In Central-Europe, a large proportion of people live in prefabricated tower blocks (most commonly called as panel buildings) which were built in the past 30-50 years. These buildings are now obsolete in terms of their energy performance and their condition has deteriorated, therefore they need to be refurbished, which action already started in the past couple of years. One of the most common building construction of tower blocks is the double-skin ventilated cold deck flat roof, which consist

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... the inner reinforced concrete roof panel, some thermal insulation (expanded polystyrene or mineral wool), a ventilated air layer and the reinforced concrete outer shell which holds the bituminous waterproofing membrane. In our research, we investigated these roofs during a refurbishment of a tower-block building with blown-in glass wool thermal insulation in Budapest, Hungary. We examined the real built-in hygrothermal performance of the double-skin flat roof with blown-in thermal insulation, and how thermal insulation capability changes during the year in these structures. The insulation was designed to not fully fill the ventilated air layer, therefore the cold roof continued to function as a ventilated/slightly ventilated roof. During the refurbishment, the external reinforced concrete shell of the roof was drilled by using a core drill in specified locations and then the glass wool was blown-in. During the process, we have set up a hygrothermal monitoring system in the roof. We placed temperature and relative humidity measuring sensors between the layers of the roof construction at four distinct places in the roof, as well as on the internal and external sides. The measurement procedure lasted for a year. The weather data for the calculations and simulations were obtained from a nearby weather station. In our paper, we analyse the monitoring measurement results, as well as demonstrate the building physical aspects of the refurbishment process and show the real hygrothermal performance of the materials built-in the roof construction.