Thermal insulation and evaporative resistance of protective clothing are basic parameters that influence thermal stress and human performance. During training, the firefighter instructors wear an accessory (bolero) to discriminate them from the exercising personnel. Such boleros may be available at training sites in one size only. At one training site an instructor repeatedly reported pain sensation and first degree burns, while at another site problems did not occur, while similar amount of fuel in fires and comparable weather conditions were expected to lead to the same heat and radiation load. The other suspected reasons for pain and burns could be the aging effect on clothing properties, different materials in turnout gear compared to the materials in original procurement or the effect of the size of the added bolero. This limited study measured the thermal properties of the protective ensemble used by the firefighters today and focused on the case of the influence of clothing and bolero size on thermal insulation by the means of a thermal manikin. The tests complemented test series on firefighter ensembles commonly used in different incident scenarios. Additionally, a correct size and one size bigger ensemble were tested both with and without bolero. Use of bolero showed 3.6-6.7% insulation increase for size L, and no change or slight reduction for XL, indicating increasing thermal risks during exposure to heat and flames when protective layers get compressed by using wrong sizes.