The aim of this article is to address the influence of air humidity and testing temperature on the thermal conductivity coefficient (λ) of various thermal insulation materials. This group includes wood-based materials, rock wools, heat-insulating renders, climate boards, and lightweight cellular concretes. These materials are used both indoors and outdoors in buildings. Over the course of several years, data were collected from laboratory tests to determine the thermal conductivity coefficient (λ) in relation to increases in temperature and humidity. The obtained results were compared with values provided by the manufacturers of the insulation materials. The aforementioned research was carried out due to the rather high sorption of most materials and thus the possibility of them becoming humid at high air humidity. Because of the very large difference in the thermal conductivity coefficient of water and air, a relatively small increase in the mass moisture content of the materials results in a loss of insulation.