*3.1. Buildings under Investigation*

Relatively new buildings constructed in the period between 2016 and 2019 were chosen for the research. At this time, the new requirements demanding not lower than class A energy performance for newly designed and built buildings were introduced, and airtightness measurements became mandatory in Lithuania. More than 200 measurements were implemented in this research in sum (Figure 1).

All the buildings and flats were divided into groups using several factors:


The main characteristics of the buildings are provided in Table 1.


**Table 1.** Properties of buildings.

**Figure 1.**

Architectural

 plan of building with specified types of the flats.

All the buildings were equipped with energy-efficient plastic windows having two insulated glass units (IGU) with selective glass coating. All the windows had appropriate construction inserts positioning window frames in the range of the wall insulation layer and in that way minimizing the linear thermal bridges of the window jambs. The roof load-bearing structures were made of hollow prefabricated reinforced concrete slabs insulated with polystyrene (EPS), the thickness of which was determined by building design energy class. The floor structures consisted of the most commonly applied layers: reinforced concrete, insulation, and damp proofing. All the buildings were two-level houses. Their heights ranged from 6.25 m to 6.35 m, although the internal ceiling height of the premises remained constant at 2.7 m. Therefore, this geometric peculiarity had no significant impact in our opinion neither on heat energy loss nor on the airtightness of the buildings. The buildings chosen for the research had the same engineering system equipment: the heat source was the heat pump with the floor heating system, all the flats had the same natural ventilation system. These choices allowed to eliminate the occurrence of possible airtightness defects in different equipment mounting places or installations, such as intersections of ventilating equipment piping with the walls or different heat sources. To reduce the influence of construction works quality to airtightness measurements as much as possible [21–25,31,33], only the buildings constructed by the same construction enterprise were chosen.
