The ability for water vapor to diffuse through material is expressed as the material’s permeance or permeability. Permeability factors have been established for a variety of common building materials through testing that measures how grains of water vapor diffuse through materials. Each grain of water vapor that passes through the material is called a “perm.” Materials with higher “perms” have greater permeability.

Click the link to the right titled “Permeance of Common Wall Assemblies” to view a brief list of common wall assemblies and their permeance.

Materials with a permeance rating of less than 1.0 are referred to as vapor retarding materials (or vapor barriers). Vapor retarding materials impede the drying process, because water trapped beneath them can cause secondary damage when not removed. Therefore, vapor retardant materials may contribute to lack of drying progress. If so, the materials need to be removed. Common vapor barriers found in structures include vinyl sheet goods, some paints and sealers, polyethylene vapor barriers, and foil backing found on some insulations and wall papers.

Adapted from the New Guide to Restorative Drying

Standing water on the inside of a basement or crawlspace exerts pressure on the walls and/or foundation of a structure. If the water table of the surrounding ground has risen along with the interior water, equal pressure pushes toward the inside of the structure—called hydrostatic pressure. Removing all of the water too quickly, while the pressure outside still exists, can result in structural failure or damage to walls. To prevent this, you need to know how to calculate the amount of water present and divide that by the flow rate of the submersible pump you’re using.

Click the link to the right titled "Calculating Standing Water" for five steps about how to determine the amount of standing water present on a loss.

Adapted from the New Guide to Restorative Drying