The aim of the present manuscript is to contribute to the understanding of material performance of machine insulation film at high temperatures and changing relative humidity (RH). This represents a common stress under normal operation and may lower the material insulation performance. PET film is used as the example material and the complex dielectric permittivity is used as a marker to investigate changes in the dielectric properties. Measurements are carried out with a high precision dielectric spectrometer in a range of 10 −2 ..10 5 Hz at different temperatures (10..125 °C) and RH levels (0..80 %). In addition, evaluation of absorbed water at different RH levels (0..80 %) is done gravimetrically and exhibits a diffusion character according to Fick's law. Moreover, the proportionality of maximum absorption and RH level becomes apparent. Variation of temperature shows altered \alpha and \beta loss mechanisms as well as changed DC conductivity proportions. The continuous permittivity increase during water absorption indicates the higher (moisture-induced) polar content in the polymer. However, a small portion of water seems to be bound to terminal -OH groups of PET, thus leading to an altered \beta relaxation. The observed increase of DC conductivity, not only with higher temperature but also with higher water content, points out the humidity-driven charge transport through the dielectric. A further, frequency-dependent loss mechanism is only visible above a certain RH level that indicates pure water formation within the polymer. This study shows that even though the level of absorbed water for PET is below 0.4 %, the dielectric spectra reveal significant changes.