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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.

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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. The vapor pressure depends on the relative humidity of the air.

The moisture flow can be expressed as

qm = (p1 - p2) / Σ Z                                                     (1)


qm = moisture flow (kg/sm2)

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:

MaterialDiffusion Coefficient - Z
GPa s m2/kgGPa 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
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  • Engineering ToolBox, (2003). Moisture Transport through Walls and Roofs. [online] Available at: [Accessed Day Mo. Year].

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