Surface heat transfer coefficients in building envelopes: Uncertainty levels in experimental methods

Abstract

There are several research methods for the on-site assessment of U-values that aim to avoid the use of surface heat flux measurements and rely on tabulated or empirically developed correlations to define this parameter. This works performs a detailed process to estimate indoor surface heat transfer coefficients based on several experimental campaigns over building walls. Data is filtered out to remove periods with large temperature variations and/or unstable convective conditions due to HVAC. A statistical analysis is conducted, and the outcomes used to test the validity of U-value estimation approaches. The outcomes show that the actual surface heat transfer coefficients are in the range of reference works, but variations in the range of up to 2 W/m2 K are found. Uncertainty levels associated to the estimation of surface heat transfer coefficient are in the range 60% for instantaneous values while this is reduced down to 12%–20% for 8-h averages. Variations and uncertainty levels are higher for low temperature gradient situations, which are considered to be very likely for modern insulation levels. It is concluded that methods seeking to avoid the use of surface heat flux measurements need to develop much deeper knowledge in this field to gain accuracy and reliability.

Influence of cooling surface area on indoor air and surface heat transfer characteristics of a thermally activated building system in warm and humid zones: An Experimental study

“…The findings revealed that the wall assembly, in which the thermally massive layer was exposed to both interior and exterior climatic conditions, outperformed the other cases investigated in terms of energy efficiency. Garay-Martinez et al (2023) investigated the process undertaken involves a thorough analysis to determine the indoor surface heat transfer coefficients. The results indicate that the actual surface heat transfer coefficients fall within the range specified in relevant works, although certain variations of up to 2 W/m 2 K were observed.…”