IR thermography is a common tool for monitoring heat losses of building facades. The surface temperature evaluation depends on several influencing parameters, especially the surface emissivity and the contribution of surrounding radiation. In this article, we will quantify the errors on surface temperature measurements due to these influencing parameters values. We will also illustrate the difficulty to obtain an accurate surface temperature on a low emissivity specular surface of a building and the need for an adequate modelling of the environment contribution.