Comparison of four in-situ methods for the determination of walls thermal resistance in free-running buildings with alternating heat flux in different seasons

Abstract

In recent years, buildings energy consumption has become a major global issue. In order to assess the actual energy performance of buildings, a reliable in-situ estimation of the thermal resistance of walls (R-value) is required. In conditioned buildings there are many well-known quasi-steady and dynamic methods to determine R-value. Nevertheless, in free-running buildings there is no much information about the usefulness of the different available methods, the required measuring periods, convergence, etc., especially in mild and warm climates where both, stable non-inverting heat flux and high thermal gradients, do not naturally occur. This paper evaluates the ability of four different in-situ methods (ISO 9869, Modified Average, RC-network and Pentaur methods) to estimate the R-value of four different wall types of a free-running building in a mild climate, under alternating heat flux condition, for different seasons and orientations of the walls. Both simulated and experimental data sets of surface temperatures and heat fluxes were used to analyze the effect of the measuring conditions (surface temperature difference and direction of the heat flow) and the duration of the survey period on the convergence and final R-values. The results showed that the Pentaur method produced the most accurate results in all cases, thus supporting the idea that the dynamical methods are most suitable for free-running buildings. This research provides guidelines to select the most appropriate method and the required survey duration for in-situ estimation of the thermal resistance in free-running buildings, depending on the season, the heat flux characteristics, and the wall stratigraphy.