**1. Introduction**

To fulfil the requirements of the European Energy Performance of Buildings Directive (EPBD) [1,2] related to the reduction of energy consumption by using high-quality materials and implementing efficient solutions for structural connections and joints, the national requirements for thermal properties in building envelope were formulated and building energy efficiency calculation methodology was developed. Many European countries have developed national methodologies for the assessment of building energy efficiency according to DIN 18599 [3] in Germany, DOCET in Italy, CALENER in Spain, etc. [4]. The above-mentioned methods vary depending on the type of buildings, climatic zone, minimal thermal requirements, and certification indexing [4].

The main evaluation criteria used in these methodologies are CO2 emissions and primary energy or heat energy consumption in buildings. All the methodologies pursue the main aim to reduce energy consumption in buildings. To this end, not only are efficient engineering systems that improve the thermal properties of the building required, but also appropriate technological solutions to assure the high quality of work and good airtightness of the buildings. A properly insulated building together with efficient heating and ventilating systems can save up to 50% of heating energy and assure comfortable conditions in the premises [5,6].

**Citation:** Paukštys, V.; Cinelis, G.; Mockiene, J.; Daukšys, M. ˙ Airtightness and Heat Energy Loss of Mid-Size Terraced Houses Built of Different Construction Materials. *Energies* **2021**, *14*, 6367. https:// doi.org/10.3390/en14196367

Academic Editor: Marcin Kami ´nski

Received: 3 August 2021 Accepted: 29 September 2021 Published: 5 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The airtightness of and energy efficiency of the buildings can be determined by different methods [7]: theoretical investigation [8,9], empirical research [10,11], modelling of general building characteristics, or modelling of one component of the building [12]. The analysis of the related literature revealed that building energy performance calculations are precise only if building airtightness is defined by measurements. The measurements help to assess the construction workmanship and define the airtightness level, which is used to calculate the energy consumption of the building according to the 2010/31/EU Directive [2]. The most widely used method of airtightness measurement is the blowing door test method, prescribed by EN ISO 9972 2015 [13,14].

The main index of airtightness used in Lithuania is n50, which indicates the part of internal air volume having changed in one hour at the set pressure of 50 Pa. The measurements of this kind are performed in many countries aiming to assess the general airtightness level of buildings using various criteria like the building type, its height, geometric forms, envelope structure, the ratio of the envelope, the floor area, etc. [15–18].

There are several main ways of air infiltration. One of the reasons is improper structural connections in the building due to using low quality insulation materials or not using them at all. In this case, the outside air penetrates through structural joints. The other path of air leakage is the building construction material. In this case, the air can infiltrate through the voids and cracks of construction elements.

The level of building airtightness can be determined and air infiltration paths in the building envelope can be detected by means of non-destructive tests using an infrared camera and observing the cold air movement in the external structures [19], or measuring the air movement speed near the splits with the anemometer sensors and calculating the approximate area of the split [20], or even measuring the sound of penetrating air.

The research objectives were: (1) experimental assessment of the flat airtightness distribution in terraced houses made of different materials, (2) theoretical heat energy loss calculation and finding out the differences in the heat loss values between the flats in different places in the building plan, (3) assessment of the compliance of flats in different places in the building with the design energy performance class.
