Thermal characteristics

3.1 Thermal insulation

Both layers of the double facade have specific parameters of thermal insulation. The value of the thermal coefficient (U) of the double facade as a whole cannot be precisely determined, because it is not a constant. Schuco, one of the producers of glass double facades, commissioned a study on how the impact of sun radiation affects the facade’s heat permeability [5]. This research was carried out in winter, when the value of this coefficient is important for the building’s demand for heating energy. It follows from the study that during periods of intensive radiation of the sun this coefficient can even double. When there is no intensive sun operation, the improvement in this coefficient is slight. Facades facing north lead to no noticeable thermal gains. The facade’s thermal effectiveness is therefore greatly dependent on climatic conditions, orientation with respect to the sun and possible shading from the surrounding buildings. Thermal gains from sun radiation can only be achieved from facades exposed to the sun — southern and to less extent south-eastern and south-western.

The estimated average data for sun radiation falling on double facade was used to calculate the mean value of its thermal coefficient. For the wall in question, with open ventilation ducts, this value was 20% lower than for the internal double-pane wall. Temperature readings from the inter­facade area showed that it was about 10°C higher that the external temperature under intensive sun radiation and some 2°C higher under slight sun operation. The closing of ventilation flaps reduces the U (k) coefficient by 30%.

A study of the double facade in the office building of Stadttor in Dusseldorf (designed by Petzinka and Partners) points to an even more advantageous influence of an additional layer of glass for the thermal coefficient for the whole building [7]. This coefficient for the external layer is rated at 1,5W/m2K. When ventilation flaps are closed in the external wall, the value of U coefficient falls to 1,0W/m2K, which leads to favourable temperatures of the internal wall when compared to room temperatures on cold days [7].