Quantitative infrared thermography imaging of the density of heat flow rate through a building element surface

•Improved thermography measurement of the heat flow rate under natural convection.•Measurement bias was estimated against reference method to be 2.6Wm−2.•Measurement uncertainty was estimated as a function of density of heat flow rate.•Uncertainty budget allows for quantification of main error sources.•Above 6Wm−2 the method yields less than 10% measurement uncertainty.