Determining overall heat transfer coefficient (U-Value) of wood-framed wall assemblies in Canada using external infrared thermography
Quantitative thermography is considered as a reliable method to measure the thermal transmittance U-values of opaque building envelopes. Previously developed external infrared thermography (IRT) methodologies mainly focused on comparison of measured U-values with nominal U-values of wall assemblies in European construction. This study attempted to develop an external IRT method to determine clear wall U-values, where the impacts of repeating members (studs) were considered. The proposed method was compared with two established practices in Canada, namely the parallel path method and 3D thermal simulations. The IRT measurements were conducted on a conditioned at-scale insulated wood-framed wall structure. Besides the importance of environmental conditions on the thermal images, two thermal imaging artefacts were assessed and discussed, including nonlinear characteristics of infrared (IR) camera focal array, a.k.a. non-uniformity corrections (NUC) and vignetting.
The results demonstrated that the location of the region of interest (ROI) plays a key role in U-value measurement due to the vignetting effect and colder thermal bridges at corners. It was also found that NUC should be considered during the survey. Furthermore, U-value measurement with IRT in the best-case scenario (depending on the location of ROI) deviated from nominal U-values by 6.25%–25.00%. The clear wall U-value results with IRT were validated with three-dimensional (3D) finite element analysis software, Siemens NX, which differed by −11.53%–10.00% (in the best case scenario). Additionally, the clear wall U-values obtained with parallel path method were comparable with simulation values for walls without highly conductive materials such as metal.
Authors: Milad Mahmoodzadeh, Voytek Gretka, Katie Hay, Casey Steele, Phalguni Mukhopadhyaya
Date Published: 2021