Details:SurfaceCoatings

Surface Coatings

Walls may have a surface 'coating' which does not affect their thermal behaviour but impedes water vapor diffusion, e.g. paint coats, wallpapers, vapor retarders, laminations, an impregnation which alters the pore space close to the surface etc.

Instead of including a separate, often extremely thin layer in the component assembly, you may allow for its effect on water vapor transport in these cases by only applying a separate diffusion resistance at the surface. In WUFI's SI mode this resistance is specified in terms of the thickness of a stagnant air layer with the same diffusion resistance, i.e. you enter the corresponding vapor diffusion thickness (sd-value).

Thus, you may simulate the effect of a vapor retarder by setting the v.d.t. to 10 m (e.g.) at the respective surface. But more massive components like gypsum board (sd ≈ 0.1 m) or renderings may also be replaced by their sd-value, as long as their effect on the hygrothermal behaviour of the entire building component is limited to their diffusion resistance.

If you nevertheless want to explicitly include membranes or other thin surface layers in the component assembly, please refer to the notes in reference: Membranes.

Because of the boundary air layer which exists at every wall surface, there is always some surface transfer resistance to the water vapor diffusion flow (cf. reference: Water Vapor Transfer Coefficients. The sd-value discussed here only describes an additional resistance that is caused by factors like those enumerated above. That is, WUFI adds the resistance resulting from the internally computed natural water vapor transfer coefficient and the sd-value due to some coating (if any) to arrive at the total water vapor transfer resistance used in the calculation. So for an ordinary naked wall surface, simply set sd = 0 m.
 

The definition of the vapor diffusion thickness (sd-value) is discussed in reference: Water Vapor Diffusion.

The vapor diffusion thicknesses of surface coatings (if any) are entered in the dialog "Surface Transfer Coefficients". There, WUFI offers a list of predefined coatings with their respective vapor diffusion thicknesses for selection; a user-defined value may be entered as well.

Please note that if you select, say, a metal foil from the list, only the sd-value at the component surface is set accordingly; other properties of the foil, such as its low radiation absorptivity or its impermeability to rain, must be specified separately by setting the appropriate surface properties.