Two new methods for the in-situ measurement of the overall thermal transmittance of cold frame lightweight steel-framed walls

•Measurement of U-value for lightweight steel frame (LSF) walls, Uoverall.•Two new methods for in-situ measurement of the Uoverall for cold frame walls.•Theoretical analysis of the methods on cold and hybrid LSF walls.•Experimental validation of methods on a cold frame LSF wall. The in-situ measurement of the overall thermal transmittance (Uoverall) of lightweight steel-framed (LSF) walls, including the effect of thermal bridges due to metal structure, is a challenge for designers, engineers and energy audits. The present study introduces two methods for the in-situ measurement of the overall thermal transmittance of cold frame lightweight steel-framed walls. The two methods, Representative Points Method (RPM) and Weighted Area Method (WAM), focus on the fast and accurate in-situ measurement of the Uoverall at the central part of an existing LSF wall. The methods combine the analysis of the examined wall using thermal images with the recording and processing of indoor/outdoor air temperature and heat flux. The requirements and the procedure for the employment of the methods on an existing wall are extensively described. The methods concern cold frame LSF walls, however numerical analysis shows that the methods can also be employed on hybrid frame LSF walls. The methods were experimentally validated on a well-known cold frame LSF wall. The results of the in-situ measurements were in a good agreement with the theoretical values, determined in accordance with the international standard ISO 10211 ensuring the validity of methods.