Analysis of Retrofit Strategies of Mid-20th-Century Modern, Concrete Buildings ^†

Abstract

Reusing existing buildings is a valid response to the architectural challenge associated with addressing climate change and can aid the regeneration of the historic built environment. This demands sensitive architectural conservation strategies that improve thermal comfort, indoor environmental quality, and energy efficiency. In addition, energy retrofit solutions that balance performance improvements with the conservation of cultural and architectural values are needed to achieve higher performance while preserving cultural heritage, architectural features, and identity. Energy retrofits of post-war, mid-20th-century buildings pose particular challenges, including low ceiling heights, full-height windows, external decorative components, and other structural aspects, as these features hinder thermal upgrades. Concrete buildings from this period are frequently demolished due to limited guidance on effective retrofit methods. This study explores the most effective energy retrofit strategies for balancing energy efficiency with conservation requirements in such buildings, and assesses the risks associated with condensation and thermal bridging arising from internal insulation strategies. This paper examines internal insulation as a retrofit solution, where external insulation is not feasible. Internal wall insulation (IWI) reduces overall heat loss but concentrates thermal transfer at uninsulated junctions, thereby increasing the risk of condensation. In the simulated case, a relatively thin, short strip of slab insulation, combined with wall insulation, significantly reduced condensation and mould risk, suggesting a potential solution for mid-century building types. The analysis shows that applying insulation asymmetrically worsens conditions on the uninsulated side. Full-height window replacement, coupled with internal slab insulation, results in the most significant improvement; however, slab insulation alone can mitigate condensation risks where window replacement is not permitted. Findings highlight that partial insulation at balconies, parapets, and roof junctions is minimally effective, reinforcing the importance of integrated internal strategies for successful retrofits.

1. Introduction

The built environment is responsible for a significant share of the world’s carbon emissions [1]. Inefficient construction and operation practices lead to high energy demand. In response to this, many countries have introduced several energy efficiency programmes and set targets to align with various international and national frameworks, such as the EU’s 2030 energy efficiency goals [2]. In the EU, buildings account for 40% of final energy consumption and 36% of its energy-related greenhouse gas (GHG) emissions, while 75% of the buildings in the EU are still not energy-efficient [2]. Space heating remains the dominant end-use in buildings, accounting for 50%, highlighting a key opportunity for energy reduction. Non-residential buildings are approximately 40% more energy-intensive than residential ones, with an annual energy consumption of 250 kWh/m² compared to 180 kWh/m² for residential buildings [3].

As previous studies pointed out, retrofits and renovations can result in approximately 53–75% reductions across various environmental impact categories compared to new construction [4]; therefore, retrofitting the existing building stock is part of this effort, as it can reduce emissions associated with operational energy use by reducing heat loss through the building envelope [5]. In recent years, research attention has shifted from purely operational energy use toward a whole-life carbon perspective, recognising the significant contribution of embodied emissions to buildings’ environmental impact [6,7]. Retaining and upgrading existing structures can offer substantial carbon savings, since extending the lifespan of existing buildings delivers immediate carbon benefits within the timeframe most critical for climate change mitigation [8]. The EU Energy Efficiency Directive [9] and the recast of the Energy Performance of Buildings Directive (EPBD), published in 2024 [2], have progressively introduced stricter regulations and requirements on thermal transmittance through the building envelope, as well as more standardised and comprehensive calculation methods in most European countries. These directives are implemented through national building regulations [10,11,12,13], which typically specify maximum allowable U-values for elements such as roofs, walls, and floors. While the exact limits vary between countries, this approach reflects a common regulatory effort to improve insulation standards and drive energy efficiency.

In most countries, the public sector is expected to lead the way in building retrofits, as regulations often require them to meet new energy efficiency standards ahead of the private sector. This is not only to meet national energy targets but also to demonstrate best practices and encourage wider adoption across the building stock.

Many public buildings that fall under these energy improvement targets were constructed during the mid-20th century, a period marked by rapid development and evolving construction techniques [14]. However, energy retrofits provide significant challenges for mid-20th-century modern structures, due to their design characteristics, construction methods and material limitations. As a result, achieving deep retrofit targets is often deemed unfeasible, leading to the demolition of these buildings [15,16]. Across Europe, this period produced a large number of schools, hospitals, administrative buildings, and housing developments that were designed with functional simplicity and cost-efficiency in mind, often at the expense of energy performance [17]. These buildings typically feature thermally inefficient envelopes, extensive glazing, and limited insulation, leading to high operational energy use and poor comfort conditions by current standards [18]. While these buildings are often affected by design and structural constraints associated with their original construction, they are also of architectural, cultural and heritage significance [19].

Therefore, a balance should be struck to improve the building’s performance without impacting its architectural qualities. This study focuses on buildings with design and structural constraints, such as low ceiling heights, full-height windows, external decorative components, and other structural aspects, because these features hinder thermal upgrades. In particular, low ceiling heights are not only a barrier to retrofit but are also considered undesirable from a real estate perspective, contributing to increased market pressures for demolition and replacement [20].

Previous studies on energy retrofits in heritage contexts have focused primarily on traditional masonry buildings [21,22,23], where material behaviour, moisture transport, and insulation strategies differ significantly from those in modern concrete structures. This study extends that discussion to mid-20th-century modern buildings, addressing a recognised gap in the literature by analysing how energy retrofit strategies perform in reinforced-concrete construction.

This study aims to identify energy retrofit strategies that effectively balance energy efficiency improvements with conservation requirements in modern concrete buildings. While not all modern buildings are formally protected in the EU and Ireland, many share construction characteristics, such as exposed concrete façades, full-height glazing, or integrated decorative components, that make the conventional and often preferred external insulation methods unfeasible or inappropriate [24]. Consequently, even non-protected modern buildings often face similar retrofit constraints to listed or culturally significant ones [25]. This study aims to address the following questions: (1) Which energy retrofit strategies are most effective in balancing energy efficiency with conservation requirements in mid-20th-century concrete buildings? (2) What are the condensation and thermal bridging risks of internal insulation strategies in these structures?

2. Literature Review

Prior to the 1970s, society paid little attention to energy and usage costs. Heat loss due to air leakage and lack of insulation was seen as more of an inconvenience than a significant concern [26]. Extensive thermal insulation was not standard practice at the time, although climate, indoor atmospheres, and users’ thermal comfort were being considered by some during the design phase of buildings [27]. However, with growing awareness of climate change and the introduction of stricter energy regulations, these inefficiencies have become a major concern in recent years [28,29]. The preservation of mid-20th-century buildings is often hindered by limited public appreciation and the perception that they lack cultural or functional value, leaving them vulnerable to demolition, often justified by poor energy performance [30,31]. Furthermore, unlike traditional heritage buildings, which are often addressed in retrofit studies, these buildings present distinct challenges. These include flat roofs, cantilevers, thin slabs, low floor-to-ceiling heights, full-height windows and curtain wall systems [32].

Addressing uninsulated building envelopes and thermal bridges often plays an important role in effective retrofitting, and mitigating these problems can significantly enhance performance and indoor comfort [33].

Overcoming these issues is particularly challenging in modern buildings when external insulation is often not an option due to architectural or, in certain cases, regulatory constraints. Previous studies have noted that the application of external insulation to modern buildings is often perceived unfavourably because of its substantial visual impact, even though it has been shown to be one of the simplest and most reliable methods for reducing thermal transmittance [34]. Alternative insulation strategies, focused on internal insulation, should instead be explored and carefully designed to minimise heat loss while maintaining façade character and avoiding the moisture risk frequently associated with internal insulating solutions [35]. This study employs Finite Element Method (FEM) [36] modelling for detailed thermal analysis and the WUFI method [37] for hygrothermal assessment, enabling the optimisation of retrofit strategies and the evaluation of condensation risk.

2.1. Retrofit Challenges and Importance in Mid-20th-Century Buildings

Retrofit refers to the process of modifying and upgrading a building or group of buildings to enhance their energy efficiency, functionality, or structural integrity [38]. Retrofitting has become increasingly common and is now required by regulation in many countries when altering a building, as it significantly boosts energy efficiency and helps lower greenhouse gas emissions related to building operation [39]. Studies have shown that retrofitting large-scale public buildings can lead to about 50% energy savings for heating [40]. However, assessment on a building level is also important. This involves evaluating the building’s heritage significance, environmental context, construction methods, fabric performance, and current conditions, including any defects affecting energy efficiency [41].

Modern buildings from the mid-20th century, many of which were originally built for public or institutional use, now face uncertain futures [14]. Some have been successfully retrofitted, repurposed, or adapted, while others were demolished [42]. The most significant barrier to retrofitting public and modern buildings in Ireland is the complexity of upgrading ageing concrete structures while maintaining architectural and heritage integrity, to meet modern energy standards [19].

Adaptive reuse is particularly relevant in this context, as mid-20th-century structures are now at an age where many of them need significant upgrades. It offers a way to extend their life while preserving architectural character, along with reducing environmental impact. This means rethinking their layout, upgrading services and improving energy performance, without erasing the building’s identity. These buildings are especially suitable for adaptive reuse due to their flexibility, including generous spatial layouts and structural systems that allow for new uses, such as housing, offices or mixed-use [43]. Some studies point out that while adaptive reuse is widely practised, a comprehensive methodological discussion is still lacking, which they consider a gap that needs addressing [44,45,46]. This underscores the need for clear, actionable guidance and highlights the relevance of retrofit-focused research. These buildings represent a significant chapter in architectural and social history and were part of an era of ambition, innovation, and large-scale urban development. They also hold a substantial amount of embodied carbon that further reinforces the case for retrofitting.

Figure 1 shows examples of buildings representative of the era and types discussed in this paper. Preserving and upgrading modern buildings involves both sustainability and the recognition of cultural and historical significance. Thoughtful renovation strategies, including adaptive reuse, can ensure their continued relevance. However, improving their thermal performance often requires addressing the prevalence of thermal bridges and insufficient insulation, which are characteristic challenges of buildings of this era.

Figure 1. Examples of mid-century buildings in Dublin. (Photo 1: The Central Bank (Architect: Stephenson Gibney, built: 1978, Image source: [47]), Photo 2: Busaras (Architect: Michael Scott, built: 1953 Image Source: [48]), Photo 3: US Embassy (Architect: John M Johansen, built: 1964, Image Source: [14], Photo 4: Fitzwilton House (Architect: Ronald Lyon Estate, built: 1969, demolished: 2018), Photo 5: Donnybrook House (Built: 1973, Retrofitted by: Henry J Lyons in 2018, Image Source: [49], Photo 6: ESB Headquarters (Architect: Stephenson Gibney, built: 1965, demolished in 2017, Image Source: [50]).

2.2. Thermal Bridging, Heat Loss and Insulation

Thermal bridges are areas within the building envelope where heat flow is locally intensified due to geometric junctions, boundary conditions, the presence of materials with higher thermal transmittance, or discontinuities in insulation layers [51]. Even in relatively homogeneous concrete structures, junctions such as wall-to-floor or wall-to-roof interfaces can exhibit increased heat transfer because heat can flow in multiple directions at these locations [52]. These concentrated heat flows can compromise a building’s overall thermal performance, often resulting in higher energy demands for heating and cooling [53]. The issue of thermal bridging is well-documented in the scientific literature [54,55,56,57,58].

Thermal bridges are often quantified using linear thermal transmittance (Ψ-value). Linear thermal transmittance is the heat flow rate in the steady state compared to a reference heat flow rate calculated without considering the thermal bridge, divided by junction length and the temperature difference between the environments on either side of the bridge. It describes the influence of a linear thermal bridge on the total heat flow through the building envelope [51]. The Ψ-value therefore expresses the influence of linear bridges, such as wall-to-floor, wall-to-roof, or balcony junctions, on total heat loss through the building envelope. This parameter is particularly relevant for concrete structures, where the continuity of dense, conductive materials increases the potential for heat transfer at junctions [59].

Buildings of the 20th century were generally built to low thermal standards compared to today’s expectations. Although insulation practices improved as the century progressed, many buildings constructed in the 1950s and ‘60s have limited insulation and exhibit multiple thermal bridges [60]. These problems can often be solved to a large extent through enhanced and targeted building fabric insulation [61].

Thermal bridges can significantly impact a building’s energy performance, depending on building type, construction system and climate [61,62,63]. Several studies have shown that applying external insulation at junctions, such as wall-to-floor or façade connections, can reduce thermal transmittance by 20–63% [64,65]. Similarly, improved wall insulation in balcony connections can lower linear thermal transmittance by 60% and heating demand by around 8% [66]. Numerical analyses confirm that insulating window frames and reveals reduces localised heat fluxes at linear junctions [67]. At the same time, FEM simulations indicate that external insulation decreases heat losses by roughly 50% and performs about 8% better than internal insulation [68,69]. However, as mentioned before, external insulation is often unfeasible for mid-20th-century buildings. In these cases, if the fabric requires improvement, internal solutions are necessary. In such cases, the use of vapour-control layers and careful material selection are essential to avoid condensation and mould formation [70,71,72,73,74].

While insulation is a vital component in reducing heat transfer and enhancing thermal performance, it is also important to find a point where the incremental benefits of additional insulation become marginal compared to the associated costs and environmental impact. A study that included in situ U-value monitoring revealed that 9 out of 10 tests on envelopes with extremely low U-values exhibit underperformance [75]. This discovery holds dual significance. Firstly, it suggests that certain insulation measures are redundant, contributing to the building’s carbon footprint without improving thermal resistance. Secondly, it indicates a risk of undersized heating systems in low-energy buildings, leading to decreased efficiency and heightened overall energy consumption [75].

Windows in modern buildings represent a major component of the building envelope and have a substantial influence on overall energy performance. Retaining existing windows can help preserve the original architectural appearance and result in less waste, but their replacement may be justified when the reduction in operational carbon outweighs the embodied carbon associated with new materials [76,77]. If windows are to be replaced, matching the original proportions and detailing remains important in architecturally sensitive façades. Replacing single glazing (typically with a U-value of about 5.2 W/m²K) with modern double glazing (around 1.2 W/m²K) can improve thermal performance by up to 75%, potentially reducing total building energy use by as much as 50% [78]. Therefore, it plays a significant role in the overall energy improvement of these structures. Furthermore, with these measures, user comfort can be increased, drafts can be prevented, and cold radiation from glazing can be reduced. However, some studies suggest that when only the windows are replaced, the surrounding wall reveals often remain less insulated than the new window units. This can create localised cold spots where condensation and mould may form, making reveal insulation necessary in some cases [74,79].

While existing studies have addressed traditional masonry buildings, contemporary low-energy designs, and other general retrofit approaches, a notable gap remains in research focusing on modern concrete structures, particularly in the Irish context. Unlike traditional masonry buildings or historically significant structures, these buildings fall into a grey area, neither old enough to be considered heritage nor new enough to meet current energy standards. Despite their prevalence, limited data exist on effective insulation strategies for these structures, making this research a valuable contribution to the field.

2.3. Hygrothermal Properties of Retrofitting

As discussed above, improving the energy efficiency of existing buildings is one of the most comprehensive mitigation strategies for reducing energy consumption and CO₂ emissions [80]. However, these energy-saving interventions often lead to unintended impacts, such as interstitial condensation, dampness and mould growth associated with poor hygrothermal behaviour [81]. Energy upgrades, such as adding insulation on the internal side of the external wall, can also increase the risk of condensation [35,82,83]. Recent research has examined the hygrothermal performance of traditional wall constructions under retrofit conditions, providing valuable insights into moisture transfer, condensation risk, and material compatibility that also inform the assessment of modern concrete envelopes [84]. Thermal insulation mitigates condensation by raising internal temperatures, which lowers relative humidity and mould risk. It also reduces surface condensation by minimising temperature difference, as seen with double-glazed windows [85]. Many studies have reported a reduction in the risk of visible mould in retrofitted buildings [86,87,88].

When sufficient internal insulation is applied, the interior side of the external wall no longer experiences heating, reducing thermal stresses and façade durability [89,90]. A hydrophobic, diffusion-open treatment on the external façade is often recommended to protect against moisture ingress [91,92]. The need for a vapour control layer should be evaluated based on wall composition and material permeability [93,94].

The WUFI method, used in this paper, enables designers to assess the likelihood and risks of condensation and optimise designs for the longevity of the building fabric. This paper used this method to assess the correct position for vapour control and vapour-permeable membranes in building components by assessing water content and relative humidity [95].

To assess hygrothermal risks, two key parameters are used in this study: the temperature factor (fRsi) and relative humidity (RH). These indicators are widely used in hygrothermal analysis to evaluate the potential for mould growth and surface condensation in the vicinity of thermal bridges [96]. The temperature factor represents a dimensionless ratio that expresses the relationship between the internal surface temperature and the indoor and outdoor ambient temperatures [97]. According to ISO 13788:2012 [98], the indicative critical temperature factor for limiting surface condensation in “offices and retail premises” is 0.5, a category encompassing most public and modern buildings. When the temperature factor falls below this threshold, surfaces become cold enough to permit condensation, particularly where indoor relative humidity exceeds 70–80%, conditions shown to promote mould growth [96,99]. Higher temperature factors indicate improved surface thermal performance, reducing the risk of condensation even under elevated humidity levels [100].

In summary, internal insulation interventions alter the temperature and moisture balance of existing walls, increasing the risk of interstitial condensation, surface mould growth and freeze–thaw damage if not carefully detailed. The reduction in inwards drying potential can trap moisture between the structure and insulation, especially in solid concrete walls with limited vapour permeability. These findings highlight the importance of hygrothermal assessment prior to design, ensuring long-term durability and indoor comfort.

2.4. Finite Element Method

Finite Element (FE) models are mathematical representations that simulate and analyse heat transfer in physical systems [101]. Based on differential equations, they account for heat flow, boundary conditions, and material behaviour. This study examines prevalent thermal bridges in modern concrete building structures and their mitigation strategies, using commercial Finite Element Method (FEM) software, COMSOL 5.6 [102]. It analyses heat flux, surface temperatures, temperature factors, and linear thermal transmittance at critical junctions. The placement of insulating materials is investigated as a means to reduce heat transfer through the building envelope.

FEM provides a practical and cost-effective alternative to physical testing, allowing detailed simulation of thermal performance and material interaction in complex geometries [103,104]. It enables the accurate prediction of how insulation and junction design influence energy use and condensation risk, supporting the design of efficient and durable retrofit strategies [101,105].

The novelty of this study lies in its focus on mid-20th-century modern concrete buildings, a group often overlooked in retrofit research. These structures form an important part of architectural heritage but face challenges in meeting modern energy standards. By applying FEM to typical structural details, the study identifies solutions that balance energy efficiency, hygrothermal performance, and preservation of architectural integrity, while seeking to minimise insulation thickness to avoid reductions in ceiling height that could compromise the feasibility of retrofitting.

Ultimately, the study contributes to the broader discourse on sustainable retrofit practices, showcasing how modern technologies and methods can be applied to historical contexts. This could serve as a model for similar studies in other regions or for different architectural styles.

3. Methodology

This study integrates numerical simulation and comparative analysis, combining FEM thermal modelling and WUFI hygrothermal assessment to evaluate retrofit strategies for representative mid-20th-century concrete building details. COMSOL Multiphysics [102] software, version 5.6 was used to model key interface details in this context and determine the optimal energy retrofit strategies. WUFI Pro 6 [106] software was used to assess surface and interstitial condensation risks. The thermal bridges were analysed using the Finite Element Method (FEM) [101] and mitigated with thermal insulation. This paper analyses the reduction in linear thermal transmittance, heat flux, and increase in surface temperatures, as well as surface temperature factor, confirming the benefits of alternative insulation strategies when conventional insulation methods and placements are not feasible.

As each detail was different, the insulation thicknesses were introduced based on current building regulation standards in the UK [107] and Ireland [108] to achieve the required U-values in every case, shown in Table 1.

Table 1. U-value (W/m²K) threshold per country for existing buildings.

Element	Ireland	UK
External wall	0.35	0.3–0.7
Roof	0.25	0.16–0.35
External windows	1.4	1.6

The most widespread method today for reducing thermal bridging is to insulate the floor and/or ceiling of the connection area with 300–500 mm wide thermal insulation boards [109]. In this case, 30 mm and 50 mm thick insulation boards were tested, as they are standard in Ireland [110].

3.1. Finite Element Method

This research’s experimental energy analysis study investigates the thermal bridging impact in key details. The finite element approach’s accuracy is approximately 5%, whereas the simplified methods for thermal bridging computation have a typical accuracy of approximately 20%, according to ISO 14683 [111]. The COMSOL Multiphysics software programme, version 5.6 was used to create 2D heat transfer models to simulate the temperature changes in structural components under actual temperature profiles.

It was used to create thermal images and heat flux results for four different problematic modern architectural details, assessing the energy-saving benefits of insulation, as per ISO 10211 [51]. The characteristic dimension of the floor (B) was assumed to be 8 m.

A thermal steady-state simulation was conducted on details representing some of the buildings’ most widely used construction methods. FEM software, COMSOL 5.6 monitored the elements’ heat flux and surface temperature, similar to a study published in 2020 [112]. Temperature factor and relative humidity were also simulated to assess the risk of mould growth and surface condensation in the vicinity of thermal bridges.

The numerical model results enable the calculation of average hot and cold surface temperatures ($T_{in,av}$) and ($T_{ex,av}$), heat flux ($q_{av}$), temperature factor ($f_{Rsi}$) and relative humidity ($RH$) by integrating temperature and heat flux values through the external surface of each detail. Heat flux is often used to measure thermal resistance because it provides a direct and quantitative measure of heat flow through a material or assembly.

Subsequently, the average U-values of each detail are determined using the equation below.

$$U_{av}={\displaystyle \frac{q_{av}}{T_{in,av}-T_{ex,av}}}$$

The full external surface of each detail was used to compare the results of the insulating scenarios and determine the heat flux changes in the overall structure. Based on initial testing, it became apparent that the change in heat flux varies at different locations and sections. It was concluded that measuring the heat flux on the full external surface at every instance and scenario would give a more understandable and conclusive result of the changes made to the detail.

3.2. Computing Method and Boundary Conditions

The details were modelled in COMSOL Multiphysics software. Insulation was added in stages, and heat flux, minimum surface temperature, linear thermal transmittance, and temperature factor were measured at the same locations at every instance to observe the overall thermal performance change in detail.

The convective heat flux ($q$) is given by:

$$q=h(T_s-T_{\infty }),$$

where

$q$ = heat flux (W/m²);

$h$ = convective heat transfer coefficient (W/m²K);

$T_s$ = surface temperature (K);

$T_{\mathrm{\infty }}$ = ambient temperature (K).

The convective heat transfer coefficient ($h$) is affected by surface orientation (horizontal or vertical), temperature difference ($T_s-T_{\infty }$), fluid properties (thermal conductivity ($k$), viscosity ($v$), thermal expansion coefficient ($\beta$)) and surface roughness [113].

To determine the heat transfer coefficient, a sensitivity analysis was conducted using a parametric sweep on the convective heat transfer coefficient ($h$) to assess its impact on surface temperatures and the temperature factor ($f_{Rsi}$). Since natural convection leads to a variable $h$ depending on surface conditions and temperature differences, an explicit value is not always known, especially in studies that assess general details. By varying $h$ within a realistic range (e.g., 5–25 W/m²·K), the analysis ensures that the predicted minimum temperature and $f_{Rsi}$ remain within acceptable limits, reducing uncertainty in the thermal performance evaluation. It was found that the discrepancy between the lowest and highest of the realistic range is approximately 4%.

For heat flux boundaries, Table 2 presents the assumptions made.

Table 2. Heat flux boundary conditions.

	External Boundary	Internal Boundary
Heat transfer coefficient (h)	18 W/m2K	10 W/m2K
Wall height (m)	3	3
Plate distance (m)	-	4
Fluid	Air	Air
Absolute pressure	Ambient absolute pressure for Ireland	Ambient absolute pressure for Ireland
Ambient temperature (°C)	0	20

The temperature factor ($f_{Rsi}$) is a dimensionless parameter used to assess thermal bridges, insulation performance and condensation risk assessment. As described in Technical Guidance Document L (TGD L), in Buildings other than Dwellings [108], the ($f_{Rsi}$) is defined as

$$f_{Rsi}={\displaystyle \frac{T_{si}{-T}_e}{T_i{-T}_e}},$$

where

$T_{si }$ = minimum internal surface temperature;

$T_{e }$ = external temperature;

$T_i$ = internal temperature.

Linear thermal transmittance is the additional heat flow to the assembly if a linear anomaly were present. The linear transmittance is a measure of heat flow per unit length and is represented by $\Psi$ [114]. As per ISO 10211 [51], it is calculated as follows:

$$\Psi =L_{2D}-\left(u_1\ast l_1\right)-\left(u_2\ast l_2\right)$$

where

$L_{2D}$ = the thermal coupling coefficient in W/mK obtained from a 2D calculation of the component separating the two environments being considered;

$u$ = thermal transmittance in W/m²K of the 1D component separating the two environments being considered;

$l$ = length over which the value $u$ applies in metres.

$$L_{2D}={\displaystyle \frac{Q}{\Delta T}},$$

where

$Q$ = total heat loss in W/m;

$\Delta T$ = difference between external and internal temperature in K.

• Temperature boundaries

The internal temperature was set to a constant 20 °C ($T_{in,av}$), while the external temperature was a constant 0 °C $\left(T_{ex,av}\right)$, following recommendations by relevant standards and local climate. According to ISO 13788 [98], external conditions should be representative of the building’s location, taking account of altitude where appropriate. Based on Met Éireann records, the lowest mean minimum air temperature in Ireland over the past 30 years is 2.3 °C. [115] To assess the sensitivity of the $f_{Rsi}$ to external temperature variation, simulations were conducted using external temperatures ranging from −10 °C to 10 °C. Results showed that the $f_{Rsi}$ value remained essentially constant, as it is a normalised ratio and therefore insensitive to external temperature changes. While condensation risk assessments are more sensitive to temperature variations than U-value calculations, the stability of $f_{Rsi}$ results across a realistic temperature range confirms that 0 °C is an appropriate and conservative reference for both simulations. This also made it possible to express the energy efficiency of different scenarios in simple terms.

• Horizontal insulation

For additional insulation to reduce thermal bridging, the most widespread method today is to insulate the floor and/or ceiling of the connection area with 300–500 mm wide thermal insulation boards. Most suppliers of interior insulation systems offer specially cut boards in cuboid and/or wedge-shaped form. This allows the surface temperatures to be permanently raised without an additional energy source, to such an extent that comfort is provided and mould growth is prevented. This study utilised 30 mm and 50 mm thick internal insulation boards on floors and ceilings, similar to a study on reducing thermal bridges caused by internal wall junctions [109].

3.3. WUFI Method

This method was used to assess the need for vapour control layers when applying internal wall insulation. Eight insulation materials were tested in thicknesses sufficient to achieve the target U-values, as shown in Table 3. A parametric analysis was also undertaken using varying concrete types and wall thicknesses to examine their impact on hygrothermal behaviour.

Table 3. Tested insulations and their properties.

Material	Density ρ [kg/m3]	Thickness on Wall [mm]	Thickness on Roof [mm]	Thermal Conductivity k [W/m·K]	Specific Heat Capacity c [J/kg·K]	Vapour Resistance Factor
Expanded Polystyrene Board (EPS)	15	100	100	0.031	1500	57
Mineral Wool	60	120	140	0.04	850	1.3
PIR	26.5	70	100	0.024	1470	51.5
Cork	150	120	140	0.04	1400	10
Sheep’s wool	23	120	140	0.38	1760	1
Wood fibre	165	120	140	0.04	1400	2.9
Cellulose	70	120	140	0.04	1400	1.5
Aerogel	146	40	50	0.014	1000	4.7

3.4. Material Properties

The same materials were used in each detail to give a more reliable and comparative result. During this stage, several insulation materials were tested using FEM simulations. The comparative analyses showed that the materials performed very similarly under the same boundary conditions and for the same junction details, with only minor variations in thermal behaviour. EPS was therefore selected as a representative material for detailed presentation, as it effectively illustrates the study’s main findings without unnecessary repetition. Moreover, EPS is one of the most widely used insulation materials in Ireland [116] and is compatible with concrete structures.

All the material properties used in the study are presented in Table 4.

Table 4. Materials used in the simulations and their properties.

Material	Density ρ kg/m3	Thermal Conductivity k W/(m·K)	Specific Heat Capacity c J/(kg·K)	Vapour Resistance Factor (-)
Reinforced concrete	2300	1.6	840	180
Granite	2600	2.9	850
Expanded polystyrene board (EPS)	15	0.031	1500	57
Adhesive	1397	0.517	1122	15.4
Plasterboard	850	0.2	850	8.3
Glass	2210	1.4	730	-
Wood	532	0.15	2700	-
Argon (gas in glazing cavity)	1.78 [g/cm3]	0.02	0.52 [J/g·K]	-
Silicone (Filling)	1.5 [g/cm3]	0.02	0.71 [J/g·K]	-

3.5. Detail Analysis

This study analyses four details representative of critical thermal-bridge locations in mid-20th-century buildings with solid concrete walls. They were selected because they can strongly influence overall thermal performance, energy demand, and moisture risk, making them key considerations in any retrofit strategy for this era. A graphic illustration of the details analysed are presented in Figure 2.

Figure 2. Graphic illustration of the details analysed. (1) Wall-to-floor junction, (2) Bulkhead detail with full height windows, (3) Balcony detail, (4) Parapet detail.

• Wall-to-floor junction

As previously mentioned, external insulation is not always feasible, especially in modern structures, where exposed concrete often serves as an essential aesthetic feature. A wall-to-floor junction (Figure 3), or the junction between external and internal walls, where external insulation is not an option and internal insulation must be used, represents a common weak point that can lead to thermal bridges, cold spots, increased heat loss, and potential condensation risks. Finding an optimal solution for this detail is therefore important.

Figure 3. Wall-to-slab detail. 1. Concrete wall, 2. Reinforced concrete slab, 3. Internal wall insulation, 4. Plasterboard, 5. Floor insulation.

• Bulkhead detail with full-height windows

Full-height windows (Figure 4), a defining feature of modern architecture, often lack adequate insulation and introduce thermal discontinuities; therefore, they are critical to address. In many cases, the structural elements above the windows act as thermal bridges, which can cause downdraughts, increased heat loss, and internal condensation risks. Upgrading the windows can improve thermal performance, but it requires careful coordination between bulkhead/reveal insulation and the overall window performance.

Figure 4. Full-height window detail. 1. Window, 2. Bulkhead, 3. Reinforced concrete slab, 4. Bulkhead insulation, 5. Internal slab insulation strip, 6. Internal slab insulation strip.

• Balcony detail

Cantilevered balconies (Figure 5) are a well-known source of thermal bridging in modern buildings. In contemporary construction, thermal breaks are used in these details, but in the mid-20th century, the concrete slab extended from the interior to the exterior without interruption. The slab acts as a thermal bridge, which can lead to surface condensation and even structural degradation due to freeze–thaw cycles in extreme climates. Possible retrofit options can include structural thermal breaks or insulated balcony connectors, but that is not always feasible. Other solutions might be able to address these issues without the need for invasive construction of the balcony cantilevers, which this paper aims to determine.

Figure 5. Balcony detail. 1. External concrete wall, 2. Internal wall insulation, 3. Plasterboard, 4. Reinforced concrete balcony, 5. Reinforced concrete slab, 6. Internal slab insulation strip.

• Parapet detail

Parapets (Figure 6) are often uninsulated or poorly insulated, creating a strong thermal bridge at the roof-wall interface. Modern buildings often feature thin, continuous parapet walls that extend from the exterior to the interior, making insulation continuity challenging. Poorly insulated parapets increase heat loss and lower interior surface temperatures, creating a risk of condensation and mould growth. This paper assesses several possible solutions to find the optimal recommendation.

Figure 6. Parapet detail. 1. Concrete parapet, 2. Flat roof insulation, 3. Internal wall insulation, 4. Concrete external wall, 5. Internal slab insulation strip.

4. Results

The following section presents the results of FEM and WUFI simulations for the four typical junction details studied: wall-to-floor, full-height window, balcony slab, and parapet, representing common conditions in mid-20th-century concrete buildings. The analysis compares steady-state temperature factors ($f_{Rsi}$), heat fluxes (q), and condensation risk patterns under baseline and retrofit scenarios.

4.1. Vapour Control Layer

A parametric study in WUFI Pro was conducted to determine whether an air-vapour control layer (AVCL) is necessary on the warm face of insulation to avoid surface and interstitial condensation. It was concluded that AVCL is required when introducing any of the studied internal wall insulation (IWI) materials.

4.2. The Role of Horizontal Insulation

This study presented a scenario where external wall insulation was not feasible; therefore, internal insulation was applied to the walls. The concrete slab interrupts the thermal barrier and remains partially exposed to external temperatures. As a result, the calculated U-value across the full elevation is higher than the theoretical U-value that assumes uniform conditions. The U-value was derived from the simulated heat flux through the entire detail, representing the overall performance of the assembly, which can be applied to a full building façade.

Internal insulation on the wall alone reduces the calculated heat flux significantly, thereby reducing the structure’s U-value, but the $f_{Rsi}$ increases only slightly. This alone might not suit every building typology due to different $f_{Rsi}$ guidelines as per TGD.

As seen in Figure 7, introducing slab insulation, even as minimal as 30 mm thick and 300 mm long, significantly impacts the $f_{Rsi}$, reducing the risk of surface condensation and mould.

Figure 7. FEM simulation results showing steady-state temperature distribution for the wall-slab junction using EPS insulation. The minimum surface temperature (min $T_s$), ($f_{Rsi}$), and linear thermal transmittance ($\Psi$) value with different insulation lengths are presented.

At a length of 300 mm, the minimum temperature (min $T_s$) and the lowest $f_{Rsi}$ are located at the end of the insulation, rather than near the corner of the detail. However, at a length of 500 mm, the min $T_s$ is no longer at the end of the insulation; this suggests that a 500 mm length is sufficient to significantly reduce the risk of condensation and mould in details such as wall-to-floor junctions and balcony details.

Introducing internal wall insulation does not necessarily increase the minimum surface temperature (min $T_s$) or temperature factor ($f_{Rsi}$); in some cases, it may even slightly reduce them. However, it significantly decreases heat flux (q) and linear thermal transmittance ($\Psi$). Therefore, energy savings can still be achieved, even if condensation risks are high. The $f_{Rsi}$ under the studied circumstances is above the minimum required for offices; however, careful consideration should be given if the affected building belongs to a different typology.

The parametric study examining different wall thicknesses revealed that the thickness of the concrete did not significantly affect the results, as shown in Figure 8. Therefore, the findings can be applied to most wall thicknesses associated with the modern era. This simulation also highlighted the increased heat flux through the critical junction, as thermal transfer is no longer distributed evenly across the entire wall surface but is concentrated at the junction.

Figure 8. FEM simulation results showing steady-state temperature distribution for the wall-slab junction using EPS insulation. The minimum surface temperature (min $T_s$), ($f_{Rsi}$), and linear thermal transmittance ($\Psi$) value of details are shown with different concrete wall thicknesses.

When inspecting the effect of insulation locations (Figure 9), it was determined that if the slab is to be insulated, insulation must be applied to both the top and bottom of the slab. This is also true for internal walls. If the insulation is only installed on one side of the slab, the other side becomes highly susceptible to condensation and mould risk; this risk is even higher than if the slab were not insulated at all.

Figure 9. FEM simulation results showing steady-state temperature distribution for the balcony junction using EPS insulation. Minimum surface temperature (min $T_s$) and temperature factor ($f_{Rsi}$) of details with different horizontal insulation locations are presented.

The above applies to all details involving slab or wall junctions where part of the detail is not exposed to external temperatures. However, when analysing a wall-to-flat roof detail, where both the external wall and roof are exposed to external conditions, a short strip of insulation was insufficient to reduce condensation risks. It was found that insulating the entire roof slab was necessary to achieve a significant improvement in minimum surface temperature and temperature factor ($f_{Rsi}$). Heat flux reductions were also measured in this case and were found to decrease slightly with the introduction of thicker insulation, as shown in Figure 10.

Figure 10. FEM simulation results showing steady-state temperature distribution for the parapet junction using EPS insulation. Minimum surface temperature (min $T_s$), temperature factor ($f_{Rsi}$) and heat flux ($q$) of details with different horizontal insulation thicknesses are shown.

Analyses of the parapet and balcony details revealed that the height of the parapet and the length of the balcony did not affect heat transfer. Additionally, insulating the parapet and balcony structures did not improve thermal performance, and only internal insulation solutions had an impact.

Replacing the windows had the greatest impact on a full-height window/bulkhead detail, as seen in Figure 11 and Figure 12. However, if this is not possible due to conservation requirements or other constraints, insulating the slab, similarly to other details, provides sufficient improvement in the temperature factor ($f_{Rsi}$) to suggest a reduced risk of condensation and mould (Figure 11). Heat flux, however, can only be significantly reduced if the windows are replaced; therefore, from an energy-saving perspective, that is the most effective solution (Figure 12).

Figure 11. FEM simulation results showing steady-state temperature distribution for the full-height window/bulkhead junction using EPS insulation when single-glazed window is retained. Minimum surface temperature (min $T_s$), temperature factor ($f_{Rsi}$) and heat flux ($q$) of detail are presented.

Figure 12. FEM simulation results showing steady-state temperature distribution for the full-height window/bulkhead junction using EPS insulation with upgraded, double-glazed window. Minimum surface temperature (min $T_s$), temperature factor ($f_{Rsi}$) and heat flux ($q$) results are presented.

5. Discussion

Preservation or demolition decisions are often shaped by factors, such as architectural value, heritage significance, embodied carbon and economic considerations. In the context of current EU climate policy and carbon reduction goals, this perspective is increasingly significant: it may no longer be environmentally sustainable to demolish functional buildings simply because they are undervalued or aesthetically unfashionable. Consequently, the retention and improvement of existing structures can be viewed as both an environmentally responsible and culturally valuable approach.

The findings of this study address the complexities of retrofitting with internal insulation in modern concrete buildings. IWI significantly reduces heat flux and improves energy performance, but its impact on condensation and mould risk depends on insulation placement and extent. This study highlighted this issue using the temperature factor ($f_{Rsi}$) as a comparison tool. The results indicate that IWI alone may not sufficiently increase the $f_{Rsi}$ and leaves condensation and mould risks partly unmitigated. While it provides energy savings by reducing heat flux, it may not always meet the minimum moisture safety requirements for all building typologies [108]. This is particularly relevant for buildings with stricter indoor climate regulations, such as residential, leisure or healthcare facilities, where the risk of condensation should be carefully managed.

The key observation is that thermal bridging remains a challenge that requires attention when internal insulation is only applied to the walls. Uninsulated concrete slabs or internal walls that penetrate the thermal barrier show a higher U-value in simulation than the theoretical value assuming uniform conditions, due to the effects of thermal bridging. This suggests that standard energy models could underestimate heat loss in such scenarios, highlighting the importance of more detailed thermal analysis in retrofits.

One effective strategy identified in this study is the introduction of slab insulation to reduce thermal bridging and associated surface temperature drops that can lead to condensation. While this approach may slightly reduce floor-to-ceiling height, even a relatively thin insulation strip (30 mm thick and 300 mm long) can have a measurable impact on condensation risks by shifting the coldest point away from the corner junction. Longer insulation lengths (>500 mm) can further improve hygrothermal performance at critical points. These results suggest that extending insulation coverage to both walls and slabs in retrofit projects generally yields better overall thermal and hygrothermal performance than wall insulation alone.

In full-height window details, replacing the windows had the most significant impact on reducing heat flux. Substantial reductions were achieved only when the windows were replaced, which may also enhance user comfort by reducing cold radiation and draughts. However, this may not always be feasible due to conservation requirements or other constraints [41]. Similarly to other details, insulating the slab improved the $f_{Rsi}$, suggesting a reduced risk of condensation and mould. This approach also avoids visible alteration and supports sustainability by reducing the need for new materials. In such cases, additional care should be given to insulating reveals, as uninsulated connections around deep-set windows can remain vulnerable to mould growth due to low surface temperatures.

Findings from the parapet and balcony analyses may challenge some conventional assumptions. The study indicates that neither the height of the parapet nor the length of the balcony has a significant effect on heat transfer. Applying insulation to these elements alone made little difference, unless coupled with external insulation solutions. Only internally applied insulation yielded notable improvements when under IWI conditions.

The analysis of insulation location and length provided further insights:

• If slab insulation is applied asymmetrically, such as on one side only or at differing lengths, the condensation risk on the uninsulated side may increase. This highlights the importance of addressing thermal bridges holistically.
• Unlike other scenarios where targeted insulation strips improved thermal and hygrothermal performance, a short insulation strip at the wall-to-roof junction was found to be insufficient. This is due to the nature of the detail, as both components are exposed to external temperatures. Continuous roof slab insulation appears necessary to increase surface temperatures and reduce the risk of condensation significantly.

The results of this study are broadly applicable to modern concrete buildings, as variations in wall thickness did not significantly alter thermal or hygrothermal performance.

The findings underscore the importance of an integrated and detail-specific approach to energy retrofits. While internal insulation is often the only feasible option in heritage and conservation contexts, its design requires careful consideration to prevent unintended moisture risks.

Limitations

This study only uses 2D FEM modelling and, therefore, does not address point thermal bridges. The potential influence of point thermal bridges was evaluated during the modelling process. Given the scale and construction type of the analysed details, these effects appeared minor and were unlikely to influence the results or conclusions substantially. Consequently, 2D FEM modelling was considered appropriate for capturing the primary linear thermal bridges of interest, in line with ISO 10211 and accepted research practice.

Different climatic conditions were analysed in an earlier version of the research, covering temperature ranges from –20 °C to +40 °C. The results indicated that, while absolute temperature and humidity values varied, the relative performance and risk patterns remained broadly consistent. Because the study focuses on the Irish climate, where extremes are uncommon, these variations were not considered significant enough to warrant inclusion in the final version. However, the research method could be extended to encompass a wider temperature range in future studies.

Due to the study’s scope and access limitations, in situ monitoring was beyond the present research. This paper aims to provide a broadly applicable analysis of retrofit performance in typical modern concrete junctions, rather than site-specific case studies. Future research could complement the current simulations with empirical validation in representative case studies.

Retrofit strategies should ideally also be evaluated in terms of economic feasibility, practicality of installation, and embodied carbon. While these aspects were beyond the scope of this technical study, they are recognised as important parameters for future integrated assessments to support decision-making.

6. Conclusions

This study aimed to address two key questions: Which retrofit strategies are most effective in balancing energy efficiency with conservation requirements in mid-20th-century concrete buildings? What are the condensation and thermal bridging risks associated with internal insulation strategies in these structures?

In exploring effective retrofit strategies that balance energy efficiency with conservation requirements, the findings suggest that for projects where external wall insulation is not feasible, internal insulation can be an effective approach to reduce thermal loss. It was shown that IWI can significantly decrease the total normal heat flux through the structure; however, local increases may occur at critical junctions, as the thermal transfer is not distributed across the whole surface of the detail and is concentrated at these points.

Full-height window replacement was shown to significantly reduce heat flux by approximately 72%, indicating that window upgrades can be among the most effective energy-saving interventions.

In examining the condensation and thermal bridging risks associated with internal insulation, the study highlights that IWI, while beneficial in reducing overall heat flux, introduces potential challenges related to condensation and mould risks. Therefore, IWI on the walls alone may not sufficiently mitigate condensation risks in specific building typologies, indicating the potential need for additional interventions. In this study, a minimum slab insulation length of 500 mm was found to significantly reduce the risk of surface condensation and mould, increasing $f_{Rsi}$ by 27% and surface temperature by 29%, under the modelled conditions. While this result cannot be assumed to apply across all building typologies, it suggests that extending insulation along the slab could serve as an effective mitigation strategy in similar mid-century construction scenarios. This may be achieved with a relatively small thickness of 30 mm. It was also found that insulating only one side of the slab or internal wall can potentially worsen condensation risks on the uninsulated side, even compared to the uninsulated scenario.

The effectiveness of additional slab insulation was found to vary from detail to detail. A short insulation strip appeared inadequate for wall-to-flat roof junctions, and more continuous roof insulation seems to be required for meaningful $f_{Rsi}$ and surface temperature improvements. It was also found that insulating the external side of the parapet and balcony offered no measurable thermal benefits, which suggests the importance of internal insulation strategies in these cases, such as insulation strips along the internal side of the slab.

In projects where window replacement is constrained due to conservation requirements, slab insulation appears to improve the $f_{Rsi}$, reducing the risk of condensation and mould. In the simulated scenario, the $f_{Rsi}$ increased by approximately 22% and the surface temperature by 21% at window to slab/bulkhead junctions, making the detail potentially suitable for most building typologies other than swimming pools or similar leisure facilities. It was also found that insulating only the bulkhead may worsen condensation risks on the uninsulated side of the slab.

Overall, these results are broadly applicable to modern concrete buildings, regardless of wall thickness, and emphasise the importance of holistic insulation strategies to balance energy efficiency with moisture control.

These findings highlight the value of an integrated and detail-specific approach to energy efficiency. While internal insulation is often the only viable option in heritage and conservation contexts, its application should be carefully designed to prevent unintended moisture consequences. Future work could aim to combine simulation-based analysis with in situ monitoring to validate these results and refine practical guidance. More broadly, improving the energy performance of modern concrete buildings represents an important opportunity, both environmentally and culturally, to ensure their continued use as part of a sustainable built environment.