Experimental Measurements on Thermal Transmittance of the Opaque Vertical Walls in the Historical Buildings

This paper presents the results of an experimental research focused on the development of a method for evaluating energy performance of historical building. Regarding the thermal performance of the opaque envelope, the difficulties arise because of not having appropriate information about the most important parameters those affect the thermal transmittance (U-value) of the envelope components. The national laws propose standard data of U-value suitable for Italian context and particularly for new constructions. The laws consider only few historical constructive technologies and inappropriate wall thickness and also does not include the effect of the presence of humidity inside the component. In the present work, a series of experimental measurements have been carried out on the thermal transmittance, confronting different non-invasive techniques, historical analyses, geometric surveys, infrared technologies, heat flow-meter measurements and sonic trial. The method has been applied on several ancient walls made of stone, bricks and mixed materials. It has been observed that in case of masonries, the difference among the standard and measured U-values varies considerably (6 to 56%). Moreover, there are substantial differences (2 to 56%.) between calculated and measured U-values due to the presence of different thermal conductivity of bricks. Therefore, for the brick walls, it is not convenient to utilize the standard and calculated U-values because they overestimate excessively the thermal loses of opaque envelope. In case of stone walls, the difference varies considerably from 16 to 52%. However, as in the case of bricks, the measured values are better than the standard data. The differences among calculated and measured U-values vary from 22 to 58% (with 90% stone e 10% mortar) and from 13 to 54% (with 80% stone e 20% mortar). It has been also noted that the presence of internal air greatly affects the final U-value of the walls. This kind of measurements permit to calibrate better the effective thermal performance of ancient walls and to consider the influences of recurring elements. The data constitute a strong base for a correct energy diagnosis and performance in historical buildings, particularly for the evaluation of thermal transmission through the opaque envelope.