it in policy and training programs. The results
and lessons learned can be exploited to create tailored guidance for different stakeholders
(owners, planners, authorities). The HiBERatlas can also be a tool to promote successful
projects through awards and publications. The database has already proven to be a valuable
resource, but growing it into a comprehensive living resource will further foster the energy
retrofit of heritage buildings.

Author Contributions: Conceptualization, D.H.-A. and A.T.; methodology, D.H.-A., J.R., K.E.T., F.H.,
G.L., T.B. and A.T.; data curation, D.H.-A., J.R., K.E.T., F.H., G.L., T.B. and A.T.; writing—original
draft preparation, D.H.-A., J.R., K.E.T., F.H., G.L., T.B. and A.T.; writing—review and editing, D.H.-A.;
visualization, D.H.-A. and A.T.; All figures included have been created by the authors. All authors
have read and agreed to the published version of the manuscript.
Funding: Data elaboration was carried out within the PNRR research activities of the consortium
iNEST (Interconnected North-Est Innovation Ecosystem) funded by the European Union Next-
GenerationEU (Piano Nazionale di Ripresa e Resilienza (PNRR)—Missione 4 Componente 2, In-
vestimento 1.5—D.D. 1058 23/06/2022, ECS_00000043). This manuscript reflects only the authors’
views and opinions, neither the European Union nor the European Commission can be considered
responsible for them.
Data Availability Statement: Data analysed and presented in this study can be found in the online
database www.HiBERatlas.com (Historic Building Energy Retrofit atlas—accessed on 19 Decem-
ber 2023).
Acknowledgments: The authors would also like to acknowledge the IEA-SHC and EBC Executive
Committees for supporting the Task59/Annex76 and the financial support from the European
Regional Development Fund under the Interreg Alpine Space programme to the Project ATLAS (ID:
ASP644) and the Swedish National Agency under the E2B2 programme, also wish to thank all the
experts in the Task59/Annex76 for their valuable contributions.
Conflicts of Interest: The authors declare no conflicts of interest.

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