The partial vapour pressure of water vapour, which varies with temperature and relative humidity, encounters a resistance as it moves from high to low. The 1 m surface of all building materials resists vapour diffusion depending on its thickness. The ratio of this resistance to the vapour diffusion resistance of air is called the vapour diffusion resistance coefficient. In thermal insulation materials, although it varies according to the detail, it is generally ideal to have high vapour diffusion resistance.
Factors affecting this coefficient:
- Temperature that does not depend on the material
- Depending on the material
- Cell wall thickness,
- Cohesion in cell walls
- Closed cell
- Small cell size
- Homogeneity
