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Building ventilation systems must get much better

The reduction of energy consumption in buildings is the focus of the European strategy to ensure that future climate and energy targets are reached. This paper focus on the definition of Nearly zero energy buildings (NZEBs) that represent one of the greatest opportunities to increase energy savings in Europe. As this term appears subject to different interpretations, the paper explores the NZEB literature to provide an overview of definitions.The analysis underlines inconsistencies and critical issues among them. The paper also assesses the progress of the NZEB implementation in Europe and it evaluates the main debates arisen around NZEBs. Among these topics, there are:the distinction between energy and primary energy, and between energy sources and energy carriers. Special attention is given in defining primary energy factors for energy carriers produced from renewable energy sources on site, nearby or far. After specifying the role of "plus" buildings, a complementary energy index is proposed to overcome the questioning on the "negative" primary energy index that can be achieved using some of current net ZEB definitions. A proposal for clarifying the meaning of near zero, zero and plus energy buildings is also formulated. The analysis enlightens how, although the attention given to NZEBs increased over the last years, the NZEB topic is still under discussion and not uniformly implemented.

The building sector is one of the key consumers of energy in Europe; consequently, European Union has enacted several Directives dealing, directly and indirectly, with energy efficiency in building aiming to reduce the buildings energy use. Those directives, while dealing with existing buildings, do not take care of the Architectural Heritage in a specific uniform way adopting the derogation regime: exceptions are available at the national level to exclude from their application buildings listed in the Architectural Heritage as historic buildings. Thus any country can adopt its own rules to include or exclude buildings from respecting the energy efficiency requirements for existing buildings. Consequently, up to now no general rules, codes and standards are available for energy retrofit of historical and architectural valuable buildings. On the other side, no international act, in the field Architectural Heritage conservation, deals with energy and energy retrofit. Furthermore, the European Union Treaty does not comprise the Cultural Heritage as matter of European legislation. Thus to cover this gap between historic/historical building and energy retrofit a lobbying action is needed, managed by the national Cultural Heritage authorities, which can steers EU policy in a more effective way towards energy retrofit of historic/historical buildings.

Breathing Walls are envelope components, based on porous materials, crossed by a natural or forced airflow. Since they behave both as recovery heat exchangers and active insulation, reducing the conductive heat flux, they represent a promising envelope technology, allowing to reduce energy consumption in buildings.From the modeling point of view, an analytical model can be found in literature, describing heat and mass transfer across Breathing Walls in steady state conditions. However, to the best of the authors' knowledge, the model lacks an exhaustive experimental validation. Therefore, in this paper, the novel laboratory apparatus named Dual Air Vented Thermal Box developed at Politecnico of Milano is presented. The apparatus is used to experimentally investigate the steady state behavior of a 1 m 2 Air Permeable Concrete sample, crossed by an airflow at different velocities up to 12 mm/s.The temperature profile inside the sample, measured in different positions, is compared with the model predictions. While in the central portion of the wall a very good agreement is found, the experimental results at the top and at the bottom of the wall suggest a non-uniform velocity field entering the sample. A qualitative confirmation of this hypothesis is provided by CFD simulations on the apparatus, clearly showing a mixed convection regime on both sides of the wall. The results lead to state the validity of the one-dimensional analytical model in literature, although a careful application should take into account adjusted boundary conditions, consisting in an airflow velocity possibly variable with height.