What Kind of Moisture Barrier Lowers the Water Table Under a House?

Water Tables

• As groundwater extends down into the Earth at varying depths, the topmost surface of groundwater constitutes the water table level for an area. This level marks the dividing line between water-saturated soil and non-saturated soil. As the effects of rain and snow can add to existing groundwater amounts, water table levels can fluctuate depending on area weather conditions, soil textures and the type of terrain (in terms of flat versus sloped terrains). Moisture barriers help to protect a home or building’s foundation from absorbing groundwater moisture.

Moisture Migration

• Even in areas where water tables remain within a certain range, physical processes within the soil can cause water table levels to rise. One process — known as capillary action — involves natural suction forces that pull water up through small passageways within the soil. Another process — known as diffusion — results from the natural processes that take place when water evaporates into vapor. In effect, water vapor moves from areas of high pressure to low pressure, which causes vapors to rise up through the soil.

Vapor Barriers

• As water and vapor can migrate through soil levels, moisture barriers with vapor resistant properties can help reduce moisture migration through the soil. In effect, vapor barriers reduce the vapor pressure difference within the soil layers that lay between the water table and a building’s foundation. This process keeps water vapors from rising through soil layers. Vapor barriers can also affect capillary action by reducing the suction forces that pull water up through soil layers. These conditions help to lower water table levels that might otherwise rise well above their normal range.

Air Sealing

• Moisture-laden air poses an added threat to foundation surfaces as well as building and home interiors. In effect, the moisture content of the air can cause considerable damage to building surfaces as changes in temperature turn vapors to water and ice. Air leakage into foundation slabs and wall cavities can eventually make its way up into a home's interior walls. To prevent these conditions from developing, the use of continuous vapor barriers can create an air-sealing effect that prevents moisture-laden air from entering or leaking through foundation and wall surfaces.