Thermal effects of air movement

An additional reason for controlling air movement in the building envelope is the effect of air flows on the energy efficiency of buildings. Many researchers in building and thermal insulation engineering have studied the mechanisms of convection heat transfer. 

Application Air leakage, mVm /h (75 Pa) Air permeance, mVmVs/Pa Diffusion thickness wind barrier, m 

Since wind barriers are located at the cold side of the thermal insulation (in cold climates) they should combine a sufficient air and water tightness with a high vapour permeance. Materials which combine these properties and are often recommended as wind barriers are the spunbonded plastic films (also called house-wraps). These films are composed of rolled synthetic filaments (typically polypropylene or polyethylene fibres) that are welded together to form a continuous porous fabric. Measuring procedures and results of vapour transfer properties of spunbonded plastic films are reported in detail by Janssens and Hens [36]. The diffusion thicknesses of the films are a few centimetres and of the same order of magnitude as an air boundary layer. 

---