Moisture Transport through Walls and Roofs
Moisture will flow through a building wall or roof if the vapor partial pressure is higher on one of the sides.
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. The vapor pressure depends on the relative humidity of the air.
The moisture flow can be expressed as
qm = (p1 - p2) / Σ Z (1)
where
q m = moisture flow (kg/s m2)
p1 = vapor partial pressure on one side of the wall or roof (Pa)
p2 = vapor partial pressure on the other side of the wall or roof (Pa)
Z = diffusion coefficient for the wall or roof (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 | |
- GPa = 109 Pa
