Moisture Movement Mechanisms
Capillary Suction
Capillary suction moves moisture through porous materials. The water can be drawn upward through small pores in the concrete footing and slab and laterally through walls. This effect creates the ring of dampness seen at the base of many basement walls. This is very common at cold joints. Water can rise by capillary draw significantly as shown below:
| Soil type |
Capillary rise |
| Gravel |
Less than a few inches |
| Sand |
1 to 8 feet |
| Silt |
12 to 16 feet |
| Clay |
12 to 20 feet |
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Air Leakage Through Walls and Floor
In most houses, a stack effect is created because warm air rises. This induces a negative pressure on the basement and draws moist air in through any cracks or openings in the foundation including open sump pits. For this reason, sumps should have an airtight cover. With a concrete block foundation, moist air is drawn through the block cores, especially if they are left open at the top course. |
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Vapor Diffusion Through Foundation Walls and Floor
Vapor diffusion is the movement of moisture in the vapor state through a material. It is the function of the permeability of the material and the driving force of vapor pressure differential. In a basement, vapor can diffuse from the wetter ground through concrete walls and floors toward the dryer basement interior. Vapor retarders such as foundation waterproofing and polyethylene slow down this process. |
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