1. Introduction
The global mean temperature has increased by about 1 °C during the last 150 years [
1]. Furthermore, in the future, heatwaves, “periods of extreme heat, which are differently defined in health, agriculture and climatology based studies” [
2] will be more frequent, more serious, much longer [
3], and associated with more environmental hazards than presently [
4].
Heatwaves affect lung functions and blood flow in the human body, cause cardiovascular diseases and increase population mortality [
5]. The mortality effects of heatwaves are higher in urban areas than the rural area. In Helsinki as a representative of urban areas in the Nordic countries, during four recent intensive heatwaves, the heat-related mortality risk was 2.5 times higher than in the ambient rural areas [
6]. Buildings are at the forefront of the challenges of warming climate from two points of view. First, it is estimated that, for a developed nation, about 25%–40% of energy-related emissions of carbon dioxide which leads to climate change can be attributed to buildings. Thus, the energy efficiency of buildings is an important factor for mitigating climate change in the political and technical European agenda [
7]. Second, due to the negative effects of climate change on human health, it is crucially essential to investigate the risk of overheating under climate change and see its effects on the thermal comfort and well-being of the occupants. Since using cooling systems is not common in residential buildings in Nordic countries [
6], the risk of overheating is higher than in commercial or public buildings.
Previous studies about the energy efficiency and overheating risk of residential buildings under changing climate and the efficiency of different strategies for reducing the demand for energy or peak power can be categorized into two groups. The first group has considered average climatic conditions using test reference year (TRY) climatic data, while the latter has focused on heatwaves of the current climate. Both groups, reviewed below, include studies for a range of current and projected future climatic zones [
8,
9].
In the first group of studies in which average climatic data is used, in the hot and humid climate, research by Invidiata and Ghisi [
10] in Brazil has used TRY climatic data of the 2020s, 2050s, and 2080s of three cities in Brazil to investigate the impact of climate change on heating and cooling energy demand in dwellings. The results showed that passive strategies can reduce up to 50% of the future annual cooling and heating energy demand. Future trends of cooling energy of residential buildings in Taiwan are investigated in the study by Huang and Hwang [
11], using TRY climatic data of the 2020s, 2050s, and 2080s. It showed that the combination of several passive strategies can significantly reduce the increases in the cooling energy usage caused by the effect of climate change. In Hong Kong, Sheng Liu et al. [
12] examined the passive design strategies for residential buildings under the different future climate change scenarios. The results showed that the combination of passive strategies can reduce annual and peak cooling load up to 56.7% and 64.5% under climate change, respectively. May Zune et al. [
13] investigated the overheating risk in modern dwellings in Myanmar under changing climate. They found a high overheating risk in the dwellings by 2070.
In the desert climate of the United Arab Emirates, warming by 5.9 °C was estimated to result in an increase of 23.5% in the energy used for cooling buildings [
14]. Karimpour et al. [
15] in Australia, investigated the effects of different envelope designs on the energy efficiency of residential buildings in an area having a hot-summer Mediterranean climate. They concluded that with climate change, heating becomes significantly less important in modern better-insulated buildings. Thus, to cope with the impact of climate change, it is important to focus more on measures that reduce cooling load. Investigating indoor thermal conditions under changing climate, the adaptation of dwellings of Argentina to climate change has been examined in a study by Filippin et al. [
16]. The results showed that in 2040, it is necessary to revise the dimensions of the glazed exterior façade areas and improve solar protection to deal with overheating.
In the temperate climate of central Europe, with mild winters and warm summers, Salthammer et al. [
17] indicated that current thermal insulation standards do not protect indoor spaces from overheating in the future climate of Germany and, therefore, smart solutions for mechanical and natural ventilation should be introduced. A recent study in the UK by Elsharkawy et al. [
18] about the effects of retrofit under three different scenarios of future climate showed that occupancy patterns have a significant impact on the predicted duration of overheating. Moreover, in the Italian Apennine Mountains, de Rubeis et al. [
19] focused on heating energy consumption and estimated it to decrease by 44.8% by 2080.
Heatwaves of the current climate and their effects on indoor temperature conditions and energy demand of residential buildings have been investigated in the other group of studies. In the Mediterranean climate of southern Europe, Barbosa et al. [
20] evaluated the vulnerability of existing dwellings of Lisbon to the heatwave of summer 2003 and the future projected climate. They found that both night ventilation in bedrooms and increased insulation can decrease indoor temperatures during heatwaves. Another study in Portugal showed that changes in average outdoor temperature do not affect electricity consumption significantly, but extreme temperatures during heatwaves have a stronger impact on the peak power demand [
21]. In the temperate climate of Switzerland, Xiaohai Zhou et al. [
22] analyzed the performance of two mitigation measures, hygroscopic materials and precooling, for the heatwave in the summer of 2018. A combination of these two mitigation measures was found to have a large potential of reducing the heat stress of people during heatwaves while minimizing the energy consumption of buildings. A study in the cold climate of Canada by Laouadi et al. [
23] proposed a simulation method to define overheating and evaluate the risk of thermal discomfort and health problems during heatwaves. By using the method to an example building in the heatwave of 2010 it was discovered that highly insulated and airtight buildings are at a higher risk of overheating than older buildings, and opening windows can significantly reduce the overheating risk.
In Northern European countries, with cold winters and rather warm summers, a field study by Maivel et al. [
24] examined the impact of ventilation, the orientation of the building, and window size on recorded overheating in 100 Estonian apartments. It was shown that without any passive solar protection solutions, modern buildings were regularly overheated already in the current climate of northern European countries. Three other simulation studies have focused on climate change and its effect on energy efficiency and thermal conditions of residential buildings in the cold climate of Nordic countries. In Sweden, Dodoo, and Gustavsson [
25] simulated buildings of three different energy regulatory regimes. They used three different future climate scenarios. Overheating risk was analyzed by comparing the percentage of time with indoor temperatures over 28 °C and between 26 °C and 28 °C, respectively. Cooling and heating energy were also calculated. Based on the results, passive cooling measures, e.g., solar shading will become more important under future climate scenarios. Investigating the effects of climate change on the energy demand and primary energy use of a residential building in Sweden, Tettey et al. [
26] indicated that with suitable strategies, e.g., window shading significant primary energy reductions are achievable under climate change. In Finland, Jylhä et al. [
27] estimated that for a typical detached house, the cooling energy use in the summer will increase but the annual purchased energy for heating and cooling will decrease by 20–35% by 2100, depending on the magnitude of the climate change.
Despite the extensive studies on overheating and energy demand under changing climate, more work for buildings in cold climates is needed to quantify the risk of overheating under climate change and the associated heatwaves. This research gap is summarized for future consideration as follows:
Most studies have focused on the actual data of previous heatwaves, while due to climate change, more severe and longer heatwaves are expected. Therefore, it is important to assess the performance of the residential buildings under those extreme weather conditions by using synthetic future weather data.
Current overheating assessments are mostly done in warm and hot climates, while climate change is more rapid in Nordic countries, and most of the residential buildings in this area are not equipped with cooling systems, so the overheating risk would be higher in this area and should be taken seriously. In Nordic old and new apartments, a systematic analysis of the effect of different passive and mechanical cooling solutions on indoor climate and energy consumption is essential in future climates.
In this paper, we assessed overheating risk and the energy efficiency of a typical old and new apartment building in southern Finland under climate change using both average and extreme weather conditions of exceptionally hot summers. The work was based on a dynamic building energy simulation and climate change projections derived from a multi-member ensemble of global climate models. We addressed the following research questions: (a) What are the simulated current indoor temperature conditions and energy demand of Finnish residential buildings? (b) How much will the energy demand and overheating risk increase by 2050 due to climate change? (c) How much can passive measures affect the overheating risk in the current and future climates? (d) How do the buildings respond to heatwaves, and how serious are the past and future heatwaves in comparison to the current and future average weather conditions? (e) How much will the overheating risk and cooling demand increase during the future heatwaves associated with climate change?
The novelty of this study is the usage of future projected data set of summer heatwaves alongside the actual climate data set of summer heatwave 2018 for describing their impacts on indoor temperature conditions and energy demand. The results of the simulations were compared to the results of the current and future average climate conditions to understand how the severity of the event would be different in the thermal comfort and well-being of the occupants and how much the energy demand is influenced. Furthermore, a more systematic and detailed analysis of the impact of active and passive cooling measures was done, and their effects on overheating risk and energy demand of residential buildings were discussed.
4. Discussion
Several factors should be considered when interpreting the quantitative results. One of them is that there is uncertainty in the scenarios for climate change, and only one of the available alternative RCP greenhouse gas emission scenarios is used in this study. However, the impact of the choice of RCP4.5 on the results is relatively small. Moreover, there are other sources of uncertainty in TRY2050, arising from internal climate variability and climate modeling. In our study, the climate change scenarios for the future are derived from a large sample of climate models [
35] which increases confidence in the scenarios in comparison to using just a single model. Furthermore, technical development, improvements in building practices, and socio-economic changes are hard to be considered for the future. Another factor in the building simulation model is occupancy profiles which affect the accuracy of the results. In this study, the presence of occupants has been assumed in a way that is as close as possible to the modern lifestyle of families. Using the fixed and continuous heat gains may cause different results. Regarding the overheating risk, Finnish regulations for the temperature limits are used in this paper, and in other Nordic countries, the temperature limits may differ. For example, in Denmark, the degree hours above 26 °C are recommended to be less than 100 Kh [
48]. These limitations must be taken into account when the results are interpreted internationally.
Despite these limitations, the severity of the effects of climate change on the health and thermal comfort of residential buildings occupants are not questionable, especially in the Nordic countries where most of the residential buildings are not typically equipped with mechanical cooling systems. According to the present simulations, the degree hours above 27 °C will increase in the future climate by more than 50% in both the old and new buildings. During the current and future heatwaves, the degree hours above 27 °C increase up to 10,000 Kh and 14,000 Kh, respectively, in both old and new buildings. In almost all the cases without mechanical cooling in the new building under the current and future climate, the degree hours above 27 °C are significantly higher than the required number of 150 Kh in the Finnish building code [
27]. This shows that except for the openable windows, other building measures are not able to guarantee to fulfill the building code requirement, and only with the usage of the split cooling unit, the new building may fulfill the 150 Kh requirement.
Although the national requirements do not apply to existing buildings, exemplified here by the old building, results for it are worth to be considered. The degree hours above 27 °C in the cases without mechanical cooling in the old building are significantly higher than 150 Kh in the current and future climate. Moreover, none of the building measures introduced a guarantee that the old building can fulfill the requirements. Thus, this indicates a high risk of overheating in the old building. In the future climate and during extreme weather conditions, the situation will be even worse in both old and new buildings. Thus, it seems to be important to revise the regulations of building stock renovation and design practice considering climate change and its effects on the health and comfort of occupants. Moreover, providing common requirements for residential buildings in Nordic countries or the whole of Europe would be worth considering while there are globally common standards and design practices for commercial buildings [
49,
50].
In most cases, the maximum indoor temperatures exceed 32 °C without mechanical cooling. The threshold value of 32 °C is the upper limit for the health of occupants in all the living spaces for both existing and new buildings in Finland [
28]. For elderly people who are cared for in the residential living spaces, this maximum value is reduced to 30 °C [
28]. The new building has the lowest degree hours above 32 °C with 1 Kh in the current climate and 39 Kh in the future climate. These degree hours will reach more than 1000 Kh in both building types during heatwave summers of 2018 and 2050. This overheating may cause serious health problems for the occupants. A study in Finland showed that the heatwaves in recent decades have caused about 200 to 400 excess deaths in Finland [
6]. Thus, Solar protection windows, openable windows, split cooling units, and ventilation cooling and ventilation boost can effectively reduce the degree hours above 32 °C in both building types in the current and future climate. Hence, to avoid the health risks of overheating, the building stock needs to change in design and renovation phases to cope with the future extreme and average climate conditions.
Orientation, blinds, and site shadings are not significantly effective on overheating risk and energy demand. Other passive strategies like solar protection windows improve indoor temperature conditions but cannot alone fulfill the temperature requirements. Openable windows are the most effective studied passive strategy in the new building, being able to reduce the degree hours above 27 °C to the acceptable range in the current and future climate. However, they are not effective enough for the fulfillment of the temperature requirements in the old buildings.
It is also important to note that based on the regulations, openable windows are not allowed to be used for the fulfillment of indoor temperature requirements in the design phase of new buildings [
29]. Although opening the windows can bring fresh air inside and reduce the overheating risk, it should be considered that by opening the window, indoor air quality and indoor acoustic conditions will be affected by the outdoor conditions. Moreover, there are security issues with opening the windows. Overall, building measures may improve the indoor temperature conditions under changing climate but they seem not to be sufficiently effective.
The maximum indoor air temperatures are significantly higher during the heatwave summer of 2018 than during the average present and future climates (TRY2020 and TRY2050). The increase in the indoor air temperature is even a more serious issue in the heatwave summer of 2050. The results show the high risk of overheating in the residential buildings in the current and future climate of Finland. To decrease the indoor temperature during the heatwaves, mechanical cooling systems should be implemented. Based on our simulations, the space cooling electricity consumption increased during the heatwave summer 2018 in a way that it was higher than the amount in the current and future average climate in both building types, and in the heatwave summer of 2050, the increase would be even higher.
Mechanical cooling systems, like split cooling units or cooling of ventilation supply air, are required to reduce the indoor air temperature effectively in the future climate. According to the present results, the energy consumption of split cooling units is below 3 kWh/m2 in the current climate and will reach 4 kWh/m2 in future climate. Typically, in Finland, the cost of a split unit is about 2000 euro for the equipment and installation. However, this cost level is reasonable for reaching the indoor temperature target values. Since the benefits of acceptable indoor temperature on the well-being and comfort of the occupants are not questionable.
Moreover, the energy needed for cooling can be partly provided by free cooling from outdoor air or ground-coupled systems. However, the efficiency loss of these free sources due to the high outdoor air temperature during the heatwaves should be considered. From the dimensioning point of view, the used dimensioning cooling power of space cooling (45 W/m2) seems to be enough in average and extreme climate conditions, and there is no need to increase the peak power of space cooling due to climate change.
In this study, average ground temperature and therefore domestic cold water temperature is assumed to be the same in the future regardless of global warming. The reason is to ease the analysis of the parameters affected more strongly by climate change. The system losses, efficiencies, and COP of the cooling systems are assumed to be the same now and in the future. This is done to facilitate the comparison and to show clearly the effect of climate change. The performance of HVAC systems may improve in the future, as according to studies performed during recent decades, COP of heat pumps increased by 2% annually [
51]. Moreover, in the 2000s, COP of commercial air/water heat pumps increased almost linearly from 3.5 to 5.1 [
52]. Any improvement in the energy efficiency of heating and cooling systems affects the purchased energy (district heating and electricity) consumption, and the amount of increase in cooling electricity and decrease in district heating may vary in the future. Thus, even if the cooling demand increase, the technical development of systems may partly compensate the increase.
Concerning the increase in the cooling electricity consumption in the future climate, a change from the common practice of split cooling units in apartment buildings to district cooling may be considered. In that case, fan coil units or radiant cooling (ceiling or floor) may be in use. Fan coil units are not limited by power and capacity. For the usage of radiant cooling systems, these limitations must be noted. However, different cooling systems should be considered to carry the increase of cooling demand in the future.
5. Conclusions
The impacts of climate change on overheating risk and energy consumption were assessed for old and new apartment buildings in Finland. Moreover, the effects of heatwaves on space cooling demand and overheating are analyzed. Test Reference Year (TRY) hourly weather data of 2020 and 2050 under the RCP4.5 scenario, weather data of heatwave summer 2018, and synthetic future weather data of heatwave summer 2050 under the RCP4.5 are used as the input of IDA ICE simulations.
The impacts of the changing climate on the indoor temperature conditions were assessed using different passive measures for reducing the risk of overheating, including orientations, blinds, site shading, window properties, openable windows in the old and new buildings. Furthermore, the effects of cooling coil in the air handling unit together with ventilation boost on overheating risk were analyzed in the new building.
Although the space cooling electricity consumption in the old building is higher than the new building, the annual electricity consumption is higher in the new building due to the HVAC aux electricity consumption.
According to the present simulations with the assumption that the efficiency of the systems is the same now and, in the future, the annual district heating demand will decrease by 8–12%, while the annual electricity demand will increase by 2–4%. However, the absolute value of the space cooling electricity is small compared to the annual space heating demand. Consequently, the amount of decrease in district heating consumption is much higher than the increase in electricity consumption in the future. In the heatwave summers, the space cooling electricity increases significantly.
Regarding the indoor temperature conditions even in the current climate, in the old building, the amount of degree hours above 32 °C is noticeable, and the risk of overheating is higher than in the new building. In the future climate, the maximum indoor temperature exceeds the health risk threshold value of 32 °C in both new and old buildings. During the heatwave summers of 2018 and 2050, the amount of degree hours above 27 °C is about 10,000 Kh and 14,000 Kh, and the degree hours above 32 °C about 1000 Kh and 3000 Kh, respectively. Thus, not only the overheating risk in both buildings is high in future climate and during heatwave summers, but also the indoor conditions would be harmful to the health of occupants and it would cause an increase in mortality rate.
Orientation, blinds, and site shadings are not significantly effective in alleviating the overheating risk. Solar protection windows improve indoor temperature conditions significantly. Although openable windows are the most effective studied passive strategy which can reduce the degree hours above 27 °C to the acceptable range and decrease the degree hours above 32 °C to zero in the new building, they cannot completely fulfill the regulation temperature requirements in the old building. Despite the effectiveness of the openable windows on reducing the overheating risk, issues like outdoor pollutants spreading and noise control should be considered when the windows are open.
Cooling coils of the air handling units together with ventilation boost can reduce the indoor air temperature effectively in current and future climate, and the cooling power is not high enough because the specified supply airflow rate is small. However, the split cooling unit installed only in the living room can prevent overheating in the current and future climate and also during the heatwaves in all the spaces by using a small amount of extra energy.