Airtightness performance of the building envelope of dwellings in spain. Characterisation and energy impact of air infiltration

Resumo

Only after the oil crisis in the 1970s has there been an undeniable growth in the level of interest in reducing the use of fossil fuels. In this regard, the countries of the European Union predict a sustainable, competitive and decarbonised energy system by 2050. For that purpose, it is crucial to address the energy efficiency of buildings, which are responsible for a significant percentage of energy consumption, and, therefore, greenhouse gas emissions. The potential seems obvious, considering that, according to the European Commission, 75% of the buildings in Europe are energy inefficient. In this context, uncontrolled airflow through the building envelope, or air infiltration, is a phenomenon that involves energy loss since it needs to be conditioned at the interior comfort conditions. Air infiltration is also related to the ventilation of buildings and, thus, has an impact on indoor air quality. In this sense, a relationship is established between air infiltration, energy impact and ventilation. Since airtightness is the main characteristic of the envelope that determines air infiltration, its study is essential to understand the performance of the building envelope. However, this issue has not been addressed in depth in most countries with temperate climates, and, specifically, in Spain, given the traditional dependence of air renewal on air infiltration. Currently, the implementation of controlled ventilation systems guarantees adequate indoor air quality in residential buildings. Therefore, air infiltration is no longer necessary as an air renewal source and, consequently, its reduction is a priority to achieve nearly zero-energy buildings (nZEB). In this sense, the airtightness characterisation of the envelope of existing buildings is essential to prioritise efforts and determine strategies for the renovation of the existing building stock according to the decarbonisation objectives which have been set at a national and international level. To meet these needs, the general objective of this work focuses on the characterisation of the airtightness of the envelope of residential buildings in Spain, framed within the INFILES research project (BIA2015-64321-R), funded by the Ministry of Economy and Competitiveness of the Government of Spain. As a starting point, the state of the art was addressed and the airtightness regulatory frameworks in Europe and North America were compared. Additionally, the main databases and protocols for evaluating the airtightness of the envelope were assessed. Its analysis allowed the identification of the weaknesses, threats, strengths, and opportunities of these practices. Likewise, the main keys for the development of a common outline at the international level were detailed. The characterisation of the airtightness of the envelope of the residential building stock in Spain was carried out in a representative sample of cases that allowed the extrapolation of the information obtained. A quota sampling scheme defined by representative variables regarding the airtightness of the envelope such as the climate zone, location, year of construction, and building typology, was proposed. In short, 401 cases proportionally distributed in single-family and multi-family dwellings, built in different periods, and located in several cities, were analysed. This sample generated a database that collects information on the complete characterisation of both construction features and the airtightness of buildings. To facilitate data management, a specific application was developed, which enabled the sequential and standardised storage of information, whose operation could be replicable in other contexts. The airtightness of the case studies was evaluated by means of fan pressurisation tests, according to the international standard UNE-EN 13829. In addition, a specific methodology was developed to guarantee the uniformity and reliability of the information gathered. Airtightness results were wide-ranging, which indicates the uneven performance of the buildings tested. Nevertheless, the distribution of the average values was in line with previously reported findings both in Spain and in other European countries. Different trends were identified by climate zone: the envelope of the cases located in areas with Oceanic and Continental climates was more airtight than that of the dwellings located in areas with a Mediterranean climate with a temperate cold season. On the other hand, contrary to what might be expected, no clear trend of airtightness improvement was observed with regard to the year of construction. The different regulations regarding the energy efficiency of buildings did not lead to a significant improvement of the airtightness of the building envelope. Therefore, it seems clear that the existing residential stock in Spain remains far from the standards and practices that are already a reality in other European countries. However, the recent regulatory limitation of the global air permeability of the envelope opens a window to a change in this trend and represents an opportunity for real achievement of nZEB. The relationship between different building characteristics and the degree of airtightness was addressed through statistical analysis. Although the limitation of the sample did not allow the development of a detailed study, a relationship was identified between the airtightness and the type of windows, the rolling-shutter system used, and the degree of renovation of the envelope. In this sense, the main leakage paths were located using infrared thermography, which allowed the identification of typical deficiencies of residential buildings in Spain. Turbulent flows through cracks located around windows and rolling shutters concentrated the main leakages. The inadequate design of the constructive solutions, as well as the careless workmanship of the joints between different elements, were pointed out as the main causes that prevent airtight envelopes, and, consequently, they pose the main challenges to face in the future. Finally, the energy impact of air infiltration on the heating and cooling demand in the cases studied was addressed, applying a simplified model for estimating the average air change rate. The results demonstrate the high energy impact of air infiltration, which reaches values of up to 25% of the heating demand. This confirms the substantial potential for energy-saving if the air permeability of existing buildings was reduced. The characterisation of the airtightness of the envelope of residential buildings in Spain contributed to filling the knowledge gap identified. Only in this way will future design criteria and renovation strategies of existing energy-inefficient buildings be approached in a realistic way, in accordance with the established decarbonisation targets.