As an alternative to adopting active architectural systems (mechanical systems) and taking advantage of the resources provided by nature, natural ventilation contributes interesting solutions to control the thermal balance and the air quality,
and it is applicable in a variety of climate zones. Natural ventilation also solves some of the more common problems of mechanical systems, such as the noise factor and installation and maintenance costs. However, it is not sensible to pretend
that natural ventilation on its own can satisfy all of the air quality needs of a dwelling. Its greatest limitation is that its effects are random, variable, and difficult to control and regulate because the airflow varies as a function of
the environmental conditions at any given time. The result is that the minimum required airflow rate for an acceptable level of air quality occasionally cannot be obtained. To solve these lacks, it is possible to take advantage of the characteristics
of hybrid ventilation (since these installations combine natural and mechanic ventilation systems) and demand-based regulation (in accordance with a specific concentration of contaminants or human presence). The application of this design
option to new construction is widely disseminated. However, its integration in retrofitting residential neighbourhoods, particularly those built in the second half of the twentieth century, is especially important and represents an added complication.
This complication requires implementing a methodology to analyse its different components and design alternate construction solutions. The methodology is then applied to the case study of CITyFiED European project (RepliCable and InnovaTive
Future Efficient Districts and cities), in the case of Torrelago district, in the municipality of Laguna de Duero, surrounding the metropolitan area of Valladolid. The retrofitting actions involve 31 residential buildings in use nowadays and
counting 1,488 dwellings. Based on characterizing the temperature and wind factors that influence natural ventilation processes, the available flow rates for the case being studied are estimated. For this case, there are procedures based on
complex numerical simulations using CFD software in combination with experimental tests. Combined with experimental data (in situ measurements and pressurization tests to determine the air tightness of the enclosures), the values obtained
in this way allow the quantification of the air renewal cycles in the different dwellings and the estimation of their ability to provide quality air to their users. Those results must be subject to a critical review and to the development
of new design alternatives, which may be incorporated in retrofitting works so as to guarantee at all times the indoor air quality by means of the most efficient option as regards energy.
Palabras clave: ventilation; building retrofitting; IAQ; ventilation efficiency; pressurization test.