Nowadays, people spend an average of 87% of their time inside buildings, and about 69% at home [1
]. Hence, it is essential to ensure the highest possible level of indoor air quality (IAQ), which is affected by indoor and outdoor pollution sources. Providing that the quality of outdoor air is acceptable or that filters are used, the IAQ level is usually improved by increasing the ventilation rates. However, depending on the season, this means that a larger volume of air must be cooled down or warmed up to ensure the same level of thermal comfort, leading to higher energy consumption and associated costs. Energy consumption in residential buildings already accounts for more than 25% of the entire energy consumption in the EU [2
], and this figure is likely to increase. Therefore, it is important to identify means to revise this upward trend without jeopardizing the indoor environmental quality (IEQ).
IAQ is one of the four IEQ components (the others are thermal, visual, and acoustic comfort) and it refers to the nature of air that affects the health and well-being of occupants. ASHRAE Standard 62.1–2016 [3
] defines acceptable IAQ as the “air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction”.
The World Health Organization (WHO) estimated that 23% of global deaths are linked to an unhealthy living or working environment [4
]. The health effects of indoor air pollutants might emerge immediately after exposure as well as several years later. In addition to the duration of exposure, it is essential to consider the corresponding exposure limit value (ELV) in order to determine the harmfulness of a pollutant in a certain environment [5
]. In other words, the pollutant level and exposure time have to be considered together to evaluate the level of risk.
Air is a complex mixture typically composed of more than 200 to 300 pollutants [6
]. The European Respiratory Society (ERS) has identified the most important pollutants of indoor air, namely carbon monoxide (CO), carbon dioxide (CO2
), nitrogen dioxide or trioxide (NO2
), polycyclic aromatic hydrocarbons (PHAs), particulate matter (PM2.5
, etc.), volatile organic compounds (VOCs), allergens, formaldehyde (H−CHO), radon, biological contaminants, and ozone (O3
All these compounds are usually present in indoor air, with different concentrations depending on the emission rate of the relative sources and the ventilation rate of the room. Thus, an analysis of the presence of specific sources is essential to assess the risk of exposure to certain contaminants, and to suggest solutions to decrease the contaminant concentration below acceptable thresholds.
Most indoor pollutants derive from anthropogenic sources [7
] that can be divided into indoor and outdoor ones. The former category comprises building materials (construction materials, furniture, paintings, ventilation ducts, etc.), cleaning products, heating and cooking appliances, people, and clothes while the most diffused outdoor sources are related to combustion processes in industrial and domestic plants and vehicles.
Focusing on indoor pollution sources and substances and residential buildings, two key pollutants are formaldehyde and CO2
. Formaldehyde is a volatile organic compound (VOC) that has been discussed for decades as a typical indoor pollutant [8
], which derives from construction materials or furniture, and is very harmful as it is carcinogenic. The analysis of a collection of formaldehyde measurement data from residential buildings in different countries shows concentrations often close to the short-term guideline value of 100 µg/m3
proposed by the WHO Guideline for IAQ [9
is generated during the respiration processes of living aerobic organisms. CO2
is considerably less harmful than formaldehyde, but it is typically used to control ventilation as a proxy for the other contaminants, or as an indicator of the occupancy of the indoor environment.
Low-cost furniture (and often low-cost building materials, in general) is a common source of formaldehyde in domestic buildings, and the spread of this more affordable type of furniture is constantly increasing. To solve this problem, two possible routes are ventilating more or purchasing less pollutant products. The former option means higher running costs for energy, the latter a higher capital cost. However, to date, too little is known about the economic comparability of these two routes. Indeed, previous studies on IAQ, ventilation, and economics [10
] focused on other approaches and metrics, such as disability-adjusted life years (DALYs), that provide a method of monetizing contaminant exposure.
The aim of this study was therefore to conduct a cost–benefit analysis of the IAQ in residential buildings. A case-study building was defined, and three sets of materials with different pollution emission levels were chosen: High, low, and very low. For each option, the ventilation rates required to have an acceptable IAQ level were calculated, and the consequent energy consumption (and associated costs) were estimated by means of dynamic thermal simulation in EnergyPlus. In general, it was found that less polluting elements are more expensive but less ventilation, and hence costs, are then needed.
The results of the cost–benefit analysis show that the potential savings range from a few hundred Euro to over 13,500€. This means that there could be a significant extra budget that could be spent at time zero to purchase less-emitting materials but that this would happen only under a few circumstances.
Considering the thermal comfort results during the warm season, the use of very low-emitting materials becomes less appealing. Unless a cooling system is installed, the building occupants would indeed experience a considerably overheated building. IAQ would be acceptable as the ventilation rates would be adequate to keep contaminants below harmful concentrations, but thermal comfort is not guaranteed for the largest part of the period from mid-May to the end of September.
Thermal comfort could be improved by adopting ventilated cooling strategies [25
], which use the cooling capacity of outdoor air to reduce or even eliminate the cooling loads. This would result in temperature-controlled ventilation to increase the ventilation rate when the outdoor air has an effective cooling capacity.
A complementary solution to improve the thermal comfort is relying on air movement to improve thermal comfort during the warm season [26
]. The EN16798:1 adaptive model does not include an explicit method for including air movement as done in other adaptive models, such as ASHRAE 55 [15
] and the Indian model for adaptive comfort (IMAC) [27
]. The former allows an increase of the acceptability upper limit according to the available air speed: 1.2 °C for 0.6 m/s, 1.8 °C for 0.9 m/s, and 2.2 °C for 1.2 m/s. The latter uses different equations in the presence of air movement. The ASHRAE 55 approach is explicit, and hence it might be applied to EN16798:1 limits as shown in Figure 13
. However, this would not solve the thermal comfort issue for the whole period but would slightly increase both the capital cost and running costs, and it is questionable from a scientific perspective. The ASHRAE 55 and EN16798:1 equations are derived from different studies and based on different assumptions. Hence, further research is needed to assess whether the same set-point increases are valid for both models.
Another technically feasible solution is installing an air-conditioning system, such as a multi-split system, that would enable use of it only in a certain part of the building and when needed. However, this solution increases both the capital cost (to purchase the device) and the running costs, which are electivity costs, and hence would have a high impact on the PV calculation. Moreover, a major issue is the significant negative effect of this technology on global warming. Hence, the use of air conditioning should be discouraged from both a financial and environmental perspective.
If the VLE configuration is therefore not feasible for thermal comfort reasons, the maximum available extra budget at time zero (i.e., the maximum PV for the HE − LE scenario) is cut down to approximately 8000€. As a result, it seems that only the 10-year scenario has a median PV value of 4485€ (Figure 7
), which enables a non-negligible upgrade of the materials. Considering furniture, there is an extremely wide range of products available on the market with very different prices. This extra budget might enable some improvements, but it is arguable that it is sufficient to use only low-emitting furniture for a house, such as this case study. This means that the total capital cost required to use only low-emitting materials is likely to be higher than 4485€ (and also than 8000€), and hence a pure cost–benefit analysis would not support this choice as the NPV would become negative.
However, the use of low-emitting materials leads to other benefits that are not considered in a cost–benefit analysis. Firstly, thermal comfort is enhanced and IAQ is similar to the HE scenario but using lower ventilation rates. This means that if, for any reasons, higher ventilation rates cannot be guaranteed, the house would still be thermally comfortable and healthy, being more resilient. This is a major advantage as the occupants of a comfortable and healthy house are more likely to be healthy too, generating a benefit for themselves and for society. Secondly, the use of a lower ventilation rate means less energy consumption, and hence a reduction of CO2 emissions, which is again a benefit for the whole society. Lastly, the disposal of low-emitting materials is also likely to be easier from an environmental point of view. All these points together can be classified as co-benefits, which could be monetized, but the methods to do so are complex and hence more studies are needed for an accurate evidence-based co-benefits analysis. This analysis would explain why and the extent to which people are willing to accept an extra cost for having a better indoor (IEQ) and outdoor (less CO2 emitted and fewer contaminants released from houses to the atmosphere) environment. Field studies are essential to build a robust framework for the co-benefits’ evaluation.
In South Tyrol, according to the database of the local energy certification agency Agenzia per l’Energia Alto Adige–CasaClima (this database is not public and it can be accessed via the agency only), over 60% of new buildings built in 2016 and 2017 were single- or double-family houses. Therefore, the study focused on this building typology. Due to the large floor area per person available in single-family houses, the results cannot be directly extended to other building typologies, such as apartments, since these are characterized by an elevated occupant density within the residential unit, and therefore by higher CO2 concentrations. However, the results of this study are applicable to recent and new single- and double-family houses located in the Mediterranean and continental climate with a number of degree days between 2100 and 3000 (called zone E in the Italian climatic zones’ definition).
This research presented a cost–benefit analysis of the IAQ in residential buildings using a case-study building, and three sets of materials with different pollution emission levels, namely high, low, and very low emitting. The main conclusions of this study are as follows.
Depending on the scenario, the use of low- and very low-emitting materials enables an up to 13,500€ running cost reduction over a 10-year period, which results in extra in the budget that could be used to purchase these higher quality materials.
In the cost–benefit analysis, the variables that have the largest effect on the present value are the number of periods (i.e., years) and those related to electricity (cost and efficiency).
In Bolzano climatic conditions, the use of a ventilation strategy based only on IAQ does not ensure the thermal comfort requirements are met during summer unless some cooling strategies are adopted.
An analysis of the co-benefits is essential to fully understand why and the extent to which people are willing to accept extra costs to have a better indoor and outdoor environment.
There are also some main limitations of this study. Firstly, the analysis could be conducted on a larger sample of building archetypes. Secondly, natural ventilation was not considered, but further work should consider this possibility as it is widely used in South Tyrol. Thirdly, in residential buildings, indoor and outdoor noise and sound also affect the use of ventilation devices and openings, but this aspect was not included in this research. Then, in the financial analysis, inflation was not considered as its value has been very low in recent years. This might not be true in longer term projects. Finally, in this study, it was assumed that materials have the same emission rates throughout their lifetime. Anecdotal evidence suggests that this is not the case, but further work is needed to characterize the long-term emission rate and the facts that modify it (e.g., time, direct sunlight exposure).