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Moisture will flow 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 will flow from one side of a wall or roof, if the vapor partial pressure is higher than the other side of the wall. The vapor pressure depends on the relative humidity in 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 can be found 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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