**2. Literature Review**

T. Kalamees [21] conducted laboratory tests of various structural timber framework connections and compared the obtained results with airtightness results of real-built houses. The researchers concluded that it was difficult to ensure the quality of airtightening works on site in the installation of both structural connections and engineering systems (water supply, electricity).

The authors of the paper [22] discussed the airtightness estimation procedure applicable in the design phase. The methodology being in its early phase included quantitative characterization of expected leaks, evaluation of building airtightness in-situ using fan pressurization, component testing for air permeability in laboratory conditions with the completion of air leakage values obtained from the published database, and correction/validation of airtightness values. The investigation of several building parts showed that ventilation ridge was responsible for the highest percentage (61%) of airflow (the air leakage values were as follows: 11.0 m3/(h·m2) for ventilation ridge, 0.66 m3/(h·m2) for window frame and connection of steel columns with the floor, and 1.15 m3/(h·m2) for panel joints).

Another article [23] discusses the air leakage problem, considering the national building energy-related regulations and the methodology of energy performance calculation. The authors investigated the construction type, the age, design details, and retrofitting of the building as airtightness factors and found better quality of newly-built dwellings, good design, high-quality workmanship, and proper quality control during the construction period contribute to energy efficiency of buildings the most. The inclusion of the airtightness factor during the energy performance assessment process could improve the energy consumption by up to 7%.

Another paper [24] presents and discusses the results of measuring the airtightness of 170 single-family houses and 56 apartments. The construction method, insulation materials, joint insulation materials, and the ceiling structure were studied in the research as the factors related to airtightness. Good airtightness of individual houses was reached in all house groups regardless of the choice of structure, number of stories, ventilation system, or technology of construction. This fact pointed out the importance of construction quality.

The research paper [25] includes a proposal for the development of a rough predictive model of the degree of envelope airtightness as a regional tool for energy efficiency assessment and tailored to southern European construction stock. The results were assessed as widely scattered due to the impact of the random component of manual construction. The paper presents the results of statistical analysis and describes the protocol used both for the identification and quantification of air leakage pathways and for construction quality management.

The authors of the paper [26] conducted a study on the relation between the airtightness of a building envelope, air infiltration, and energy use of a typical modern Finnish detached house with an IDA-ICE simulation model also considering the stack-induced infiltration. An adapted model for the rough estimation of the annual air infiltration was determined from the numerical simulation results. The dependency of both the infiltration rate and heat energy use is nearly linear on the building's leakage rate, measured as n50. This research showed that infiltration induces about 15–30% of the energy used for space heating, together with the ventilation in the prototypical detached house.

The authors of the work [27] performed the univariate analysis and multiple linear regression of the Canadian airtightness database to reveal the important trends. Two airtightness model classes with 3 variables and 8 variables (building volume, climate, building age, building height, and insulation levels for basements, walls, roofs, and windows) using two airtightness metrics (ACH and NL) were developed. The models referred to the round half airtightness variation of the building. The study set a feasible lower boundary of perspective models for regression-based airtightness prediction.

Tests were carried out in five flats of the same building in order to characterize the air permeability and to improve the design of buildings [28]. Although the flats tested were of the same size, with the same components, and were erected using the same construction processes, their overall air permeability showed a wide variation. The authors assumed this was mainly due to the change of the width of the gaps around the roller shutter boxes and the gaps in the bottom opening joint of the doors. The quality of windows, entrance doors, and kitchen external doors also had an impact.

The results presented in [29] give some ideas for how to decrease the measurement uncertainty in the blowing door test and to better detect energy and environmental issues in the audits of buildings. The chimney and the windows, without sealing and natural ventilation systems, were discovered to be the critical causes in the building's over-ventilation. The most critical uncertainty contributions were found to be the operative test conditions and metrological performances (e.g., internal–external temperature and the wind velocity difference) of the pressure measuring device.

The research [30] empirically investigated factors that should be considered while using pressure difference measurement values and airflow rate to derive more accurate airtightness values for large buildings. The distribution of vertical pressure across the whole building envelope can differ considerably when the building is pressurized. A method to measure airtightness was proposed where the pressure difference on each level of the building is measured and a medium value of pressure difference is defined.

Two problems related to design solutions of building airtightness were revealed in the work [31]: contemporary airtightness predictive models are too complex to be used for everyday design practice, and existing airtightness predictive models do not meet the needs of contractors and designers. More detailed issues in this context could be addressed: the lack of standardization, including factors classification, parameters definition, their impact quantification and significance assessment, metric analysis, the influence of supervision and workmanship, the classification of the air leakage paths, and research of significant air penetration areas/points.

The air permeability measurement results of 287 post-2006 new-built UK dwellings averaged 5.97 m3/(h·m2) at 50 Pa were studied in the paper [32]. Relationships between the airtightness and management context, building method, and dwelling type as the influencing factors were investigated. The superior airtightness was achieved in buildings with the self-build procurement route as a result of more innovative construction practice, prefabricated concrete panel systems, etc.; the houses built using site-based labour-intensive methods were the most air leaky. The predictive regression model was developed to predict the potential impacts of the air leakage-related factors of dwellings and improving energy efficiency.

Airtightness testing is described in [33] as a highly informative tool of the dwelling retrofit process. The authors refer to the statement that air infiltration through apertures in the building envelope can make up to one-third of the total heat loss. Particularly in this project, it was possible to reduce the measured air permeability (from 15.57 to 4.74 m3/(h·m2) @ 50 Pa) during the dwelling retrofit. This improvement was achieved through the use of usual draught-proofing means (a decrease in air permeability more than 30%), close attention to installation detail, workmanship, and sealing of the floor/wall joints at the skirting board connection (air permeability reduction of 3.6 m3/(h·m2) @ 50 Pa). Airtightness measures alone contributed to around 9% of the forecasted total reduction of heat energy demand. The effectiveness of fabric measures was very good (64% reduction considering the case of the uninsulated house), although the installation of double glazed units combined with the roof and wall insulation showed minimal improvement of airtightness (approximately 1.26 m3/(h·m2).

The paper [34] investigates the building's airtightness in terms of location and exposure of the building. The authors state that energy-efficient buildings situated in windy areas and at exposed locations could constitute up to 10% of the total heat consumption. The altitude, strength, and speed of the wind have a significant impact on the building by determining the amount of airflow through gaps, cracks, and leaks in the envelope. The possible impact of main parameters of location on the ultimate airtightness of the building envelope was verified while investigating 150 low-energy houses constructed in 2004–2014. The altitude's contribution to airtightness is 0.06%, whereas 99.94% of the airtightness is influenced by other factors.

A statistical method is presented in the work [14] investigating relevant factors related to the airtightness of the dwellings: climate zone, year of construction, and typology. The proposed methodology and its results were compared to the extracted database values. An open to expanding quota sampling scheme consisting of 411 representative cases was built to extrapolate the infiltration rates for Spanish buildings using typical constructive solutions. In the case study, leakage paths were located mainly around shutter boxes, window joints, and frames. The research of the infiltration impact on the ventilation and energy performance of the dwellings has been planned on this basis.

The authors of [35] developed a simplified method to evaluate energy savings from enhanced airtightness. This method was aimed to facilitate the use of energy savings estimates available to building designers and owners and expand the possibilities of the existing governmental online calculator. It expanded the ability to examine energy savings in commercial buildings for all cities in the USA. A simplified approach including energy savings predicting equations was developed to estimate annual and hourly heating energy savings. The equations predicting the percental energy savings for retrofitted buildings only require their expected air leakage rates before the retrofit and after it. Annual energy savings estimated using the online calculator and the proposed approach differed by 15% to 24%.

In the study [36], a model equation was obtained that uses statistical analysis based on empirical models to predict the apartment airtightness of reinforced concrete buildings with the data from 486 units. Two groups of variables were used in the airtightness prediction model equations along with correlation dependence analysis and multiple regression analysis. The model with the area variables was more accurate in predicting airtightness out of the two models. This approach has a limitation because the prediction results may differ depending on the characteristics and the data type collected by various countries. Nevertheless, the methodology presented in this work contributes to similar studies for finding influential variables with better applicability in the future.

The paper [37] investigates the problem of the seeming airtightness of partitions constructed in buildings. The study deals with the wind effect which is the washing reason of fibrous and porous materials of the envelopes. The authors explain how the disintegration of insulation material by forming empty areas determining local discontinuities of material in the envelope reduces thermal resistance. Appropriate areas were proved by the dynamic infrared detection method. The results show that thermal resistance of such envelopes is reduced to 87% with an absence of wind protection. The authors recommend considering the decomposition of this type while calculating the heat transfer coefficient.

In the study [12], an alternative approach was advanced to evaluate the air infiltration rate and air leakage area in building envelope parts such as exterior and interior floors and walls. Physical and acoustical methods were applied in measuring the sound reduction index to determine the leakage area. Therewith, the airflow rate through air leaks was determined using pressure difference over the floor or behind the wall and the values of leakage area. Subsequently, the calculated air infiltration rate also enabled evaluating the convective moisture rate through leaks and heat losses of the building.

The study [38] examined the airtightness performance of container houses and the impact of airtightness on their energy efficiency comparing the measurement and calculation results before and after building treatment. The identified weak places (thermal bridges, air leakages, and condensation) were mainly as junctions of walls, slabs, roof panels, and the edges of the openings. Significant improvement of the airtightness (81%) led to a certain reduction of annual energy demand (9.3%). Airtight joints and thermal brakes are essential for junction details seeking to avoid thermal problems and improve the energy performance of the building.

The authors of the work [39] studied the leakage–infiltration ratio by implementing the tests of more than twenty houses in the UK. The existing rule of thumb of the divideby-20 (the error of using ranged from 3% to 175%) was revised and a new rule divide-by-37 as a more representative of the leakage–infiltration ratio was proposed. The mismatch of the assessment using the existing Standard Assessment Procedure (SAP) was particularly noticeable after adding the modification factors for local wind and sheltering: the overestimated infiltration rate values reached 500% and more, especially in airtight houses.

In the research [40], the airtightness role in the context of thermal insulation performance of traditional double-glazed air-filled windows was analysed. Tests were conducted in a typical dwelling in the UK by comparing the windows that are fitted with a special transparent cover improving airtightness and standard windows. The average U-value of the window sash with air-filled double-glazing was calculated to be 2.67 W/m2·K, as it was 1.79 W/m2·K for the airtight window sash which resulted in a 33% decrease in heat losses. Windows are still important in the energy demand of buildings, and effective solutions such as retrofitting windows with covers can notably contribute to decreasing the windows-related energy losses in buildings.

Performing Blower Door Tests in large buildings [41] requires airflow rates that are impractical to achieve using available equipment and because of the necessity to test only the individual zones of buildings. The Lstiburek method and the Love and Passmore method were adapted for use in multi-unit high-rise residential buildings. The results showed that neither of the proposed methods could be finally recommended as a replacement for the pressure neutralization method in traditional residential buildings. The first method was unacceptable in the accuracy estimation for exterior boundary leakage (estimation error exceeded 108%). The second method showed a small error of 0.2% for the exterior boundary leakage estimation, though the pressure neutralization method was less sensitive to measurement noise compared to the alternative Lstiburek method. There is still a need for new methods that can accurately represent the external boundary airflow while still being less labour-demanding than the pressure neutralization method.

The paper [42] describes the validation of the new model for prediction of the airtightness of buildings utilising a neural network and using four corrective factors related to the building envelope. The model was obtained based on measurements in the field at 58 units in Croatia. The model, which requires a reduced amount of data and therefore is more economical and faster than the field measurements, was validated both in the local field and outside the native country conditions. The proposed model is supposed to be appropriate for predicting airtightness values at the early design phase, as well as for the planning of regular energy refurbishment of dwellings.

Based on the literature analysis and the use of around 300 dwellings' empirical data the study [43] analysis the relationships between the airtightness of building and eight individual variables. Correlation analyses indicated the significant relationship of the construction method, roof type, year of construction, and construction typology with building airtightness. Regression analysis showed that only the year of construction and the total leakage affect the airtightness. ANOVA tests revealed that both variables have a notable influence on the airtightness, in terms of specific leakage rate. Both variables could hardly help to assess the specific air leakage in advance because the year of construction correlates with many other variables and the building leakages can only be assessed when the construction is over.

The paper [44] concerns measurements of airtightness of 16 single-family houses with natural ventilation built from 1880 to 2007 (the measurement values ranged from 1.1 to 5.8 L/(s·m2) at 50 Pa). The results of the ventilation measurements (from 0.09 to 0.28 L/(s·m2) per heated floor area) did not meet the requirement established in the Danish Building Regulations (0.3 L/(s·m2)). The typical places of leaks were identified: the penetrations of electrical installations, exhaust ducts, chimneys, contours of older doors and windows, attic hatches, and connections with wooden ceilings. The findings are relevant for the renovation projects of the older small building stock, especially where mechanical ventilation systems are planned to be installed.

In the article [45], the research of the airtightness level of single-family energy-efficient houses was measured and compared with the requirements of Polish norms and European standards. The different wall structures of the buildings did not significantly affect the level of airtightness (ranged within n50 = 0.17 to 5.33 h−1): the buildings with the worst and the best tightness had the same brickwork wall construction. As the reason for the insufficient tightness, the human factor was referred: a lack of experience and inaccurate performance of coatings, not airtight insulating layer, the mistakes made in porous insulation of transition systems, and the leaks of vapour barrier at connections.

The study [46] focused on the infiltration rate prediction of public buildings in China by implementing the in-situ tests and simulating the infiltration rates for 1800 cases. The main factors influencing the air infiltration were described as meteorological parameters, architectural structure, infiltration path characteristics. The construction period was not useable individually as a separate factor: zones that were built later (2007) had even worse airtightness than zones built earlier (1990). The airtightness of public buildings was found to be much worse than that of traditional dwellings. The centralized HVAC system had more elements in the building envelope than the split HVAC system, and the outer windows' airtightness was worse than the wall. For buildings with a mechanical fresh air system, the airtightness needs to be strengthened in order to reduce the impact of air infiltration. The conclusion was that the influence of air infiltration on public buildings should be acknowledged by policymakers in defining more energy-reasoned design standards.

The authors of the research [47] aimed to reveal the impact of local conditions by evaluating relations of infiltration rate and individual location and heat demand of residential buildings. Depending on the airtightness of buildings the differences in energy consumption between two different locations from the same climatic zone were evaluated

in a rather wide range (from 70% to 90%) and could reach even 200% considering sheltered environmental conditions. The general conclusion of the research was that the building location and its level of exposure were recommended to be considered in forthcoming airtightness regulations.

While investigating the airtightness through the light concrete chimney elements, T.O. Relander [48] found out that better airtightness results can be achieved if the chimney is installed near the wall or in the wall corner because the external surface of the chimney through which air can penetrate will be reduced. External surface finishing workmanship and the materials used also influence the airtightness.

In the study [49], the energy performance of a school building before major renovation planning was modelled using the energy simulation software IDA ICE. The annual simulation indicated the following renovation measures with the best potential: improved envelope airtightness, changing to energy-efficient windows, new controls of the HVAC system, and improved outer wall thermal insulation.

Some articles have weaker relation to our research because the airtightness problem appears there as one among the other research aspects. The researchers investigate the association of the building envelope tightness, its improvements, and ventilation with relative humidity and air distribution in buildings [50,51], discuss the reasonable building airtightness level to seek for [52], the airtightness and thermal defect detection using thermographic research and image processing [53], the impact of airtightness of window and door openings, more stringent requirements for the products [54,55], point out very contrasting air leakage rates of some structural joints [56], the effect of airtightness when investigating the relation of the energy performance, and the indoor air quality performance [57].

The review of the recent studies helped to shed some light on the research hypothesis and formulate an adequate approach to the problem of airtightness influence on the energy performance of the particularly widely spread type of buildings. What did we expect, what did we find in the publications on the one hand, and what was subsequently visually observed, instrumentally measured, recorded, and computed from the field on the other hand speaking more generally? After the extensive review of research results, one can safely assert that the characteristics of the building airtightness or air permeability have a significant influence on the building's energy behaviour. At the same time, it was evident both from the theoretical review and from the field measurements that the nature of these properties is characterized by a rather wide distribution of the values, despite the same construction and material of the building. One of the main reasons revealed in most of the papers and confirmed in the field is the quality of the workmanship. This generalization led to the idea of limiting the diversity of the workforce on the construction site by choosing for the investigation the buildings constructed only by the same company. Furthermore, previous studies have covered a wide range of technical factors with the discussion about their influence on airtightness (as power supply installation). The analysis of recent studies in this regard helped to focus on the aspects discussed in the next chapters.

The literature review encouraged the formation of the research methodology, as well as the logic of its process. It was apparent that the starting point should be the experimental airtightness measurements of separate flats, as the logical architectural building parts with the aim to check the hypothesis that the flats in different locations of the building could have different airtightness values. The literature provided no definite answer to this question. Airtightness-related heat loss values (expressed in percentage) provided in the papers were presented in a rather wide range (not exact), or the data came from buildings of different structures, materials, and typology. Afterwards, it would be possible to theoretically calculate the heat loss of the flats with their subsequent evaluation of compliance with the design energy performance class. More details about the research process are provided in Section 3.2.

The standard methodology of energy performance calculation was also modified based on the analysis of literature sources in the part of the heat loss differences evaluation between the equal floor area flats situated in different parts of the building. It was appropriate to undervalue the formula member for solar radiation, considering the environmental factors described in Section 3.3 in more detail.

#### **3. Methods**
