
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12924-6
10.1016/j.heliyon.2024.e36893
e36893
Research Article
Application of a cost-effective methodology for the refurbishment of DOCOMOMO buildings. A case study in Northern Spain
González-Martínez Purificación pgmarti@unav.es
a⁎
Monge-Barrio Aurora a
Domingo-Irigoyen Silvia b
Sánchez-Ostiz Gutiérrez Ana a
a School of Architecture, Universidad de Navarra, Campus Universitario, Pamplona, 31009, Navarra, Spain
b Lucerne University of Applied Sciences and Arts, Lucerne School of Engineering and Architecture, Werftestrasse 4. 6002, Luzern, Switzerland
⁎ Corresponding author. pgmarti@unav.es
24 8 2024
15 9 2024
24 8 2024
10 17 e3689316 8 2023
21 8 2024
23 8 2024
© 2024 The Authors. Published by Elsevier Ltd.
2024

https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
The energy rehabilitation of listed buildings guaranteeing heritage values but allowing a use that contributes to their conservation, supposes a challenge with the need of a holistic approach. Buildings of the Modern Movement, many of which are registered as DOCOMOMO (DOcumentation and COnservation of buildings, sites and neighbourhoods of the MOdern Movement), are a particular case since many of them are not yet listed or are under unclear requirements. This paper explores the inclusion of a cost-effective methodology as part of the decision-making in the energy rehabilitation of these DOCOMOMO buildings, applying it to the case study of an office building located in the north of Spain. Different scenarios were studied balancing cost-effectiveness, energy efficiency and rehabilitation requirements. In this case study, the analysis may allow policymakers to have supportive arguments to subsidize certain elements (as e.g., steel frames), or allow the use of alternative options with similar aesthetic characteristics, but at a much lower cost. This second option will constitute the unique cost-effective scenario, with energy savings of between 25.36 % and 38.8 %. The inclusion of a cost-effective methodology as part of the mechanics for decision-making in the energy refurbishment of DOCOMOMO buildings permits the optimisation of the intervention guaranteeing their use and the conservation of heritage values.
==== Body
pmcNomenclature

a Annuity for constant real prices

BEM Building Energy Model

EU European Union

GHG Greenhouse gases

i Interest rate (%)

λ Thermal conductivity coefficient (W/m K)

m General average adjustment factor

r Rate of yearly increase of energy prices (%)

RH Relative humidity (%)

t Time range of cost evaluation

Ta Temperature (°C)

U Thermal transmittance (W/m2 K)

1 Introduction

Buildings’ refurbishment has a great potential for energy-saving and the reduction of greenhouse gas emissions with the aim of reaching the EU objectives, outstanding among which is the decarbonisation of the European Union (EU) energy system in order to reach the 2030 climate targets and net-zero carbon neutral emissions by 2050 [[1], [2], [3], [4]]. To do so, the EU countries must establish a long-term strategy to facilitate the economically efficient transformation of existing buildings into nearly zero energy ones. The directive on the energy performance of buildings (EPBD) [5] has set out the minimum requirements for energy efficiency when the thermal envelope of existing buildings has changed or improved. However, historic, monumental, listed or protected buildings are exempt. In Spain, the Código Técnico de la Edificación (Spanish Technical Building Code) establishes that the following types of buildings are excluded in the area of application of the Documento Básico de Ahorro de Energía (Basic document: Energy saving): “officially protected historic buildings as part of a declared area or because of their particular architectural or historic value, inasmuch as fulfilling certain basic energy efficiency demands could unacceptably alter their character or appearance; this official protection will be determined by the authorities who decide on the unalterable elements” [6].

Lidelöw et al. indicate that the traditional theory on the restoration and conservation of historic buildings is open to debate [7]. In this regard, in the last few years it has been considered that the energy rehabilitation of historic buildings does not clash with their cultural values but can, on the other hand, be an instrument for their protection [8,9].

One way of protecting these buildings is making them useful, fulfilling the basic requisites for functionality, safety, accessibility and habitability which are compatible with the conservation of their heritage values [[10], [11], [12]], while also taking into account climate change scenarios [13]. Webb believes that the refurbishment of historic buildings must take into account environmental (energy-saving, reduction of GHG emissions, etc.), heritage conservation, indoor environment quality and economic criteria [14]. The holistic vision of cultural, economic, environmental and social sustainability, must be part of the interventions on historic buildings [15,16].

The enormous variety of existing buildings makes it difficult to apply one sole methodology for energy-efficient rehabilitation. This problem is even greater in the case of historic buildings which must be treated case-by-case [17]. Domingo et al. set out a methodology in four phases: identification of the historical features of the building, analysis of the envelope's performance, establishing it's diagnosis and a pre-assessment intervention proposals [18]. A historic building in the Aegean region of Turkey was studied with a building energy simulation tool to assess its thermal performance through the measurement of its indoor temperatures and, finally, several intervention scenarios were applied in order to make potential thermal improvements [11]. Franco et al. applied a four-step methodology: on site analysis of the case study, building's energy modelling and calculations, smart management of historic architecture through energy performance enhancement and analysis of efficient power generation solutions [19]. Ascione et al. use a cost-optimal methodology to show that the refurbishment of historic buildings under energy consumption targets is feasible from a technical and economic perspective [20].

Within historic buildings, those included in the DOCOMOMO register are a special case. The fact that buildings are registered as DOCOMOMO does not guarantee their preservation. DOCOMOMO International is a non-profit organization dedicated to the DOcumentation and COnservation of buildings, sites and neighbourhoods of the MOdern MOvement, initiated in 1988 [21]. Three of the aims of DOCOMOMO (Eindhoven-Seoul Statement 2014) are to promote the conservation and (re)use of buildings and sites of the Modern Movement, oppose destruction and disfigurement of significant works, and foster and disseminate the development of appropriate techniques and methods of conservation and adaptive (re)use [22]. As an example of the energy refurbishment of a DOCOMOMO building, Blazquez et al. present the retrofitting of a protected housing block in Seville, with the aim of improving the comfort of the users and the energy effectiveness of the building by means of passive measures [23]. Domingo-Irigoyen [24] applies the cost-effectiveness methodology to four DOCOMOMO residential buildings in Pamplona, concluding that the energy refurbishment of catalogued residential buildings is possible and cost effective without damaging their architectural value, and achieving an approximate 45 % reduction in energy demand.

This paper explores a methodology that contemplates a cost-effective analysis together with the conservation of historic heritage values, as a tool for holistic decision-making for the conservation of DOCOMOMO buildings. Unlike other studies [25], this work introduces the novelty of applying the cost-effectiveness methodology to DOCOMOMO buildings. On the other hand, the cost of electricity has increased in recent years [26], so its influence on the results of the cost-effective analysis is explored. The methodology is applied in a case study of an office building in Northern Spain (Andoain- Gipuzkoa).

The research questions posed are the following.- Can the refurbishment of a DOCOMOMO protected building be cost-effective if the original appearance of the building is maintained?

- Will the increase in energy prices allow the renovation of the thermal envelope of a DOCOMOMO building to be cost-effective?

2 Case study: Laborde Factory Pavilion (Gipuzkoa-Spain)

2.1 Historical and architectural features

The building which is the object of our study is included in “The architecture of the industry. Iberian DOCOMOMO Register” under the name Laborde Hermanos [27]. It was built in 1939 as a pavilion with canteens, showers and a library for the workers at the Laborde factory. From an architectural perspective, it has outstanding blocks of overhanging balconies with painted steel-tube railings, large-scale windows to achieve the natural light and hygiene demanded at the time of construction, and a pronounced contrast between the horizontality of the balconies and the verticality of the staircases, together with the pergolas and shading elements on the second-floor (Fig. 1).Fig. 1 Main facade of Laborde Factory Pavilion.

Fig. 1

In addition, the building is classified as a Cultural Interest Asset, in the category of Monument [28]. The protected elements are its volumetry and its spatial organisation; the general organisation of the structure and the building's envelope. In the envelope, the objects under protection are the exterior facades and the diverse elements which complete them, together with the elements that make up the roof as a fifth facade of the building including the eaves, the pergola, and the use of the roof as a terrace.

At present, the building is being used as rented office space. On the ground floor (Fig. 2a and d) there are four office premises with independent access from the outside. The first floor (Fig. 2b and d) is divided into three offices, which share access from the outside through a foyer. On the second floor (Fig. 2c and d) there are two offices, each of which can be accessed from the balcony. Currently, the only offices in use are Off. 01 and Off. 09 (Shaded in Fig. 2), due to the lack of thermal comfort, costs and other shortcomings, which have led the tenants to gradually abandon the building. It should be noted that this was the reason why the owner initiated the process of energy and functional refurbishment of the building, maintaining its architectural features.Fig. 2 Laborde Factory Pavilion: a. Ground floor; b. First floor; c. Second floor; d. Section.

Fig. 2

2.2 Construction features

In 2002, a partial restoration was carried out on the thermal envelope together with maintenance work. Among other work, the facades were insulated under the window sills on the inside, and the roofs and balcony of the second-floor were waterproofed and insulated. The window frames were dismantled for repair, and double glazing with an air cavity was put in place while maintaining the original steel framework. Table 1 gives a summary of the makeup of the various elements of the envelope. The data obtained from the inspection and the available graphic documentation were completed, where there was doubt, with a thermographic study, by inspecting the cavity of the facade by means of a borescope and with the data obtained from a heat flow measurement test.Table 1 Envelope's construction elements composition. Current state.

Table 1Envelope	Description (from the outside to the inside)	Thermal transmittance
U, W/m2K	
Facade
Second floor (F1)	Rendering of painted cement mortar + double hollow brick + polyurethane insulation (2 cm) + air cavity + double hollow brick + gypsum plaster. Total thickness 34 cm.	0.65	
Facade
First and ground floor (F2)	Rendering of painted cement mortar + double hollow brick + polyurethane insulation (2 cm) + air cavity + double hollow brick + gypsum plaster. Total thickness 39–40 cm.	0.64	
Roof
Second floor roof (R1)	Flat roof composed of slab + concrete slope layer + waterproof layer + extruded polystyrene insulation (3 cm) + geotextile sheet + gravel.	1.07	
Roof
First floor roof (R2)	Concrete slab + thermal insulation (2 cm) + waterproof layer + bond mortar + ceramic flooring.	1.45	
Ground floor (GF)	Concrete slab + cement screed + bond mortar + ceramic or wood flooring	2.6	
Windows(W)	Painted steel frame without thermal break and double glazing 3 + 3/10/6	Frame, 5.7
Glass, 2.8	
Exterior door (D)	Painted steel frame without thermal break	3.2	
Thermal bridge beam and column (TB)	Rendering of painted cement mortar + concrete + gypsum plaster	2.9	
Cantilever above ground floor (C)	Rendering of painted cement mortar + concrete + suspended ceiling	1.07	

2.3 Characteristics of the air-conditioning system

In each office, there are air-to-air heat pumps with a variable refrigeration volume (VRV), as well as external evaporators individually regulated per office. The external machines are on the south-east facade on the first-floor rooftop. The details of the heat pumps for each floor can be seen in Table 2.Table 2 Heat pumps per floor.

Table 2Floor	No. heat pumps	Brand. Code	Power (kW)	Power per square meter (W/m2)	
Ground floor	1	Carrier, code Alpine 120B	12.4 kW heating & 11.6 kW cooling	142 (cooling)
150 (heating)	
2	Carrier, code Alpine 90B	8,7 kW heating & 8.5 kW cooling	
2	Carrier, code Alpine 70B	7.8 kW heating &7.3 kW cooling	
First floor	3	Carrier, code Alpine 120B	12.4 kW heating & 11.6 kW cooling	140 (cooling)
149 (heating)	
2	Carrier, code Alpine 70B	7.8 kW heating & 7.3 kW cooling	
Second floor	2	Carrier, code Alpine 120B	12.4 kW heating & 11.6 kW cooling	203 (cooling)
217 (heating)	

The present system using heat pumps is not enough to achieve thermal comfort due to the inadequate distribution of the air and its high discharge speed. In addition, the premises in use have extra electric radiators to ward off the winter cold: four on Office 01 and one on Office 09.

2.4 Climate analysis

The building is situated in Andoain, 15 km from San Sebastian, in the C1 climate zone according to the Spanish Technical Building Code [6]. In accordance with the Köppen Geiger classification, it has a Cfb climate: an oceanic climate, temperate and humid with mild summers. Fig. 3 gives the climate parameters of the San Sebastian weather station provided by the Spanish Agency of Methodology, for the period 1980–2010 [29].Fig. 3 Climate parameters of the San Sebastian weather station for climate series 1980–2010.

Fig. 3

3 Methodology

To act on a DOCOMOMO building, a five-step methodology is used: diagnosis of the current state of the building [30], construction of an adjusted building energy model (BEM), definition of refurbishment scenarios and simulation, cost-effective analysis and analysis of results and decision-making. In addition, the influence in recent years of the increase of electricity costs on the results of the cost-effective analysis is explored [31].

3.1 Current state diagnosis

In view of the damage seen in the building, the probable causes and repair alternatives may be decisive when it comes to defining the optimum intervention scenario [32]. The guidelines for improving the energy performance of historic buildings of the European Committee of Standardization [33] indicate that the state of the building envelope must be assessed before considering an upgrade of its energy performance, carrying out necessary repairs before or simultaneous with the improvement of the energy efficiency.

The assessment of the current thermal performance of the building is necessary in order to create a simulation model conforming to it [[34], [35], [36]]. Thus, different studies are carried out: thermographic study [30], inspections by borescope and heat flow tests [37]; a blower door test [38], and measurement of temperature and humidity in different spaces [39]. Finally, surveys were carried out with the building users on the suitability of the temperatures in their workplaces. Ascione et al. [31] consider that interviews of managers and/or occupants can complete the necessary information for the energy audit of buildings.

3.2 Building energy model (BEM)

In this research, the target of the energy simulation is to detect the current energy performance [40] and assess the reduction of demand and energy consumption obtained by means of different refurbishment strategies of the thermal envelope. Various authors [23,[41], [42], [43]] have carried out adjusted energy simulations on historic buildings based on the data obtained from the audit.

The BEM was set up with Energy Plus and DesignBuilder software. The BEM was made based on the existing documentation and the data obtained from current state diagnosis. On Fig. 4 the numerical model of the building is visible. The adjustment was carried out firstly considering temperature data of the unoccupied areas in which there were no internal loads and the temperature was in free oscillation (Off. 03, Off. 06 & Off. 07 located in different floors and orientations, see Fig. 2). Secondly, the model was adjusted on premises in use (Off. 01 & Off. 09, see Fig. 2) and the energy consumption from monthly energy bills. Table 3 shows a summary of available data for the adjustment of the model.Fig. 4 Numerical model of the building.

Fig. 4

Table 3 Conditions of premises in use. Adjusted model.

Table 3Conditions of use and loads	Parameter	Source	
Ventilation	45 m3/person*h	Considered to guarantee the air quality in IDA 2 offices (RITE [44])	
Infiltrations	0.45 r/h	Blower door test	
Shading	Open	Considered open as in winter period	
Internal loads	
 - Lighting	12.7 W/m2	Inspection	
 - Equipment	4.5 W/m2	Inspection	
 - People	6 people. Off. 01
3 people. Off. 09.	Questionnaire	
Schedule and setpoint of	
 - Office 1 (heating)	22.5° - 09:00 to 18:00	Monitoring (Dec 2nd, 2016 to Feb 20th, 2017, 10-min data)	
 - Office 9 (heating)	23.5 °C - 10:00 to 17:00	Monitoring (Dec 2nd, 2016 to Feb 20th, 2017, 10-min data)	
Energy	Electrical consumption (Monthly kWh)	Bills of one year (2016)	
Weather	Ta, H, irradiation	Weather station Zizurkil, https://www.euskalmet.euskadi.eus/observacion/datos-de-estaciones/)	

According to the results of the energy simulation of the previous adjusted model, the thermal losses of the two rooms in use and the other premises through indoor floors, ceilings and partitions, suppose 16–51 % and through the facade and the glazing supposes 41–58 %. (Including infiltrations).

Once the adjusted BEM has been modelled, the building is simulated as if being fully occupied. The conditions of use and loads are shown in Table 4. The Energy Plus climate file for the city of San Sebastián located 15 km from the building site, with 30-year data, has been used (https://energyplus.net/weather/sources).Table 4 Conditions for the refurbished building model.

Table 4Conditions of use and loads	Parameter	
Ventilation	45 m3/person*h	
Infiltrations	0.45 r/h	
Solar protection	Automatic activation with radiation over 200W/m2	
Internal loads.	
 - People	1 person/m2	
 - Lighting	12.7 W/m2	
 - Equipment	4.5 W/m2	
Timetable climate control use and setting	
 - Winter (heating set-point)	21 °C	
 - Summer (air conditioning set-point)	25 °C	
 - Schedule	09:00 a 18:00	
Climate control system	
Existing system	COP: 2	EER: 1.8	

The results of the building simulation in its current state show that thermal losses through the facade, thermal bridges, windows and infiltrations represent 54 %, through the slabs on the ground floor 13 % and through the roof 11 %.

3.3 Energy retrofit scenarios

In order to decide on the optimum energy retrofit and considering the need to preserve the outer appearance of the building, several intervention scenarios were contemplated [45] with individual measures which were combined to be assessed jointly [33]. An energy simulation of the scenarios was made [46].

The information obtained in the simulation of the current state of the building, the need to replace the windows detected in the diagnosis of the current state of the building and the maintenance of the heritage values, have made it possible to design the different intervention scenarios.

Table 5 shows the different retrofitting scenarios. The scenario of reference is the current starting point and permits comparison of the energy savings in the different scenarios. It includes the necessary measures to restore the functionality of the building with its current characteristics, without improving its energy efficiency. This case contemplates replacing the existing windows and doors with new ones, with no improvement from the thermal perspective, together with the repairs necessary to maintain the functionality of the facade.Table 5 Table summarising intervention scenarios.

Table 5	Intervention	
Reference Scenario	Intervention to guarantee the functional conditions. Window replacement.
Windows: Uframe: 5.7 W/m2K Uglass: 2.8 W/m2K; g: 0.75; Udoor: 3.2 W/m2K.	
Scenario 1 (S1)	Window replacement (W) (1)
Uframe: 3.2 W/m2K. Uglass: 1.1 W/m2K g: 0.58	
Scenario 2 (S2)	Installation of thermal insulation on facade (F1 & F2; TB), interior.- Cavity filling below windows and opaque sections of facade (6 cm). U facade: 0.54–0.32 W/m2K, respectively.

- Direct cladding of laminated plasterboard with 4 cm of mineral wool above windows (TB) and on columns. U: 0.64 W/m2K

	
Scenario 3 (S3)	Installation of thermal insulation on facade (F1 & F2; C), interior and exterior.- Scenario 2

- ETICS (4 cm EPS). U facade: 0.33–0.23 W/m2K, respectively

- Installation of 4 cm EPS in false ceiling below cantilever of first floor (C). U cantilever: 0.30 W/m2K

	
Scenario 4 (S4)	Installation of insulation under roofs: in suspended ceilings and under slabs (R1, R2)- Installation of 8 cm mineral wool under roofs, resting on the false ceiling. Second floor ceiling: U = 0.24 W/m2K, first floor: U = 0.34 W/m2K.

- Insulation of beams on the first floor and second floor with 2 cm of insulating mortar (λ = 0.042W/mK).

	
Scenario 5 (S5)	Installation of insulation on ground floor slabs (GF) (4 cm XPS).
U: 0.55 W/m2K	
Scenario 6 (S6)	Scenario 1 + Scenario 2 (2)	
Scenario 7 (S7)	Scenario 1 + Scenario 3 (1)	
Scenario 8 (S8)	Scenario 1 + Scenario 2 + Scenario 4 (1)	
Scenario 9 (S9)	Scenario 1 + Scenario 3 + Scenario 4 (1)	
Scenario 10 (S10)	Scenario 1 + Scenario 2 + Scenario 4 + Scenario 5 (1)	
Scenario 11 (S11)	Scenario 1 + Scenario 3 + Scenario 4 + Scenario 5 (1)	
(1) The airtightness of the building will be improved by 50 %.	

3.4 Cost-effective analysis

The cost-effective analysis was carried out, following the procedure as developed in Annex 56 of the International Energy Agency, “Methodology for Cost-Effective Energy and Carbon Emissions Optimisation in Building Renovation”. This methodology obtains the optimum-cost scenario (the renovation scenario that allows for the lowest global costs considering investment costs, maintenance and running costs during the lifetime of the renovated building) and the cost-effective solution (solutions going beyond the cost-optimal level which allow the achievement of more ambitious energy and carbon emission targets, with global costs higher than those related to the cost-optimal level but below the costs associated to the reference scenario) [47,48].

The steps are the following.1. Establish the work items of different intervention scenarios in order to obtain the contracting costs.

2. Measurement of the building envelope's surface: facades, roofs, windows and floors in contact with the ground.

3. Cost of works for the different scenarios, estimated through construction costs databases or real cost estimates.

4. Calculation of global annualized costs for each scenario.

5. Obtention of cost-effective graphs in which all intervention scenarios are represented. The abscissa represents the consumption of each scenario in kWh/m2y, and the ordinate shows the annualized global costs in €/m2y.

For the calculation of the global costs of the scenarios, the annuity method has been used [49,50]. This method transforms investment cost into average annualized costs, obtaining constant annual cost during the lifespan considered. The annualized cost includes the investment costs of the retrofit measures and their maintenance, the investment costs of the functional retrofit of those elements that are not energy upgraded and their maintenance, and the costs of heating and cooling energy during the considered lifetime. The annualized coefficient (a) (equation (1)) has been considered taking inflation into account, with an interest rate of 2 % [51] (i) for a 30-year period (t). To calculate energy costs, the coefficient (m) (equation (2)) has been considered to adjust them with the annual price rise for energy (r = 3 %), for 30 years (t). It is based on an electricity price level of 0.1805 €/kWh, year 2017 [26].(1) a=[i(1+i)ˆt]/[(1+i)ˆt−1]

(2) m=((1+(i−r)/(1+r))ˆt−1)/((i−r)/(1+r)*(1+(1−r)/(1+r))ˆt)a

a, annuity for constant real prices (costs); m, general average adjustment factor; t, time range of cost evaluation; i, real interest rate; r, rate of yearly increase of energy prices.

4 Results

4.1 Current state diagnosis

The main damages observed in the envelope are related with windows: high level of oxidation and lack of watertightness (Fig. 5) (Table 6).Fig. 5 State of conservation of exterior steel frame.

Fig. 5

Table 6 Damage detected in the envelope (symptom-cause) and actions proposed.

Table 6	Damage: symptoms/cause	Proposed action	
FACADES	Rising damp on GF facade, due to the accumulation of water touching it, caused by an inadequate sloping of the exterior pavement	Placement of a drain at the facade, to be connected to the existing downpipe. Increase of the slope towards the drain.	
Damp around the windows with possible origin in:
- Infiltration dampness around the windows, from the frame-masonry join.
- Infiltration dampness due to lack of watertightness in casement windows
- Possible damp from condensation in the glazing.	Replacement of window frames and glazing. Special care will be taken with the frame/masonry join, the gasket to be made of flexible material (EPDM).	
Occasional rust stains around the windows from the oxidation of the window frame.	
WINDOWS (Fig. 4)	Oxidation of window frames.
Deterioration of seals and glazing stop.	
RAILINGS	Oxidation.	Stripping, anticorrosive protection, two coats of paint, thickness to be decided depending on environment (maritime and industrial).	
EXPOSED CONCRETE STRUCTURE	Occasional infiltrations (second floor slabs).	Water-repellent protection.	
Oxidation of concrete reinforcement.
Concrete cracking and spalling.	Cleaning of visible reinforcements, passivation of rebar and filling with epoxy mortar or synthetic mortar with low shrinkage.	
Dirt.	Cleaning and water-repellent protection.	

The thermographic study [52] was carried out with a FLIR model B250 camera. It detected the location of the thermal bridges and that the facades were only partially insulated (Fig. 6). The heat flow measurement test [53] was carried out with AHLBORN equipment, series ALMEMO, model 119. The thermal transmittance obtained was 0.65 W/m2K, with a standard deviation of 0.002 W/m2K.Fig. 6 Thermographic image and photographic image of the north-west facade (Authors, 2017).

Fig. 6

The airtightness test by blower door (ISO 9972 [54]) was carried out on the complete ground floor (7.00 r/h) and in Office 09 on the second floor (4.39 r/h). The main infiltrations occurred at the joint of the wall with the structural girder, at the window joints, and at the joints between the windows and the wall and the lintel. The use of a borescope showed the existence of a variable thickness cavity with 2 cm projected polyurethane thermal insulation.

In addition, the monitoring of indoor and outdoor environment temperatures and relative humidity was carried out. Madge Tech compact temperature loggers, model RHTemp101A, which register the environment temperature (Ta) and relative humidity (RH) were used, and programmed to gather data every 10 min. This equipment has a precision of ±0.3 °C for temperature and ±3 % for relative humidity. Indoor sensors were placed at 0.25 m and 1.70 m from the floor level.

Fig. 7 shows the data corresponding to the coldest week of the winter (2017) in the heated premises (Off. 01-Off. 09). During office hours, the users heated the premises to 22–24 °C. When the heating was switched off at the end of office hours, the temperature in Office 01 dropped to 13 °C and that of Office 09 to 9 °C. The non-heated premises had mean temperatures during the coldest week of the winter of between 2.7 °C and 8.4 °C.Fig. 7 Indoor and outdoor temperatures in the coldest week of the winter (2017).

Fig. 7

The analysis of the surveys with the nine workers in the two offices in use indicate that their degree of satisfaction with the temperature of the workplace is low, with “dissatisfied” (45 %) and “very dissatisfied” (45 %) as the most frequently chosen options. A 67 % consider that the thermal sensation in winter is “cold”. The time period during which the users felt the greatest discomfort in winter is in “the morning before 11.00” (55 %) and identify the facade as the point of origin of the thermal discomfort. All of the users work at a distance of less than 4.5 m from the facade.

4.2 Simulation of energy retrofit scenarios

A simulation was carried out for each scenario considered, with the current adjusted model. Table 7 shows the results of the different intervention simulations and the energy savings compared with the reference scenario. The consumption has been itemised for heating and cooling. The proposed scenarios allow for savings from 4 % when there is improvement in the insulation of the roof (S4) or the insulation of the ground floor slab (S5), and up to 65 % when the intervention improves the whole envelope (S11). The small savings made by changing the windows (S1) (12 %) should be highlighted; this is because double glazing was already installed. The intervention on the inner and outer facade (S3) is a simple scenario that makes for the greatest savings (41.7 %). If, in addition, windows are improved (S7), savings of 52.5 % can be achieved, and if roof improvement was added (S9), the saving is 58.9 %.Table 7 Results of simulation under different actions scenarios.

Table 7SCENARIO	HEATING DEMAND (kWh/m2y)	COOLING DEMAND (kWh/m2y)	TOTAL DEMAND (kWh/m2y)	HEATING CONSUMPTION (kWh/m2y)	COOLING CONSUMPTION (kWh/m2y)	TOTAL CONSUMPTION (kWh/m2y)	ENERGY SAVING	
Reference Scenario	68.6	0.0	68.7	34.3	0.0	34.3		
S1	W	59.8	0.1	59.9	29.9	0.0	30.0	12.5 %	
S2	Facade/interior	51.1	0.1	51.2	25.5	0.0	25.6	25.4 %	
S3	Facade Int + ext	39.8	0.2	40.0	19.9	0.1	20.0	41.7 %	
S4	Roof	65.4	0.1	65.5	32.7	0.1	32.8	4.4 %	
S5	Slab	65.2	0.1	65.3	32.6	0.1	32.7	4.7 %	
S6	S1+S2	41.8	0.1	42.0	20.9	0.1	21.0	38.8 %	
S7	S1+S3	32.3	0.3	32.6	16.2	0.1	16.3	52.5 %	
S8	S1+S2+S4	39.8	0.3	40.0	19.9	0.1	20.0	41.7 %	
S9	S1+S3+S4	27.5	0.8	28.2	13.7	0.4	14.1	58.9 %	
S10	S1+S2+S4+S5	37.6	0.4	38.0	18.8	0.2	19.0	44.6 %	
S11	S1+S3+S4+S5	22.9	1.3	24.1	11.4	0.6	12.1	64.7 %	

4.3 Cost effective assessment

Cost-effective analysis relates total energy consumption (Table 7) with the global annualized cost of the different intervention scenarios (Table 8). In this case study, it is mandatory that frames were steel painted white (material and finish of the original frame). The cost of steel frames is extremely high; therefore, the use of aluminium frames with similar characteristics is contemplated so as not to change the appearance of the building. From the point of view of energy simulation, there is no difference between the two types of windows. Investment costs, the cost of 30-year maintenance and energy costs are shown in Table 8. Also given are the annualized costs for 30 years.Table 8 Costs of investment, maintenance and energy, and annualized cost of the different action scenarios (year 2017).

Table 8SCENARIO	INVESTMENT COSTS	MAINTENANCE COSTS (30 YEARS)	ENERGY COSTS
€/year	ANNUALIZED COSTS
€/m2 y	
€	€	
Window	Facade	Roof	Slab	TOTAL	Window	Facade	Roof	TOTAL	
Reference scenario	41043	11955	0	0	52998	2052	26513	36693	65258	4898	16.8	
S1	W Alum (A)	68404	11955	0	0	80359	2052	26513	36693	65258	4273	17.1	
S1	W Steel (S)	194796	11955	0	0	206751	2052	26513	36693	65258	4273	24.0	
S2	Facade interior	41043	48244	0	0	89287	2052	26513	36693	65258	3651	16.3	
S3	Facade int.-ext.	116594	0	0	157637	2052	23075	36693	61820	2855	18.5	
S4	Roof	11955	13895	0	66893	2052	26513	36693	65258	4673	17.1	
S5	Slab	0	24335	77333	2052	26513	36693	65258	4660	17.7	
S6	S1(A) + S2	68404	48244	0	0	116648	2052	26513	36693	65258	2994	16.6	
S6	S1(S) + S2	194796	48244	0	0	243040	2052	26513	36693	65258	2994	23.9	
S7	S1(A) + S3	68404	116594	0	0	184998	2052	23075	36693	61820	2324	19.0	
S7	S1(S) + S3	194796	116594	0	0	311390	2052	23075	36693	61820	2324	26.3	
S8	S1(A) + S2+S4	68404	48244	13895	0	130543	2052	26513	36693	65258	2858	17.1	
S8	S1(S) + S2+S4	194796	48244	13895	0	256935	2052	26513	36693	65258	2858	24.4	
S9	S1(A) + S3+S4	68404	116594	13895	0	198893	2052	23075	36693	61820	2021	19.2	
S9	S1(S) + S3+S4	194796	116594	13895	0	325285	2052	23075	36693	61820	2021	26.5	
S10	S1(A) + S2+S4+S5	68404	48244	13895	24335	154878	2052	26513	36693	65258	2711	18.2	
S10	S1(S) + S2+S4+S5	194796	48244	13895	24335	281270	2052	26513	36693	65258	2711	25.5	
S11	S1(A) + S3+S4+S5	68404	116594	13895	24335	223228	2052	23075	36693	61820	1731	20.0	
S11	S1(S) + S3+S4+S5	194796	116594	13895	24335	349620	2052	23075	36693	61820	1731	27.3	

Maintenance of facades, roofs and windows is carried out once in the 30-year lifespan, both in the reference scenario and in the intervention scenarios [55]. This includes, among other things, repairing the facade cladding, checking window fittings and glazing beads (this maintenance is the same for aluminium and steel windows), and cleaning and repairing any water leaks problems on the roof. No replacement of interventions is considered as they have a life span of over 30 years with proper maintenance.

The results of the cost-effective analysis for each individual scenario and the combined scenarios are shown in Fig. 8. The reference scenario is the starting point (being the scenario with the highest energy consumption). The lower the energy consumption resulting from the various scenarios and the lower their corresponding global costs, the more profitable the intervention will be. This scenario is the optimum-cost scenario.Fig. 8 Cost-effectiveness graphs. Scenarios with steel and aluminium windows (2017).

Fig. 8

In the case of using steel windows the optimal cost is insulating the facade on the inside (S2), which makes for 25.4 % energy saving and a 3 % reduction in global costs. There is no cost-effective scenario. In the case of using aluminium windows, the optimal cost renovation scenario corresponds to the intervention placing insulation on the inner facade (S2). However, in that case, a cost-effective intervention is replacing the windows together with the inside insulation of the facade, which makes for a 38.8 % energy saving and a 1.2 % reduction in global costs.

4.4 Cost effective assessment. Increase in energy prices

Since 2017, there has been a significant increase in electricity prices. To answer the second research question, it has been simulated that the intervention on the building would take place in 2018, 2019, 2020, the first half of 2021 and the second half of 2021. To this end, electricity prices and construction prices have been updated. All other parameters have been maintained. Fig. 9 shows the increase in electricity and construction prices compared to 2017.Fig. 9 Percentage increase in the cost of electricity and construction, compared to 2017.

Fig. 9

Table 9 shows the results of the cost-effective simulation for the various scenarios with steel and aluminium windows, for the years 2018, 2019, 2020, the first six-month period of 2021 and the second six months of 2021, updating electricity prices [26] and construction prices for each year [56]. The results for 2018, 2019, 2020 and the first six-month period of 2021 are similar to those obtained in 2017 (Fig. 8). The optimal cost scenario is S2 (interior and cavity insulation) in all years with both aluminium and steel windows. With steel windows, there is no cost-effective scenario. The replacement of the windows together with interior and cavity insulation is cost-effective using aluminium frame (S6). This scenario implies an increase of the annualized investment costs of between 1 % (2018) and 1.9 % (first six months 2021), with a 53 % reduction in consumption, compared to the optimal cost scenario (S2).Table 9 Results of cost-effective assessment. Years 2018, 2019, 2020, 2021_1 and 2021_2.

Table 9

It can be highlighted that electricity prices increased considerably in the second six months of 2021 (31 % compared to the first six months and 36 % compared to the year 2017) (Fig. 9). Construction costs increased 6 % compared to the first six months and 14 % compared to the year 2017. In the case of steel windows, optimal cost scenario continues to be S2, and no cost-effective scenario appears (Table 9. Fig. 10). In the case of aluminium windows, we have the first combined optimal cost scenario, S6 (replacement of windows and interior and cavity insulation), which corresponds to a 38.8 % saving in energy consumption and a 4.9 % reduction in global costs compared to the reference scenario. Also, scenarios S2 (interior insulation) and S8 (interior insulation, window replacement and roof insulation) are cost effective (Table 9. Fig. 10). In these cases, energy saving is 24.5 % and 41.7 %, and the reduction in global costs is 4.9 % and 2.3 % respectively, compared to the reference scenario.Fig. 10 Cost-effective graphs. Second semester 2021.

Fig. 10

5 Discussion

Guaranteeing the use of historic and traditional buildings by adapting them to present-day requirements is a means of protecting them and avoiding their deterioration and demolition [9,57]. The analysed case study corresponds to a DOCOMOMO building which is only in partial use as it lacks comfort, has high energy costs and other shortcomings. It is an example of the need to intervene in a building in order to maintain its use as rented office space, and avoid its being abandoned by the tenants.

The first decision to be made to choose between the different intervention strategies compatible with the conservation of its heritage values [58]. The characteristics of the building itself, the conclusions of monitoring and user surveys, together with its heritage values, are the aspects which defined the scenarios. In this case study the replacement of windows is necessary, allowing for an improvement in the characteristics of the frame and the glazing. In addition, the building is protected [28] and so, there is a tight requirement over the material for window frames (in this case study, steel).

Secondly, adjusted BEM allows to obtain the energy savings of the different scenarios. The best solution was a total intervention in the envelope (S11). It is interesting to highlight that the replacement and improvement of the windows (S1) only carries a reduction in consumption of approximately 12 %, a value coherent with the fact that the windows were already double glazed. Stanojević et al. [59] highlight that, at average protection levels, intervention on the interior of the envelope is possible, and in the case of low-level protection, intervention on the roof, on the exterior of the facade, and changing the frame and glazing is appropriate, provided that the appearance of the building is not changed. According to this, S11 seems to be the best solution.

Following the proposed methodology for decision making, cost-effective analysis provides the economic approach [60]. In this case study, the optimal cost scenario is the placement of interior insulation (S2, 25.36 % energy saving). With steel windows, there is no cost-effective scenario due to their high cost (almost 3 times higher than aluminium ones). The cost-effective measures with aluminium windows are the replacement of windows together with interior insulation (S6, 38.8 % energy saving). It must be pointed out that these scenarios do not match with those that achieve the greatest energy saving (S11 has 42 % and 52 % less consumption than S6 and S2 respectively). Zangheri et al. [61] consider that, in general, optimal cost solutions applied to refurbishment are not very ambitious from the perspective of energy efficiency, a fact which can also be seen in the results of this case study.

The final phase of the methodology is decision-making based on the information obtained in the previous phases. In this case study, it is necessary to change the windows, but it is mandatory to use steel windows (not cost effective). However, the owners cannot assume this cost. The only optimal cost solution is to insulate the interior.

From this perspective, it could be of interest to offer greater flexibility in the choice of frame material, and allow the much more economical use of aluminium frames. Otherwise, the investment and conservation of the building through its use may be endangered. In this case study, after a cost-effective analysis, a solution for the use of steel windows could be that the government subsidized these elements.

The increase in electricity prices, especially in the second half of 2021, reaffirms the above conclusions. These circumstances make it even more important to introduce cost-effective analysis in the methodology of intervention in a DOCOMOMO building.

6 Conclusions

The energy refurbishment of listed buildings is extremely complex as energy efficiency improvements must be combined with the conservation of heritage values. However, this is fundamental to keep them in use and reduce their energy consumption. DOCOMOMO buildings are a special case as many of them, despite being catalogued as such, do not have their conservation guaranteed, as they are not “officially listed buildings”. Thus, the refurbishment must ensure their heritage values, update their use in accordance with current demands and be worthwhile for the owners.

The inclusion of cost-effective methodology as part of the mechanics for decision-making in the energy refurbishment of DOCOMOMO buildings allows for the optimisation of the intervention guaranteeing their use and the conservation of heritage values.

In this case study, the windows need to be changed due to a poor state of conservation. In addition, the use of steel windows is mandatory. The consequence is that any scenario that involves the replacement of steel windows with new windows of the same material is not cost-effective. In this case, cost-effective analysis may influence the government to subsidize these windows, or allows the use of aluminium windows with the same aesthetic characteristics, but at a much lower cost. In that case, cost-effective scenarios emerge, with energy savings of between 25.36 % (interior insulation of facade, a cost-optimal solution) and 38.8 % (replacement of windows together with interior insulation of facade, a cost-effective solution).

The increase in electricity prices, especially in the second half of 2021, reaffirms previous conclusions. The use of steel windows is still not cost-effective in any case. With aluminium windows, up to a 41.7 % energy-saving would be achieved with a cost-effective solution (window replacement, interior insulation of facade and under roof insulation). These circumstances make it even more important to introduce cost-effective analysis in the methodology of intervention in a DOCOMOMO building.

The last decision on the intervention will depend on the degree of conservation required for the building according to conservation theory, the need to preserve heritage values and the economic viability of the intervention. All the proposed interventions guarantee the conservation of the building as an icon of the Modern Movement.

Finally, in this case study the replacement of energy systems and the potential use of renewable energies has not been analysed; nor have the environmental conditions monitored after the rehabilitation. These points constitute a limit for this study and are open to subsequent research for going in deep in the studies of the conservation of Modern Movement Buildings.

Data availability statement

Data will be made available on request.

CRediT authorship contribution statement

Purificación González-Martínez: Writing – review & editing, Methodology, Investigation, Conceptualization. Aurora Monge-Barrio: Writing – review & editing, Methodology, Investigation, Formal analysis, Conceptualization. Silvia Domingo-Irigoyen: Software, Methodology, Investigation, Conceptualization. Ana Sánchez-Ostiz Gutiérrez: Supervision, Methodology, Investigation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A Supplementary data

The following is/are the supplementary data to this article:Multimedia component 1

Multimedia component 1

Acknowledgements

The authors would like to acknowledge the support of the Servicio de Patrimonio Histórico-artístico y Archivos, de la Diputación de Guipuzkoa (provincial government) in the building intervention, and also thank the owners of the building for all the information offered, and the users of same for their assistance in the monitoring of the building.

Appendix A Supplementary data to this article can be found online at https://doi.org/10.1016/j.heliyon.2024.e36893.
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