While heat acts as an essential component of various hair care processes, the thermal properties of hair are little studied and these properties dictate how heat is spread and stored in hair. Here, the infrared microscopy enhanced Ångström’s method is introduced for accurate measurement of thermal diffusivity of hair. Three factors that could influence the thermal diffusivity were statistically tested in vacuum: (1) hair type, (2) the specific fiber from the same hair type, and (3) the specific locations along the same hair fiber (i.e., near the root vs. near the tip). The average thermal diffusivity of hair across the types measured in vacuum is 0.15 mm²/s, which is in good agreement with the published data. Hair type, fiber, and location had no statistically significant impact on the thermal diffusivity. Hair is particularly sensitive to moisture content and while absolute measurements of thermal diffusivity in air are not reported here, we show that increasing humidity level reduced the apparent thermal diffusivity of the samples. Understanding the variations in thermal properties with humidity is crucial to understanding how treatment processes impact hair health.