The physical properties of bark are virtually uninvestigated, and the resulting lack of knowledge has relegated bark to the role of residue. Significant among these properties are thermal characteristics, which are basic to the use of bark as thermal insulation. This paper presents the results of an investigation on three thermal properties of bark. Thermal conductivity of bark from several tree species was measured by the heated probe method. Based on these results, an adequate prediction of bark conductivity can be made from knowledge of its density, moisture content, and temperature. Bark is a somewhat better insulator than wood of the same density and also exhibits less anisotropy than does wood. Specific heat and elevation of specific heat due to moisture sorption were measured using a standard laboratory procedure. Bark and wood are shown to be nearly equal in their ability to absorb heat. Thermal diffusivity of bark was calculated from measured values of thermal conductivity, specific heat, and density. Over wide ranges of bark density, moisture content, and temperature little variation of thermal diffusivity occurs.