*4.2. Thermographic Photo Research*

To determine the reasons for the rather low airtightness of the buildings, thermographic photo research was performed. It revealed the defects related to improper construc-

tion works and wrong structural solutions. The most frequently met defects are presented in the diagrams (Figure 6).

**Figure 6.** Frequency of defects for hollow clay masonry buildings (**a**) and sand–lime block masonry buildings (**b**).

The analysis of research results showed that the most popular defect type in the buildings can be associated with improper installation of windows and their technical adjustment. During the assessment process of the buildings of the energy class A, the defects of that kind were recorded in 90% of the cases and in 75–80% of A+ and A++ energy class buildings. The most likely reason and explanation of this finding could be the thicker insulation layer of the envelopes and the opening jambs of higher energy class buildings. A thick insulation layer creates a lengthy way between the internal and external surfaces of construction, and thus stronger resistance to the moving airflow.

The joints of external walls with other parts of the building, such as floor or roof structures, can also be described as important and defect-sensitive and adding to the airtightness of entire structure. This factor can be related to the flats at different locations in the building and having different lengths of joints of these types. It also influences the differences in the airtightness measurement values of differently situated flats.

Evaluation of the junctions and details of electric installation and water pipes showed significant differences in recorded results. In structures made of hollow clay masonry units, the risk of defects in the above-mentioned junctions grows up to 30%. In the envelope structure constructed of hollow clay elements, the external layer of the building products is destroyed when electric outlets are installed and cables are routed. In this way, the interlinked hollows of the building envelope through which air can flow easily are reached. Installation and repair of these elements and their junctions must involve careful insulation, otherwise defects cannot be avoided (Figure 7).

**Figure 7.** Defects of electric installation influencing the airtightness of construction.

#### *4.3. Analysis of the Heat Loss*

Total heat loss through the building envelopes of the flats with different floor areas and various building energy classes calculated per 1 m2 of the heated floor area expressed in kWh/m2 per year depending on the location of the flat in the building plan are presented in Table 4.

**Table 4.** Total heat energy loss kWh per 1 m<sup>2</sup> of the floor area per year of the flats of various size and energy classes, considering where the flat is situated in the building plan.


The analysis of obtained results revealed that a bigger heated floor area leads to higher values of the total heat loss, regardless of the building energy performance class. The explanation could be that the envelope areas increase together with the floor area of the flats and the heat loss is directly related to the size of the envelope area.

The assessment of the influence of different locations in the building plan of flats with the same floor area showed that the total heat loss through the building envelopes calculated per 1 m<sup>2</sup> of heated floor area and expressed in kWh/(m2·year) is around 9–12% higher for the end units compared to the middle units (Figure 8). The distance between the chart lines for lower energy performance building of class A (blue colour) is bigger than respective distances for the buildings of higher classes A+ and A++ (red and green colours). Accordingly, the heat loss increases calculated as differences between the values considering air infiltration and despite air infiltration are different: for the class A it makes approximately 12% and for the classes A+ and A++ it makes about 4%. This fact is logical evidence that better thermal insulation of the building contributes to higher airtightness values.

**Figure 8.** Heat loss differences reflecting the increase of values for end units in comparison with inside units.

The two above-mentioned tendencies remain, regardless of the material of the flat wall structure.

Generally, the total heat loss difference considering air infiltration per 1 m2 of heated floor area (kWh/(m<sup>2</sup> per year)) between the end units and inside units can exceed 15% because of the different airtightness of these flats.

Currently, the compliance with the allowable value of heat loss is assessed by examining the volume of the entire building in its design stage. The heat loss criterion is difficult to meet in the process of energy certification when there is a need or opportunity to assess individual flats or other logical architectural parts.

Figure 9 shows the average design values of heat energy loss for different flats and their comparison with the corresponding limit values prescribed by the regulation. The dwellings that exceed these limit values should be assigned to a lower energy performance class, i.e., moved one class down in the classification.

**Figure 9.** Comparison of heat energy loss values with limit values of different flat types.

The results also show that all inside units of the investigated buildings meet the heat loss requirement, regardless of their design class. Therefore, the assessment of the end units shows that some of them would exceed the allowable limit, which would lead to downshifting their energy class. To avoid these problems, it would be reasonable to plan improvement measures for end units, which include both additional airtightening and thermal insulation, already on the design stage.

#### **5. Conclusions**

Airtightness as an important factor, together with other complex design solutions, can reduce heat energy expenses, increase thermal comfort, and ensure a healthy building environment and its longevity. Airtightness as a property is dependent on human factors, technical solutions, and materials, therefore it will differ in every single case.

Only the buildings constructed by the same construction company were investigated in the research. Nevertheless, the difference of airtightness values measured in the flats of the same category was twice as high. Most researchers underline the aspects related to the construction work quality. Therefore, the average values of the entire building group, but not separate measurements, should be used for the assessment of airtightness values of separate building groups.

The average airtightness value differences collating the smallest and the largest flats exceeded approximately 25%. This can be explained by the fact that local air leakages or minor construction defects of larger flats statistically had less influence on the general airtightness, understood as the air exchange speed in the premises.

Evaluating the buildings constructed of different types of brickwork, it is safe to state that the building's airtightness values depend on the material structure of the chosen brickwork as well as on bricklaying technology and proper installation of engineering systems. When the construction of hollow clay masonry units is chosen where the bricklaying technology involves the filling of horizontal brickwork seams with mortar, the air can circulate through many open voids in the wall. The comparison of the hollow clay unit masonry structure with the solid sand–lime block masonry, the seams of which are filled with mortar both vertically and horizontally, revealed the airtightness reduction of ceramic structure around 7–11% on average.

The comparison of the airtightness measurement results for the flats of equal floor area located at different places of the buildings showed up to 20% higher airtightness measurement values for end units than in inside units, which is a significant difference. The reasons for these value differences could be explained by a larger length of structural joints in the end units. The longer structural joints and additional windows in the walls of the end units cause the higher probability of the emergence of defects worsening the general result.

The obtained results show that all the dwellings surveyed did not exceed the allowable heat loss limits when the total heat loss of the inside units was assessed. As for the end units, we see that most of them, especially the ones in the buildings belonging to higher energy classes A+ and A++, exceed the heat loss limits prescribed for these energy classes. In the further process of real estate development and design of terraced houses, they should be assessed not as a single object, but as a whole consisting of separate units, where each unit should meet the heat loss requirements.

Continuing the research, the role of airtightness should be extended to overall building energy performance assessment by combining and incorporating comprehensive experimental test results, database data, and simulation that could lead to more precise and reliable results and give the opportunity to verify them.

**Author Contributions:** Conceptualization, V.P., G.C., J.M. and M.D.; methodology, V.P. and M.D.; software, V.P., G.C. and J.M.; validation, V.P., G.C. and J.M.; formal analysis, V.P. and G.C.; investigation, V.P.; resources, V.P. and M.D.; data curation, V.P.; writing—original draft preparation, V.P. and G.C.; writing—review and editing, V.P., G.C. and J.M.; visualization, V.P.; supervision, V.P., G.C. and J.M.; project administration, V.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
