Transfer processes in animal coats. III. Water vapour diffusion

The diffusion of water vapour through samples of cured and uncured fleece and fibreglass wool was measured. The diffusion resistance of the fibreglass was close to the value expected for still air, i.e. 4.2 s cm-1 per centimetre for samples ranging in depth from 1 to 7 cm. The resistance for natural fleece was similar to the resistance for still air up to a depth of 4 cm but at 7 cm deep was only 2.5 s cm-1 per centimetre. The difference in behaviour of the three materials was interpreted in terms of liquid movement. By appeal to principles of similarity, an equation for sensible heat transfer by free convection from an isolated sample of fleece is used to estimate corresponding rates of latent heat transfer when the skin is wetted by sweat. When a sheep is exposed to air at a temperature close to deep body temperature the exchange of sensible heat between the skin and the air may be a trivial component of the heat balance but provided the skin is wet, the evaporative heat flux from the skin may reach 200-300 W m-2.