*3.2. Measurement Methods*

The principal scheme of the whole research process is provided below (Figure 2), followed by a detailed explanation of the steps.

The airtightness measurements were performed in all flats of the terraced houses analysed. The airtightness values of the premises were determined according to the standard measurement method (LST EN 13829). As stated by this method, all windows of the building were fully closed, the natural ventilation channels were properly glued, and all internal doors were opened to let the air inside and distribute easily in the flat. The measurements were implemented using Blower Door Model 4 equipment with the following technical specifications: measurement precision ±3%, measurement uncertainty 8.3%. The obtained results were statistically processed to get the average values for separate building groups and define possible dependencies on the flat location in the building.

There are mandatory requirements for the airtightness value of buildings, and the energy performance class of every newly designed building cannot exceed the predefined value. In case the building does not meet the airtightness requirements, it should be classified as belonging to the lower energy performance class. As the required airtightness cannot be achieved in a smaller part of buildings, the related defects must be recorded and rectified. For this purpose, the infrared (IR) research was performed using the FLIR ThermaCAM B640 infrared camera with measurement precision of 2% or 2 ◦C. All research was performed in the winter period when the temperature difference between the internal and external air was about 15–20 ◦C.

To examine the leaks of the building envelopes the infrared camera research was done twice. At the initial stage, there the temperature measurements were obtained on the surface in the natural conditions without creating an additional pressure difference. Afterward, in order to identify the main leakage locations a 50 Pa pressure difference between the outside and inside air in the rooms was created by means of airtightness equipment and the internal wall surface temperature was measured. There was an alteration of the internal surface temperatures compared to assess the tightness of the structures and to find out if the temperature differences are normal. There were two goals to perform the infrared research: first, to identify the problematic places that do not meet the tightness requirements in the buildings, and subsequently to implement corrective actions by repairing the defects and achieving the desired airtightness level, and second, to statistically evaluate the obtained results in order to determine in what type of buildings the most frequent problems were met.

**Figure 2.** Principal scheme of the research process.
