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Moisture Transport through Building Walls and Roofs
Moisture flows from one side of a wall or roof if the vapor partial pressure is higher on one side than the other
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Moisture flows from one side of a wall or roof if the vapor partial pressure is higher on one side of the wall or roof than the other. Vapor pressure depends on the relative humidity of the air.
The moisture flow can be expressed as
qm = (p1 - p2) / Σ Z (1)
where
qm = moisture flow (kg/sm2)
p1 = vapor partial pressure (Pa)
p2 = vapor partial pressure (Pa)
Z = diffusion coefficient (Pa s m2/kg)
Diffusion coefficients for some common building materials are indicated in the table below:
| Material | Diffusion Coefficient - Z | |
| GPa s m2/kg | GPa s m2/kg m | |
| Aluminum foil | 5000 | |
| Asphalt 1 coat | 150 | |
| Asphalt 2 coats | 2000 | |
| Asphalt paper, 0.5 kg/m2 | 750 | |
| Asphalt paper, 2.5 kg/m2 | 1700 | |
| Concrete | 150 | |
| Gypsum | 40 | |
| Glass | infinite | |
| Mortar | 80 | |
| Cork | 100 | |
| Wallpaper | 0.3 | |
| Air, still | 5 | |
| Metals | infinite | |
| Rock wool | 5 | |
| Polyethylene | 125 | |
| Polystyrene | 75 | |
| Bricks | 25 | |
| Wallboard, hard | 500 | |
| Wallboard, soft | 15 | |
| Paint for walls, matt | 2-5 | |
| Paint, alkyd | 20 - 40 | |
| Paint, oil | 10 | |
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