Introduction

For decades, energy renovation in European housing has been strongly associated with reducing heat losses and winter heating demand. In Montenegro, this priority is understandable: a large part of the residential stock is older and energy inefficient, and thermal insulation remains one of the most effective ways to reduce annual energy needs. Yet the climatic conditions in which renovated buildings will operate are changing faster than the buildings themselves, with hotter summers and longer overheating periods are shifting the renovation target. A building improved mainly for winter performance can still face increasing summer discomfort and cooling demand if solar gains are not controlled and accumulated heat cannot be released. This changes the question from ‘how can we save heating energy?’ to ‘how can a renovated home remain comfortable and efficient throughout warmer years?’.

Four connected studies on Montenegrin residential buildings provide evidence at different scales: climate projections for Podgorica, the capital of Montenegro, parametric simulations of a multi-apartment building, an occupant survey and a national assessment of overheating-prevention capacity. Together, they suggest that cooling readiness should emerge from the interaction between future climate, building performance and occupant behaviour. Montenegro is a useful case because its compact territory includes warm Mediterranean and inland conditions as well as much colder mountainous locations, a contrast that also reflects a wider European challenge.

From heating-led to cooling-ready renovation

Thermal renovation often begins with the envelope, which is fully justified. In a simulation study by Pajek et al. (2023) of a representative multi-apartment building in Podgorica, thermally insulating the opaque envelope had by far the greatest individual effect on total annual heating and cooling energy need. The same study, however, showed that the next most influential actions were related to summer operation: earlier activation of shading and increased natural ventilation. 

It is not an argument against conventional energy-efficiency measures, nor a proposal to replace insulation with a list of passive measures; it is an argument for assessing overheating mitigation measures as a coordinated system. Insulation affects heat transfer throughout the year; shading determines how much solar heat enters the building; windows, layout and ventilation opportunities determine whether heat can be removed when outdoor conditions are favourable, and mechanical cooling then responds to the load that remains. 

A further consideration is the climate basis used to inform renovation decisions. The relevant design horizon is not the climate of the year in which a renovation is carried out, but the climatic conditions expected over the remaining service life of the building and the retrofit measures. This distinction is particularly important for deep renovations intended to perform for several decades. The first signal comes from the changing bioclimatic potential of Podgorica. Pajek and Košir (2022) analysed present conditions and future RCP4.5 and RCP8.5 scenarios for three periods up to the end of the century. Under the two scenarios, the average annual temperature was projected to rise by 2.8 K and 5.0 K, respectively, by 2080-2099. The more relevant result for renovation is the shift in the duration of conditions that require different passive responses. Based on the median projections, efficient shading would be needed 40 days longer under RCP4.5 and 62 days longer under RCP8.5 than under the present climate. In the most adverse RCP8.5 case analysed, the increase reached 74 days. The number of days requiring additional passive overheating-prevention measures, including high thermal mass, night ventilation and evaporative cooling, increased by up to 60 days. At the same time, heat retention and conventional heating were projected to be needed for 25 to 60 fewer days (Figure 1).

_{Figure 1. Projected shift in Podgorica's average daily temperatures under RCP4.5 and RCP8.5 across three future periods. Source: Figure 2, adapted from Pajek and Košir (2022).}

These numbers describe more than a warmer summer: they show a moving design target. Measures that were once secondary can become relevant for a substantial part of the year, while strategies historically prioritised for cold conditions gradually lose part of their relative importance. A renovation decision taken today can therefore be exposed to a very different balance between heating and cooling before the end of its service life. 

Pajek et al. (2023) later tested what this moving climate target could mean for one multi-apartment building in Podgorica. The research combined an occupant survey with 4,536 parametric energy simulations and evaluated retrofit measures under the present climate and future RCP4.5 and RCP8.5 scenarios. In the existing state and present climate, the building was still heating-dominated, but cooling already represented about 32% of the combined annual heating and cooling energy need. Without retrofit, the balance changed sharply under the most severe end-of-century scenario. Heating energy need fell by 56%, while cooling energy need became 3.6 times higher than under the present climate. Total annual heating and cooling energy need could rise by up to 45%. The simulated cooling season also expanded from approximately June to mid-September at present to roughly mid-April to the end of October under RCP8.5 in 2080-2099.

The same study further assessed the relative effectiveness of individual retrofit measures in reducing total annual heating and cooling energy need. Thermal insulation remained the strongest measure, followed by earlier and more consistent activation of shading, while natural ventilation ranked third. In the simulations, changing the shading activation set point from 300 to 100 W/m² reduced total annual heating and cooling energy need by about 12% under the present climate. The benefit rose to approximately 18% under RCP4.5 and 19% under RCP8.5 at the end of the century. Natural ventilation also produced a meaningful reduction, although its relative effectiveness declined in the most extreme RCP8.5 conditions as outdoor temperatures increased. The best combined retrofit cases reduced total annual heating and cooling energy need by 75% under the present climate. The reduction fell to 66% under end-of-century RCP4.5 conditions and 59% under RCP8.5 (Figure 2). This does not mean that renovation becomes ineffective. It shows that climate change can erode part of the expected performance gain, even when the same building and the same package of measures are considered. Testing renovation options against projected climatic conditions is therefore a form of performance risk management.

_{Figure 2. Relative impact of individual retrofit measures on total annual heating and cooling energy need. Source: Pajek et al. (2023), Figure 8.}

The study also highlighted a practical point. High investment cost was identified as the main renovation barrier by 80% of surveyed occupants, while external shading and natural ventilation were underused. Some of the measures with a strong simulated effect are therefore not necessarily the most capital intensive. Their effectiveness can depend on whether existing building features are used at the right time and in the right way.

The occupant is part of the cooling system

Cooling performance depends not only on the measures installed but also on how occupants interact with them. Shading, window opening and night-time ventilation are inherently user-dependent, making occupant behaviour a central part of any human-centred cooling strategy. Understanding how residents actually operate their homes is therefore essential when assessing the real-world capacity of buildings to prevent overheating. A nationwide e-survey carried out in Montenegro in 2023 produced 1,029 valid responses from all 25 municipalities. Košir et al. (2025) used the full survey in the national assessment discussed below, while an initial analysis by Pajek et al. (2024) presented results for Podgorica, Nikšić and Pljevlja, representing three different climate zones in Montenegro.

The Podgorica results illustrate the gap between the presence of equipment or building features and the comfort actually experienced. Air conditioning was reported in 77% of surveyed homes and external blinds in 71%. Nevertheless, 22% of respondents considered daytime summer thermal comfort unacceptable and 26% reported unacceptable comfort at night. Fewer than 40% used night-time natural ventilation for cooling, although it is one of the most accessible passive heat-dissipation measures where outdoor conditions and the building context allow it. These findings suggest that passive cooling opportunities remain underused in practice. This is particularly important where households cannot afford costly renovation measures, as better use of existing shading and ventilation options may still offer a low-cost means of reducing overheating and improving comfort.

Passive cooling is often described through physical components: blinds, thermal mass, openings and glazing. In practice, several of these measures are operational: shading is more effective when it limits solar gains before they enter the room; opening windows during the hottest part of the day can introduce additional heat, while night ventilation can help purge heat stored in the building when outdoor air is sufficiently cooler. Therefore, the same physical building can perform differently depending on timing, control and occupant understanding. This is why human-centred cooling should not be reduced to asking occupants to ‘behave better’. Renovation should make effective operation easier. Shading controls need to be intuitive; occupants need clear information on when natural ventilation is useful; openable windows must be practical and safe; and noise, security, outdoor air quality and local wind conditions can all constrain ventilation. Fans or automated window controls can support ventilative cooling in some contexts. In multi-apartment buildings, shared rules and responsibilities may also affect what can be changed at façade level.

The evidence also cautions against presenting behaviour as a substitute for adequate building performance. During prolonged heatwaves, in homes with vulnerable occupants or where night-time temperatures remain high, passive measures may not be sufficient to maintain safe conditions. Human-centred cooling means designing the building, its controls and any active systems around real users and real constraints, rather than assuming idealised operation.

The cold-climate blind spot

The national assessment of overheating-prevention capacity adds another important perspective: future cooling vulnerability is not limited to places that are already hot.

Košir et al. (2025) developed a method that connects the overheating-prevention measures reported by households with the passive responses required by bioclimatic analysis under current and projected climate conditions. The survey indicated that, on average, 79% of buildings had some form of solar protection capacity. The picture changed when more complex combinations of measures were considered. Only 19% had the capacity for ventilation combined with shading where this combination was relevant; 29% had the combination of high thermal mass, ventilation and shading; and 17% had the more demanding combination of passive measures for hot and arid conditions, high thermal mass, ventilation and shading (Figure 3).

_{Figure 3. Overheating-prevention capacity of surveyed residential buildings and the reported presence of air conditioning across Montenegrin municipalities. 
Source: Košir et al. (2025), Figure 9, CC BY 4.0.}

Under the SSP5-8.5 projection for 2066-2095, municipalities in the colder climate developed a need for passive overheating-prevention strategies that is nearly absent under current climatic conditions. At the same time, passive overheating prevention capacities were reported at broadly similar levels in colder and warmer municipalities, while air conditioning was much more prevalent in warmer locations. This creates a cold-climate blind spot. A historical absence of strong cooling demand can become a preparedness problem when buildings, residents and renovation practice have not treated summer resilience as a priority. In the same national analysis, some warmer locations were projected to become almost exclusively cooling-driven, while conditions in the intermediate climate zone moved closer to those currently associated with warmer areas.

The lesson extends beyond Montenegro. European regions that traditionally concentrated on heating may face a rapid shift in seasonal priorities within the lifetime of buildings being renovated now. Because their building stocks and renovation practices have been shaped primarily around winter performance, they may be less prepared for a rapid increase in summer overheating. The capacity assessment also has limitations that are relevant for practice. It is based partly on self-reported building characteristics and the presence of measures, which does not guarantee their correct design or effective use. A blind, an openable window or thermal mass should not automatically be counted as verified overheating performance. For renovation projects, screening at building-stock level needs to be followed by building-specific assessment.

A practical logic for cooling-ready renovation

The four studies and the Montenegro example point towards a simple sequence for cooling-ready residential renovation:

• Limit avoidable heat gains. External shading is particularly important because it intercepts solar radiation before it enters the building. The design and control of shading should be considered together with glazing area, orientation and solar properties. In appropriate contexts, cool roofs, higher-albedo external surfaces, vegetation and green roofs or façades can also reduce heat gains or moderate the local thermal environment.
• Use available passive heat dissipation. Night-time natural ventilation can remove stored heat when outdoor conditions are favourable. Cross ventilation can improve airflow, but it depends on the arrangement of rooms and openings; single-sided apartments have more limited potential. Thermal mass is most useful for summer comfort when accumulated heat can subsequently be released, so it should be considered together with night cooling rather than as an isolated measure.
• Make the measures usable. Occupants should understand the purpose of shading and ventilative cooling, but the building should not depend on constant expert behaviour. Intuitive controls, simple guidance and, where appropriate, automation can reduce the performance gap between simulated and real operation. Renovation assessments should also acknowledge constraints such as noise, security and outdoor air quality. In multi-apartment buildings, organisational and ownership barriers need to be addressed alongside technical design.
• Add efficient mechanical cooling where passive measures are insufficient. Cooling-ready does not mean passive-only. Air conditioning or other efficient active cooling remains essential during extreme heat, for vulnerable people and in situations where climate or building constraints prevent sufficient passive heat removal. The objective is to reduce avoidable cooling loads before systems are selected and sized, rather than to rely on mechanical cooling as a first and only response.

This sequence is neither exhaustive nor intended to imply a fixed order of priority; it reflects the measures examined in the studies discussed here. The suitable package will depend on the local and projected climate, the existing building, its orientation and layout, household needs and the feasibility of collective action. The essential change is methodological: renovation decisions should evaluate heating and cooling together and should test performance against future climatic conditions, while recognising that occupants are active participants in building performance.

Conclusions

Evidence from Montenegro shows why residential renovation needs to become cooling-ready. Thermal insulation remains central, but future performance increasingly depends on coordinated solar control, passive heat dissipation and effective operation by occupants, with efficient active cooling available when passive measures cannot ensure safe comfort. Retrofit options should be tested against projected climate conditions, not only historical weather, and measures should be selected as complementary packages rather than isolated upgrades. The wider European lesson is especially relevant for traditionally cooler regions: buildings renovated today may face summer conditions for which neither the building stock nor its occupants were prepared. Renovation decisions made today should not create greater cooling challenges in the future.

References

1. Pajek, L. & Košir, M. (2022). Implications of projected RCP4.5 and RCP8.5 climate change scenarios for the bioclimatic potential of Podgorica. GNP 2022 Proceedings, pp. 763-770.

2. Pajek, L., Jevrić, M., Ćipranić, I. & Košir, M. (2023). A multi-aspect approach to energy retrofitting under global warming: A case of a multi-apartment building in Montenegro. Journal of Building Engineering, 63, 105462.

3. Pajek, L., Potočnik, J., Košir, M., Ćipranić, I. & Jevrić, M. (2024). An overview of overheating prevention measures in Montenegrin residential buildings based on occupant survey results. GNP 2024 Proceedings, pp. 859-866.

4. Košir, M., Ćipranić, I., Jevrić, M., Potočnik, J. & Pajek, L. (2025). Climate-change-induced overheating prevention capacity of Montenegrin residential buildings. Building and Environment, 269, 112458.