The theoretical background for the effects of moisture on heat transfer, which is given here, is valid for all types of materials, but the applications and detailed guidelines have been developed with a specific type of material in mind: masonry with medium or high thermal conductivity. This document is restricted to moisture contents in the hygroscopic range, where moisture content is in equilibrium with 95 % relative humidity or lower. For most building materials, moisture content in equilibrium with air having up to 95 % relative humidity is usually below the threshold value where the flow of liquid takes place. This value is often called critical satura