This study describes a quantitative method using thermography to measure the thermal properties of building fabrics that are subjected to non-steady state heat flow due to consistently changing meteorological conditions. The method includes two parts. First, the convection heat transfer coefficient is measured by thermography and heat flux meters on a small segment of the examined building fabric with uniform surface temperature. Then, thermal properties of large building fabrics are evaluated by thermography. The two parts are measured simultaneously. The method was tested on 140/160/190 mm thick massive laminated spruce timber walls of a test facility cabin located in Östersund, Sweden. The results varied by only a few percent in comparison to validation measurements performed with heat flux meters and in comparison, to values from the literature. Due to rapid changes in weather conditions the measured values had large disparity, but still a linear regression with low confidence interval was obtained. Obtaining an accurate value of convection heat transfer was important for achieving high measurement accuracy and, therefore, the value of this parameter should be measured. Other important factors to consider are solar radiation, reflected infrared (IR) radiation from nearby objects and the number of thermal images.