The influence of the number of thermal layers on the clothing insulation of a cold-protective ensemble

The required thermal insulation in cold conditions and time limits for cold exposure can be predicted by the thermal index IREQ/DLE (Insulation REQuired), described in ISO/TR-11079. The thermal insulation of a clothing ensemble can be measured with a thermal manikin or estimated according to ISO 9920. The basic thermal insulation of single garments (Iclu) can be summated to estimate the basic insulation (Icl) of a whole clothing ensemble. Air that is trapped in the garments may escape when layers are added to a single layer. Then, the insulation would be reduced. Addition of thermal layers to an ensemble is therefore not likely to have a simple additive effect on the thermal insulation of a multi-layer ensemble. The purpose of the study was to investigate the resulting thermal insulation of clothing ensembles with different numbers of thermal underwear and the relative contribution of the underwear to the insulation of a cold-protection clothing ensemble. The basic insulation, Icl, of different combinations of knitted sweaters and pants manufactured from various qualities of "Ullfrotte" (wool and synthetic fibres mixed), a heavy insulated coverall (HIC) and a less insulated coverall were measured on a standing thermal manikin at low air velocity. Seven pieces of clothing, sweaters, pants and coveralls were combined in different numbers of layers (1-6 layers). The garments were measured together with double pairs of thick socks, gloves and a hood. The resulting Icl of layers of knitted underwear was not purely the sum of Icl of the layers measured and worn as single layers. For example, the sum of Icl for U2 sweater/U2 pants and U4 sweater /U4 pants was 0.29 clo (0.045 m2°C /W) higher than Icl of the combination of U2+U4 sweaters/U2+U4 pants, which corresponded to 22 % of the insulation of the single layers added together. A third layer worn under CHI on the upper body barely contributed to Icl, only by 0.05 clo compared with two layers. A corresponding relevant table value would be about 0.30 clo (0.046 m2°C /W). In conclusion, the basic insulation of knitted wear combined in layers were 22-31 % lower than Icl of the sum of the garments in single layer. The insulation values of multi-layered clothing ensembles would be overestimated by summation of the Iclu of garments according to ISO 9920. The overestimation leads to wrong recommendations of cold-protective clothing or working time limits, at least during standing, but probably also during physical activity. (Less)