The below grade water control layer (often referred to as the moisture barrier) controls the infiltration of moisture into the building from the below grade exterior walls and floors.

The moisture barrier protects the wall cavity from moisture that escapes from the concrete walls or slab.

Moisture can move through concrete walls and slabs through the following three methods:

• Capillary action: Water travels through the pores/openings within the concrete (Concrete is hydrophilic; it loves water). The control layer working against capillary action has to be higher than grade as water can travel higher.
• Diffusion: Water vapour travels from a high moisture concentration to a low moisture concentration
• Gravity: Cracks in the concrete foundation can allow bulk water to seep through to the interior

A moisture barrier is a material, membrane or coating that is installed to prevent water moving by these three methods from reaching the interior wall structure. The moisture barrier is installed on the interior below grade portion of the foundation walls or floor slabs or where insulation or wood may come into contact with the concrete. It is necessary to install a moisture barrier within a house that is below ground level. The soil surrounding a home will almost always contain moisture, therefore, there will always be a requirement for a moisture barrier unless it can be proven that one is not necessary. A moisture barrier under the slab is also recommended to reduce moisture diffusion into the house.

Criteria for the below grade water barrier system:

• Installed on the interior (moisture barrier) or exterior (damp-proofing) of the foundation
• Resistant to moisture flow
• Not necessarily resistant to air flow
• Durable during construction and for the expected life of the building

Materials:

• Polyethylene
• Rigid foam insulation. (Closed cell or other rigid foam insulation that is unaffected by moisture)
• Paint or spray applied asphalt emulsions
• Sheet materials, such as some house wraps and building papers
• Specially designed/engineered foundation membranes or products

Placement/Location:

• Foundation walls and slabs
  

Moisture can easily penetrate a foundation wall or floor without proper installation of a moisture barrier. Issues such as; concrete deterioration, water damage, mould and mildew, and damage to partition wall studs can arise from the moisture that penetrates the walls and floor. Insulation's thermal effectiveness is greatly reduced when insulation becomes damp or wet.