Opening the Snow/Wind Load dialog and clicking the Permeability tab displays the following dialog. 
The dialog defines the permeability of structure walls. Wall permeability has a direct effect on generating the coefficients of wind pressure inside a structure. When you select an option its diagram displays in the upper-right corner of the dialog. 
The dialog box is divided into 2 fields with certain option groups: 
- The Side wall permeability field has the following options: 
- Left side - Specifies the permeability of the left side wall. 
 
- Right side - Specifies the permeability of the right side wall. 
 
  
- The Front and rear wall permeability field has the following options: 
- Front - Specifies the permeability of the front wall. 
 
- Rear - Specifies the permeability of the rear wall. 
 
  
Note: Permeability is always expressed in %. For a completely open wall, the permeability value is 100%; whereas, for a completely closed wall it is 0%.