Thermal envelope quality versus nZEB parameters and long-term economics: the Eco-Silver House case in Ljubljana

In 2014 the first multi storey residential building planned and constructed to meet the Passivhaus Institute (Darmstadt) criteria was put in operation in Ljubljana, the capital of Slovenia. This massive-structure building is part of the FP7 EE-Highrise project, aiming to demonstrate nearly zero energy building (nZEB) technologies, an integrated design concept, and advanced systems for sustainable construction. The aim was to test and assess the technological and economic feasibility of innovative energy solutions on a large-scale real project (www.ee-highrise.eu). The construction phases had to be meticulously prepared in order to reach the goals and meet the targets defined during the integrated planning process and selection of most viable design options. This included among other special on-site training and coaching of construction workers, preparation of how-to written protocols, and performing various non-destructive tests and measurements within the established quality assurance scheme.
As two of the most important objectives were to reach the passive house standard according to the PHPP methodology and to possibly comply with the national nZEB boundary values while securing a high level of thermal comfort special attention was put also on the airtightness and avoidance of thermal bridges on the thermal envelope, both in planning and implementation phase. The air exchange rate (n50) strongly influences the energy use and affects the efficiency of ventilation system with heat recovery, conditioning of occupied spaces and overall performance of smart control units in each apartment. A high level of airtightness and absence of thermal bridges are two of the vital preconditions for reaching the nZEB standard after having had other parameters successfully fine-tuned.
Through a step-by step process of repetitive Blower door tests in consecutive construction and installation phases all weak point were identified, solutions for sealing proposed and implemented, corrective measures tested and the final state approved. The same approach was used for identification and remedy of thermal bridges using infrared thermography. The goal was not just to achieve remarkable energy indicators on paper, but on the actual building. The paper describes the main principles and implementation of the above mentioned approach.
Another point of consideration was the long-term economics of selected solutions, i.e. an analytical study of the influence of various building quality parameters (like air exchange rate and thermal bridges). The question was how is the energy use according to different scenarios reflected in the long-term costs. An LCC analysis based on parametric sensitivity studies about the influence of individual thermal envelope features was conducted to find out which of them is the most significant. The aim was twofold, namely to see what is the impact level of the quality of specific work procedures and, indirectly, skills of workers on operational costs, and to see if common guidance for future similar projects can be extracted from the results. The paper explains selected findings and point out the main lessons learnt to be considered on the way towards nZEB standards and sustainable buildings.