Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

# 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/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:

Moisture Transport through Walls and Roofs - Diffusion Coefficients
MaterialDiffusion 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

## Related Topics

• ### HVAC Systems

Design and sizing of Heating, Ventilation and Air Conditioning systems.

## Related Documents

• ### Gases Solved in Water - Diffusion Coefficients

Diffusion flux [kg/m2s] tells how fast a substanse solved in another substance flows due to concentration gradients. Diffusion constants [m2/s] for several gases in water.
• ### Roof Framing

Run, roof slope, gable height and gable area.
• ### Roofing Materials - Weights

Weight calculator for roofing materials.
• ### Walls - No. of Studs

Calculate required number of studs in a wall.

## Search

Search is the most efficient way to navigate the Engineering ToolBox.

## Engineering ToolBox - SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with older versions of the amazing SketchUp Make and the newer "up to date" SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp Make/Pro from the Extension Warehouse !

We don't collect information from our users. More about

We use a third-party to provide monetization technologies for our site. You can review their privacy and cookie policy here.

You can change your privacy settings by clicking the following button: .

## Citation

• The Engineering ToolBox (2003). Moisture Transport through Walls and Roofs. [online] Available at: https://www.engineeringtoolbox.com/moisture-transport-walls-d_163.html [Accessed Day Month Year].

Modify the access date according your visit.

7.8.15

.