Illustration Source: CHBA Manual, Fig. 9.1, Pg.133

How Does It Work?

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:

• 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