1. Introduction
Buildings and building construction account for almost one third of global final energy consumption, as well as for nearly 40% of total direct and indirect greenhouse gas (carbon dioxide (CO
2)) emissions [
1].
Similarly, in the European Union, buildings are responsible for the largest proportion of energy consumption and associated CO
2 emissions [
2], and residential buildings account for 23% of total energy consumption in the European Union [
3]. Also, two thirds of the buildings in the European Union were built when energy efficiency requirements were limited, and the majority of these buildings will still exist in 2050 [
4,
5]. As space heating represents a 60–80% proportion of total energy consumption in buildings in a cold climate [
6] and as renovation can substantially decrease the demand for space heating, buildings play a key role in achieving long-term climate and energy targets; that is, buildings can play a key role by implementing basic renovations. Renovation not only impacts on the energy consumption of buildings, it also considerably influences the indoor climate conditions. This has been recognised by the European Union [
7,
8,
9], which—besides emphasising the low energy consumption, reductions in CO
2 emissions and improved energy efficiency in the building sector [
10]—emphasises the indirect benefits achieved by renovations. One of the factors highlighted by the Energy Performance of Buildings Directive (EPBD) [
7], is indoor climate conditions, such as indoor air temperature, air humidity, the CO
2 level and the sound environment. To achieve this essential factor of the EPBD, a building should meet the energy requirements and should have proper heating and ventilation systems.
Although indoor climate can refer to the thermal, chemical (air quality), actinic, acoustic or mechanical climate, this study concentrates on thermal climate and air quality, for which the World Health Organization (WHO) has given recommendations [
11]. Thermal (climate) comfort studies have a long history [
12]. Wu et al., described thermal comfort as the thermal experience, air temperature change rate and temperature ramp direction in all conditions [
13]. In many studies, thermal comfort has been widely demonstrated to be an important factor for human preferences [
14,
15,
16,
17,
18,
19]. Berglund and Cain [
18], studied the perception of air quality and the thermal environment, concluding that physical properties (such as temperature and humidity) and chemical properties (such as contaminants) together determine the level of comfort. However, in their experimental study, Wang and Liu [
19], discovered that different emotional states impact on people’s thermal comfort. The results indicated that whether sitting or standing, emotional states significantly impact thermal comfort and physiological parameters [
19]. It is also widely acknowledged that females have around a 1.5 °C higher comfort temperature than males [
20,
21].
Several studies have demonstrated the great potential for energy saving by installing ventilation heat recovery systems [
16,
22,
23,
24,
25,
26,
27] and improving the thermal quality of the climate envelope (i.e., insulating attics, walls and foundations, as well as installing double or triple glazing) [
16,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32]. More than half of these studies reported on deep renovation projects in similar climatic conditions to those of this study [
22,
23,
24,
25,
26,
27,
30]. Liu et al. [
23], studied multi-family buildings in Sweden in order to identify the most cost-effective retrofitting measures and to realise energy and CO
2 reductions of 50% and 75% respectively by 2050 [
33]. The study showed that at current energy prices and renovation costs, an approximately 7–27% reduction in energy consumption is still possible. Similarly, the study by Fleur et al. [
24], investigated life cycle costing (LCC) and optimal energy measures by using an optimisation approach as part of the renovation of a multi-family building in Sweden. The study demonstrated that it is not cost-effective from an LCC perspective to invest in major energy-efficiency measures to reduce the space heating demand. Considering the 40-year life cycle and given framework conditions, the improvement in the thermal performance of the building envelope or the implementation of heat recovery ventilation measures taken to reduce the space heating demand are not regarded as cost-effective measures. However, with a 40% saving target, a balanced mechanical ventilation system with heat recovery has been verified as being cost-effective. Nevertheless, the lowest LCC was achieved when modern windows were installed that had better
U-values and a longer technical lifetime than the original windows [
24]. In addition to that, the study of La Fleur et al. [
24], highlighted that thermal comfort is usually better in a building that has undergone a major energy renovation than it is in a building with poor thermal performance.
It has been widely acknowledged that user behaviour, such as the actions users take or do not take, is one reason for the differences in the expected, modelled and measured performance of buildings [
34,
35,
36,
37,
38,
39]. The study by Dar et al. [
34], demonstrated that an approximately 12% deviation from predicted energy consumption can be affected by user behaviour. However, in Sunikka-Blank and Galvin’s study [
35], they indicated that occupants consume 30% less heating energy than the calculated evaluation. In old buildings the deviation could be the result of the loss of ventilation being compensated for by using an exhaust fan or heat recovery, for example. Another reason for deviation could be fairly low occupancy per floor area, such as is the case in single-family homes. Additionally, the performance gap could be the result of uncertainties in the planning stage of the renovation, as De Wilde claimed [
39]. The results of the study by La Fleur et al. [
30], indicated that the energy-saving potential and the choice of renovation measures are significantly affected by assumptions of user behaviour which make the energy renovation planning complex.
On average, people spend around 80–90% of their lives inside buildings [
40]. Nevertheless, relatively few studies have investigated the impact of deep energy renovations on the indoor climate quality in residential buildings located in a cold climate [
26,
27,
30,
41,
42]. However, in California, deep energy renovation, including improved mechanical ventilation systems and air cleaners, resulted in improved indoor climate conditions [
43]. Improved thermal conditions were also reported in New Zealand after simply installing standard insulation [
44]. Földváry et al. [
28], studied the temperature, relative humidity (RH) and concentration of CO
2 in 94 apartments before and after basic energy renovation in Slovakia. In contrast, the study discovered that the CO
2 concentration was significantly higher in the renovated buildings, resulting in lower occupant satisfaction with indoor air quality (IAQ). As a result. Földváry et al. [
28], pointed out that in energy-renovated residential buildings in Central and Eastern Europe, the installation of controlled ventilation systems is recommended.
However, at least five similar studies have investigated the effects of energy renovation on the IAQ in a cold climate. Liu et al. [
26], demonstrated the positive effects of energy renovation on the indoor environmental quality and occupant satisfaction in a mechanically ventilated residential building in Sweden. The building also achieved a 39% reduction in space heating demand [
26]. The Swedish case study by La Fleur et al. [
30,
41], demonstrated that thermal comfort improved as a result of the selected renovation strategies while the use of district energy heating was reduced by 35–44%, including the domestic hot water (DHW). Another study by Fleur et al. [
41], stated that significant improvements to indoor climate conditions can be achieved by the building’s energy efficiency (e.g., adding thermal insulation and using a heated supply air system) [
41]. Both of the studies [
30,
41] utilised the simulation software IDA-ICE [
45], to simulate energy consumption, heating loads and the indoor climate.
Thomsen et al. [
42], described a Danish apartment complex that had undergone a comprehensive energy retrofit including a photovoltaic installation on the roof. This case study [
42], noted that by improving the envelope and the air conditioning in the studied building (built in 1968) the indoor climate improved significantly and there was a 31% reduction in energy consumption. The earlier complications with insufficient temperature, draughts (a noticeable current of air inside the building) and cold areas in the apartment were reduced, which improved the temperature conditions during the winter. As a result, 88% of the residents were satisfied with the indoor climate conditions after the renovation, while prior to the renovation, only 20% of the residents were satisfied. However, the thermal bridges created a number of problems for the indoor climate. All the above-mentioned studies describe a major energy renovation and its results. However, Kuusk et al. [
27], presented a case study analysis of the low-budget renovation of an apartment building in Estonia. The study investigated and measured energy consumption, indoor climate, CO
2 concentration, air leakage rate, the thermal transmittance of thermal bridges and the thermal transmittance of the building envelope before and after the renovation. The low-budget renovation reduced the purchased energy by 40% and the heating energy requirement by 50%. However, the performance of the ventilation systems, thermal bridges in external walls or window jambs and the economic viability required further investigations as estimations set before the renovation failed [
27]. Besides the four above-mentioned studies, in another study, Kuusk et al. [
46], stated that not enough attention was being paid to energy loss through thermal bridges. As a result, they stated that designers should use calculations for thermal bridges because building owners could reduce construction costs [
46].
To give an overview and to compare the results of the projects conducted in this field,
Table 1 presents some details of these case studies.
Whereas most of the above-mentioned studies have primarily concentrated on measuring the impact of thermal insulation, new and improved insulated windows and a heat recovery system on indoor climate conditions, there is a gap in the current knowledge regarding the field testing of indoor climate conditions affected by supply air windows and exhaust air heat pumps (EAHPs) as a part of a major energy renovation. In the supply air window, the outdoor air is led through the window frame (see
Figure 1).
When the air flows through the window structure, the air flow is heated. To meet the purity requirements of the supply air and to keep the windows cleaner, the outdoor air is usually filtered. To avoid excessive room temperatures, the better performance of windows should be considered when calculating heating demand.
Considering that the energy balance and performance of supply air windows have been presented in multiple studies [
47,
48,
49,
50,
51,
52,
53,
54,
55], the lack of experimental results is surprising. However, EAHPs can be used for space heating [
56], a combination of space and DHW heating, or for heating the DHW only. This study focuses on EAHPs used for space and DHW heating as DHW heat pumps using outdoor air as a heat source are uncommon in the Nordic countries [
57]. To the author’s knowledge, studies on EAHPs used for space and DHW heating combined with the installation of supply air windows are scarce.
To fill the knowledge gap described above, this article presents the results of a field test relating to the indoor climate quality of a typical Finnish apartment building that has undergone a major energy renovation that included the installation of supply air windows and EAHPs, among other renovation measures. The combination of supply air windows and an EAHP is a quite new and low-budget way to renovate old, prefabricated apartment buildings. The market potential is great, and it is important to ensure the performance of the renovation concept. To date, the emphasis has only been on energy saving; this study raises the issue of the impact on IAQ alongside it. In particular, the long testing period gives specificity and credibility to the study.
4. Discussion
Housing cooperatives need basic, cost-effective solutions in order to save energy without compromising indoor climate quality. If the building has mechanical extract air ventilation, installing an EAHP and supply air windows is a viable option. This study examines the impact of these measures on the indoor air climate.
Five different measuring methods were used to study the indoor climate of a residential building before and after a retrofit. By combining a blower door, SwemaFlow 3000, and a Tinytag TGP-4500 logger, the indoor environment was studied for human comfort. The measured features comprise temperature, humidity, CO2 concentration, draughts, air flow rates and the tightness of the building envelope. Energy consumption and outdoor conditions were subject to real-time monitoring.
As a result of the energy-technical measures of the renovation, the energy consumption of the studied building decreased by 45%. The results of this field study are similar to a Danish study by Thomsen et al. [
42], as well as a Swedish study by La Fleur et al. [
30,
41], and Liu et al. [
26]. In all of these field studies, significant savings in energy consumption were achieved. Thomsen et al. reported a 31% energy consumption reduction (in both heat and DHW energy consumption) and significant improvements in the indoor climate, while La Fleur et al., demonstrated a 34.7% reduction in district heating and DHW demand (a 44% reduction without DHW) due to the increase in the indoor air temperature after the renovation. Similarly, the study by Liu et al. resulted in a 39% reduction in space heating demand. In addition to this, the indoor climate similarly improved in all of the above-mentioned studies, mostly as an indirect effect of the improvement in energy efficiency. However, in this field study the savings in energy consumption were mainly attributable to the EAHP, although the indoor climate was considerably improved by the renovation of the building envelope and the ventilation. The tightness of the building envelope improved by approximately 40%, the draughts (air flow) decreased by 24% and the floor-level temperatures increased. These results are approximations as the outdoor temperature, and wind speed and direction were not identical at different measurement times. Using the same improvements but adding thermal insulation and using a heated supply air system, Fleur et al. [
41], had similar kinds of results for draughts, but instead of field study measurements, a questionnaire and IDA-ICE simulation were used. In this study, the air flow remained at the same level as with the mechanical exhaust ventilation after installation of the EAHP. The measured air flows were low and the indoor temperatures of the apartments were high for part of the heating season. Thus, the heating system was balanced again after the renovation during the heating season in 2018 and the room temperatures decreased to a comfortable level. Studies like [
30,
42], had similar results regarding the higher and more comfortable indoor temperature. However, the temperature rise was not as significant as in this study. One solution for excessive room temperature could be dynamic control of the heating system in order to keep the room temperature at a comfortable level during the heating season. According to Ala-Kotila et al. [
68], it would also be possible to achieve further energy savings.
Besides the energy savings, the efficient tightness of the building envelope has other benefits. It avoids the condensation of water in the structures, preventing impurities from flowing into the indoor air from non-compact spaces and avoids the draughts caused by air leaks. The draughts can be also compensated for by increasing the room temperature and adjusting the air conditioning. By reducing the draughts, a convenient indoor air temperature and better energy efficiency of the building can be achieved. The efficient tightness of the building also improves the soundproofing and acoustics, which is desirable in an urban environment. Liu et al. [
26], demonstrated reduced noise levels from outside in their study, and this study indicated similar results, even though the building is located along a busy main road. However, this study received feedback from the residents through conversations, while Liu et al. used a structured questionnaire with the response rate of 53%, of which 11 answers came from residents in the retrofitted building and the remaining 31 answers came from other buildings in the area. Conversely, Thomsen et al. [
42], did not achieve a similar change for apartment noise compared with the change found in these studies.
In this study the energy consumption of the building decreased by 45% (the annual energy saving is about 75 kWh/m
2) with the mentioned measures [
63]. La Fleur et al. [
24], similarly stated that a 40% saving target is verified to be cost-effective when a balanced mechanical ventilation system with heat recovery has been taken into operation. In this study both of the above-mentioned measures were implemented and the payback time for the energy-related investment and its financing is about ten years, after which it becomes profitable. This study, as well as the study of La Fleur et al. [
24], demonstrates that by implementing simple renovation measures, it is possible to accomplish both profitability and thermal comfort.
Many studies have demonstrated that user behaviour is one reason for the differences in the performance of buildings [
34,
35,
36,
37,
38,
39]. Likewise, in this study, a further consideration is that residents should be well instructed about the seasonal supply air window adjustment. In this field study, many apartments had the winter settings switched on during the summer, which caused a heated air flow into the apartments through the window air valves. Property managers, housing committees or maintenance services should be responsible for issuing reminders as well as changing the filters of the window air valves. Overall, the residents of the studied apartments were satisfied with the renovation process, outcome and were particularly satisfied with the attractive appearance of the façade. As an additional benefit of the renovation, the residents communicated a reduction in incoming traffic noise. However, some dissatisfaction was expressed about draughts entering through the vents of the supply air windows.
Although the study included a small number of housing units, the exceptionally long testing period with the extensive number of samples is the existent strength of the study. As an observation, the running period of several years and adjustment of the technical systems were essential for achieving all the benefits. In future studies, the measuring period before the renovation should be 12 months instead of the 4–5 months that was used in order to cover all the seasons.
As this study exposed the challenge of stabilising the indoor temperature in order to avoid the excessive room temperatures, further studies should concentrate on the profitable dynamic control system’s [
68], suitability for reducing the power peaks in cases like this. In addition to the better indoor temperature, the building owners who pay the energy bill can gain financial benefits if power peaks are reduced. Also, physically shading windows to achieve lower indoor temperatures should be considered. It is also apparent that any future energy retrofitting studies that include supply air windows, ensuring adequate fresh air by having large enough trickle vents is the most notable technical challenge and must be thoroughly investigated.
5. Conclusions
Finland, as well as the rest of the European Union, is confronting the challenge of renovating an aging building stock. This study offers a field test approach to understanding the effect of deep renovation on the indoor climate quality and its profitability in an apartment building in Finland. The deep renovation included façade repair with extra insulation, new windows with trickle vents, new balcony glass and doors, and the installation of an EAHP into the existing mechanical exhaust air ventilation.
By combining the measurements and occasional occupant conversations, the study has investigated how the building’s energy use and indoor environment have been reformed by comparing the retrofitted and non-retrofitted building.
The retrofit has shown positive effects on energy performance in the building as well as provided a better indoor environment in the building where residents spend a significant amount of their time. However, the study revealed the challenge of excessive room temperatures. In order to avoid that challenge in deep renovations in general, one option could be to reduce the heating power accurately. Instead of a traditional static heating control system, dynamic control that takes into account real-time room temperature could be a solution, avoiding excess temperatures as well as achieving additional energy savings. Another noteworthy outcome of the study was that in order to achieve all the benefits, there needs to be a running period of several years, as well as adjustment of the technical systems.
This study demonstrates that, in addition to energy reduction in the building sector overall, deep renovation also has a role in providing a better indoor environment in residential buildings.
In conclusion, by deep renovation, considerable savings can be achieved compared to the actual investment. Regardless of the challenges that were faced, the residents of the apartments were satisfied with the improved indoor climate. The results of this study can be generalised to those countries where similar prefabricated apartment buildings with mechanical exhaust ventilation are common.