The article concerns the widespread issue of thermal comfort; investigations into textiles and thermal insulation problems are presented. Materials that were tested include double-layer knitted fabrics with potential application in multi-layer garments addressed to a specific group of users. The investigated materials were constructed with the following raw materials: cotton, polypropylene, polyester, polyamide, bamboo, and viscose. The textiles with a comparable geometric structure and different composition were tested for their thermal insulation. In the experimental section the temperature gradients in specific constant ambient conditions using a thermal imaging camera were obtained. In the simulation section three-dimensional models of actual textiles were designed and the temperature gradients on the basis of performed simulations were calculated. Both measurements and simulations yielded comparable results and showed that the comparatively thick knitted fabrics’ thermal insulation strongly depends on the raw materials from which they were made and less on the parameters of the yarn.